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Project Specific Discussions => Dynamic Processors => Topic started by: Falk on June 22, 2020, 01:57:43 PM

Title: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on June 22, 2020, 01:57:43 PM
Good evening gentlemen,
I am planning a diy take on this one and gathered many information. Still missing are the information on the transformers. Especially ratios and impedances are of my interest. Also some information on the three inductors used in the tube stages and psu would be great.

I am located in Berlin, Germany. If anyone around has a unit at hand and would allow for measurements I d be extremely thankful.

Kind regards,
Falk // blankroomaudio.com

Title: Re: Rohde und Schwarz U23
Post by: Winston O'Boogie on June 22, 2020, 06:48:19 PM
Good evening Gentleman,
I am planning a diy take on this one and gathered many information.   

Hi Falk,
Ooh, ooh, that's a nice unit.   I don't have any useful info at hand, just wanted to give a thumbs up  :)

Do you have a basic schema you're working from that you could post?
Transformer ratios and inductor values could probably be approximated as place holders for now at least.
Title: Re: Rohde und Schwarz U23
Post by: Rob Flinn on June 22, 2020, 06:58:21 PM
Hi Falk,
Ooh, ooh, that's a nice unit.   I don't have any useful info at hand, just wanted to give a thumbs up  :)

Do you have a basic schema you're working from that you could post?
Transformer ratios and inductor values could probably be approximated as place holders for now at least.


See attached
Title: Re: Rohde und Schwarz U23
Post by: Winston O'Boogie on June 22, 2020, 07:50:13 PM
Cool, thanks Rob.
Shooting entirely from the hip, given I don't know any of the passive component values or what valves are in this:
I would guess the interstage and  input transformers are 1:1
Output transformer  4:1 or 8:1, side-chain output 4;1
Anode load choke won't need to be as big as say a V72 choke (700+ H) since the pentode here is strapped for triode so, 200H?
With the values of the resistors in hand, a  better approximation could probably be made based on the throughput gain

Isn't there a dutch company that was building a new/improved version of this at one point?  Might be worth checking to see.

 Edit - revised opinion on ratios and also found the company making new units here:
http://vacuvox.com/vacuvox-u23-tube-compressor-limiter/

 
Title: Re: Rohde und Schwarz U23
Post by: Rob Flinn on June 23, 2020, 03:49:22 AM
Isn't there a dutch company that was building a new/improved version of this at one point?  Might be worth checking to see.

 Edit - revised opinion on ratios and also found the company making new units here:
http://vacuvox.com/vacuvox-u23-tube-compressor-limiter/

Maybe it doesn't sound so much like a U23 ?

"The circuit was also modified resulting in a more stable operation over a much broader range. The lowest possible distortion was not a design goal during development and reducing the gain results in some higher second and third harmonics, which give the effect of making the sound appear louder and more euphoric."
Title: Re: Rohde und Schwarz U23
Post by: gyraf on June 23, 2020, 03:52:37 AM
Yes.

their insistent  emphasis on marketing and fluffy words scare me too
Title: Re: Rohde und Schwarz U23
Post by: gyraf on June 23, 2020, 03:56:08 AM
back to point, here's some of what I have on the topic..

this is an excerpt from the legendary Braunbuch, whole article is over our upload limit

/Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Winston O'Boogie on June 23, 2020, 03:57:36 AM
I admit I didn't read the blurb fellas.  I was mainly thinking that Falk might manage to extract info about transformers from them.

Is there a scanned copy of the original schematic anywhere?  I looked on the Kubarth site but that particular schem  link is broken.  Also can't seem to find the docs that were once available on the IRT site.

Edit: I see Jakob posted while I was typing.  Thanks  :)

OK, so I see there's a fairly stiff pad on the front end and the values point towards the input transformer primary being for a low impedance source so, maybe that transformer isn't a 1:1 but more along the lines of a mic to push-pull grids ratio.
Title: Re: Rohde und Schwarz U23
Post by: Falk on June 24, 2020, 02:34:15 AM
Good Morning,

thank you everybody for the valuable information. I am working on a spice model at the moment. At this point I only achieved a model of the three EF86 / EF804s stage and the PSU. With the values of the original schematic this stage has a gain of 1 with a 10k : 600r OT. Using 200H inductors for the plate choke and anode works well. Even though a 100Hz sine already looks slightly distorted. 1kHz looks clean.  I will work out this model as far as possible.

With respect, I assume the Vacuvox U23 design mainly allows for settings on the front panel that were originally on the inside and not needed at hand in broadcast use.

My plan is to use Edcore TXs and Hammond chokes. I will build a first version with turret boards and point to point wiring. As soon as I get it running I will layout a pcb.

Would you rather skip the input pad and use a 1:1 TX on the input? Could you explain the advantage of having the four secondaries of the interstage TX wired in series rather than having a single winding?

Best regards,
Falk

Edit: Got the OP stage fixed in spice now. Works like a charm. Plotted is input vs output at 100Hz. Frequency response is okay but could need fine tuning at some point.

Title: Re: Rohde und Schwarz U23
Post by: Winston O'Boogie on June 24, 2020, 03:45:01 AM
 
I am working on a spice model at the moment. At this point I only achieved a model of the three EF86 / EF804s stage and the PSU. With the values of the original schematic this stage has a gain of 1 with a 10k : 600r OT.

OK nice.  So 4:1 works from a gain standpoint and, with the follower beforehand, probably no need for anything like an 8:1.  Most of the designed for NWDR units I've played with seem to have really quite low output impedance, in the region of 30R if that helps.
 
Using 200H inductors for the plate choke and anode works well. Even though a 100Hz sine already looks slightly distorted. 1kHz looks clean.  I will work out this model as far as possible.

I'm not sure what the impedance of an EF86 strapped for triode is off hand, but 200H seems instinctively big enough to not cause too many problems at the low end but?  I tend not to trust distortion results from a sim myself and it's very possible you may not need to worry about it in real life. 
 
With respect, I assume the Vacuvox U23 design mainly allows for settings on the front panel that were originally on the inside and not needed at hand in broadcast use.

Ah OK, good to know, makes sense.


My plan is to use Edcore TXs and Hammond chokes. I will build a first version with turret boards and point to point wiring. As soon as I get it running I will layout a pcb.


Sounds good.  I don't know where you're located but, if in Europe, there might be some alternatives from Carnhill worth a shot.  I noticed recently there was an inductor in the £20 region that might be a good candidate for anode loading and folks have used their 10K:600 with good results. 
 
Would you rather skip the input pad and use a 1:1 TX on the input? Could you explain the advantage of having the four secondaries of the interstage TX wired in series rather than having a single winding?
 


It might be that Rohde & Schwarz were just using the best transformers available to them then...  Could be the mic to p-p grids had  better balance than a line to grids?   I would bet that interstage they used too just required that each secondary be terminated for good response rather than it being a purposeful design decision.   

There's usually a design evolution you can see with these NWDR units and a look at the U73 schematic which came later might give you some clues.   Speaking of which, I think someone on here had the 1:1 transformers  from that unit analysed and copied.  I remember them being quite reasonable in price for what they were, and pretty much spot on to the original coils.

Your spice looks good :)
Title: Re: Rohde und Schwarz U23
Post by: Falk on June 24, 2020, 10:25:07 AM
Good evening everyone,

Thanks for your feedback on my ideas. I simulated the frontend of the audio path today and substituted the EAF42 with EF85 / EF89 radio tubes which works fine and maybe I just stick with them.

I will simulate the side chain during the next day or next week and include the features I am wishing for. The attack time is defined by the resistance of the rectifier tube. Any suggestion how to make the attack time variable?

@Winston: I will look at the Carnhill TXs but for some reason I really like the Edcors. Your totally right. The original unit has an output impedance of 30r. You think I should go down that far?

Best regards,
Falk

Ps. Attached is the the i/o plot @100Hz and the spice.


Title: Re: Rohde und Schwarz U23
Post by: Winston O'Boogie on June 24, 2020, 02:07:58 PM
 Hey Falk,

I will simulate the side chain during the next day or next week and include the features I am wishing for. The attack time is defined by the resistance of the rectifier tube. Any suggestion how to make the attack time variable?

Switching in different value capacitors on some additional 'Begrenzung' switch  positions would alter the attack time.  The release time resistor in parallel with the cap could be a separate control   
I wasn't sure what 'Ableich' meant so looked it up on Google translate as 'comparison'.   I'm assuming  this means test in this context yes? 


@Winston: I will look at the Carnhill TXs but for some reason I really like the Edcors. Your totally right. The original unit has an output impedance of 30r. You think I should go down that far?


 

OK yep, Edcors have been used with success on tons of projects.   
30R output z is spot on with other units I've seen so, no surprise there then :D
I don't think it's necessary to worry about it too much. The lowest load that these units saw when in use within old broadcast installations was about half  of the lowest load you're ever going to put on the output, they were designed to drive 300 ohm circuits without loading losses and you'll, maybe, put 600 ohms on it worst case.

Not got my head around all the switch positions yet, there's a fair bit of stuff happening there for sure!

Title: Re: Rohde und Schwarz U23
Post by: Falk on June 25, 2020, 02:45:19 AM
@Winston

Switching in different value capacitors on some additional 'Begrenzung' switch  positions would alter the attack time.  The release time resistor in parallel with the cap could be a separate control   
I wasn't sure what 'Ableich' meant so looked it up on Google translate as 'comparison'.   I'm assuming  this means test in this context yes?

Thank you. Sounding great with having a switch for a Cap (Attack) and another for R (Release) in parallel.  Abgleich bypasses the side chain and connects a 12V sine to the CT of the Input TX secondary winding. Allowing for tube matching on the meter which monitors the output in this position.

OK yep, Edcors have been used with success on tons of projects.   
30R output z is spot on with other units I've seen so, no surprise there then :D
I don't think it's necessary to worry about it too much. The lowest load that these units saw when in use within old broadcast installations was about half  of the lowest load you're ever going to put on the output, they were designed to drive 300 ohm circuits without loading losses and you'll, maybe, put 600 ohms on it worst case.

Great, so I will stick with this output Z for the first try.

Btw. really appreciate your well organized style of replying and tried to do so as well.
Best, Falk
Title: Re: Rohde und Schwarz U23
Post by: Winston O'Boogie on June 25, 2020, 01:46:41 PM

 Abgleich bypasses the side chain and connects a 12V sine to the CT of the Input TX secondary winding. Allowing for tube matching on the meter which monitors the output in this position.


Thanks, figured it was that but hadn't traced through the various bits.
 

Great, so I will stick with this output Z for the first try. 
Btw. really appreciate your well organized style of replying and tried to do so as well.


Haha, cool thanks.  I'm usually not so organised so, decided it'd be better this way to cut down on clutter.

Does the spice you posted have the correct ratios attached to the input and interstage transformers or are the labels just wrong? 

 
Title: Re: Rohde und Schwarz U23
Post by: Falk on June 28, 2020, 03:56:04 AM
Good Morning,

after a day of optimizing and modifying everything works and all features are implemented such as: Threshold, Attack, Release, Ratio, Gain Makeup. It was quite a bit of pain to get the frequency response right and I am not too experienced with designing circuits but I figured the contributing elements and learned a lot during the past day.  Thank you again.

In the ende the Transformers will be:
Input: 10k:2k4
Interstage: 10k:10k
Output:10k:600r
Sidechain: 10k: 10k

What time constants would you implement? I want to have 5 positions switches. Also for the side chain hipass. What frequencies would you choose?

I attach the frequency response this time. For now I will start to layout a pcb.
Best regards,
Falk

Title: Re: Rohde und Schwarz U23
Post by: echoplex on June 30, 2020, 07:31:10 AM
Amazing project :) Following
Title: Re: Rohde und Schwarz U23
Post by: gyraf on June 30, 2020, 08:23:58 AM
..If it was me, I wouldn't try to decide on the A/R timings before actually having the unit functional - then trying out what was feeling right in real-life situations. then last extrapolating these figures slightly to allow for "extreme-case-use"..

/Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Winston O'Boogie on June 30, 2020, 09:18:54 AM
 +1  on the A/R settings
Title: Re: Rohde und Schwarz U23
Post by: mjrippe on June 30, 2020, 01:29:58 PM
Get a couple of capacitor and resistor decade boxes.  They allow you to change values on the fly without desoldering.  Hook them up to your timing circuit and tweak til you find combinations you like.
Title: Re: Rohde und Schwarz U23
Post by: Falk on July 05, 2020, 12:55:03 AM
Good morning Gentleman,

thank you for your advice. Its highly appreciated. I will get myself those decade boxes and hook them up to the circuit. At the moment I am designing a PCB which I thought was a fun exercise but turns into a time killing monstrous project.

Its a complete new perspective I am gaining by doing all the stuff from scratch (thank God I could copy paste the basic circuit). There is a lot of knowledge needed be gained around the microcosmos of DIY audio that is not to been touched when building kits.

I got somewhat stuck with the trace width and clearance for the plate supply lines (300V 10mA). I decided for 30mills width and 100mills clearance now. I hope this will do the trick.

Have a nice Sunday. Thank you for your advice and thoughts on this build.
Falk
Title: Re: Rohde und Schwarz U23
Post by: Falk on July 16, 2020, 07:58:13 AM
Hello,

just got notified that the pcbs were produced today. So next week I will work on it. Totally curious if it will work somehow. I am still afraid I messed it up. Meanwhile I made a sketch of the front panel. Which color do you prefer? Any comments regarding the design?

Thank you!
Falk

Title: Re: Rohde und Schwarz U23
Post by: gyraf on July 16, 2020, 08:38:09 AM
Black.

But are you sure about those meters - that type are traditionally VERY averaging (slow-reacting), and usually pretty impossible to read for GR.. Yes, you can get used to it, but as an indicator of what is going on, you can do MUCH better..

(yes, I'm aware that a well-known company uses these,  ::) )

/Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Falk on July 16, 2020, 09:51:06 AM
Hi Jakob,

thank you.  To be honest, I didnt think much about the integration time of the meters and I totally get your point. I will think about it now. Thank you for favoring black. Its my first choice too. These Meters are from a German company called Weigel. You think Thermionic uses them too?

Regards,
Falk

Title: Re: Rohde und Schwarz U23
Post by: Winston O'Boogie on July 16, 2020, 02:03:52 PM
I vote black too. 
Title: Re: Rohde und Schwarz U23
Post by: gyraf on July 17, 2020, 06:01:04 AM
(emailed you)
Title: Re: Rohde und Schwarz U23
Post by: Falk on July 25, 2020, 06:35:42 AM
Hi Jakob, Hi Winston, thanks for your vote. Jakob, thank you for your valuable email. The circuit is up and running. The heaters clearly introduce hum but I just wired them to work and have to redo that in a proper manner. The circuit works at unity gain but I want to have it with 6dBs of gain so I will swap the interstage for another ratio. Also the circuit is motor-boating when compressing a lot. Frequency response needs to be optimized too. Still a lot to do but it already sounds beautifully thick and vintage by the best means. I ll be on vacation now and continue in the late summer days. Regards, Falk
Title: Re: Rohde und Schwarz U23
Post by: echoplex on August 05, 2020, 06:47:16 PM
Hi Falk,

any news on this amazing project?
I would also choose black as frontpanel color.
Very curious if you got the PCB and have a running prototype?

thanks
Title: Re: Rohde und Schwarz U23
Post by: Falk on August 08, 2020, 03:29:16 AM
Hi Echoplex,

I´ve been on vacation. The prototype is running very basically. But compresses as supposed. I need to find a cure for a couple of issues.

These are:
- hum from heaters
- motorboating when threshold is very low
- frequency response

I did a lot of modifications on the tube section. I use:
EF83 instead of EAF42
EF806s instead of EF804s
6SN7 instead of ECC41
6AL5 instead of EB41

I use Edcore TXs and Hammond chokes only. I integrated Threshold, Release, Attack, Ratio and Side Chain HPF. 

I am pretty sure I get this machine up to work during the next month. I will keep you gentleman posted here. I appreciate your interest. What I can say by now is that is really sounds beautiful.

I attach a photo of the prototype pcbs.
Regards,
Falk



Title: Re: Rohde und Schwarz U23
Post by: opacheco on August 09, 2020, 05:29:17 PM
......... whole article is over our upload limit

/Jakob E.

Dear Jacob, could you share a copy of this article please?....you could upload to www.wetrnasfer.com and send us the provide link here please??

Thanks anticipated
opacheco
Title: Re: Rohde und Schwarz U23
Post by: opacheco on August 09, 2020, 05:35:41 PM
........Also can't seem to find the docs that were once available on the IRT site.....

Look like they delete all the German Gray Units docs from its website!!; Do someone have a link to this IRT docs??....it is a shame!

Thanks
opacheco 
Title: Re: Rohde und Schwarz U23
Post by: gyraf on August 10, 2020, 10:54:49 AM
Your email?
Title: Re: Rohde und Schwarz U23
Post by: echoplex on August 10, 2020, 04:17:03 PM
the board looks already beautiful.. I sure you can get it work.
Sound demos would be amazing..
and I hope if the PCBs are flawless you will make them available for us DIY nerds :)
Title: Re: Rohde und Schwarz U23
Post by: opacheco on August 11, 2020, 11:54:44 PM
Your email?

gyraf sorry;

[email protected]

Thanks anticipated,
opacheco
Title: Re: Rohde und Schwarz U23
Post by: gyraf on August 12, 2020, 04:25:30 AM
mailed
Title: Re: Rohde und Schwarz U23
Post by: opacheco on August 12, 2020, 02:12:59 PM
mailed

Thanks a lot Jakob!

opacheco
Title: Re: Rohde und Schwarz U23
Post by: echoplex on August 14, 2020, 05:11:50 AM
Hi Falk,

are you planning or willing to sell some of your PCBs to us folks as well?
Once you have a working version
Title: Re: Rohde und Schwarz U23
Post by: Falk on August 16, 2020, 11:43:14 AM
Good evening,

today I found time again to work on the unit. Pretty much solved all problems. It works as it should. Frequency response is attached. Noise Level is at -75dB (FS) at unity gain. Mainly 50Hz hum.

Whats a good noise level for tube studio gear?
I still do not have a case for the unit. It just sits on my work bench.

@echoplex - I am not sure how I will move on after having two perfectly running units.

Before selling pcbs I would need to design a revision of the boards. They arent good to be used by anyone else. The have some minor flaws.

It can be that I will run a single batch of pcbs. Maybe not. I didnt think of it and I also wont before everything has fallen in place. I will keep everybody posted here.

Thanks for your interest and support.
Best Regards,
Falk

Title: Re: Rohde und Schwarz U23
Post by: JMan on August 16, 2020, 01:27:55 PM
I think you have plenty of interest here if you offer pcbs.  I know you haven’t decided yet, and that you’d have to do some revision work, but if you do, I will add my name to the list of people interested.
Title: Re: Rohde und Schwarz U23
Post by: gyraf on August 17, 2020, 03:08:38 AM
Start out by getting rid of the hum, you can do that.

Identify where it comes from - is it radiated into transformers, is it psu regulation, is it from AC-heating tubes, is it ground loops

Then evaluate NF again - I don't think it'll be all that bad (at least with the original tube set)..

/Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Falk on August 17, 2020, 05:17:17 AM
Good Morning,

@Jakob, I will do so. What SNR would you aim for? 90dB?

BTW: The original manual stated the noise level as 0.4% of the nominal level. That would equal a SNR of 68dB.

I suspect my PCB layout to be one of the issues. It has two GND layers. One on the top and one on the bottom layer. Another flaw is that it has one plane for all grounds. The revised PCB will have single planes for signal gnd and psu gnd. (+ chassis gnd). Furthermore PSU and signal PCBs will be separated.
To continue with my worries - Vb fluctuates about 0.5V  over several seconds. I have a plan for debugging but dont really know when to move on.

@JMan
Thank you for your interest.

I accidentaly found out that the later version of the U23 which was called ABR uses the EF83 too. Thats a coincidence I cant believe. I chose them because they have a noval socket and very low distortion when compressing.

Best regards,
Falk
Title: Re: Rohde und Schwarz U23
Post by: gyraf on August 17, 2020, 07:29:16 AM
Falk,

I would aim for first getting rid of all traces of hum, that part is possible.

From there it would be a matter of IRL evaluation what noise distance is really needed. Depends on quite a few factors, including how much of the noise gets modulated by GR, what it's sweet spot is, etc.. I'm not holding out on you, it's just something that can't be quantified in absentia.


Oscar,

Unfortunately no info at all about the transformers or their parts.. And your inbox is full..

Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Falk on August 17, 2020, 11:30:07 AM
Hello,


@Jakob
thank you so much for giving me this motivating critical view.

I just turned my two power transformers (one for the 4A  of initial filament current and one for the 250V and 12V) 90° and the hum is almost gone. I could also try to isolate the Input TX and Interstage TX + cables since they are sensitive when I touch them.

The good news is: Noise is at -88dB FS now.  Thats almost fine for my mastering use. I am not doing any music that would need a dynamic range like this.

I attach a photo of the noise spectrum.
The 50 Hz is from mains TX the 160Hz comes from the heater TX. 

Apparently my two ground planes aren't a problem. How do you gentleman layout pcbs? Would you recommend to have one ground plane on the bottom or do you fill everything apart from the traces with a ground plane?

Best regards,
Falk
Title: Re: Rohde und Schwarz U23
Post by: Michael Tibes on August 17, 2020, 02:44:54 PM
I've been following this thread loosely. In case you haven't seen it yet: I did some more digging now and came across this company http://vacuvox.com/ (http://vacuvox.com/). They build replicas with some enhancements. They linked a review of their older unit here http://vacuvox.com/wp-content/uploads/2018/10/Vacuvox-U23M-Promo.pdf (http://vacuvox.com/wp-content/uploads/2018/10/Vacuvox-U23M-Promo.pdf) and the measured noise spectrum there looks very much like yours...

Nevertheless I believe it should be possible to get rid of the hum.

Michael (also from Berlin  ;))
Title: Re: Rohde und Schwarz U23
Post by: opacheco on August 17, 2020, 05:27:44 PM
Oscar,

Unfortunately no info at all about the transformers or their parts.. And your inbox is full..

Jakob E.

Jacob,
That's a really shame!....It will be a great help have some measurements for real transformers used in the U23 from a real unit!! but this is more difficult than find a original unit  in good shape!

I will check my inbox thanks!(you have my email address if will be necesary at any time!)

Thanks a apprecite your help and time so much!
opacheco
Title: Re: Rohde und Schwarz U23
Post by: EmRR on August 17, 2020, 05:59:16 PM
Two thoughts.

The IT with separate resistive loading of each coil is interesting.  I've only seen that in European designs, never an American one I can think of. 

An input pad like that is common when the unit is used in a bridging scenario with a an unknown number of other parallel amps.   Besides establishing bridging Z, it also gives isolation.  Allows use of a more common step up transformer, and can be removed if bridging isn't needed, as opposed to amps with dedicated bridging transformers that only work in that service. 
Title: Re: Rohde und Schwarz U23
Post by: opacheco on August 28, 2020, 10:08:12 PM
Two thoughts.

The IT with separate resistive loading of each coil is interesting.  I've only seen that in European designs, never an American one I can think of. 

An input pad like that is common when the unit is used in a bridging scenario with a an unknown number of other parallel amps.   Besides establishing bridging Z, it also gives isolation.  Allows use of a more common step up transformer, and can be removed if bridging isn't needed, as opposed to amps with dedicated bridging transformers that only work in that service.

EmRR could you talk us about the Bridging Input Section type used in this unit please? What it does exactly? How this works?

Thanks in advance!
opacheco.
Title: Re: Rohde und Schwarz U23
Post by: Falk on September 02, 2020, 05:38:08 AM
Hello everybody.

@Michael Tibes, thanks for the Link it was a great motivation to work on the SNR performance of the Unit.
Attached is a screen shot of the noise spectrum. The dB scale equals a dB(V) reading. So my unit is a good bit quiter in the low end than the unit from the comparison above. But its also noisier in the highs freq range.

The noisefloor of the unit in my mastering setup is -88dB(FS) RMS now.  Thats completely fine for me. I will shield some more cables and I am sure it will improve the performance.

Also I ordered a custom power transformers. This will be another advancement too.

Title: Re: Rohde und Schwarz U23
Post by: Falk on September 02, 2020, 01:13:57 PM
The machine is almost working as I wish for it to be. I corrected the frequency response today. Before and after in the plot attached
Title: Re: Rohde und Schwarz U23
Post by: echoplex on September 03, 2020, 05:54:45 AM
Hey Falk,

I'm impressed with what you have reached in the meantime.
-88dB(FS) RMS is pretty amazing and should suffice for even mastering applications.
And then the frequency graph you posted.. it's pretty amazing as well.

I rarely feel so hyped about a project on GDIY as I do with this one.
Please keep us updated and please remain so motivated.
This is really the most interesting project 2020 and I would love to see it proceeding.
Also eager to hear some samples and to see the PCBs :)

cheers
Title: Re: Rohde und Schwarz U23
Post by: gyraf on September 23, 2020, 12:33:48 PM
bumpin' for support and interest.. you can't leave us hanging like this :-)
Title: Re: Rohde und Schwarz U23
Post by: Falk on September 23, 2020, 12:43:43 PM
Good evening gentleman,

I havent had time during the past weeks to go on with the project. I am also waiting for parts to arrive.
I am not satisfied with the SNR yet, so I redesigned the whole pcb and psu.

I keep you posted as soon as there are news.

@echoplex and @gyraf - Thanks for your support and motivation.

Falk
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 01, 2020, 09:29:51 AM
Hello Gentleman,

the new pcbs were dispatched today. The custom power tx will be dispatched tomorrow, mouser has already delivered the parts. Next week I will assembly the revision 1 board and see how things go.

What I did in revision 1 is to divide the PSU from the signal boards. Physically and regarding ground connections. That has two reasons:

1. I had the impression that the noise floor [-88dB(FS)] arises from having the same ground plane for psu and signal.
2. If the noise floor is still bad, I can put the PSU in a single case.

I also had some flaws on the first pcbs:
1. a wrong routing in the side chain
2. a ground plane on the top of the PCB and another on the bottom layer. That caused to have literally tons of ground loops.
3. also I decided to have EF83 as compression tube. The first boards could wether have EF83(audio remote cutoff) or EF89(radio remote cutoff). Both were great and sonically did not make too much of a difference.

I also played a lot with my spice model and got it fixed to be totally exact with the specs from the Rhode and Schwarz data sheet from 1954. This was a very tiring but also valuable process of deeply understanding the interaction between the positive diode bias in the side chain and the threshold control. This outcome will be the starting point for the calibration process. It allows for having the unit at a maximum ratio of a bit more than 4:1.

This ratio is what the original data sheet states. It says: The unit has a gain of -1dB (when input is +6dB; output +5dB) When signal rises for 10dB (input +16dB) the output signal is 7.5dB. Since the overall gain is -1dB the output actually rises 2.5dB what equals a ratio of 4:1.

I will integrate the following ratios: 1.25: 1; 1.5:1; 2:1; 3:1; 4:1

Any comments are highly appreciated.
Best regards,
Falk





Title: Re: Rohde und Schwarz U23
Post by: gyraf on October 02, 2020, 03:26:09 AM
Very good job getting the simulation under control - however, don't be alarmed when this does not reflect upon reality (!). The important thing is that it shows you the expected interactions in the circuit.

Getting the ratios perfect may or may not be important for the subjective qualities of this unit, don't sweat it until you have a working unit at hand AND have a chance to evaluate in company with known-good ears. Do you have any mastering studios at hand where you are*? These sorts of people are often very good at determining the real impact of details like that - and it's my impression that a vast majority of them are genuinely interested in first-hand experiences of what is possible in electronic design. This is also the situation where you will want to explore the ranges of attack- and release timings. You can usually instigate genuine interest in perfecting your particular project by letting them have the prototype or an early production model at cost..

For tube selection - what are the reasons to stray away from the original EAF42's? I thought them to still be plentiful enough in the wild?

-88dB noise floor is not at all bad UNLESS it is hum products. A real-life evaluation will again tell you more than a thousand measurements.

Physically separating the PSU is a good idea, but please don't put it in an external box. Connecting high DC voltages is hard. If needed, perhaps go with bolting the "external" PSU enclosure to the back of the unit so cabling can be permanent.

/Jakob E.

*if not, perhaps I can set you up with someone, depending on where you are?
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 02, 2020, 04:24:41 AM
Hello Jacob,

thank you for another valuable bunch of hints on how to move on.

Getting the ratios perfect may or may not be important for the subjective qualities of this unit, don't sweat it until you have a working unit at hand AND have a chance to evaluate in company with known-good ears. Do you have any mastering studios at hand where you are*? These sorts of people are often very good at determining the real impact of details like that - and it's my impression that a vast majority of them are genuinely interested in first-hand experiences of what is possible in electronic design. This is also the situation where you will want to explore the ranges of attack- and release timings. You can usually instigate genuine interest in perfecting your particular project by letting them have the prototype or an early production model at cost..
I am located in the middle of Berlin and as soon as I have two running units I will hit the road to ring the bell at some downtown mastering studios to find company to get these machines on spot.

For tube selection - what are the reasons to stray away from the original EAF42's? I thought them to still be plentiful enough in the wild?
On the latter revisions of the U23 (actually the U23 Type ABR) Rhode und Schwarz themselves used the EF83 instead of the EAF42. Since I never planned to build a perfect clone of a historical piece of gear I simply went with the EF83 because of the much more easily available socket and tubes.

-88dB noise floor is not at all bad UNLESS it is hum products. A real-life evaluation will again tell you more than a thousand measurements.
Indeed its hum products and I think in the very best case the noise floor can go down to about -100dB(FS)
Thank you for the idea with attaching the PSU to the back. I will see if that is needed. The PSU is pretty big because of the regulator tube (108c1) and a big inductance of 7H.

*if not, perhaps I can set you up with someone, depending on where you are?
I appreciate this offer and would get back to this once the devices are up and running.

I will post more about the progress next week.
Best regards,
Falk


Title: Re: Rohde und Schwarz U23
Post by: jans on October 05, 2020, 05:05:47 PM
Hi Falk,
I am also very much interested in your project. Hope you succeed!
Greetings from Amsterdam,
Jan
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 08, 2020, 05:24:46 AM
Good Morning,

@Jans thank you for your interest. I will keep everybody posted here.

What I achieved until now is: I skipped the input pad so the machine has a total gain of 20dB and highest possible SNR in my configuration. Maybe it would be possible to further step up internal levels but I sounds pretty damn cool like it is. The highest threshold is at +18dB(U).

I am still waiting for the power toroidal to arrive and am hoping that the nasty hum at 50Hz is going to get lower. Redesigning the PCB made the build less sensitive to noise and lowered the overall noise a bit but not tremendously. By redesigning the pcb I gained loads of space in the case which is super helpful for finding  alternate positions for all the iron.

I am figuring out some cool ratio settings with a pot to implement discrete values later. I like the attack and release time as is. Whats impessive about this unit is - It nails the rms level on spot but leaves most of the transients pass. Its truely a leveling amplifier.

I attach a picture from the bench.
Thank you everybody.
Falk

Title: Re: Rohde und Schwarz U23
Post by: Erik2345 on October 08, 2020, 07:58:24 AM
Nice work!
Title: Re: Rohde und Schwarz U23
Post by: gyraf on October 08, 2020, 11:10:57 AM
Really good work there.

Have you tried re-orienting your audio transformers, as to get them to pick up less of your PSU hum? turning in one of the three possible axis's could perhaps get you further away from the hum?

/Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 08, 2020, 03:26:09 PM
Thank you Jakob, thank you Erik.

Unfortunately the new toroidal are noisy like crazy BUT! i had that mu-metal sheet flying around and wrapped it around the power tx - it was the cure. Noise spectrum at unity gain in my mastering chain is attached. While I am writing this I am listening music through the unit and it sounds ace.

I will make a break until next week and record some drums at the studio. Next week I will make a plan how to continue. Now that everything works, I need to build a second unit to see if I can reproduce whats been achieved.

Once the second unit works. I will finalize the front panel design. Still a good mile to go. Thanks everyone for hanging in here. Its great motivation to keep on working.

The 100Hz noise peak comes from the power supply a guess. Its unrelated to the transformers and their positions. Its always there and steady. Any idea on this?

Have a nice evening.
Falk

PS! The dB Scale is dB(FS) with FS = +16dB(U)

Title: Re: Rohde und Schwarz U23
Post by: volker on October 08, 2020, 03:29:59 PM
> 100Hz noise

You could try more B+ filtering, just put in another bigger cap with alligator clips to see if it helps.
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 09, 2020, 03:14:23 AM
@Volker

great idea! A 470uF cap reduced 100Hz noise by 6dB. Gonna expand the psu by two of these and hope for the fuse to not blow :)
Title: Re: Rohde und Schwarz U23
Post by: gyraf on October 09, 2020, 07:55:45 AM
Good one Volker!

Falk:

Perhaps try with C/R/C - two capacitors with a resistor in between? resistor value depending on how much voltage you have to spare - I often use 1K here, which looses me 1V per mA consumption on HT line

/Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 10, 2020, 06:20:40 PM
Hi Jacob,

I decided to use a π-Filter (CLC) with two 470uF caps and a 7H choke. Before and after the caps will be 390 ohms resistors.
For now its the easiest way to treat this hum issue. Once it works I will redesign the psu pcb again.
Thanks for your help.

Best regards,
Falk

Title: Re: Rohde und Schwarz U23
Post by: Erik2345 on October 11, 2020, 04:22:30 AM
Regarding the Braunbuch U23 schematic Jakob posted on page 1, the resistors are drawn with markings.
Do these markings reflect the wattage of the resistors? Or the tolerance?
Can anyone explain? Markings can be longitudinal, across, combined.

Cheers, Erik
Title: Re: Rohde und Schwarz U23
Post by: SKJGProject on October 11, 2020, 09:07:50 AM
Subscribed
Title: Re: Rohde und Schwarz U23
Post by: mjrippe on October 11, 2020, 06:43:15 PM
Regarding the Braunbuch U23 schematic Jakob posted on page 1, the resistors are drawn with markings.
Do these markings reflect the wattage of the resistors? Or the tolerance?
Can anyone explain? Markings can be longitudinal, across, combined.

Cheers, Erik

They indicate wattage.  There are a few different styles of markings that were used.  Neumann schematics usually had a legend explaining them.
Title: Re: Rohde und Schwarz U23
Post by: Erik2345 on October 12, 2020, 11:56:17 AM
Thanks mjrippe!
Title: Re: Rohde und Schwarz U23
Post by: gyraf on October 15, 2020, 12:06:59 PM
Yes.

their insistent  emphasis on marketing and fluffy words scare me too

Is this such a bad thing to say?

I just got off the phone from a very unpleasant conversation with a guy claiming to be vacuvox

He basically told me that he thinks I am attacking and trying to damage his product. Mentioned several times that he is not the kind of person that talks badly about other companies. And btw that I use cheap and bad switches in my products (well, but I only ever had to replace power switches so far) where he uses expensive ones from *name* company. and so on.

I was pretty certain that I mainly commented on the marketing BS surroundings, not the product itself. Returning here to check, I don't see a big problem in what I wrote?.

Conversation got gradually more and more unpleasant and threatening, the guy working himself up to yelling at me in the phone - and I eventually had to hang up on him.

If I was a psychologist I'd say that with a defense like that, you have to have very little confidence in your own product. Makes me kinda hope he wasn't who he claimed to be.

Always freaks me out dealing with such dark sides of business - so I'm here to ask for your opinion and timestamp the incidence.

And should I now be scared about someone starting to talk badly about my products? I'm not big on marketing, my customer base builds very slowly from hands-on - could that be impacted?

/Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: jarvis on October 15, 2020, 01:46:47 PM
Is this such a bad thing to say?

Hey Jakob,
I have no idea, where that guy is coming from. What you said is not even remotely a comment on his products, merely the way he markets them. Sorry, you had to have such an unpleasant conversation.

If I was a psychologist I'd say that with a defense like that, you have to have very little confidence in your own product. Makes me kinda hope he wasn't who he claimed to be.

Haha, yes, if only you had a degree in psychology and were teaching at a renowned university...  ;D

And should I now be scared about someone starting to talk badly about my products? I'm not big on marketing, my customer base builds very slowly from hands-on - could that be impacted?
I'm not trying to be rude here but no one knows the guy and I doubt his opinion would influence anyone interested in buying your products. I mean, I can't image that anyone has had a bad encounter with you or your products so I wouldn't worry for a second about some random guy all of a sudden tarnishing the great reputation, that you've been building up over the years with great products and support.

Best
Jannis
Title: Re: Rohde und Schwarz U23
Post by: Heikki on October 16, 2020, 12:53:02 AM
Is this such a bad thing to say?

Definitely not a bad thing to say. The guy is probably just frustrated. There's COVID going on and maybe his products aren't selling well and he just needed someone to vent his frustration to.

I used to work for a boutique guitar amp company and there were at least two times when somebody started going crazy and badmouthing the company on internet forums. I don't remember if these were real dissatisfied customers or just some crazy trolls but it probably did hurt the sales. Then there was one time that some guitar amp service shop did a complaint to the Finnish Safety and Chemicals Agency, so they came around checking that we had the S3 installation repair qualification that is needed to repair electronics.

People can make your life difficult if you're a small company in audio business, but in your case I wouldn't worry too much. Like jarvis said, you have excellent reputation and great products and it would be very difficult to tarnish your reputation.
Title: Re: Rohde und Schwarz U23
Post by: echoplex on October 16, 2020, 07:03:18 AM
Hey Jakob,

I don't want to hijack this thread and go too far off-topic but the fact that someone from Vacouvox called you and yelled at you, just once again confirms my impression: this company is unattractive!

I don't like their marketing either .. and I think you said nothing wrong. A company like them should be able to take this kind of critics anyways..

Maybe at this stage they are sh*tting their pants because someone from the DIY community is working on proper U23 clone boards and shows this thing is not about magic but about decent electronic engineering.
For me personally Falk's project is the most interesting project on GDIY this year and I'm super excited to see this proceeding.
I guess and hope soon there will be no reason anymore to dump 15k€ on a stereo pair of Vacuvox U23.
Deal with that, Vacuvox!
Title: Re: Rohde und Schwarz U23
Post by: jensenmann on October 16, 2020, 09:51:32 AM
First of all, Falk, that´s an impressive work!!! Congrats and big respect.

The 470uF PSU cap is a hefty load on the bridge rectifier as the charging current will be very high since it has to pass the rectifier in a very short time. That´s something diodes don´t like in the long run. It´d be more economical to lower the capacity to a point where you still have around the same amount of hum reduction than before but life is going to be easier on the rectifier. The other option of course is to use a high current HT bridge rectifier.

It needs to be highlighted again what Jakob said about the orientation of transformers. You can get rid of crosstalk and hum by turning trannies in all 3 dimensions. The 50Hz peak in your FFT will benefit from that. Distance and shielding is another option and probably the best of all is to use an external PSU. If you don´t want that for some reason then get a well shielded toroidal mains trannsformer for internal use. I´m using Müller-Rondo for my tube projects as well as for gear modifications. They will wind whatever you want for you, including static and magnetic shield, even with shielded wire.

@Jakob
I haven´t found any offence against vacuvox in your posts. Let´s forget about that crap and make Falk´s U23 better than every other U23 on earth.  8)

Title: Re: Rohde und Schwarz U23
Post by: gyraf on October 17, 2020, 03:38:37 AM
Thanks guys. I'll worry less about the episode. I wonder if Falk gets a dose of it as well for his interest?

Jens - very good point on the peak-charge-current and how to limit it. I had a problem once with too big electrolytics for heaters, injecting rectifier switching currents everywhere. Can often be helped with a small-ish power resistor in series with transformer secondary..

/Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 17, 2020, 01:26:55 PM
Good evening, gents!

@Jakob - My phone rang too. But that was at least two weeks ago. I understand his worry about a DIY approach on the U23 but he has a product on the market which is high end and aims on a totally different group of users than my approach could ever do. I am sorry for the discomfort that arises from your great and highly appreciated support here. I did not perceive your comment as offensive. A technical product needs marketing that evokes emotions to sell. Every good marketing campaign needs exaggeration to be vivid. Thats normal but also makes (DIY-) engineers and technicians as we are suspicious. One needs to deal with this.

I do not plan to build a historically correct replica of the Rohde und Schwarz U23. Neither do I want to build something like the dutch high end recreation. For me its just a piece of gear that shall facilitate my work. Furthermore its a feasibility study and a personal challenge. I rather have a bread and butter approach than needing a silver spoon to eat.

I really hope emotions calm down again.

@jensenmann Thank you for the nice words on my build. I really appreciate it. I still use my Jensamps pretty often and love them every single time. I will consider hi current diodes for the next revision of the power supply. I am using 1N4007 now. I will see how much I can lower the capacity to still have enough hum suppression.

@echoplex Thanks for your interest. I ll keep documenting every step I take.


Shall I maybe close this thread and give it a name that is not directly connected to the U23? Would that prevent any more discomfort?

Back on the thing:
The π-Filter boards arrived and I will implement them tomorrow and see if there is the desired benefit on the 100Hz hum.

Furthermore I optimized some resistor values to have a better reading for the meter when in calibration mode. Also the threshold has a wider range now and the ratio works like a charm. I am constantly updating the boards for the second and hopefully final revision. Finally I found out a sweet solution for the heater wiring. I do not think this project will be finalized before mid / end of next year. I really want to build this machine to be secure and down to LVD standard.

Thanks for hanging in here. Your support and interest is priceless.
Falk
Title: Re: Rohde und Schwarz U23
Post by: echoplex on October 17, 2020, 05:52:16 PM
Shall I maybe close this thread and give it a name that is not directly connected to the U23? Would that prevent any more discomfort?

Absolutely not! .. I think it's a cheek that Vacuvox called you guys at all!
Rhode&Schwarz are still operating.. so Vacuvox clones a unit by a company who's still in business and then freak out when some else is cloning it? Crazy !
Title: Re: Rohde und Schwarz U23
Post by: JMan on October 17, 2020, 08:32:37 PM
...Vacuvox clones a unit by a company who's still in business and then freak out when some else is cloning it? Crazy !

Right???  I guess they were upset at what they perceived as a slight toward them directly.  Still, if they had any worry that Gyraf’s comment was going to ding their reputation or sales among the people here, being aggressive and making a**es out of themselves has surely put the final nail in that coffin.

Falk, I agree with the earlier statement that this is one of the most exciting threads of the year (for the build, not the drama).  Keep going!  I just wish I had more to contribute.
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 18, 2020, 10:15:13 AM
Hey everybody. I hope you are having a nice moody sunday as it is here in Berlin.

News from the workbench: The pi-filter does its job. 100Hz hum disappeared somewhere in the noisefloor.
The machine has a SNR of 92dB at unity gain now and I will stop to further improve the noise issue for now.

The dB scale in the plot attached equals a dB(u) reading.

@jensenmann: I put a 1k resistor between the diode bridge and the big capacitors. The voltage rises quite slowly now and the simulation looks well too. So now the time has to show how the diodes pair up with the caps.

Gotta work the next days so I wont be able to make progress. I ll kepp you posted.
Falk

Title: Re: Rohde und Schwarz U23
Post by: volker on October 18, 2020, 10:29:49 AM
I would make the first capacitor 100-200µF, and then increase the subsequent ones after the choke and the following resistor. There you already have enough isolation from the rectifier, and you don't need to introduce more voltage drop.
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 18, 2020, 10:52:57 AM
Hi Volker,

I will try this as soon as I order new parts. I dont have any caps at hand to do so. Thank you for your advice. I am not too concerned because even the 100mA fuse (right before the diode bridge ) in the Vb circuit works well when powering up. I designed the pi Filter to have a cutoff frequency below 3 Hz. When I lower the capacitance the frequency will rise. I will keep an eye on the PSU but for now its works. :)

Thank you.
Falk
Title: Re: Rohde und Schwarz U23
Post by: gyraf on October 19, 2020, 04:26:47 AM
Supply ok, now the interesting part starts - figuring out what parameters to put under user control, user interface, and usable control ranges.

Quote
I do not plan to build a historically correct replica of the Rohde und Schwarz U23.

 ;D nor did they actually, if you read through their marketing blurps: It's claimed to be "built upon" the U23, then lists a loong list of "upgrades", "modifications" and "improvements". Think of it for a second: trying to "improve" something made by R&S in the 50'es.. I'm 100% with the modification(s) and even additions, these are very welcome for modern-day use

Quote
modification of the output gain and the thresholds and 2 extra gain stages

I wonder how much u23 is really left in there at a closer look?

/Jakob E.



Title: Re: Rohde und Schwarz U23
Post by: Michael Tibes on October 19, 2020, 05:21:18 AM
...My phone rang too. But that was at least two weeks ago. I understand his worry about a DIY approach on the U23 but he has a product on the market which is high end and aims on a totally different group of users than my approach could ever do...

Wow, this makes me angry! He did copy the old U23 design - just like you - and made some more or less obvious improvements. So how is his approach justifying that you shouldn't do what you are doing?? I really dislike all these 'experts' who claim to have found the holy grail and secret information about the old german tube stuff. When I see the spektrogram of his unit's measurements from the link I posted earlier I can clearly see the hum in there. I do not understand why a unit being built today can still have a measurable hum...

On your unit, in case it hasn't come up and kinda nearly too obvious to mention it: Why don't you use regulated supplies for B+ and heaters?

Michael

Title: Re: Rohde und Schwarz U23
Post by: Falk on October 19, 2020, 03:23:42 PM
@Jakob

yeah! Now starts the fun part. The attack / release circuit I am using is pretty much like the Altec 436 / RS124 circuit with a fixed capacitance. So the release time also rises when the attack time rises. I like the sound of that circuit much more than the more precise designs. The ratio circuit is something I came up with from scratch and is based on a voltage divider. I will integrate 1.25:1; 1.5:1; 2:1, 3:1; 4:1 (maximum what the compressor can deliver - also the original U23 never did more than this due to the circuit design).

I again attach a scratch of the front panel. Is the meter too big? I like it actually but am curious what you guys think.

@Michael Tibes
I agree. There is no myth about all the great machines. They are decent and beautiful engineering but there is definitely not so much magic as some might say. From my very first days in music production it was always the myths of marketing that bothered me the most. Its mean to tell young engineers they would need deadly expensive machines to make a great recording. There are so much more important things to take care of.

Anyways.. regulated power supply. I didnt think of it. To be honest. Not a single time. I cant tell why. The U23 clone (I call it LA-23) is the first clone I ever did. I built diy kits like Volkers D-LA2A, the Porusha TG-Limiter, a Sontec Clone, a RS124 clone and a Mastering Console. But I always had PCBs and circuits that would work. The LA-23 is a different thing. Its  a deadly time killer. Sooner or later I will think of a regulated power supply. Thank you for the advice. But for now I want to move on. The noise of the LA-23 in my mastering setup is at -92dB(FS) that is much quiter than many other pieces of gear in my rack and I don´t do classical music. I dont do masters with less than -10LUFS usually. Nobody will every notice the noise. 

I am curious if you have any ideas considering the front panel.
Best regards,
Falk

Title: Re: Rohde und Schwarz U23
Post by: gyraf on October 20, 2020, 03:06:31 AM
Off the top of my head, in no particular order. Not a complete list. No offense meant, only brainstorming here

- you'll probably want legend/fonts with less sharp edges if going for engraving
- hard bypass needed
- meter is OK, but you'll probably be able to find some more classy ones (that one screams "1979!!! 1979!!!")*
- Why not rotary switch for the ratio? Would look more coherent and less "scary in the 1967 Japanese way"
- The S6 (Vorverzerrung) - looks like a sidechain emphasis on high frequencies.. Shouldn't that have a place?
- chikenheads are un-european and thus only serves to builds mistrust, unless you're guitarist
- try some dual-time-constants like the GSSL "auto" - this often works really nice with low ratios
- put in more time constants - you're only using one deck of each switch, just go for all 11 possible settings you have anyway. exact timings are very important for perceived "musicality" of a comp.
- on the other hand, you very very very rarely want more than two sidechain hpf freq's, like off, 80 and 160
- consider doing it as stereo-in-one-box, makes it much much easier in every-day use
- ("LINK UNIT") you probably don't want this small toggle switch type, it breaks, and until then it screams "cheap" (even if not)
- if you have the "cal" setting for the meter brought out to front panel, you will want the "abgleich" trimmers out here as well.

Again, for now, just mock up the circuit with whatever metal you have. Make it electrically safe but not beautiful. Then get it to people with good ears and explanation skills - the important thing is to get it tried IRL and harvest feedback on what is good, what is bad, and what we want more of

/Jakob E.

*somewhere I have a stash of meters for my G-III (a compressor that was sadly banned under european law) - ballistics nice for vari-mu comp use. Could send you a handful of these if you want to try that route?

Title: Re: Rohde und Schwarz U23
Post by: Falk on October 20, 2020, 05:48:55 AM
Hi Jakob, what a great feedback. Thank you. Many of your ideas are amazing but hard to implement now. I will reconsider all this for the 2nd revision of the boards. This is going to be much better thought than this first try.

- you'll probably want legend/fonts with less sharp edges if going for engraving
This faceplate is going to be printed. Its the first revision and will only be made for home use. Also saving some money

- hard bypass needed
This can only be included in a later version. I need at least one more change to the PSU to have 12V DC for relays

- meter is OK, but you'll probably be able to find some more classy ones (that one screams "1979!!! 1979!!!")*
Okay. Good to know how its looks from a different angle.

- Why not rotary switch for the ratio? Would look more coherent and less "scary in the 1967 Japanese way"
For now I will stick with it but I will consider a rotary for Revision 2

- The S6 (Vorverzerrung) - looks like a sidechain emphasis on high frequencies.. Shouldn't that have a place?
I did not include this feature since I dont see me using it. I could at lease make an option for it on Revision 2 boards.

- chikenheads are un-european and thus only serves to builds mistrust, unless you're guitarist
Okay. Got it! Any suggestion what to use?

- try some dual-time-constants like the GSSL "auto" - this often works really nice with low ratios
I am totally not into this auto function. Never worked well for me.

- put in more time constants - you're only using one deck of each switch, just go for all 11 possible settings you have anyway. exact timings are very important for perceived "musicality" of a comp.
Also noticed for Revision 2

- on the other hand, you very very very rarely want more than two sidechain hpf freq's, like off, 80 and 160
If I experience this during the usage, I ll kick them off again

- consider doing it as stereo-in-one-box, makes it much much easier in every-day use
This will only be possible with a different PSU design and at least 4HE space. I could try it, but I dont think it will work too well.

- ("LINK UNIT") you probably don't want this small toggle switch type, it breaks, and until then it screams "cheap" (even if not)
I will think about it. I guess you re totally right.

- if you have the "cal" setting for the meter brought out to front panel, you will want the "abgleich" trimmers out here as well.
I totally agree and its planned for revision 2.

By the way, I coulndt resist to start working on a regulated PSU. The 2nd revision will definitely get one.
Best regards,
Falk
Title: Re: Rohde und Schwarz U23
Post by: JMan on October 20, 2020, 01:24:39 PM
- chikenheads are un-european and thus only serves to builds mistrust, unless you're guitarist

As an American/guitarist/audio gear enthusiast, this comment cracked me up!   ;D  ;D  Chickenheads, to me, really do look out of place almost anywhere except on guitar amps, but there is the occasional exception.
Title: Re: Rohde und Schwarz U23
Post by: echoplex on October 20, 2020, 05:21:09 PM
@Falk:

I basically agree to all points Jakob mentioned!

Usually you want sidechain-filter on or off .. I never felt the need to set particular frequencies tbh

Secondly about the metering: huge analog VU meters are definitely sexy..but also super slow and in my opinion obsolete. It's 2020 and we have great solutions of LED metering. They might not be as "romantic" as VU meters but super precise and especially FAST !
I can imagine a very modern looking take on a U23 would be super sexy too ;-)
But of course, it's your project and you can do whatever you want to.
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 20, 2020, 07:29:22 PM
Aiaiai. I see. Many things to do.
As said, I will do a second revision which is not so much boiled down to fit into my setup. Thank you for your advice.

I could throw an arduino or raspberry pi in there to drive an OLED display VU Meter?! This could be fast and / or slow and suuuper modern. Or would you rather see a purely analog circuit?

I need to think about it. :)
Good night.
Title: Re: Rohde und Schwarz U23
Post by: echoplex on October 21, 2020, 04:45:30 AM
I could throw an arduino or raspberry pi in there to drive an OLED display VU Meter?! This could be fast and / or slow and suuuper modern. Or would you rather see a purely analog circuit?

Hm noo I think that would be too much and give another big source for issues ;)
What I meant is a simple LED Metering. As for example on the DBC-20 here, or the Vertigo Sound VSC-3:
https://www.youtube.com/watch?v=BHe7-TzTlbI&ab_channel=BuzzAudio

Look at the LM3916 for example. A very simple but yet effective LED Meter driver IC.
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 21, 2020, 05:07:00 AM
Okay.. I will check this LED driver. What would be easy to implement with not more than 40€ more is a LCD with a peak metering. Could look like this

Title: Re: Rohde und Schwarz U23
Post by: jensenmann on October 21, 2020, 05:10:43 AM
Another metering option is the old Sifam PPM. They are ridiculously fast and very accurate. I have them in my desk, Compexes, etc and they are a joy to work with. Not to compare with the chinese crap available today.

As for LED metering have a look at JLM Audio´s LED meter kit: https://www.jlmaudio.com/shop/40-led-ppm/vu/gr-meter-kit.html?display_tax_prices=1 VU, PPM and CV all in one meter
Title: Re: Rohde und Schwarz U23
Post by: Michael Tibes on October 21, 2020, 05:26:39 AM
Another metering option is the old Sifam PPM. They are ridiculously fast and very accurate. I have them in my desk, Compexes, etc and they are a joy to work with. Not to compare with the chinese crap available today.
I totally agree, but apart from being quite expensive - are they still available? I recall searching for the original SSL bus Comp Meter some years ago and it seemed out of production. Could be that I didn't look hard enough... I had an original big Sifam PPM on my SSL bus compressor for testing and it was quite a difference to the chinese lookalike-clone I bought to fit the case (which wasn't really cheap either).

Michael
Title: Re: Rohde und Schwarz U23
Post by: jensenmann on October 21, 2020, 02:58:13 PM
They are obsolete, but can be found used on ebay. Since most people are looking for the Sifam PPM14 to fit in their GSSL the larger meters are not so sought after.
Title: Re: Rohde und Schwarz U23
Post by: gyraf on October 22, 2020, 02:56:16 AM
For metering, we could also consider this

Precise readout of actual gain reduction is probably never needed - but a predictable readout of relative behavior and reactions to audible events are very important to monitor

What about simply using a stack of the cheap and readily-available chinese 6E2 / EM87 / 6UH6 "magic eye" type indicators?

https://www.aliexpress.com/item/33038866524.html

(https://ae01.alicdn.com/kf/HTB1D31BSFXXXXbIaXXXq6xXFXXXg.jpg_480x480q90.jpg)
(https://ae01.alicdn.com/kf/HTB1E1tmKFXXXXb1XpXXq6xXFXXXB/Magic-Eye-6E2-EM87-6UH6-EM84-Tube-Audio-Indicator-VU-Meter-Driver-LOW-Level-Kits-diy.jpg_220x220xz.jpg)

..imagine having three or four of these besides each other - the first set to max out at 3dB, the next at 6dB, then 9dB etc. That is, just setting them to different sensitivities, and allowing the "lower" ones to max out..

I think this would give you all the information you'd really need, in a form that would (probably) be readable "out of the corner of your eye" like you ultimately want it to be..

That said, classic moving-coil meters still has a place in vari-mu monitoring imo - trying to get Falk to try a type I had good results with before..

/Jakob E.

Title: Re: Rohde und Schwarz U23
Post by: Falk on October 24, 2020, 05:20:07 AM
Good Morning Gentlemen,

I need some time to sort out how to move on with all these ideas. They are really valuable and I want to reconsider them with a bit of time.

First of all: I think I cant get rid of the VU Meter. Its a must and will be included.

2nd: I totally get your point of a peak meter but I really dislike these traffic lights color scheme and I have the impression it will just destroy the look of the front panel. So maybe it could be a one colored peak meter.

3rd: I actually would like to implement an LCD with different meter options but I would need another regulated 5V and it would take this project to a whole different level which I doubt is appreciated.

4th: @Jakob - these magic eyes are just great. I will definitely keep an (magic) eye on them and seriously think about it. I totally get your point on the qualitative requirements of metering.

Beside of these metering issues I need to figure out what user controls are really needed.

Other than that: I met a really nice guy from Berlin who develops tube gear since more than 40 years. He built tube consoles and Fairchild clones. We met just to talk about security standards and basic security requirements of gear in these days. Its been an impressive talk and I need to make up an resume of all the hints from the GDIY community and the talk to come up with a concept.

Further more: I fiddled around with a circuit I had on my mind for quite some time. Its a pretty simple yet amazingly fast diode limiter circuitry. I think: What would make this comp really special would be to expand it with a one knob <1ms limiter circuit. This way the machine would be perfect to become the go to 2 bus partner.

What do you think?

Best regards,
Falk
Title: Re: Rohde und Schwarz U23
Post by: SKJGProject on October 24, 2020, 04:47:17 PM
If you ask me....definitely add the limiter circuit!
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 27, 2020, 02:26:14 PM
I mounted a dummy front panel today to get a better impression of how things are and it actually brought me to the following. I will finish this build now and build a second unit after the same specs. Once accomplished, this will lead to an evaluation phase with other engineers to figure out whats really needed. From this point a second and final revision will arise. Then we have to talk again. I will take a break from this thread now. Thanks for all your feedback until now. I ll be back as soon as there are important news. Feel free to text me via: [email protected]

Best,
Falk

Title: Re: Rohde und Schwarz U23
Post by: Falk on October 29, 2020, 06:39:46 AM
I evaluated the I/O curve yesterday. My maximum ratio is still too low. I aimed for a 4:1 but only achive a 3:1. I need to optimize this.

Thank you,
Falk

Title: Re: Rohde und Schwarz U23
Post by: gyraf on October 29, 2020, 08:31:33 AM
You'll need higher gain in the sidechain

..what would happen if you replace the R63 "Regelspannung Verstarkung" with 250K Ohm in stead of the 25K ohm in the schematic? The attenuation against R64 (500K) would be less..

Otherwise perhaps set the TR4 sidechain signal drive transformer to a higher ratio (with original transformer by omitting one or more of the primary windings, giving higher transformation ratio and thus higher Ratio+

(yes, I haven't been able to ship meters yet - will go out with our next shipment)

/Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Falk on October 29, 2020, 12:21:14 PM
Hi Jakob, thank you. I will try another transformer for this. Lets see how it goes then. A never ending story. I am really curious if the rationality will somehow stay consistent when the threshold is changed. I cant really imagine this to be - unfortunately. If so, I will change the front panel to some less quantitive description of the ratio into something more qualitative. I am thinking of expanding the knee term a bit.

Speak soon and thanks for the meters again.
Falk
Title: Re: Rohde und Schwarz U23
Post by: boji on October 29, 2020, 03:09:14 PM
Late to the party, but I enjoyed catching up and reading on your progress Falk. Excellent work!
Title: Re: Rohde und Schwarz U23
Post by: gyraf on November 07, 2020, 10:31:06 AM
Plot thickening..  :o

It turned out that the mastering engineer I work with in Germany (it was his U23's I tried) was also called and interrogated sharply about if, when and how I'd been there and tried it out...  fortunately it wasn't his fault at all that I got to hear the unit, it just happened to be set up in the studio downtown Frankfurt when I visited..

There sure is some strangeness going on

/Jakob E.

Title: Re: Rohde und Schwarz U23
Post by: JMan on November 07, 2020, 04:45:22 PM
This project is turning out to be equal parts awesome geek-out engineering and soap opera drama.  I love it.  (Assuming the drama side of it is as silly and harmless as it seems).
Title: Re: Rohde und Schwarz U23
Post by: jensenmann on November 08, 2020, 12:40:16 PM
The drama part slowly convinces me that despite having more than enough compressors I have to DIY a U23.
Title: Re: Rohde und Schwarz U23
Post by: Falk on November 09, 2020, 09:15:47 AM
Hello Gentlemen,

so I fiddled around with the operating point of the compression pentodes again and got the unit to work as supposed. Maximum ratio is at 5.8 : 1 now. This is perfect for me. I ll keep things as they are. This operation point also allows for a slight saturation when pushing the signal higher than +12dBu.

This might cause some current at the input grid. Since the current through the tubes rises by 10uA  when no compression is applied and the input level increases from 12dBu to 18dBu.

I think I am almost done with the prototype. On more bug to solve is the frequency response of the side chain.
See graph attached. There is a serious lowpass filtering going on. This might be because of the trannies I am using. I ll figure that out this week.

@Jakob - sorry again for the inconvenience. Is there anything I could to? There is still the option of moving this thread or to rename it. I dont really want to cause any personal trouble just because of a DIY compressor. Thats totally not worth it. And more than this: THANK YOU for your support and the super generously packed package that arrive at my door last week. I am still thrilled.

Best regards.
Stay safe everyone.
Title: Re: Rohde und Schwarz U23
Post by: Falk on November 09, 2020, 10:07:40 AM
Just found, that the frequency response of the side chain can be easily smoothed out with a 10p cap there. Solved!

Best,
Falk
Title: Re: Rohde und Schwarz U23
Post by: Falk on November 10, 2020, 08:39:25 AM
Got the ratios fixed today so I am finally able to solder the right resistors to the board.
Aside of this I fixed the circuitry around the meter which wasnt right anymore after changing the operation point of the tubes. I ll try to measure the harmonic distortion soon but still have no software which could do it. But I ll figure something out. Maybe I ll take some day off of this super time consuming beast. :)

Speak soon.

Title: Re: Rohde und Schwarz U23
Post by: Falk on November 11, 2020, 08:14:49 AM
Hei!

Finally found some free Plugin to evaluate THD.
Its attached. Any comments are highly appreciated.

Some more Data:
THD of original U23:
[email protected](U): <0,3%

THD of LA-23:
[email protected](U): <0,6%

THD of original U23:
[email protected](U): <0,8%

THD of LA-23:
[email protected](U): <0,9%

Regards,
Falk

EDIT:
blue - no compression applied
red - maximum gain reduction applied (>25dB GR at +18dB(U))
Title: Re: Rohde und Schwarz U23
Post by: gyraf on November 11, 2020, 10:07:32 AM
Looks decent enough - is this measured with everything trimmed for lowest distortion and gainreduction disabled?

..remember that distortion figures are all but void when it comes to predict subjective listening impression

Did you measure a real u23, or are figures from published specs?

/Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Falk on November 11, 2020, 11:12:17 AM
The blue line is with no compression applied. The unit is in amp-mode and the side chain is not connected. The red graph is with compression at maximum. This means >25dB gain reduction at +18dB(U) input.

The unit sounds really clean in general. But I will need to do more listening. A while ago I built an RS124 clone with Edcor trannies and really like them since they were not too clean. Thats why I wanted them in this build too. For me its perfectly fine.

Shell I do some sound examples? What would you like? Drums?

The figures are from the Rohde and Schwarz manual.
Title: Re: Rohde und Schwarz U23
Post by: Winston O'Boogie on November 11, 2020, 12:26:27 PM
I've been AWOL on this thread for ages so I'm a bit late in congratulating you Falk, well done sir  :)
I like that you've used relatively inexpensive transformers in your build with good results, it's encouraging.

I need to catch up by reading the complete thread but, on the soap opera part, sorry you guys had drama. 

The builder of the commercial unit is always welcome on here if he wishes to communicate anything.  I think there's plenty room for DIY and commercial units to coexist, certainly no need for making enemies. 

 
Title: Re: Rohde und Schwarz U23
Post by: Falk on November 11, 2020, 05:57:05 PM
Hi Winston,

welcome to the thread. I appreciate it. I didn´t wrap my head around transformers much yet. I just heard an Interview with Paul Wolff ( former API) recently and he said, the apparently not too well old trannies were an important part of the sound. He also committed that it took years to regain this sound after moving to (better) Jensen TXs in the 80ies. However. I am not a bit experienced in designing audio circuitry but somehow managed to get LT SPICE and KICAD running and of course had an amazing support here on GDIY especially from Jakob. And here we are.

I found two of the original Rhode & Schwarz meters on Ebay for 20€. They are quite small but the ballistics are very good. I have never seen such a meter movement before. Extremely accurate with almost no overshooting.

I ll make some pictures later.
Speak soon.

Title: Re: Rohde und Schwarz U23
Post by: Winston O'Boogie on November 11, 2020, 06:32:47 PM
Nice catch on getting original R&S meters, yep take a picture when you get 'em.

On transformers, I think it's 'whatever works' really. 

My experience with coils built for IRT Braunbuch units was that they were VERY well designed for the job.

I did the original re-engineering for the Mercury Recording Equipment M72 & M76 pre amps based on historic V72 & V76 amps and:

I auditioned the late Oliver Archut's copies of the original transformers against some Cinemag custom units I had ordered and Oliver's were indeed something special, especially the input.   
At the end of the day though, price was somewhat an issue and as Oliver's were much more expensive (not at all saying they weren't worth it) , we went with Cinemag.

Anyway, sorry for the divert.   Again, congrats  :)


Edit: Just to clarify on the M72 & M76 - I was happy with the Cinemag transformers.  It was basically like you say, a little "better", or, different?


Title: Re: Rohde und Schwarz U23
Post by: warpie on November 12, 2020, 03:55:04 AM
Got the ratios fixed today so I am finally able to solder the right resistors to the board.
Aside of this I fixed the circuitry around the meter which wasnt right anymore after changing the operation point of the tubes. I ll try to measure the harmonic distortion soon but still have no software which could do it. But I ll figure something out. Maybe I ll take some day off of this super time consuming beast. :)

Speak soon.

What software did you use to get the ratio plots?

BTW, awesome project!
Title: Re: Rohde und Schwarz U23
Post by: Falk on November 12, 2020, 04:33:04 AM
@warpie - thank you. To measure the ratio response I simply took ten levels over threshold and wrote them into a spreadsheet Input vs Output and plotted a graph with Libre Office. There is some smoothing applied but it does not make too much of a difference. Ten points is more than enough to interpolate. Even five measurements would be good for a qualitative impression of whats going on.

@winston - My conclusion until now os the same. What works is right. As already said - I rather have a bread and butter approach than doing boutique stuff. I need a device that does the job for me. If it looks good - great. If it doesnt - its alright too. But I understand your impression of the IRT engineering. The stuff from that time is just amazingly good. Actually better than what capitalism was able to produce in later decades. BUT: without this exact development I could not build an all tube compressor for significantly less than 1k.

Attached are the historical R+S meters with matching scale for an exact GR reading.
Best,
Falk



Title: Re: Rohde und Schwarz U23
Post by: echoplex on November 12, 2020, 05:38:06 PM
Hey Falk, first of all congrats to the progress you made!
The figures look amazing!
Everytime I come here and see there's new posts in this thread my heart skips a beat  :D

And wow to the meters you bought on ebay! Bummer they are not available in quantity anymore, are they?
Would love to have them for a potential DIY u23 built .. especially as you said they are fast and accurate.
And they look amazing.


The unit sounds really clean in general. A while ago I built an RS124 clone with Edcor trannies and really like them since they were not too clean.

I really like the fact that it's clean to be honest. There's this widely spread misconception about tube gear being very colored. But as you guys probably know, well-designed tube gear, can be pretty much the cleanest. Some time back I heard a Knif Vari-Mu in a friends studio and I was astonished by how clean it was.
So if you consider to make your la-23 a DIY project, accessible for others too, would you mind designing the footprint for the trannies on the PCB, so that it could be used with different ones as well?

EDIT: sorry, I just had a look at your pics again and realized the trannies are not mounted onto the PCB anyways.. so ignore my point  :P

Shell I do some sound examples? What would you like? Drums?

Yes please!  ;) Some drums would be amazing as well as some mix-bus .. whatever genre. Preferably Pop/Rock/Dance sorta thing ..

Title: Re: Rohde und Schwarz U23
Post by: boji on November 13, 2020, 03:57:32 PM
Quote
Shell I do some sound examples?

If you build a lovely engine, you should treat enthusiasts to its exhaust notes  :)

Title: Re: Rohde und Schwarz U23
Post by: Falk on November 16, 2020, 03:23:14 PM
Good evening

I wont make any progress until next week cause there is too much work to do. I ll do some sound samples then and respond to the posts before. Thanks for hanging in.

Best,
Falk
Title: Re: Rohde und Schwarz U23
Post by: gyraf on November 19, 2020, 06:10:31 AM
Is this the same type meter by any chance?

https://www.surplus-elektronik.de/Ersatzteilgewinnung/Rohde-Schwarz-LF-Trennverstaerker-10-1600-kHz-Type-NV2/1::1054.html

/Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Falk on November 19, 2020, 01:17:35 PM
Hi Jakob, yes, its the exact same meter. INS 10103. 100uA DC meter. Mine came from a thermostat.
Title: Re: Rohde und Schwarz U23
Post by: gyraf on November 20, 2020, 02:31:59 AM
R&S chassis, power transformer (for ECC88 tubes), choke, meter..  in Germany - perhaps you should consider it for next version :-)

/Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Falk on November 26, 2020, 04:18:20 AM
@Jakob

thanks for the hint. The price is actually great but I can not afford to store stuff I dont need. My studio is unfortunately too small for such an approach right now.

I couldnt make sound samples yet since there is too much work to do in the studio. The LA-23 got somewhat cued BUT! I revisioned the frontpanel and will include the Moogish knobs Jacob sent. What do you think?

Best,
Falk

Title: Re: Rohde und Schwarz U23
Post by: gyraf on November 26, 2020, 04:44:48 AM
Darn, that one looks really fine imo!

If I should change anything now, it would probably be details like there perhaps being too much coloring going on in the meter scale - the red/yel/blu dots at the meter 15/12/8dB are possibly visually disturbing (unless they serve a necessary technical purpose in calibration, where you need to identify working area quickly and "out of the corner of your eye")

totally get you on the storing-stuff-for-niceness thing, this I'm so very not good at myself :-)

Jakob E.
Title: Re: Rohde und Schwarz U23
Post by: Falk on November 26, 2020, 05:03:47 AM
Hi Jacob,

thanks for the super fast feedback. I get your point on the meter scale. I will make a redesign at some point. When I colored the scale it looked great but together with the final layout it looks somewhat childish. I agree.

I am super thankful you motivated me to get rid of the chicken heads and emphasized your point by sending your favorite knobs. Those look really timeless and cool. I totally into them.

I will make another dummy plate from wood to see if everything fits now. If so, I will give Frank from frontpanels.de a call and order two sets. I cant wait to build a second unit.

In the near future I will start to write a documentation of the machine. I think in spring or early summer we can have a round table to see what features need to be included to make this comp ready for the community.

Best,
Falk
Title: Re: Rohde und Schwarz U23
Post by: Winston O'Boogie on November 26, 2020, 05:08:19 AM
I like your new panel layout Falk.

Another opinion from me  ;)  I understand where Jakob is coming from on the meter's colour markings but, looking at the photo you posted of your actual R&S meters, I  think they look nice.   Maybe because the actual colours are more pastel than on the drawing, dunno?

Anyway, whichever way you do it, I like it   :) 
Title: Re: Rohde und Schwarz U23
Post by: Falk on November 26, 2020, 06:59:55 AM
Hi Winston,

thanks for the encouraging words. I will take a photo once the unit is assembled and we ll see how it fits. Actually I thought the color scheme of the meter would help to read it since the digits are really small.

I hope for everything to get together until Christmas.
I ll keep this channel updated.

Speak soon,
Falk
Title: Re: Rohde und Schwarz U23
Post by: SKJGProject on November 26, 2020, 01:18:54 PM
i'm short of rack space....really hoping for a stereo version
Title: Re: Rohde und Schwarz U23
Post by: echoplex on November 27, 2020, 05:12:39 AM
i'm short of rack space....really hoping for a stereo version

This is a DIY forum, right?!  ;)
No one will deter your from cluttering a 2U case with 2 pcbs, a bunch of transformers and tubes.
Make a stereo-unit if you want to!
Welcome do groupDIY !
Title: Re: Rohde und Schwarz U23
Post by: Falk on December 02, 2020, 05:46:58 AM
@skjg project - I am not sure of what will happen once the both prototypes I am woking on are up and running. This means I can not promise at this time that there will be anything more than a documentation which could be used as a solid base to work from. What I can promise is that I will not design a stereo unit.
This has several reasons. Most are technical but also there are personal reasons, such as time and of course money. To stick two units in one case will need to look at this project from a totally different angle to make it work. This basically means to start all over again. Buying or building a beautiful 6 HE is so much cheaper.

I decided to build the second unit as fast as possible. So I made another front plate dummy to check for the drilling and printing again. With minor changes I am ready to order them.

@Jacob - thank you again for your feedbacks regarding the front panel. Experience is priceless and you really nailed it. See attached. I am super happy. Some of the printing will be yellowish what will look even better and less cool as now when monochrome.

Thanks for your interest anyone.



Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 04, 2021, 10:33:04 AM
Good evening Gentlemen,

I almost finished the second unit. It misses the input transformer since it was not shipped with the order I placed but it should be here Wednesday. I transplanted the input tx from unit one and did a first measurement. The units measure almost the same. There is some difference in the high end which I will tame soon but I want to have everything in place and measure again.

I will also do sound samples this week with stereo program material. I hope you are doing fine. Have a nice 2021.

Falk

Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 04, 2021, 11:09:12 AM
And some more pictures.. the link button needs some tiny pcb to light up. Its needs 12V DC and I only have 12V AC right now.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Erik2345 on January 04, 2021, 11:37:19 AM
That really looks very, very nice!!
That old VU meter does not look dated at all.
Is that a strat switch for the ratio?
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 04, 2021, 01:08:50 PM
Hi Erik, thank you. I am glad you like it too. Yes this is a five way switch known from guitars. It was on my mind from the very first scratch of the front panel and I like it a lot. I will make a video of the VUs sooner or later. They are really great.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: warpie on January 04, 2021, 02:45:00 PM
Good evening Gentlemen,

I almost finished the second unit. It misses the input transformer since it was not shipped with the order I placed but it should be here Wednesday. I transplanted the input tx from unit one and did a first measurement. The units measure almost the same. There is some difference in the high end which I will tame soon but I want to have everything in place and measure again.

I will also do sound samples this week with stereo program material. I hope you are doing fine. Have a nice 2021.

Falk

Do you have a zobel network? Looks like there's some ringing on the red trace.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 04, 2021, 03:14:00 PM
Hi Warpie,

yes there is a damping network right with the interstage. I am not really worried about it. Should be easy to fix. When I did this measurement there was also a tube with some out of spec behavior involved. I only figured that later. So I will do it again once everything works as supposed.

Best,
Falk
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 05, 2021, 07:30:36 AM
The guts:
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 05, 2021, 07:33:29 AM
and the two of them:
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: gyraf on January 05, 2021, 08:03:25 AM
And now the really important question:

Can you get them tracking well regarding gain reduction? This seems to be a weak spot in some other "versions" of these.

/Jakob E.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 05, 2021, 08:56:28 AM
Hi Jakob,

interesting. I didnt know that.

The missing input tx arrives tomorrow. I will extensively test stereo processing then and update here with full transparency. :)

I ll keep you posted.
Best,
Falk
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: mjrippe on January 05, 2021, 09:51:48 PM
Beautiful inside and out!
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 06, 2021, 06:40:16 AM
Thank you mjrippe. I appreciate it. Still waiting for the input transformer to come. I ordered a small pcb to light up the Link-LED button and switch a relay. 7 month have passed and there is still stuff that I didnt think about yet. Engineering at its best.;)
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 06, 2021, 02:16:39 PM
Okay, finally got both units playing together. Its right regarding the tracking issue. With applying the same voltage everywhere the tracking is not too well. BUT! One gets them playing together pretty well. See the following graph. Units are linked and with the same setting. Input Signal is a sweep of 10 seconds. GR is about 6dB
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 06, 2021, 02:21:48 PM
When the units are NOT linked the tracking is a bit more off in the high frequencies. This sweep is with +4dBu and maximum GR applied.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 19, 2021, 09:40:09 AM
Hi,

I made some samples for sound comparison. I used the files from the gearslutz compressor comparison and some drums sum I had laying around. If anyonw has some nice vocals I would add them.
You can find the original samples and a compressed version here.

https://www.dropbox.com/sh/rghwgzklk1wcvdu/AAB6aEX54cnTxUjhLUwzZPpHa?dl=0

I compressed the mixes with 2:1 @ -3dB GR.
I compressed the the drums with 6:1 @ -6dB GR

Really curious what you think.
Best,
Falk
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Erik2345 on January 19, 2021, 03:15:21 PM
In the LA23 drum sample those cymbals are much more present than in the original. More lively after compression. But still musical, not artificially sounding.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: JMan on January 19, 2021, 09:51:28 PM
I've only had a chance to listen on my iPhone speaker, so I can't pretend to have serious meaningful input, but in that listening context, the mix seemed to have a nice, gentle glue to it and maybe a touch more presence through the compressor.  I had a harder time picking out the difference on the drum track, but I'm 100% certain that the phone speaker is to blame for that!
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: echoplex on January 22, 2021, 08:19:51 AM
Thanks for the samples, Falk.
Are they 100% level matched? The mixes sound a tad louder. Especially obvious on the dance track.
But I like what I hear.. the compressor brings out the mids a bit and adds a nice sparkle or "expensive sound"
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 26, 2021, 04:03:41 AM
@Echoplex, yep your right, the compressed version are somewhat louder, I´ll level match them asap. Apart from that, I totally agree. The added sparkle is really nice. I did side by side comparisons with my Stamchild in some mastering sessions now and the LA-2A is a great sounding compressor. The EF86 add that Neumann-Sound of super smooth airy highs.

@Thank you JMan for taking the time. The difference might be too small to evaluate on a phone.

@Erik2345 I agree, the added high-end is the first thing thats notable. It also does something to the transients which isnt completely clear to me. But i really like how vivid the compression is.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: rock soderstrom on January 26, 2021, 04:18:12 AM
The guts:

Very impressive build. Looks really good, there is a lot going on in there! :)
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: TwentyTrees on January 26, 2021, 07:05:56 AM
Level differences aside, the LA23 sounds fabulous - tucks everything in very nicely, yet also seems to add detail. I definitely agree that something is happening to the transients, and I like it very much. Fantastic work, Falk!

Colour me interested.  ;D
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: JMan on January 26, 2021, 12:42:12 PM
@Thank you JMan for taking the time. The difference might be too small to evaluate on a phone.

I did end up listening again on monitors, and I agree with the rest of the comments here.  Nice and detailed, lovely sparkly top end, a very elegant sounding compressor.  You’ve really done some great work!
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on January 27, 2021, 03:17:02 AM
Very impressive build. Looks really good, there is a lot going on in there! :)

Thanks a lot. Yep, there is a good amount of iron involved in this one.

Level differences aside, the LA23 sounds fabulous - tucks everything in very nicely, yet also seems to add detail. I definitely agree that something is happening to the transients, and I like it very much. Fantastic work, Falk!

Colour me interested.  ;D

Thank you so much. I do really appreciate it. I keep your interest on my mind. Lets see how 2021 goes and what it brings. :)

I did end up listening again on monitors, and I agree with the rest of the comments here.  Nice and detailed, lovely sparkly top end, a very elegant sounding compressor.  You’ve really done some great work!

Thanks for taking time again and feeding back your impression. Its really the engagement of you gentleman that motivates each time to get this build further.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: echoplex on February 11, 2021, 03:02:41 PM
Hey Falk,

now that your prototypes are up and running and sounding great, do you have an idea already if you'll ever make the boards available for us DIY folks?
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on February 11, 2021, 07:10:49 PM
Hi Echoplex,

thanks for your question.

I am still not where I would need to be to make a new run of boards. The machines are still in the prototype phase. Beside everything being basically fine I still have an issue with a tad too much THD during compression. Its up to 2%. Which is especially audible with synthetic sounds. For rock music its great. :) But still.. too much. I am on it but not too sure where it comes from.

I have several ideas where the harmonic distortion arises from and will figure it out asap. Right now there is too much going on at my job to spend much time with these machines.

Another question is if its worth to make boards. 1 unit costs about 1k euro only for the parts. And the second more personal question is if I want to sell boards for a high voltage project as this. I can answer this question with "Most probably not." Maybe a few boards to some people I "know" but I can tell, this will never be a public thing. I am just not confident with this. I am clearly aware of your interest and should there accidentally be more boards than I need, I ll text you. I appreciate your steady interest in this.

Best,
Falk



Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: JMan on February 11, 2021, 07:46:22 PM
FWIW, you've got a bunch of folks on this thread (me included) who would jump at a chance to take on this project if boards were available.  Maybe once you're happy with the unit, a one-time limited batch run for those who have been following this with keen enthusiasm?  I know you're hesitant about the idea, but I think I speak for more than just myself when I say that *if* you were to do something like that, it would be a pretty exciting day on GDIY.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 11, 2021, 07:48:01 PM
...more personal question is if I want to sell boards for a high voltage project as this. I can answer this question with "Most probably not." Maybe a few boards to some people I "know" but I can tell, this will never be a public thing... 

I can personally 100% understand this and, actually, had pause to think about it recently.

In your other thread, the "RS124ish" project, I had been responding with suggestions in regard to the high voltage supply.  But then I started to worry that, if not understood clearly, there could be serious consequences for someone and I just couldn't live with the thought of that happening. 

So I ended up deleting a few replies.  I hope it didn't appear too rude as it wasn't my intention. 

Onwards and upwards:  What are your ideas re. lowering the distortion during compression?  Is the issue caused by a mismatching/drifting of the valve pair as Mu is reduced?  How well balanced is your transformer?

No rush on answers, sounds like you have your hands full   

Edit: I should have asked about transformers (plural) as, besides the anode load transformer, the input transformer balance would be important too if you're feeding the control voltage through a centre tap. 
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on February 12, 2021, 03:01:35 AM
@JMan

Maybe once you're happy with the unit, a one-time limited batch run for those who have been following this with keen enthusiasm?

This appears to be a nice approach as I also would like to give something back for your steady, supportive and motivational contribution here.

I can personally 100% understand this and, actually, had pause to think about it recently.

In your other thread, the "RS124ish" project, I had been responding with suggestions in regard to the high voltage supply.  But then I started to worry that, if not understood clearly, there could be serious consequences for someone and I just couldn't live with the thought of that happening. 

So I ended up deleting a few replies.  I hope it didn't appear too rude as it wasn't my intention. 

Onwards and upwards:  What are your ideas re. lowering the distortion during compression?  Is the issue caused by a mismatching/drifting of the valve pair as Mu is reduced?  How well balanced is your transformer?

No rush on answers, sounds like you have your hands full   

Edit: I should have asked about transformers (plural) as, besides the anode load transformer, the input transformer balance would be important too if you're feeding the control voltage through a centre tap. 

@Winston Thanks for your answer.

Regarding the distortion:

1.)My circuit is kind of on steroids regarding the side chain. To gain a higher dc level and thus higher compression ratio I am feeding the 6al5 cathode with 15V instead of 7V as per spice simulation. I will lower this and measure again.

2.) The original manual states that matching of the mu valves can be completed with the circuit itself. I am running both units with Siemens tubes now. They are not matched and from different batches. I have more than 40 Telefunken and 20 Valvo tubes here which are from the same batch. I will try them

3.) I am feeding the dc through the input transformer. This could be another issue.

4.) To even the frequency response I had to lower the value of the parallel resistors on the interstage from 12k5 to 1k5. This is due to the low inductance of the Edcor TXs.

5.) I use a side chain pretty similar to the Altec 436 with having the release resistor before the attack resitor. I use a 2u capacitor as time constant to allow for a more even dc as used in the 670. Is that capacitance too high? When I change the attack and release time Settings the distortion changes it’s character. So I assume the problem itself is the dc.

Need to get to work. Speek later.
Thanks,
Falk
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: gyraf on February 12, 2021, 04:03:02 AM
Quote
a tad too much THD during compression. Its up to 2%.

and

When I change the attack and release time Settings the distortion changes it’s character. So I assume the problem itself is the dc.

In a compressor, you're always SUPPOSED to have distortion while compressing - after all, you're modulating the signal with a poorly-lowpassed-rectification of itself. Completely expected, and of no reason for concern.

If you're unsure of actual amplifier performance under compression, feed the control grids with a variable low-noise DC voltage and check

The most important in vari-mu is controlling the "thumping", that is, the breakthrough of control voltage into the audio - this happens is the two tube branches are not set equally, so the controlling DC upsets their balance, punching an amount of the control signal through to the audio path (as the dynamic unbalance is generating kinda a halfway-differential-signal). To check and adjust for this, remove audio from unit's input and apply audio (e.g. [email protected], that's what I use) to the control grids, monitor output audio, and adjust the trimmers (Abgleich I&II) for lowest possible amount of 300hz signal breaking through to the audio output.. Note that this won't ever null completely, we're just looking for "best compromize"

/Jakob E.



Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Heikki on February 12, 2021, 04:25:30 AM

When I change the attack and release time Settings the distortion changes it’s character. So I assume the problem itself is the dc.

gyraf already got this, but here's a quote from Elements of sound recording / by John G. Frayne and Halley Wolfe.

Quote
When a more or less steady signal is applied to the input and equilibrium has been established, an alternating voltage will be superimposed on the negative d-c voltage applied to the gain-control grids owing to the imperfect filtering action of the RC combination in the control path, though of course this voltage will be much smaller than the initial pulse. Its effect is to introduce extraneous voltages in the output partly because of the unbalance discussed above, but mainly because of modulation of the signal resulting from the fluctuating grid bias voltage on the variable-mu tubes. This modulation will be present even though the amplifier is in perfect balance. The shorter the time constant (release time) the poorer is the filtering action; hence compressors are more subject to these troubles than are limiters. When amplitude distortion measurements are made with steady sine-wave signals, compressors and limiters always show increased distortion at low frequencies because of the reduced effectiveness of the RC filter.

4.) To even the frequency response I had to lower the value of the parallel resistors on the interstage from 12k5 to 1k5. This is due to the low inductance of the Edcor TXs.

1.5k plate resistors will increase distortion, but it will be mostly 2nd harmonic and largely cancelled if the tubes are balanced. Of course tubes balanced for lowest thump might not be balanced for lowest distortion.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 12, 2021, 07:43:40 AM
I just remembered the time I built a particular opto based tube compressor.   The opto was a series element in the feedback network of the tube amp and, when the unit was compressing, distortion went down.

I was very underwhelmed.   

 
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 12, 2021, 08:58:34 AM


1.5k plate resistors will increase distortion, but it will be mostly 2nd harmonic and largely cancelled if the tubes are balanced. Of course tubes balanced for lowest thump might not be balanced for lowest distortion.

True enough.  It does seem a wee bit of a cruel load for those poor valves though don't ya think?  ;)
I've only tried  1:1 Edcors as an interstage for loading a 6BC8 triode where inductance was fine, the only thing needed was a zobel on the secondary.

I do wonder if, given what Falk discovered necessary with the Edcor in this circuit, a different interstage might be better.

However, if we're talking about 2% of fairly benign distortion then I personally wouldn't overly worry.   

Nice quote from that book, thanks :)

 
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on February 12, 2021, 10:21:30 AM
You gentlemen are great.

Thank you so much. That takes me a step further. But one after another.

The most important in vari-mu is controlling the "thumping", that is, the breakthrough of control voltage into the audio - this happens is the two tube branches are not set equally, so the controlling DC upsets their balance, punching an amount of the control signal through to the audio path (as the dynamic unbalance is generating kinda a halfway-differential-signal). To check and adjust for this, remove audio from unit's input and apply audio (e.g. [email protected], that's what I use) to the control grids, monitor output audio, and adjust the trimmers (Abgleich I&II) for lowest possible amount of 300hz signal breaking through to the audio output.. Note that this won't ever null completely, we're just looking for "best compromize"

Thank you very much Jacob.
Thumping is not a problem. And the integrated calibration mode is super for getting the tubes balanced. I will try to feed the grids with a clean dc and measure again.

I am a bit undecided if I like the distortion or not. It is too audible with highly dense (frequency domain) program material as synths. I have the impression that the higher voltage on the anodes of the 6al5 leads to more distortion of the feedback signal. I will recalibrate the unit after the original R+S manual and see if that makes any difference.

1.5k plate resistors will increase distortion, but it will be mostly 2nd harmonic and largely cancelled if the tubes are balanced. Of course tubes balanced for lowest thump might not be balanced for lowest distortion.

This is where 2/3 of the distortion happens. The rest is 3rd order. The issue is that most of the 2nd and 3rd order distortion happens around 500Hz to 5k and is clearly audible.

I just remembered the time I built a particular opto based tube compressor.   The opto was a series element in the feedback network of the tube amp and, when the unit was compressing, distortion went down.

I was very underwhelmed.   


I had to laugh loud. Thank you.

True enough.  It does seem a wee bit of a cruel load for those poor valves though don't ya think?  ;)
I've only tried  1:1 Edcors as an interstage for loading a 6BC8 triode where inductance was fine, the only thing needed was a zobel on the secondary.

I do wonder if, given what Falk discovered necessary with the Edcor in this circuit, a different interstage might be better.

However, if we're talking about 2% of fairly benign distortion then I personally wouldn't overly worry.   

I agree, I will order two other interstage TXs. Any idea? I need center tapped 1:1 (10k). Any other than Carnhill VTB9071 with a <60€ price tag?

The journey goes on. Thank you everybody.
Falk
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 12, 2021, 12:41:16 PM
If the transformer is the major cause of your woes, then an alternative that comes to mind is the Haufe T188  as used in the U73B..  Haufe aren't (can't?) making it anymore but I would contact Manfred and see if he's still able to supply his copies.

Scroll down the page here:

https://www.manfreddeppe.de/menu/verkauf-for-sale/


Edit:  there are details of his reverse engineering an original 188 here which might be of use in comparing to other transformers:
https://www.manfreddeppe.de/menu/diy-u73b/
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on February 12, 2021, 01:19:51 PM
If the transformer is the major cause of your woes, then an alternative that comes to mind is the Haufe T188  as used in the U73B..  Haufe aren't (can't?) making it anymore but I would contact Manfred and see if he's still able to supply his copies.

Thats funny because I have 8 of them at home for the U73b project. Thanks for this. I ll give them a try.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 12, 2021, 01:32:54 PM
Thats funny because I have 8 of them at home for the U73b project. Thanks for this. I ll give them a try.

I knew that.

:D  Not.     

That's serendipity for ya!  Fantastic.  I guess I might say first try it with the single 10K resistor across the top and bottom of primary as in the U73 but, you know how to play around with things to make it work in your circuit so just do your stuff and let us know.

Good luck :)


Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on February 14, 2021, 11:37:43 AM
Good afternoon,

I spent most of the day with the compressors again, measuring, calibrating, measuring, changing tubes and so on. Good news is, that I could lower the distortion by changing the voltage on the 6al5. Its around 1% THD now where it was 2% and more before. I will test it again during work in the next weeks but I am hopeful its going to be good.

Thats pretty good news for me since I will not mess around with other transformers again. Doing so could have meant to change the overall gain staging (again).

Is there actually some regulated tube amp / compressor power supply which one could adapt for this project? It needs 250V and 102V.

I will order Mu-Metal cans next week. I will see if this will help. Overall noise is at -83dB(FS) rms now. Or -64dB(u) rms. Honestly, this could be better. For me its okay but I am sure you gentlemen would wish for it to be better. I think without shielding the audio transformers and / or using a dc heating it wont happen to get better. Main component of this noise is 50Hz hum @ -87dB(FS). The rest is below -95dB(FS).

Anyway, thanks for all your input lately. It was quite valuable.
Have a good start of the week,
Falk
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Heikki on February 15, 2021, 01:42:23 AM
Looking at the original U23 schematic, R9 adjust the threshold voltage for tube Rö6. Adjusting voltage on Rö6 should have the same effect on distortion as adjusting the threshold pot.

I don't like that the threshold voltage for Rö6 is taken from the Rö 1 & 2 cathode circuit. I think it's going to cause distortion when the timing cap is charging. It's probably not going to show up when measuring with steady sine waves. Timing cap charging currents flowing trough the cathode resistors doesn't seem like good idea to me, but it might not cause any audible distortion and if it does it probably doesn't matter in a sound effect.

My experience with edcor XSM series transformers is that they can't be anywhere near any power transformers or they'll pick up the hum.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 15, 2021, 06:54:57 PM
Hey Falk,
I understand 100% clearly your reasons behind not wanting to try Manfred's T188 copy.
Who knows what rabbit hole or can of worms might reveal itself once you change a major component.   
If something is working OK as is, then why change right?

On the other hand, should the mid-range distortion artifact you measure still be something that you feel is an issue, at least the T188 is there for you to try without laying out money.

Heikki's comments on time constant related artifacts reminds me of sometimes being in abject fear or paralyzed uncertainty when trying to evaluate such things. 
I have no doubt these things interact, they do.  I usually just try and find the best compromise and cross fingers.

I'm sure you'll find the correct balance.

:)   
 

   
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Heikki on February 16, 2021, 06:40:03 AM
Heikki's comments on time constant related artifacts reminds me of sometimes being in abject fear or paralyzed uncertainty when trying to evaluate such things. 
 

When the timing cap is charging there is basically full wave rectified signal that has no RC filtering injected to the remote-cutoff tube cathodes and that just seems like bad design to me. I think it might cause significant distortion that probably doesn't sound too pleasing. Or is there something else going on that I'm not seeing in the schematic? It is also injected to the screen grids, maybe that cancels it somewhat or maybe it makes it worse? I'm too stupid to think about it too hard and nobody else probably cares.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 16, 2021, 09:00:57 AM
...is there something else going on that I'm not seeing in the schematic? It is also injected to the screen grids, maybe that cancels it somewhat or maybe it makes it worse? I'm too stupid to think about it too hard and nobody else probably cares.

Haha, your words sound like the ones in my head and it's what I meant by paralyzed uncertainty.

Yes the threshold voltage for the diode valve R06 is derived from there but, with R9 being labeled Kompens. Spanng., isn't that saying 'voltage compensation rather than threshold adjust?  Yes, it IS the threshold voltage, but  I was thinking that it's compensating and minimizing control voltage feed through by feeding a pre-set percentage of the opposite polarity of that voltage in common mode to the cathodes and screen-grids.   

What does the manual and test procedure say, can anyone translate?
 
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on February 16, 2021, 07:56:51 PM

Yes the threshold voltage for the diode valve R06 is derived from there but, with R9 being labeled Kompens. Spanng., isn't that saying 'voltage compensation rather than threshold adjust?  Yes, it IS the threshold voltage, but  I was thinking that it's compensating and minimizing control voltage feed through by feeding a pre-set percentage of the opposite polarity of that voltage in common mode to the cathodes and screen-grids.   

What does the manual and test procedure say, can anyone translate?

In my circuit I am using R63 as variable threshold and R9 to match the threshold of the 2 units.
I match the units as follows which is what the manual states:

1) With no input applied I set the cathode voltage of RÖ1 and RÖ2 to 3.25V and adjust R9 to measure 7V at the cathode of RÖ6.
2) Then I adjust R5 in calibration mode so that the tubes RÖ1 and RÖ2 are balanced.
3) With +6dB(U) applied to the input I adjust R41 so that the unit works at unity gain when being set to amp mode.
4) With +16dB(u) I adjust R63 for 10dB gain reduction in compression mode.
5) I then lower the input level to +6dB(u) again and set R9 to read +5dB(u) on the output.
6) I repeat the steps 3) - 5)

When this is done both units compress almost the same (+/- 0.4dB). They can be further balanced by changing R5 on the units again slightly to match gain reduction perfectly. (+/- 0.1dB).

I have some mastering sessions later this week and will use the compressors for this. But I was listing to music through the machine on sunday for two hours and was switching them in and out of the chain and I felt for the sound to be a tad warmer / softer now. I also put in some Telefunken EF83 tubes now but did not really notice a difference to the Siemens tubes sound wise. Only thing is that the Telefunken tubes are from the same batch and the compression of the two units is perfectly synced whether linked or not. No matter if I compress 3dB or 14dB in whatever setting. They really work like a charm.

I think the wording compensation voltage derives from compensating the behavior of the different rectifier diodes. Its really perfect to get two units to play together or at least having the same regulative behavior. I can totally understand that taking the voltage from the RÖ 1&2 cathode branch is somewhat inelegantly. Maybe one could do this differently in another revision.

I ll write again later this week.
Stay safe,
Falk
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Heikki on February 17, 2021, 12:31:24 AM
I asked spice to do some thinking for me and spice seems to agree with me. I simulated to see what happens with the cathode/screen circuit in isolation. Of course tube models aren't perfect and I just used a model of whatever small signal pentode I happen to have, but I think it's pretty close to the truth. If the tubes are in perfect spice world balance, there's no distortion. Look at the attachment to see what happens when I simulate imbalance.

Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: 5v333 on February 17, 2021, 11:37:32 AM
Hej Falk!

If you are interested, I have two mains transformers for tube projekts up for sale .
Edcor XPWR223-240
Hammond 370CAX

But be sure about what you need before you buy so you dont end up with an expensive part that in the end wasnt right for you. (like i did..)

Im in Gothenburg.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 17, 2021, 02:49:38 PM
Thanks for the clarifications Falk, and the sim Heikki.

Given that it's easy to derive the 7v DC threshold by other benign means, I suppose the thing I had a hard time with was that the U23 designers/engineers tapped off where they did merely for convenience.   
Those guys absolutely knew what they were doing, and were also in a position where units were not being designed/built to a strict price point.
So I assumed there had to be good reason for what they did.   Maybe not?
 
Keep the unit(s) in perfect balance or make the distortion from imbalance  an adjustable "feature"  ;)

Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Heikki on February 17, 2021, 11:55:53 PM
Absolutely lowest distortion might not have been the design goal, if the purpose is to prevent overmodulation of AM transmitter or sound on film variable area recording. Making fairly stable low voltage and low impedance voltage reference costs money in 1955. Decade later in U73/U83 zener diodes seem to have been used.

How much of the distortion I simulated actually shows up in a real unit is hard to say. There's already distortion when the gain is changing and adding some on top of that might be inaudible. But then again the simulated waveform looks a lot like nasty sounding frequency doubling distortion that happens in some guitar amps. In 2021 zener diodes and resistors are cheap and any builder if worried about the distortion could just put a switch that changes where the 7V threshold voltage is taken. That way it would be easy to listen if there is any difference.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 18, 2021, 04:19:27 PM
Heikki,  excuses based on facts such as historical parts availability doesn't help.
My once exalted opinion of those engineers has now fallen, 'tis tainted, has been besmirched...

 
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Heikki on February 22, 2021, 04:35:50 AM
But did you guys know that somebody who clearly possesses high level of technical knowledge claims that Fairchild 660 is a copy of U23.

https://www.gearslutz.com/board/so-much-gear-so-little-time/1264821-fairchild-compressor-comparison-thread-hardware-only-4.html#post14013221

I think some of you might know this character from pleasant phone conversations you have had with him.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: gyraf on February 22, 2021, 05:08:53 AM
 :o wow, hadn't seen that post - what a load of it

Attached here, our very own CJ's interview with Rein Narma himself about the fairchild side of the subject - not much matches the dutch legitimization claims :-)

/Jakob E.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 22, 2021, 06:01:14 AM
What a load of bollocks!  So this is the guy that got into it with Jakob and Falk?

Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: volker on February 22, 2021, 06:37:15 AM
What a load of bollocks!  So this is the guy that got into it with Jakob and Falk?
Yes, seems to be...


Falk, what ratio are you using on the interstage transformer? Also, I sent you a message concerning another topic, maybe you missed it.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 22, 2021, 04:39:28 PM
Yes, seems to be...


Falk, what ratio are you using on the interstage transformer? Also, I sent you a message concerning another topic, maybe you missed it.

OK got it.  Well, I see no reason at all to invent (it is invention) a story that links the U23 to Rein Narma, the U23 stands on its own.

I'm not Falk but I believe his interstage transformer is an Edcor 10K:10  1:1
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 22, 2021, 04:55:18 PM
One thing that occurred to me earlier is a solution (modification) to help a little with potentially too low an inductance in that interstage.

Of course strapping the input valves as triodes would lower the source impedance to the interstage and relax the inductance requirements, but say you wanted to mostly keep the pentode characteristics.
Well, an ultra-linear connection would get the anode impedance down a bit, as well as lower any distortion.

If the main anode voltage is, say, 265V, a pair of simple resistive dividers from the EF83 anodes to ground.  Lets pick 165K on top and 100V on bottom for ease (scaling them up would lessen any extraneous anode load though).  At the mid point, feed that into the grid or gate of a triode or Mosfet and tie the triode/Mosfet's cathode/source to the EF83's G2 with a bit of current care of a 33K to ground.  Triode/Mosfet anode/drain is tied to actual B+.

This would be about a 37% ultra-linear feedback and would lower the anode impedance.
There is precedent for this in a compressor, albeit using cascodes rather than pentodes so it's not totally off the wall I don't think.  Values need to be adjusted and a cap possibly added, but hopefully someone gets the point.

Anyway, just a thought I had.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Heikki on February 23, 2021, 02:15:11 AM
Triode or pentode with screen feedback might introduce new problems. With pentode assuming the negative feedback lowers the plate resistance significantly. With triode or screen feedback pentode the changing plate resistance from low to high impedance needs to be considered. In gain reduction how much will the changing plate resistance affect frequency response? Is the sidechain sufficient for triode or screen feedback pentode? How much more control voltage will be needed and can the sidechain provide it?

I think lowering the plate resistors to 1.5k was the smart move. Gain goes down in the the gain reduction stage with 1.5k resistors, but it doesn't really matter because there's plenty of gain after it.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on February 23, 2021, 08:28:22 AM
Thanks Heikki.   It was more a thinking out loud for future experiment than suggesting  Falk do this on his current unit.   
I take your point about changing anode impedance, although it's something that happens with 90% of the variable mutual conductance compressors we discuss on here, and to a greater extent than I'm suggesting from a 30+ % of ultra-linear fb.
 
Edit:  Side chain voltage -  looks like, with 100V on G2, then -20V is the maximum before it poops out anyway.  Probably not a compressor for deep gain reductions whichever way it's cooked.   OK for mastering etc of course. 

I checked my scribbles from messing with the Edcor in this type of use - I had a 10K across the full primary and a zobel on the secondary.   


Cheers  :)
   




 

 



 
 
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on February 23, 2021, 06:33:45 PM
Good Evening.

Sorry for being so awfully less responsive but the studio work keeps me busy. Thank you so much for making this thread flourish. There will definitely be some compensation for all this sooner or later. I will need some more time testing but the last mastering sessions felt great and the customers didnt feel too much distortion anymore.

@Volker.. totally missed your message. Regarding the mu metal housings I had a tough one. I got a quote for about 400€ for ONE housing from the Netherlands. I have another for 10 housings for 900€ from France and a third for 10 housings for 600€ from England. Honestly thats too expensive for a DIY project which was aimed to build the compressor as cheap as possible. I will keep myself busy on finding a solution for this.

Aside of that: Input TX is 10k:10k, Interstage is 10k:10k, Output is 600r:600r, Sidechain is 600r:2k4
The transformers are the biggest compromise in this build but for the price they do a great job. This build was never aimed to become a boutique compressor rather than a unit that works well for what it is. It has some weak spots but its too early for me to evaluate if they really bother me or if they become the weak spots that every good friendship brings.

Best regards,
Falk
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: gyraf on February 24, 2021, 02:34:34 AM
You may not NEED mu-metal casings at all, unless you are physically limited to a certain amount of space in your build.  Remember, the main benefit of mumetal shielding is that it has some 6-10 times the magnetic softness (or magnetic conductivity) of standard soft iron

..which in the case of shielding only means that you have shielding efficiency of 6-10 times the same amount of iron. This off course paramount important for tight fits like Danner-casettes with integral power transformers - not necessarily for us

..which again means: Just put in 10-times the thickness of cheap soft iron and you're golden IF you have space for this

I have had really good results just getting some 8-10mm iron sheet lasercut to profile fitting the transformer, then stacking a handful of these with a top and bottom "lid" of same material. Yes, gets heavy - but do we care?

For smaller transformers; in building supply stores you can get off-the-shelf soft iron pipes in short lengths, threaded and with fitting end caps. Yes, you probably can't ship this solution internationally for obvious reasons  :o but it works really nice as well..


/Jakob E.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Heikki on February 25, 2021, 09:50:10 AM
Falk your compressor might have less noise and hum than a certain compressor that costs 7500€

http://vacuvox.com/wp-content/uploads/2018/10/Vacuvox-U23M-Promo.pdf

If I am not mistaken Diagramm 1 is the noise spectrum. Biggest hum spike at -70 dBu is not spectacular for a compressor that has only few dB of gain. It's probably inaudible with modern pop music, but if I were to buy a compressor that cost more than any car I've ever owned I would want it to be dead quiet.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: gyraf on February 25, 2021, 10:48:01 AM
well spotted..!  ;D
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: JMan on February 25, 2021, 11:26:23 AM
Falk your compressor might have less noise and hum than a certain compressor that costs 7500€

We’ll be waiting patiently to hear the report of the angry and belligerent phone call you receive  ;D
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Heikki on February 25, 2021, 12:04:11 PM
We’ll be waiting patiently to hear the report of the angry and belligerent phone call you receive  ;D

I doubt he’ll find my phone number anywhere but if he does I won’t mind. He has already harassed pro audio manufacturer and studio owners, he should probably stop at that and not start harassing hobbyists.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on February 25, 2021, 04:15:03 PM
Hello,

so again distortion issues. Today I was working on some electronic music again. Deep 909 kickdrum sounding distorted when passing through the unit. It distorts in amp mode and distorts even more when compressing. The waveform looks totally fine on the oscilloscope but its too much distortion. I compared it with my Stamchild which is dead clean at the same level. So what can it be? The low frequency distortion is not too much dependent on the level. Even if I lower the input 6dB it sounds the same.

I found out, that the wave form after the 6sn7 in the side chain (originally ecc40) looks distorted. Since its rectified afterwards it should not be too bad but what do you think this could cause audible distortion at any point?

I attach some spice simulation of my side chain circuitry as soon as the upload folder got emptied.
Thank you!

EDIT:
1)Can someone tell me what C4 in the original schematic is needed for? I remember that I had problems with it in the beginning and left it out at some point.

2)And again regarding the low frequency distortion. Can it be core saturation of the interstage (plate voltage) or the input tx (control voltage)?

3)Or can it be that a cathode voltage of 3.5V at the EF83 causes the distortion? I though by raising the cathode voltage I would gain some more clean headroom. Was that right?

Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Heikki on February 26, 2021, 12:25:08 AM
C4 is the screen bypass cap. If it is left out it will reduce gain of Rö3.

High low frequency distortion when not compressing could be caused by the transformers. Are there any specs how large low frequency signals the transformers can handle and how much DC current can the interstage transformer handle? If the plate currents of RÖ1 and RÖ2 are not in perfect balance it can cause distortion if the interstage transformer isn't designed to handle dc currents.

Edit: some more thoughts.

Signals large enough to clip the input of sidechain amp can cause distortion.

If your output transformer is 1:1, driving low impedance loads will cause distortion.

Are your plate chokes hammond 156C. That's one source for distortion if they are.

Not taking B+ to interstage transformer center tap and putting a cap in series with the primary might be worth a try. Not having DC current flow in the transformer primary makes life a lot easier.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on February 26, 2021, 03:56:10 AM

Not taking B+ to interstage transformer center tap and putting a cap in series with the primary might be worth a try. Not having DC current flow in the transformer primary makes life a lot easier.


Hi Heiki,

thank you for your thoughts. I appreciate it a lot. I fed audio signal to the different stages. After the Interstage everything sounds fine. But as soon as I feed audio to the Input of the interstage distortion appears.

Do you mean to have a cap on each side between plate and primary inputs? Or once cap between center tap and B+?

EDIT:

I tried both, still the same amount of distortion. I also tried a transformer from Manfred Deppe in the instage position with regular wiring and the distortion occurred alongside with less low end.

But! I fed a symmetrical audio signal to the inputs of the instage and the sound is clean. So it appears it must be the balancing of the input tubes that cause the distortion. I will check for symmetry of B+ at the plate of the input tubes and will try to get a pair of matched tubes. Unfortunately I dont have a tube tester here.

Thanks a lot!
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Heikki on February 26, 2021, 04:27:20 AM
The upload folder is full. PM me your email address and I'll email you a drawing of what I mean.

Edit:
My suggestion was to connect the primary of the interstage transformer like the primary of T2 in this schematic
https://web.archive.org/web/20180221103320im_/http://www.michaelkingston.fi/files/M670-schematic-Kingston-web.png
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on February 27, 2021, 03:15:07 AM
Good morning,

literally spent the whole day yesterday with trying to get things solved unfortunately with no results but some knowledge gained.

I fed symmetrical external audio via a 10u caps to each side the b+ of the mu tubes et voila I had super clean audio on the output which tells me that the distortions happens on the first stage. This rises some questions about the origin of the distortion that I would appreciate your thoughts about.
Image: https://www.dropbox.com/s/pzz4psn2b4wp4p2/IMG_9450.jpg?dl=0

1) Lowering the plate resistor from 12k5 to 1k5 also lowers the maximum swing of the anode voltage, right?
May this be a reason for distortion to occur?


2) When I lower the input by about 12dB distortion seems to disappear. This is btw. the same amount of gain I lost on the interstage when lowering the plate resistors.

3) I compensated for the gain loss by changing the input pad to a 1k - 2k - 1k where it was 2k5 - 320 - 2k5 before. So yep I might be feeding too much voltage to the grids. But I can only change this when having more gain in the first stage. This is because of the overall noise that would be amplified in later stages leading to new troubles

What I think of now is:

4) Changing the interstage to a 1:1 (15k) Edcor XSM (now its WSM) which has more Henries as some forum research revealed.
-> https://groupdiy.com/index.php?topic=55122.0
This would allow me to have a larger plate resistor. This giving me more gain allowing for a stiffer pad on the input and more voltage swing on the anode.

Does this sound logical to you gentleman? Any advice is highly appreciated.
(ALERT! Tube newbie might talk nonsense :) )

Btw. is there anyone with a tube tester in Berlin?

Thank you,
Falk

Edit: One more questions and some math:

Z = impedance
XL = inductive reactance
R = resistance

When considering the following:
Z = sqrt(R^2 + X^2)

X = sqrt (Z^2 - R^2)

for high impedance transformers R is much smaller than Z so we can almost ignore R.
In this case we can consider Z = X

From this point:

L = XL / ( 2 × π × f ) where XL is the inductive reactance given with 15k
L (60Hz) = 39H

L = XL / ( 2 × π × f ) where XL is the inductive reactance given with 10k
L (60Hz) = 26H

The thing is: How I came up with lowering the plate resistor was due to my simulation which uses 8H. I am not sure if I ever tried the original values actually. When I assume 20H (still a bit conservative) the simulation works great with original values. Maybe I really should not question what the engineers invented without computers. That makes me feel even more stupid.



Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on February 28, 2021, 01:56:20 PM
I tried the original value resistors today with no good results. The search continues.

Have a good start of the week.
Falk

Edit: Some more hours of studying transformers and listening later. :) I noticed that one component of the distortion I notice is that the sub bass becomes longer. So there is some low frequency resonance. I was checking all the LCR combinations in the compressor for their resonance frequency and found out that the LCR group on the output has a resonance around 46Hz what is exactly the annoying thing I noticed with the 909 kick some posts before. I will try to dampen this resonant circuit and see what happens. If I get this thing solved I am sure its not far to get those units to work really great.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on March 13, 2021, 12:22:35 PM
After a lot of testing and trying I finally got the units to play perfectly. The resonant behavior is gone and the bass is really tight now, sitting right in place. Also the overall THD is much lower now.

0.5% THD below 150 Hz
0.4% THD above 150 Hz

under full compression THD is around

up to 1.2% THD below 150 Hz
0.4% THD above 150 Hz

The units sound even better now. Although they have gained a bit more hiss now. I changed all the transformers in the audio path. Using 15k 1:1 Edcore XSM as input tx and interstage tx and a 10k 1:1 WSM as output tx.

I will test the machines again for some time and then do a final revision of the pcbs. I am updating the pcb with everything I find out and it really got better doing so.

I attach a plot of the noise. The db scale equals a dB(u) reading. RMS noise is around -70dB(u). I am really wondering why the hiss got so prominent since I swapped the input tx. Does anyone have an idea on this?

Edit: The hiss disappears as soon as I disconnect the input XLR.

Thank you,
Falk


Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: mjrippe on March 13, 2021, 04:21:50 PM
I attach a plot of the noise. The db scale equals a dB(u) reading. RMS noise is around -70dB(u). I am really wondering why the hiss got so prominent since I swapped the input tx. Does anyone have an idea on this?

Edit: The hiss disappears as soon as I disconnect the input XLR.

Thank you,
Falk

That's not an XLR, that's a snake  ;D  Sorry, could not resist a bad audio joke.

So if all the xfmrs are 1:1 I don't see why attaching an input would add hiss that is not coming from the source.  Unless the way it is terminated causes a rising response?
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on March 14, 2021, 04:14:48 AM
Its terminated as this. C3 + C4 are NOT installed. Source is a THAT 1646 line driver with 50 ohms output impedance. As long as the units are not inserted the source is dead quiet.

Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Winston O'Boogie on March 14, 2021, 05:13:15 PM
Do the replacement transformers have significantly more dcr?  That would imply more inductance which is good, but might explain the higher hiss.

I'm convinced that, even though you're running these into a bridging input -  the output transformer should really be a 4:1 or 8:1 rather than the 1:1 you have in there but, if it's working for ya...   ;)

Edit:  It's possible I'm not understanding the hiss issue but, if it's such that the U23 with nothing plugged in is quieter than when your 50 ohm THAT driver chip is connected then:

What happens if you double the values of the U pad so, 2 X 2K series, and 1 X 4K shunt.

As is, with nothing connected there is a 2K primary termination reflected to the secondary.  As soon as your 50 ohm source is connected then you have 50R and 2 X 1K all in series, which are then all in parallel with the 2K primary term. so, a 1K source impedance total.

Doubling up on values would get you  2K source impedance with the THAT driver connected which might be closer to what the transformer and/or valves prefer.   
Just thinking out loud and rambling...
Good work  anyway Falk  :) 


Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on March 15, 2021, 05:02:29 AM
Hi Winston, thank you for your thoughts. I will give it a try later this week. I will also try 5:1 on the output then. I should have enough overall gain to go with 5:1 and have some 3 to 5dB left for gain makeup. We are really getting close to an amazing compressor. With the latest changes it handles low frequencies better as my Stamchild. I ll keep you posted. Speak soon.

Edit: New parts will only be delivered early June. Until then there will be no news. Aside of this - having a 600ohms impedance before the units instead of  a 50 ohms output cured the noise issues.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: jans on April 19, 2021, 02:37:59 PM
Very excited about this project. I checked the files, already sounding amazing. Well done, very impressive.
Title: Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
Post by: Falk on April 19, 2021, 02:46:19 PM
Dear Jans,

thank you. Since I recorded the files I learnt a lot about this device and took it some steps further. I am still waiting for the new parts to arrive. I already designed a new revision of the main boards and will build them until mid of this year. There are still some details to be figured out but its going to be a really nice compressor.

Best,
falk