Rohde und Schwarz Begrenzungsverstärker ABR / U23

[Falk]

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

 

[gyraf]

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.

 

[Falk]

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

 

[Winston OBoogie]

I vote black too. 

 

[gyraf]

(emailed you)

 

[Falk]

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

 

[echoplex]

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

 

[Falk]

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

 

[opacheco]

......... 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

 

[opacheco]

........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 

 

[gyraf]

Your email?

 

[echoplex]

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

 

[opacheco]

Your email?

gyraf sorry;

[email protected]

Thanks anticipated,
opacheco

 

[gyraf]

mailed

 

[opacheco]

mailed

Thanks a lot Jakob!

opacheco

 

[echoplex]

Hi Falk,

are you planning or willing to sell some of your PCBs to us folks as well?
Once you have a working version

 

[Falk]

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

 

[JMan]

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.

 

[gyraf]

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.

 

[Falk]

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