Rohde und Schwarz Begrenzungsverstärker ABR / U23

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Falk said:
...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

 
@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

 

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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?

 
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
 
gyraf said:
- 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.
 
@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.
 
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.
 
Falk said:
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.
 
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

 

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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
 
jensenmann said:
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
 
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.
 
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

HTB1D31BSFXXXXbIaXXXq6xXFXXXg.jpg_480x480q90.jpg

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.

 

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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
 
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

 

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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

 

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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.
 
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
 

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