Rohde und Schwarz Begrenzungsverstärker ABR / U23

[gyraf]

Plot thickening.. 

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.

 

[JMan]

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).

 

[jensenmann]

The drama part slowly convinces me that despite having more than enough compressors I have to DIY a U23.

 

[Falk]

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.

 

[Falk]

Just found, that the frequency response of the side chain can be easily smoothed out with a 10p cap there. Solved!

Best,
Falk

 

[Falk]

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.

 

[Falk]

Hei!

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

Some more Data:
THD of original U23:
1kHz@12dB(U): <0,3%

THD of LA-23:
1kHz@12dB(U): <0,6%

THD of original U23:
40Hz@12dB(U): <0,8%

THD of LA-23:
40kHz@12dB(U): <0,9%

Regards,
Falk

EDIT:
blue - no compression applied
red - maximum gain reduction applied (>25dB GR at +18dB(U))

 

[gyraf]

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.

 

[Falk]

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.

 

[Winston OBoogie]

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. 

 

[Falk]

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.

 

[Winston OBoogie]

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?

 

[warpie]

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!

 

[Falk]

@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

 

[echoplex]

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 

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 

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 ..

 

[boji]

Shell I do some sound examples?

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

 

[Falk]

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

 

[gyraf]

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.

 

[Falk]

Hi Jakob, yes, its the exact same meter. INS 10103. 100uA DC meter. Mine came from a thermostat.

 

[gyraf]

R&S chassis, power transformer (for ECC88 tubes), choke, meter..  in Germany - perhaps you should consider it for next version

/Jakob E.