Rohde und Schwarz Begrenzungsverstärker ABR / U23

[Falk]

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

 

[Falk]

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.

 

[Falk]

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

 

[Erik2345]

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.

 

[JMan]

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!

 

[echoplex]

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"

 

[Falk]

@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.

 

[rock soderstrom]

The guts:

Very impressive build. Looks really good, there is a lot going on in there!

 

[TwentyTrees]

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

 

[JMan]

@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!

 

[Falk]

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.

 

[echoplex]

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?

 

[Falk]

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

 

[JMan]

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.

 

[Winston OBoogie]

...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. 

 

[Falk]

@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

 

[gyraf]

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. 300Hz@0dBu, 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.

 

[Heikki]

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.

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.

 

[Winston OBoogie]

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. 

 

[Winston OBoogie]

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