Tube compressor circuit adapted to tube preamp

[Winston OBoogie]

The 'S' version of the V76, somewhat surprisingly called.. the V76S had the ability to send the signal from the 1st double EF804S valve input stage out to a compressor, and then bring it back into the EF804S/E83F output stage.

I've never seen the schematic for the external compressor part and relatively few were ever made - it was an EMI Cologne/Carl Lindstrom, Peter Burkowitz designed affair.

I do know that the compressor was a stripped down version of the U73.
I'd bet it used 2 of the 188 1:1 transformers for in/out of the comp section, and another 188 1:1 as output of the side chain amp.

And with possibly an additional cathode follower to drive that input T188 transformer... ? I don't really see how the output impedance as shown on the V76S 'send' line could drive the 188 transformer without a buffer but? Maybe that little cap across the 80K resistor compensated for a high end loss from the transformer but...

Anyway, it can be done.

But the U73 is feedforward and needs careful setting up etc. to get it to rock out properly so, be warned.

 

[Potato Cakes]

The U73 was in list of compressors I was studying for this project of mine. I ran into the same issue looking at trying to adapt the RS124 where there had to be two opposite polarity audio signal hitting a remote cut off tube(s) at the input, as discussed earlier here. Of course, as was determined, this is addressed with a transformer. If I could properly incorporate either the RS124 or U73 side chain methods with this preamp and in a 2U chassis I would be coolest kid in town.

The larger scheme I am trying to figure out is how to integrate a compressor circuit with the already existing two gain stages in the V76m circuit without building two separate units in the same chassis. I'm certain it can be done but as this thread goes on I learn more about what I don't know.

 

[Winston OBoogie]

At minimum, I think you'll need two interstage/1:1 transformers. Three would also give you a transformer after your side chain amp which you'd then rectify.

If you go with an RS124 type scheme (although it won't be an RS124): I'd go out of the V76 input stage into a 1:1 (again, might need a follower/lower impedance source) and use a 6ES8 valve as the vari-mu, something around 90V on the anodes via the second 'output' 1:1 transformer centre-tap looks about OK to me according to the curves.

You could make it feedback rather than feedforward by using the U73 side chain amp to derive the control voltage - but take its input from the secondary of that 6ES8 'output 1:1 transformer rather than from the compressor input.

It's likely none of us here can give you a definitive schematic as a lot of this would be figured out with a prototype build.

Edit: If you do it, you will be the coolest kid in town

 

[Potato Cakes]

I should have started the thread with why I'm pursuing this design. I have had a Retro Instruments Powerstrip for years which has been such a fantastic tool. However, I am now needing to sell it to fund a couple of circuit board projects and a commercial product. At some point I want to build something that serves the same function as a super great sounding tube channel strip. The Powerstrip seems that it is designed as a compressor (uses a 12AX7, 12AU7, 6BC8, and a 12BH7) with high input gain so it can be used as a preamp as well, which is different from what I am discussing but the concept is still the same. I have a friend who has one of these but also says he's used a V76m and the preamp on that was significantly superior (subjectively speaking) to the Powerstrip's input section. So I thought if I could fit a V76m in a 2U case, would it be possible to also fit in a passive EQ and compressor in same chassis? I think I can, but I haven't sorted out how to utilize the V76m gain stages with these additional circuits. The one thing I have determined is that once again, I'm way over my head, and it's going to be more frustration upon frustration to find a way to make this fantasy become a reality.

Thanks!

Paul

 

[Potato Cakes]

Thanks, Winston! That's some really good info to help move this along. I have just finished up a design for an 8 channel Neve summing board (discussed on another thread) and the next thing will be to workout a V76m preamp set of PCBs (also on another thread which you are also very helpful). Once that is finished I will start prototyping the compressor section to go with it.

Thanks!

Paul

 

[Winston OBoogie]

Yep, I get it Paul.
It's a big project for anyone to get their head around.

Build the V76 sections first. Adapt the V76M to the V76S in regard to the coupling of the first stage into the second stage.
Do you really need that 1dB incremental (up to +5dB) bit of the V76M?
Put more gain positions on the 1st stage feedback if you need smaller than 6dB increments.

Get that working first. Then move onto the compressor part.

For what it's worth, you can get more gain out of the V76 output stage if need be for make-up etc., that's fairly easy.
As is, on the V76M output stage - from the EF804S input grid to the secondary of the 9:1 output transformer it's minus 4dB.

That 1st EF804S biasing is bootstrapped (same as on the input stage), so the 1K cathode sets the bias. Leave that as is. Underneath that, on the V76M is a 15K resistor, and it's the combination of the 1K & 15K which is your shunt leg of the feedback, with the 80K from the E83F anode being the series leg. That determines gain.

On the V76S, that 15K is changed to a 13K which gives a tidgy bit more gain. So, it's that lower cathode resistor that you'll change to alter your 2nd stage gain.

Edit: gain difference between V76M and V76S on the output stage really is teeny, it amounts to 1dB extra gain with the V76S so, that 15K would need to be 7K to get an extra 5- 6dB etc.

 

[Potato Cakes]

I'm going to have print out those schematics and make some notes to fully see what you're saying but I do believe I am loosely following along. Thanks for share this knowledge with me. It's incredibly helpful.

I planned on scrapping the +1dB potentiometer and +5dB rotary switch that is on the V76m.

Thanks!

Paul

 

[Michael Tibes]

If I was to persue such a project, I'd first build Ian's poor man compressor, play with it and see whether I like the sound. I love the simplicity of that circuit. I would not mind using solid state in the then probably more developed compressor sidechain. I might also choose a simpler preamp schematic, though the V76 is certainly interesting. But it is also complex and pricey and I like my recording channels to have some colour while the V76 was most likely designed to be as clean as possible.

Maybe this helps as an inspiration.

Michael

 

[Potato Cakes]

JBW - The above message should be sent to me personally. This particular thread is not the proper place for soliciting services. If you would also kindly remove this message from this thread so the discussion can be focused on topic at hand.

Thanks!

Paul

 

[Winston OBoogie]

Hey Paul,
I realize you're a ways off thinking about the EQ part of your goal, and you did mention 'passive' eq..
But before I forget my thought: It would be possible to wrap some reactive elements around that V76 output stage.
There's a fair amount of open loop gain above the closed loop in that stage so, sensible amounts of boost wouldn't be an issue I don't think.

If it interests you, inspiration might be found by looking at the old Universal 1008 valve pre which could be used with the external 508 eq in its feedback loop. A simpler version of that concept was found in the later 610 modules, and then later, the same 508 eq was used with the solid-state 1108.

Just a thought, probably back burner stuff for now

 

[solkatten]

Hi I know your idea is to implement a compressor to the V76 preamp. But I think the simple solution would be to keep preamp/compressor combo in differetial mode. Just add a extra differential gain stage at the input followed by a classic PP compressor circuit... Just a thought.. But if you aim to implement EQ as well, then forget the PP-thing...

 

[industrialarts]

I've also looked at Ian Thompson's Poor Man Tube Comp/Limiter for ideas.

Just wanted to add that I am in the process of adapting this circuit into an Ampex 600. After converting it to a couple of mic pres, there are enough tube sockets left to add the compressor circuit. Not sure how it's going to work but my aim is too make a "dirty" stereo mic/ compressor for room or overhead mics.

 

[Potato Cakes]

Hello, Everyone,

I had put this project on the shelf for some time and have recently have started looking at how to implement this project. It seems that I could lose either the last EF804s or the EF83F after the passive EQ section and which ever tube was used to make up the gain from the insertion loss would feed an interstage transformer that would phase split the signal to the input tube of the AM-864 (or perhaps a 436C instead) and I could use just one output transformer for the whole circuit. For bypassing the compressor I could use a switch on the return from the 6AL5 to the output tube cathode to remove it from the circuit.

I think I will also change out the chokes on the preamp side with dropping resistors. After doing a number of other tube builds with and without them I don't notice any audible differences so I don't see the need for the extra iron and expense.

Thanks!

Paul