Build Thread:VECA MS (VCA comp)

[Gustav]

This is a project Ive been working on for a while. Its up to PCB revision 4 revision 5 (august 1 2015), and I think its close enough to finished (it never will be, as usual) to turn it lose on the public.

Its basically a VCA comp, which originated from Jakob's (Gyraf)  work on the GssL, but it lets you control side chain mid/side stereo detection balance, and it has a few other tweaks and changes such as a blend function and hard bypass.

This is a new project, so there is no FAQ and probably some possible issues I haven't even thought about, but Ill do my best to help out if you have any questions, or if some error I did not catch shows up. This is how these things usually work....

Board for sale and build documents

Gustav

 

[Majestic12]

Gustav gets alot of questions what the MS Sidechain circuit actually does and I try to do my best to explain it

In an usual GSSL circuit the sidechain signal is made by mono summing the L and R channel of the actual audio signal. You can alter the behaviour of the compressor by altering this signal with additional filters like the HPF on a super sidechain board.

On the VECA the audio signal for the sidechain isn't summed to mono in first instance. It is first converted to M/S and you'll have control over the balance between the Mid and the Side signal. After that, the signal then gets converted back to normal R / L, is summed and can be filtered like you could do in an usual GSSL style compressor.

The cool thing now is the balance option. When taking out some of the mid signal (which is usually where most of the power of the audio signal is) you can get a similar effect as you would when you add an HPF (less compression on bass / kickdrum hits)

But with the M/S balance you can also over - emphasize the side signal  so that the compressor now more looks on what happens on the outer sides of the stereo field instead of the mid and responds accordingly.

This is not the same as an M/S compression since the audio path of the unit never leaves its L/R configuration and isn't treated different on the mid or side signal. (you can of course also feed an M / S encoded audio signal into the compressor )

I really recommend to try out this unit. It was really impressed what you can do with it and how it sounds, when I passed audio through one of the early prototypes for the first time.

 

[Gustav]

Gustav gets alot of questions what the MS Sidechain circuit actually does and I try to do my best to explain it

In an usual GSSL circuit the sidechain signal is made by mono summing the L and R channel of the actual audio signal. You can alter the behaviour of the compressor by altering this signal with additional filters like the HPF on a super sidechain board.

On the VECA the audio signal for the sidechain isn't summed to mono in first instance. It is first converted to M/S and you'll have control over the balance between the Mid and the Side signal. After that, the signal then gets converted back to normal R / L, is summed and can be filtered like you could do in an usual GSSL style compressor.

The cool thing now is the balance option. When taking out some of the mid signal (which is usually where most of the power of the audio signal is) you can get a similar effect as you would when you add an HPF (less compression on bass / kickdrum hits)

But with the M/S balance you can also over - emphasize the side signal  so that the compressor now more looks on what happens on the outer sides of the stereo field instead of the mid and responds accordingly.

This is not the same as an M/S compression since the audio path of the unit never leaves its L/R configuration and isn't treated different on the mid or side signal. (You can of course also feed an M / S encoded audio signal into the compressor, but I have no I clue what would happen.... )

I really recommend to try out this unit. It was really impressed what you can do with it and how it sounds, when I passed audio through one of the early prototypes for the first time.

The technical aspect aside, I asked Markus for a comment, since I have a hard time saying a lot about the sound and use, and stay objective since I made it and want to make some money from it (Its the GREATEST THING EVER, of course, and you can't live without it!).

He built one of the prototypes as a tester for me, so he has some insight, which is better to hear than ask me by mail

Thanks Markus!

 

[bernbrue]

Aha, that's the Fletcher Munson comp. When I understood correctly it hasn't got the turbo option built in?  So it has got a single VCA for the side chain? The MS side chain seems to be fun and provides quite a lot of sonic options. I'm following this thread with great interest. Sound samples would help a lot.  Great project, Gustav.
Bernd

 

[Gustav]

When I understood correctly it hasn't got the turbo option built in?  So it has got a single VCA for the side chain?

The second VCA (turbo) is not needed.

Read Markus description again, and that should make sense. 

The turbo, in effect, gives 1 balance of the M/S content going into the SC( The result of summing DC after two VCAs). Jakob's version is the summing before one VCA ( as AC). The VECA is summed to one VCA, but only after an MS encoder/decoder , which gives you 6 different balances, which can be controlled by the gain set to the opamp buffer between the encoding/decoding.

You could easily do it with pots or a 24 step switch for more flexibility as well, but I like to be able to recall, and the lorlin is the least exotic switch I know

Gustav

 

[XAXAU]

So arter the M/S processor the SC is summed á la Århus? No Oxford available?

Looks really good Gustav!

 

[Gustav]

So arter the M/S processor the SC is summed á la Århus? No Oxford available?

Looks really good Gustav!

I would have to repeat the same answer as just above

When I understood correctly it hasn't got the turbo option built in?  So it has got a single VCA for the side chain?

The second VCA (turbo) is not needed.

Read Markus description again, and that should make sense. 

The turbo, in effect, gives 1 balance of the M/S content going into the SC( The result of summing DC after two VCAs).

Jakob's version is the summing before one VCA ( as AC).

The VECA is summed to one VCA, but only after an MS encoder/decoder , which gives you 6 different balances, which can be controlled by the gain set to the opamp buffer between the encoding/decoding.

You could easily do it with pots or a 24 step switch for more flexibility as well, but I like to be able to recall, and the lorlin is the least exotic switch I know

Gustav

Or I could ask (might make more sense to think of it like this)

Why would you care about the VCAs on each side before summing (a la turbo mod), when you have the MS balance control before summing the side chain?

Or maybe a metaphor?

Aarhus mode:Light is bright
Oxford mode:Light is dim
MS SC:Light off, light weak, light dim, light medium, light bright, light strong

The biggest confusion I see is when people think the signal is never summed (stereo the whole way through the side chain) with the turbo mod.

This is not the case, it simply sums to mono after two VCAs, which gives a different center image weighting, since summing DC is different from summing AC.

What I tried to do here was figure a way to control the weighting without just copying Keiths mod, and the solution I came up with makes it variable.

(Might be adding to the confusion here with a text heavy post, but its a recurring question, that I would like to have cleared )

Gustav

 

[XAXAU]

Thanks Gustav!

In my head I thought the SC would first have the MS knob setting the balance and then one could choose between linked or unlinked.

 

[Gustav]

and then one could choose between linked or unlinked.

Linked or unlinked what?

Gustav

 

[Majestic12]

Maybe some more explanations:

The basic GSSL circuit is a real stereo compressor that cannot be "unlinked" to use both channels of it seperately. Both audio channels have their own VCAs that are responsible for the compression, but they are getting the exact same control voltage from the sidechain circuit. 

In case of a stereo signal this totaly makes sense since you usually want to have the same amount of GR on both sides to not change the stereo image.

The GSSL  (and VECA as well) is not a "dual mono" compressor with two complete independent channels that can be "unlinked"

It of course would be possible to make a compressor with completely seperated channels, but than you would need as well two seperated sidechains with their own time constants, makeup gain etc....

The basic GSSL circuit does behave a bit different than the original unit out of a SSL 4K console. It reacts stronger to most stereo signals. The reason is that it makes its sidechain signal out of a simple stereo sum of the L and R audio channel.
But what happens if you sum two channels that have signals in it that are common on both sides (just as energy rich kickdrums and bass signals in the middle of the stereo field are)? They power level will rise up a few dB and the sidechain now sees a mono summed stereo signal where this buildup isn't compensated.

By adding the "turbo mod" sidechain, the audio signal doesn't get summed. It will be split to basically two seperated rectifiers that will make two different DC signals according to the "power" of the audio signal on each of the L and R channel.

This DC levels are then combined and fed into the rest of the sidechain which doesn't result in a buildup of the audio levels. The compressor will be compressing a bit less (with same input signal strengh) than in the standard GSSL  configuration) That's the big difference.

But the "turbo mod" isn't the only methode to change the behaviour of the compressor. Since most audio energy lies in the low bass frequencies it is also possible to simple high pass filter the audio signal that goes into the sidechain rectifier to remove a bit of the buildup. That's exactly what happens with the super sidechain board and its HP filters.

On the VECA the buildup can be compensated with the M/S circuit. You have some strong bass / kick signals in the middle of the stereo field ? Just lower the stereo mid by adjusting the M/S knob and make M smaller.

As you can see, all those different approaches will be technically different, but lead to a similar result (less compression on strong center signals)

It's not as if the the super sidechain mod / turbo mod will make the GSSL circuit "better". They will just add new colours and possibilities to it.

On the VECA you'll have much more colours than on every modded GSSL circuit since the M/S balance can do things that aren't possible on the other mods.

 

[Gustav]

I am quite surprised the concept is not more intuitive, and I think its interesting to see how this could be communicated to the GS-type end user, so this is very informative along with the mails with the same content

I still suspect it comes from a terminology I have seen, calling the stock GssL side chain "mono", and the turbo modded side chain "stereo", which is wrong, if you get the idea the whole side chain is double (dual mono, would just be two separate compressors with no common reference!)

So

1.The VCA takes in AC, and it spits out DC.

2.Summing AC and DC gives different results.

3.Oxford mode (turbo) sums DC to mono (after two VCAs)

4.Aarhus sums (stock) AC to mono (before the single VCA)

I tried making a drawing

Gustav

 

[bernbrue]

Ok, this is supposed to be a  build thread, so discussion about what it is or what it is not leads to nothing.  So let us sum up the facts:

- it's a newly designed VCA compressor with unique and very interesting m/s side chain features that can't be found elsewhere.
- its not a clone of something else
- Gustav shares his design with us although his plans were different when I asked him a few month ago.

Buy the pcb, built it and report about your findings. I'm definitely interested and want to build it, when my  recent tube compressor project becomes less time consuming.
Thanks Gustav
Bernd

 

[Gustav]

Ok, this is supposed to be a  build thread, so discussion about what it is or what it is not leads to nothing.  So let us sum up the facts:

- it's a newly designed VCA compressor with unique and very interesting m/s side chain features that can't be found elsewhere.
- its not a clone of something else
- Gustav shares his design with us although his plans were different when I asked him a few month ago.

Buy the pcb, built it and report about your findings. I'm definitely interested and want to build it, when my  recent tube compressor project becomes less time consuming.
Thanks Gustav
Bernd

I dont mind a talk about the features being part of a build thread, but I hope the drawing clears it up. Its interesting to find ways to explain something, even if the turbo question surprised me. I will still offer these as finished builds in "other" circles as one-offs  (I assume everyone here will DIY ), so its good to think about it and how to communicate about it.  The confusion is vast by mail/facebook-messaging as well.

I ramble and get incoherent when I am reaching for the essence, so its good for me to get it boiled down to a drawing. I think that works, and Ill draw it up nicely to have in my pocket

I also completely agree - Buy it and build it  ;D

Gustav

 

[bernbrue]

Sorry, I didn't want to spoil things here with my question concerning turbo mode and my "stop the discussion" approach. The reference to the GSSL is a bit misleading, since people might think they have an über gssl with all mods included. That's why I pointed out that is a new approach and not a copy of a clone with all inclusive features. Go ahead.
Bernd

 

[Gustav]

Sorry, I didn't want to spoil things here with my question concerning turbo mode and my "stop the discussion" approach. The reference to the GSSL is a bit misleading, since people might think they have an über gssl with all mods included. That's why I pointed out that is a new approach and not a copy of a clone with all inclusive features. Go ahead.
Bernd

Nothing spoiled , only illuminated, Bernd. It was an important point to make.

Gustav

 

[XAXAU]

Makes total sense now Gustav!

Btw, how would you describe the stall function? Sort of a leveling mode can mean different things!

I'm also have trouble seeing the markings on the SC filter knob. You say that it can do de-essing but can it also do the "normal" high pass stuff?

This thing looks killer!!!

 

[Ericbazaar]

Hi Gustav,

pcb arrived yesterday,  what a huge thing 8) thanks for this great project and wish me good luck. hope i can fire up soon

best
Andreas

 

[Gustav]

Hi Gustav,

pcb arrived yesterday,  what a huge thing 8) thanks for this great project and wish me good luck. hope i can fire up soon

best
Andreas

Excellent. Do report back on any issues. Ive built about a dosen of these by now, but since its me building it every time, I may have let something lame slip through every time - only had user feedback, which has been very positive . (Some "issues" any DIYers could have could also turn out to be strange preferences on my part, so good to turn every stone )

Gustav

 

[Ericbazaar]

Hi Gustav,

in a view days i finish up my veca build. at the time i waiting for all ic's and my blue frontpanel. i have a question about the four resistors named B, MR, SB, SR inv. i don't find any info about the values. can you tell me the exact function?
in your documentation two of these are zero ohm resistors, is this right?
I use in my build two 1ma dc meter, is it impossible (in future) to show in one vu the gr for mide and in other the gr for side signal?
with a little modification?

thanks

 

[Majestic12]

Hi Gustav,

in a view days i finish up my veca build. at the time i waiting for all ic's and my blue frontpanel. i have a question about the four resistors named B, MR, SB, SR inv. i don't find any info about the values. can you tell me the exact function?
in your documentation two of these are zero ohm resistors, is this right?
I use in my build two 1ma dc meter, is it impossible (in future) to show in one vu the gr for mide and in other the gr for side signal?
with a little modification?

thanks

MB inv and SB inv have to be wire bridges and MR  inv and SR inv 10K resistors if you are using the standard gain resistor table for the sidechain.

It is not possible to show different GR values on two meters since the compressor doesn't have two seperated sidechains for each channel (and also doesn't ever split the L/R audio path into M/S to treat it independently. )

You could tap of the  sidechain signal after it is converted into M / S and display these values on seperated VU meters but that wouldn't give you any useful informations about whats going on inside the compressor.