Completely lost with VCA spill

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felix le chat
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Completely lost with VCA spill

Post by felix le chat » Mon Mar 08, 2021 3:08 pm

Hello

In my modular system the VCAs don't work properly for controlling the audio amplitude. When they are supposed to be silent you can still hear some sound.

I have 3 VCAs at the moment:
- Doepfer A-130-2 (this one I got today because of this problem, and it does seem to always work)
- STG .VCA (https://www.modulargrid.net/e/stg-soundlabs-vca)
- Malekko VCA (https://www.modulargrid.net/e/malekko-h ... try-vca-v1)

I did many tests but don't understand what's going on. Nothing changed in my room nor my modular system. I am almost sure it did not happen before, or at least there was a lot less spill so that it was not a problem for recording songs. On a few occasions there was some spill from low pass gates, which is generally considered as something normal that can happen, and I did correct this using either the Malekko or STG VCAs after the LPG.

I wanted to post here because maybe someone will have an idea reading the test below. Maybe it's so obvious I am missing it.

Test patch:
* VCO output connected to VCA in
* VCA output connected to loudspeakers
* A-190-5 gate output connected to Intellijel Triatt input A
[Initially, Triatt channel B and C = zero volt, but we use channel B as an offset to the gate (not the envelope) later in the test]
* Triatt C output connected to Maths (red version) channel 1 slew input (" -->")
* Maths channel 1 output connected to VCA CV input
* Case = Doepfer suitcase with A100PSU2

Results

* If VCA = "Malekko VCA" (linear mode), there is some spill except when using these workarounds:
1) add a slight negative offset to the gate (using channel B of the Triatt: there is a threshold under which the spill suddenly disappears)
OR
2) do not put the Malekko VCA gain at maximum (there is a threshold under which the spill suddenly disappears)

* If VCA = "STG .VCA", there is some spill except when using workaround 1) above

* If VCA = "Doepfer A-130-2": No spill (well TODAY AT LEAST -- I just plugged it for the first time 1h ago)


The A-190-5 is not the problem, this happens with any positive gate. No problem with bipolar gates (eg Dixie 1 square output in LFO mode).


This situation is worst when using the Maths as amplitude envelope controlled by its "-->" input (no problem with the trig input), this is why I am describing this test with the Maths.
But when using other envelopes instead of the Maths:
* Generally
- the Malekko VCA has no spill problems (even with gain at maximum and no Triatt negative offset)
- the STG .VCA has lots of spill problems.

* Sometimes
- the Malekko VCA does have spill problems
- the .VCA works.

But most of this is surprisingly difficult to reproduce and I cannot find a logic behind it.

If someone has an idea, thanks a lot

EDITED: grammar, missing words, etc
Last edited by felix le chat on Mon Mar 08, 2021 3:40 pm, edited 1 time in total.

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Re: Completely lost with VCA spill

Post by studio460 » Mon Mar 08, 2021 3:37 pm

FYI: Another member here reported hearing bleed on the Malekko VCA.
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Re: Completely lost with VCA spill

Post by felix le chat » Mon Mar 08, 2021 3:44 pm

studio460 wrote:
Mon Mar 08, 2021 3:37 pm
FYI: Another member here reported hearing bleed on the Malekko VCA.
Thanks
Do you know in which context it happened? (which envelopes, case, etc)
Is it really with that particular Malekko vca? (see the link above)
Maybe you have a link?

Suddenly I am having lots of trouble matching gates-->envelopes-->vcas so that there is no sound leaking when the vcas are "closed"
Last edited by felix le chat on Mon Mar 08, 2021 4:17 pm, edited 2 times in total.

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Re: Completely lost with VCA spill

Post by sir stony » Mon Mar 08, 2021 3:46 pm

Sounds like your bus might have a ground issue, so there are a few mV offset among the bus boards or certain portions of the bus boards. This can happen when the common ground in the rack is insufficiently wired, or for a example, a connector is bad or a solder point half broken... something like that.

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Re: Completely lost with VCA spill

Post by jorg » Mon Mar 08, 2021 3:48 pm

Maybe something happened with your power supply? I could imagine a VCA leaking if its supply voltage is low, or possibly the performance of the envelope generator could suffer in that situation.

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Re: Completely lost with VCA spill

Post by felix le chat » Mon Mar 08, 2021 3:58 pm

sir stony wrote:
Mon Mar 08, 2021 3:46 pm
Sounds like your bus might have a ground issue, so there are a few mV offset among the bus boards or certain portions of the bus boards. This can happen when the common ground in the rack is insufficiently wired, or for a example, a connector is bad or a solder point half broken... something like that.
Thanks for you answer

Yes it looks like something related to ground. I'm not an expert of it, because it never happened to me in the past, except ground loops involving a computer and a Rhodes, that I removed using a passive transformer.

Should I remove all modules and check all pins with a multimeter?

That said actually I have two Doepfer Suitcases and two STG .VCAs, and at the moment each STG .VCA is in its own case. The problem is exactly the same with both cases and STG .VCAs

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Re: Completely lost with VCA spill

Post by felix le chat » Mon Mar 08, 2021 4:03 pm

jorg wrote:
Mon Mar 08, 2021 3:48 pm
Maybe something happened with your power supply? I could imagine a VCA leaking if its supply voltage is low, or possibly the performance of the envelope generator could suffer in that situation.
As I answered to sir stony above, it happens with 2 different cases and 2 different .VCAs

Yes I thought about it because after over 10 years these cases are almost full of modules, but these are analogue modules only (except ES-3 and A-190-5 obviously -- but all digital audio is done inside the computer here), and my system is not a bunch of 2-4 HP modules that all need lots of current, so there are not a lot of modules, I did not check but it should be well below the limits of the PSU

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Re: Completely lost with VCA spill

Post by Pelsea » Mon Mar 08, 2021 4:11 pm

This is a common problem with analog exponential VCAs. The output curve approaches 0 as a limit, but doesn't actually get there. There is usually a trim pot to fix this-- it does as you are doing and adds a bit of negative DC to the control input. It may be labeled "offset".
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Re: Completely lost with VCA spill

Post by sir stony » Mon Mar 08, 2021 4:18 pm

felix le chat wrote:
Mon Mar 08, 2021 3:58 pm
That said actually I have two Doepfer Suitcases and two STG .VCAs, and at the moment each STG .VCA is in its own case. The problem is exactly the same with both cases and STG .VCAs
This makes it much more unlikely that my guess was right. Good ground is always the best foundation for a working modular, but same issue on two different cases? Did you add these VCAs just recently, or have you been using them for a long time and only this problem is new to you? Modular troubleshooting can be a long and rocky road...

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Re: Completely lost with VCA spill

Post by felix le chat » Mon Mar 08, 2021 4:36 pm

Pelsea wrote:
Mon Mar 08, 2021 4:11 pm
This is a common problem with analog exponential VCAs. The output curve approaches 0 as a limit, but doesn't actually get there. There is usually a trim pot to fix this-- it does as you are doing and adds a bit of negative DC to the control input. It may be labeled "offset".
Thanks Peter, but I am not sure to understand. The STG .VCA is a linear VCA only, and while the Malekko VCA can be switched between exponential and linear the test above was done in linear mode.
The Malekko VCA does not have any problem in exponential mode here (well, as related to leaking: because honestly, I have yet to understand how the Malekko VCA can be used in exponential mode. For me it's just silence or huge distortion, even with the trim pot that allows to "tune" (for the lack of a better word) the exponential gain; or if you prefer it sounds like f(x)=x^16 for x between 0 and 1).
The Doepfer VCA works in any mode with any envelope.

Regarding trim pots:
* the STG .VCA has two trim pots labeled T1 and T2
* the Malekko VCA has a trim pot labeled R18

There is no documentation (which is a shame in itself by the way) and I only vaguely know from an ancient forum post that R18 is for trimming exponential response or gain

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Re: Completely lost with VCA spill

Post by studio460 » Mon Mar 08, 2021 4:40 pm

felix le chat wrote:
Mon Mar 08, 2021 3:44 pm
Thanks
Do you know in which context it happened? (which envelopes, case, etc) . . .
Sorry, I can't recall. I think it was in the "quad-VCA" thread, so I believe it was the Malekko quad-VCA. Don't remember the context, but the comment was fairly brief without many details.
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Re: Completely lost with VCA spill

Post by felix le chat » Mon Mar 08, 2021 4:48 pm

sir stony wrote:
Mon Mar 08, 2021 4:18 pm
felix le chat wrote:
Mon Mar 08, 2021 3:58 pm
That said actually I have two Doepfer Suitcases and two STG .VCAs, and at the moment each STG .VCA is in its own case. The problem is exactly the same with both cases and STG .VCAs
This makes it much more unlikely that my guess was right. Good ground is always the best foundation for a working modular, but same issue on two different cases?
Yes indeed, plus when it works (for example with the Doepfer VCA) it does work perfectly, and the electric installation is new here
Did you add these VCAs just recently, or have you been using them for a long time and only this problem is new to you?
It is a new problem. Most of this modular system is quite old and I never had such problems before (nor ANY problem actually)
Modular troubleshooting can be a long and rocky road...
Well I hope to find a solution because I have some work to do with it just now!

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Re: Completely lost with VCA spill

Post by felix le chat » Mon Mar 08, 2021 7:25 pm

Update if it helps

I have a Richter Oscillator 2 and a Wiard Anti-Oscillator (Richter too, but this is the old version), both Eurorack versions built by Malekko. If these are on the same power bus board, you can hear the Anti-Oscillator leak into the Oscillator 2 even if only the Oscillator 2 is plugged to the loudspeakers (I mean just one cable really, both oscillators are not even patched together and no other module is used). So I plugged one of them in another bus board and it solved the problem

But now I discover that the Anti-Oscillator can also be heard in the Malekko VCAs and STG VCA (the one that is in the same case as the aforementioned oscillators) when they have leaking problems. And when I tested today before writing the first post, this particular problem did not happen. Well perhaps the town electricity is different after midnight or something? :mrgreen:
The Malekko VCA is plugged in the same power board as the Anti-Oscillator and the STG VCA is plugged in the same power board as the Oscillator 2. It's late and I cannot check all power board vs half-working modules combinations just now. Maybe tomorrow if it is useful, but I'm not keen on placing modules at crazy places in the rack just because they do not work as expected

And "of course", the Doepfer VCA still works perfectly

Perhaps I should precise: this system was never used live, only very carefully at home (as this is my most expensive instrument) and it is in mint condition

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Re: Completely lost with VCA spill

Post by adaris » Mon Mar 08, 2021 7:48 pm

I've definitely had to put some oscillators / LFOs on different power supplies to avoid interference with each other and other sensitive modules, including but not limited to the Richter Oscillator II. I don't have any large cases though, just a bunch of smaller ones, so separating certain modules hasn't really been a problem for me.

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Re: Completely lost with VCA spill

Post by Pelsea » Mon Mar 08, 2021 10:07 pm

If those were my modules, I would try tweaking the trimmers (you can’t get into much trouble with a VCA, unlike with VCOs). First, ground the CV input somehow. Mark the initial position of the trim (a graphics pen is good for this). Apply an audio signal. Now turn the trim slightly. If you don’t hear any difference, put the trim back where it was and try the other one.
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Re: Completely lost with VCA spill

Post by moremagic » Tue Mar 09, 2021 4:33 am

felix le chat wrote:
Mon Mar 08, 2021 7:25 pm
Update if it helps

I have a Richter Oscillator 2 and a Wiard Anti-Oscillator (Richter too, but this is the old version), both Eurorack versions built by Malekko. If these are on the same power bus board, you can hear the Anti-Oscillator leak into the Oscillator 2 even if only the Oscillator 2 is plugged to the loudspeakers (I mean just one cable really, both oscillators are not even patched together and no other module is used). So I plugged one of them in another bus board and it solved the problem

But now I discover that the Anti-Oscillator can also be heard in the Malekko VCAs and STG VCA (the one that is in the same case as the aforementioned oscillators) when they have leaking problems. And when I tested today before writing the first post, this particular problem did not happen. Well perhaps the town electricity is different after midnight or something? :mrgreen:
The Malekko VCA is plugged in the same power board as the Anti-Oscillator and the STG VCA is plugged in the same power board as the Oscillator 2. It's late and I cannot check all power board vs half-working modules combinations just now. Maybe tomorrow if it is useful, but I'm not keen on placing modules at crazy places in the rack just because they do not work as expected

And "of course", the Doepfer VCA still works perfectly

Perhaps I should precise: this system was never used live, only very carefully at home (as this is my most expensive instrument) and it is in mint condition
my anti oscillator has bled thru the output in some configurations, eventually i rearranged my case to suit it & its been happy since, but id expect it to be the culprit instead of your vcas

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Re: Completely lost with VCA spill

Post by auditorycanvas » Tue Mar 09, 2021 5:57 am

studio460 wrote:
Mon Mar 08, 2021 4:40 pm
felix le chat wrote:
Mon Mar 08, 2021 3:44 pm
Thanks
Do you know in which context it happened? (which envelopes, case, etc) . . .
Sorry, I can't recall. I think it was in the "quad-VCA" thread, so I believe it was the Malekko quad-VCA. Don't remember the context, but the comment was fairly brief without many details.
Might have been me. I had a Quad VCA from Malekko that had bleed one 3 of the 4 channels at 0. 1 channel was a lot worse than the other 2. Tried it in different cases to rule out any power issues. Took it back to the store, they heard the same thing. Tried 2 others they had in stock, and both had similar issues. No visible trimmer to be seen unfortunately. Tried a quad there from another brand, but that also had slight leakage at 0 on one of the channels. Ended up taking home a Doepfer 8 channel VCA (A-130-8) that was silent at 0 across all channels.

The bleed on the Malekko was relatively low, and in a track with other stuff going on, you wouldn't notice it, but when using it solo, it was noticeable enough to not be usable.

That said, it was there from the start out of the box, as were the others I tried in the store. Seems odd that your issue recently developed across 2 VCAs. I'm clutching at straws, but is it possible you had a power surge or something that may have caused damage to some components?

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Re: Completely lost with VCA spill

Post by felix le chat » Tue Mar 09, 2021 11:12 am

adaris wrote:
Mon Mar 08, 2021 7:48 pm
I've definitely had to put some oscillators / LFOs on different power supplies to avoid interference with each other and other sensitive modules, including but not limited to the Richter Oscillator II. I don't have any large cases though, just a bunch of smaller ones, so separating certain modules hasn't really been a problem for me.
I have a 9U and a 6U Doepfer suitcases (A100PSU2 version) = 5x84HP
I could split it in four 3U cases and one skiff case for example, if it helps (but it would cost a lot of money)

See my answer about the Richter VCOs in my next post

I have a question about the interference you are talking about:
Is it something specific to Eurorack modular systems? Or specific to analogue systems in general? Does it also affect 5U, Moog, Buchla etc systems?

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Re: Completely lost with VCA spill

Post by felix le chat » Tue Mar 09, 2021 11:16 am

moremagic wrote:
Tue Mar 09, 2021 4:33 am
my anti oscillator has bled thru the output in some configurations, eventually i rearranged my case to suit it & its been happy since, but id expect it to be the culprit instead of your vcas
The Anti-Oscillator output leaking only happens from time to time, this is unpredictable. I have no idea how to reproduce it on purpose. Yesterday it happened after midnight and stopped about 1h later

When removing both the Anti-Oscillator and Oscillator 2 from the case, the STG and Malekko VCAs still have leaking

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Re: Completely lost with VCA spill

Post by felix le chat » Tue Mar 09, 2021 11:30 am

Pelsea wrote:
Mon Mar 08, 2021 10:07 pm
If those were my modules, I would try tweaking the trimmers (you can’t get into much trouble with a VCA, unlike with VCOs). First, ground the CV input somehow. Mark the initial position of the trim (a graphics pen is good for this). Apply an audio signal. Now turn the trim slightly. If you don’t hear any difference, put the trim back where it was and try the other one.
On the STG .VCA:
- the T1 trim pot changes the level (either output, input or CV level) and has no effect on leaking
- I could not find what the T2 trim pot does (no level change, no effect on leaking)

This means I cannot use this module anymore, or perhaps only for CV


On the Malekko VCA the only trim pot is supposed to change "the maximum that the exponential CV input will open the VCA to (to avoid clipping)" (see here: viewtopic.php?highlight=&postdays=0&pos ... =0&t=48113) and "it only affects exp mode" (see viewtopic.php?t=46335)

That said reducing the panel gain knob does the job (under about 4PM the leaking suddenly stops)

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Re: Completely lost with VCA spill

Post by felix le chat » Tue Mar 09, 2021 11:40 am

auditorycanvas wrote:
Tue Mar 09, 2021 5:57 am
Seems odd that your issue recently developed across 2 VCAs. I'm clutching at straws, but is it possible you had a power surge or something that may have caused damage to some components?
Actually it affects two Malekko VCAs and two STG .VCAs, so at the end it's 8 problematic VCAs. There is a workaround for the Malekko (see my previous post) but not for the STG.

My other modules don't have any problem

I am not familiar with power surges. How can it happen exactly? When not in use, the system is switched off and even unplugged actually. When in use I never remarked anything strange.

Also how could something damage only the 4 aforementioned dual VCAs so that they have exactly the same behavior after the damage?

Isn't it possible that these modules are simply too old and some components have to be replaced, like on old analogue mixers for example?

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Re: Completely lost with VCA spill

Post by Pelsea » Tue Mar 09, 2021 12:14 pm

Have you done anything that affects the loudness of the system? New speakers for instance? Or made your listening room quieter? It’s possible this was there all along and was not audible.
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Re: Completely lost with VCA spill

Post by adaris » Tue Mar 09, 2021 12:20 pm

felix le chat wrote:
Tue Mar 09, 2021 11:12 am
adaris wrote:
Mon Mar 08, 2021 7:48 pm
I've definitely had to put some oscillators / LFOs on different power supplies to avoid interference with each other and other sensitive modules, including but not limited to the Richter Oscillator II. I don't have any large cases though, just a bunch of smaller ones, so separating certain modules hasn't really been a problem for me.
I have a 9U and a 6U Doepfer suitcases (A100PSU2 version) = 5x84HP
I could split it in four 3U cases and one skiff case for example, if it helps (but it would cost a lot of money)

See my answer about the Richter VCOs in my next post

I have a question about the interference you are talking about:
Is it something specific to Eurorack modular systems? Or specific to analogue systems in general? Does it also affect 5U, Moog, Buchla etc systems?
I'm pretty certain this issue isn't unique to Eurorack but as to whether it's less prevalent in other types of modular systems or not I can't really say, as I don't have any experience with the other types of modular you mentioned. Also I'm definitely not an expert on power supplies, which I think are part of the equation when it comes to this kind of thing.

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Re: Completely lost with VCA spill

Post by R.U.Nuts » Tue Mar 09, 2021 1:06 pm

Another aspect to consider: You use Maths as an envelope. If you use the slew input and not the trigger input Maths is sensitive to the gate amplitude. If the gate amplitude isn't zero volts in the gate's low state Maths will put out that voltage. If the low state level is positive Maths will open the VCAs. So the problem could be related to your gate signals. The fact that you don't get spill if you use Maths' trigger input further strenghthens this assumption since when using the trigger input Maths is not sensitive to the gate's amplitude. Do you have any other envelope generator in your rack to check this?

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Re: Completely lost with VCA spill

Post by felix le chat » Tue Mar 09, 2021 1:15 pm

Pelsea wrote:
Tue Mar 09, 2021 12:14 pm
Have you done anything that affects the loudness of the system? New speakers for instance? Or made your listening room quieter?
Yes (new place, full acoustic treatment (no soundproofing though), new speakers), but it was several years ago and I did not remark anything wrong before
It’s possible this was there all along and was not audible.
It is very likely, for these reasons:

1) Normally for audio amplitude I use the Malekko VCAs, because they have a knob that allows some mixing, which is useful when running several voices in parallel (= polyphony with different sounds, which includes drums and percussion). I know this is a CV input attenuator (not output attenuator), but this is a clean linear VCA so it's ok to use it as a volume control. So most of the time when I used it the knob was not at maximum, but well under the leaking threshold I found yesterday.
As I have two of these dual Malekko VCAs I can have up to 4 synth voices, and given the size of my system I never ran out of Malekko VCAs.

2) Given the precedent point, normally I use the remaining STG VCAs for CV control. This makes sense because most of my modules have CV attenuators, otherwise I still have a Triatt, Maths, etc that can do the job. Using the STG VCAs for CV makes leaking less obvious to spot because once patched you focus on the sound and naturally compensate the leaking directly in the module you are controlling.

3) I am not recording or playing everyday, and when doing it I am not always using my modular system. It makes less opportunities for discovering new problems


So as a result I have too many VCAs: 8 brand new that work perfectly, 4 old that can work with a simple workaround, and 4 that can only be used for CV

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