renee/ken stone VCA (vca3) problems with bleedthrough

[loss1234]

I got a new ken stone/Renee schmitz vca finished this week. works good EXCEPT for a tiny (but infuriating) amount of signal bleedthrough (or whatever the term is).

when i connect say a vco to the input, connect a CV in to the cv in and connect the out to my amp or computer, even with NO cv (from envelope or whatever) i hear a tiny amount of the vco bleeding through. Its driving me nuts. Now i tried moving all the offset trimmers and the cv trimmer does what it is supposed to as far as making sure the envelope turns a note on and off. but wayyy in the background i can hear a tiny bit of the constant vco tone

advice?? is this a common vca problem? could it be that the vco output is TOO hot for the vca so some of it bleeds through?


thanks

[Blue Hell]

Some bleed is normal for analog VCAs, but too much aint ...

Ideally you should quantify this by measuring the output for the VCA fully opened and fully closed ... you could do that by recording the VCA output for those two situations and then look in the wave editor how many dB's of difference there is in the signals, that number would tell if it's good or bad

[loss1234]

well its a big difference in db. if i turn the gain up loud enough to really hear the bleed then the actual signal will distort when it plays a note that i am SUPPOSED to hear.

but it drives me nuts! this is common? are there ways to reduce it? my prophet 600 doesnt have this problem..maybe i am setting my sights too high.

anyway, thanks for the advice.

[Blue Hell]

[frijitz]

[loss1234]

fritz, so when you say isolate the audio wires, what is the best way to do this? should i use thicker gauge wire as well?

i have to get back to work tommorow but hopefully this weekend i can put your suggestions into practice (and ill let you know how they turn out)

interested in hooking the chaos up to this too.

[Rykhaard]

Loss - what are you using in the transistors section? Discrete transistors? A chip? (Can't remember the board layout at the moment and it's all downstairs.)
In my last 2 that I completed, I hand matched the trannies to 3 decimal points on my Fluke 83 and I have no bleed through at all. (That I can tell with my 25+ years of damaged ears. ) I'm far more'n happy with mine in that regard and wont use the chips for them any more.

Curious.

[Coriolis]

Hi Rykhaard, what do you match for? Hfe?

C

[loss1234]

i am using the lm394 chip...

and for the regular transistors, 2n3904's and 293906's

here is the instructions for setup. i dont get what the offset does, but it does not seem to effect this. the CV init is the trimmer which does get rid of bleed through during CV but not when there is NO CV.

thanks


"Setting up involves adjusting three trimmers.

Offset is used to set the DC offset of the output to zero. CV Reject is used to null out the effect of the control voltage on the output.

Connect a varying waveform of low frequency (triangle wave from an LFO is ideal) into the CV input of the VCA, and connect the output of the VCA to a control voltage input of a VCO. Monitor the output of the VCO. You should now be able to adjust these trimmers so that there is no modulation present on the output, and the output is at zero volts. Note that these trimmers interact to some extent, so you will need to alternately adjust them until the best result is obtained. On the prototype, CV bleed through was totally eliminated.

CV Init is used to set the VCA to zero gain at 0 volts CV input. Feed an audio signal into an input, and monitor the output. With no CV at any of the inputs, and the optional external Initial Gain pot, if used, set to its zero position, adjust this until no signal is heard. "

[Dego]

Is it still bleeding? Did you manage to fix it?

[loss1234]

stil bleeds

[jean-louise]

i have a similar problem with a VCA-2 wich i think is the predecessor. it uses a NE5543.

i have powered everything (VCO, VCF, ADSR, VCA) from a dual battey setup (+/-9V).

the VCA is very quiet and there's massive bleedthrough.

when i power only the VCA from two different batteries with only ground tied together, it is still quiet, but there's no audible bleed at all.

my conclusion would be:
the bleedthrogh creeps through the power lines.
probably something's wrong with a part of the VCA, maybe a transistor or the NE5534, i can't imagine that it's supposed to be so quiet. especially compared to the dr-110 single transistor VCA

hm

[jbaken]

Sorry to dig up an old thread but im having a similar problem. I get a bit high passed sounding bleed when I plug my vco into my cgs vca. I'm using matched transistors and separate plugs for the two modules.

[eXisteNz]

I've built the vca but i have the same problem....do you still have that bleed?