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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
renee/ken stone VCA (vca3) problems with bleedthrough
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loss1234



Joined: Jul 24, 2007
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PostPosted: Thu Jan 17, 2008 6:02 pm    Post subject: renee/ken stone VCA (vca3) problems with bleedthrough Reply with quote  Mark this post and the followings unread

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

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Blue Hell
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PostPosted: Thu Jan 17, 2008 6:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

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 Very Happy

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loss1234



Joined: Jul 24, 2007
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PostPosted: Thu Jan 17, 2008 6:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.

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Blue Hell
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PostPosted: Thu Jan 17, 2008 6:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

loss1234 wrote:
this is common?


That it drives you nuts Laughing yes that's good, it brings enlightenment Wink

From how you describe it many dB's might be over 50, assuming you can hear some street noise as well .. that'd not be really bad ... just not too good ...

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frijitz



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PostPosted: Thu Jan 17, 2008 7:47 pm    Post subject: Re: renee/ken stone VCA (vca3) problems with bleedthrough Reply with quote  Mark this post and the followings unread

loss1234 wrote:
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).


That design may not totally turn the control current off. I usually design so that there is enough leeway to provide a hard turnoff. You could try connecting a large resistor from where the three bases are connected together to the (-) supply. Something like 1M or so should help.

Also make sure your audio input signal wires are well separated from the rest of the circuit, as pickup can also be a problem.

Remember to let us know if this works.

Very Happy

Ian
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loss1234



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PostPosted: Thu Jan 17, 2008 9:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.

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Rykhaard



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PostPosted: Thu Jan 17, 2008 10:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

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. Wink ) I'm far more'n happy with mine in that regard and wont use the chips for them any more.

Curious. Smile
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Coriolis



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PostPosted: Fri Jan 18, 2008 5:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Rykhaard, what do you match for? Hfe?

C
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loss1234



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PostPosted: Fri Jan 18, 2008 5:22 am    Post subject: Reply with quote  Mark this post and the followings unread

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. "

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Dego



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PostPosted: Thu Aug 07, 2008 11:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

Is it still bleeding? Did you manage to fix it?
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loss1234



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PostPosted: Fri Aug 08, 2008 4:25 am    Post subject: Reply with quote  Mark this post and the followings unread

stil bleeds
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jean-louise



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PostPosted: Sun Aug 30, 2009 11:01 am    Post subject: Reply with quote  Mark this post and the followings unread

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
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jbaken



Joined: Feb 07, 2012
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PostPosted: Wed Apr 11, 2012 7:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.
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eXisteNz



Joined: Jun 08, 2010
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PostPosted: Fri Mar 08, 2013 4:35 am    Post subject: No solutionS Reply with quote  Mark this post and the followings unread

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



Joined: May 05, 2013
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PostPosted: Sat Jul 19, 2014 8:22 am    Post subject: Reply with quote  Mark this post and the followings unread

i just finished this VCA from Bergfotron:
http://hem.bredband.net/bersyn/VCA/lm13600%20vca%201.htm

whilst adjusting it i noticed that it's different from it's Outputs as my VCA3+ i've built...

the one VCA has about 50% of the Input signal silenced while my VCA3+ cuts nearly 100%
altough the 50% seem to be normal for that LM13700 VCA but what makes me wonder is that this Output is centered via 0V/GND while the VCA3 always put's a steady positive voltage out...
The Output of my VCA3+ is centered around +7V... is this normal? have i made a mistake?
I mean audiowise it makes not much difference but when i want to modulate a CV theres is always a positive signal of at least +7V left varying from 5,5V to 8,5V when VCA is open...
when remving the wiper of trimpot for CV rejection from +15V it is nearly 0V when closed ...
whilst the VCA is open the Amplitude swings between those ca.0V to nearly 4V or 0V to 8V depending how the Manual control pot is turned...
when i remove the wiper pot of that trimmer from +15V also the signals are a bit distorted and folded not as symetrical as with the wiper connected...

maybe important instead of a dual Transistor i used 2x BC550C i guess can't check that at the Moment as they're glued together and i had to desolder them...

is that the correct an normal behaviour of the VCA3+? Are there any mods possible to Center the Output around GND not losing the Performance in silencing the Audio signals?

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