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 Forum index » DIY Hardware and Software
rene schmitz vca 3
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rumpofsteelskin



Joined: Apr 22, 2009
Posts: 46
Location: brighton, uk

PostPosted: Mon Nov 07, 2011 1:53 pm    Post subject: rene schmitz vca 3 Reply with quote  Mark this post and the followings unread

I know it's a little difficult to help over the 'net, but this has been driving me mad, so I though I would hand it over to the learned bods at electro-music...

I built some of Rene Schmitz VCA 3's (http://www.uni-bonn.de/~uzs159/) but they have a problem - even when the "manual control" CV pot is turned all the way to ground, there is still a signal (about 0.8 of the input signal) at the output. It seems the first BC558 is always conducting, even when I directly ground the emitter because if i remove the link between the emitter and the BC548s the signal is no more..

I've simulated it and it works fine.. anyone got any idea why is this happening?

frustrated and confused,
Philip

p.s. as far as I can tell everything is connected properly but I'll keep looking...
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andrewF



Joined: Dec 29, 2006
Posts: 1174
Location: australia
Audio files: 4

PostPosted: Mon Nov 07, 2011 5:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

This VCA, like most of them, needs some setting up before use. Have a read of the set-up procedure for the CGS VCA which is based on Rene's.
http://www.cgs.synth.net/modules/cgs64_vca.html
sounds like you need to play with the offset trimpot

Smile
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rumpofsteelskin



Joined: Apr 22, 2009
Posts: 46
Location: brighton, uk

PostPosted: Tue Nov 08, 2011 2:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Andrew, thanks for your reply. As I understand it, the signal reaches zero when there is (effectively) no current through the transistor pair. This happens when the driving BC548 is not sinking any current - which happens when there is no current driving its base - which drops very fast as the voltage between the base and the emitter of the BC558 drops below one diode drop, hence the deaf-zone elimination pot. I'm not sure either the cv or offset control would affect this behaviour

I'm thinking since reading the CGS schematic maybe I need to match the driving transistors......
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andrewF



Joined: Dec 29, 2006
Posts: 1174
Location: australia
Audio files: 4

PostPosted: Tue Nov 08, 2011 5:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Perhaps try adding a trimpot wired between +/-V for 'CV initial' (as on the CGS version), as you can wind it down to a negative voltage, this should be able to turn the trannies right off!
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