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Simple VCA Amplitude?????
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mrmrshoes



Joined: Feb 19, 2011
Posts: 73
Location: Newcastle Upon Tyne
Audio files: 4

PostPosted: Sat May 31, 2014 7:19 am    Post subject: Simple VCA Amplitude????? Reply with quote  Mark this post and the followings unread

I've just built the Simple VCA and it seems to be working, I think, But I haven't done any serious configuring yet apart from trimming A2 to mute input signal when the gain control is in a CCW position. When this is configured the amplitude on the output of the VCA is about ¼ of the input amplitude. Is this normal? I haven't matched the transistor pair yet just used two from the same batch for basic tests.

I know you can change the gain of the VCA with R10 & R11. I'm thinking I’ve got a working board and just need to configure these. Has anyone experienced the same behaviour?

I've check all component values on the board as well as for other problems like Shorts/Opens/Cold Joints and everything looks good.

The VCA is set-up for AC operation.

Any input would ready help me out.

Cheers

mrmrshoes
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Benjamin AM



Joined: Nov 04, 2010
Posts: 83
Location: Boise

PostPosted: Thu Jun 05, 2014 7:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Curious what the control input voltage is.
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mrmrshoes



Joined: Feb 19, 2011
Posts: 73
Location: Newcastle Upon Tyne
Audio files: 4

PostPosted: Sun Jun 08, 2014 9:04 am    Post subject: Reply with quote  Mark this post and the followings unread

I have figured out my problem man. I spent a couple of days checking over the board, replacing parts and looking for Shorts and opens. All the wiring looked good as-well, so I was pretty stumped.

When I was testing the VCA I had all the pots and nanner sockets attached to my panel. When I eventually remove the gain pot from the panel I realized that I had the wrong value pot (10K) instead of 100k. This explains the lack of amplitude when trimming A2 trim pot.

I feel pretty stupid. Well you can't win them all. Embarassed
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Benjamin AM



Joined: Nov 04, 2010
Posts: 83
Location: Boise

PostPosted: Sun Jun 08, 2014 11:15 am    Post subject: Reply with quote  Mark this post and the followings unread

You shouldn't be to embarrassed, it happens to everyone.
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gdavis



Joined: Feb 27, 2013
Posts: 359
Location: San Diego
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PostPosted: Sun Jun 08, 2014 8:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

If it makes you feel any better, I just realized my LFO wasn't working because I put the TL074's in backwards Embarassed
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My synth build blog: http://gndsynth.blogspot.com/
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AlanP



Joined: Mar 11, 2014
Posts: 746
Location: New Zealand
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PostPosted: Mon Jun 09, 2014 1:47 am    Post subject: Reply with quote  Mark this post and the followings unread

Hah, when I was trying to dial in my Minimoog VCF, I couldn't work out why adjusting T1 did nothing to the voltage on Q1... until I noticed the resistor attached to it was 100K, not 1k8.
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