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VCA Buss Voltage
Old 13th October 2013
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VCA Buss Voltage

Is 0-8Vdc good for vca bus volage or do I need more V for the range.

BTW I'm bulding all tube VCA mixer....
Old 13th October 2013
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JohnRoberts's Avatar

I've never seen a VCA made with tubes, some tube limiters vary the gain with tube transconductance similar to FET limiters.

I am not enough of a tube guy to predict useful control voltage range for tubes. Solid state VCAs generally only require mVs so several volts may be enough for tubes.

Old 13th October 2013
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thats what I have now. so if using 0-10v logic will that be too corse?
Old 13th October 2013
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must get more parts to build ch2.... and more and more

Last edited by drtechno; 13th October 2013 at 08:55 PM.. Reason: ....and more and more....
Old 13th October 2013
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then interface 10V analog to 8bit ctrL? (256 values) like daw controller fader loop...

then send stems prefader direcly out mix in real world using existing daw automation controlling tube vca via controller bus.
Old 14th October 2013
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Valves are not a good choice for 'VCA' duties as the distortion will be extreme if you get out of around 30 - 40 dB gain variation. As a VCA for 'fader' type duties you need around 100dB gain variation which combining with low distortion and noise will be near on impossible. They are OK as a control element in a compressor as the useable gain range is usually 30dB or less.
Getting 2 or more of these things to 'track' each other is yet another game so you would probably need a 'look up table' in the digital realm before a D/A feeding the valve. This could then 'straighten out' the dB/Voltage curve.
As explained in the THAT corp papers, a 'transistor' VCA relies on basic physics for it's accuracy, which can therefore be very good (certainly in the +40 to -100 dB range used as a VCA).
Matt S
Old 14th October 2013
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ok, so I need to look at making a dc servo circuit with an error amp for tracking vca vs bias current/transconductance. high voltage op amp or some hv transistors for this purpose...

thanks matt.

I change circuit in cathode now responds @0v Vca =.002mvrms -10V vca = 20Vrms
is that good enough?

now I'll work on the error amp today.
Old 14th October 2013
Lives for gear

oops, the vca voltage is backwards. 0V is full volume.
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