My Dual Tube VCA is finished
2007-02-24 by Richard Brewster
I just finished the second VCA of my dual Tube VCA. Both modules worked
the first time with no problems. I used most of the default parts
values. Mine have two attenuated audio inputs (with the output
normalled to the second input for feedback), an attenuated CV input and
an initial gain pot. I used a 270K resistor on the manual gain pot,
which goes between zero and 15V, so that the range is about the same as
a 5V CV input. With the gain all the way up, the circuit doesn't have
much more to go anyway.
The most interesting find was that the distortion is very much related
to the level of the input audio. With low input levels (+/- 2V or so),
there is not much distortion. At this level a triangle wave input comes
out looking like a triangle. The standard gain resistors give you about
70% of the input level at the output. Now, as the input level
increases, the output becomes more squared up, eventually becoming
almost a pulse wave. This wave is chopped off at about +2V, but spikes
downward well below -5V. It's a pleasing sort of distortion. If you
mix the output back for positive feedback, the distortion is accentuated
at lower input levels. At high feedback levels there is a sub-octave
effect that changes with the CV level. All in all, a neat module.
Thanks, Ken!
I have some extra 1J24B tubes. I am offering three pairs of these for
sale to buyers in the United States. $15 for a pair of tubes, shipping
included. Contact me off the list if interested.
Richard Brewster
http://www.pugix.com
the first time with no problems. I used most of the default parts
values. Mine have two attenuated audio inputs (with the output
normalled to the second input for feedback), an attenuated CV input and
an initial gain pot. I used a 270K resistor on the manual gain pot,
which goes between zero and 15V, so that the range is about the same as
a 5V CV input. With the gain all the way up, the circuit doesn't have
much more to go anyway.
The most interesting find was that the distortion is very much related
to the level of the input audio. With low input levels (+/- 2V or so),
there is not much distortion. At this level a triangle wave input comes
out looking like a triangle. The standard gain resistors give you about
70% of the input level at the output. Now, as the input level
increases, the output becomes more squared up, eventually becoming
almost a pulse wave. This wave is chopped off at about +2V, but spikes
downward well below -5V. It's a pleasing sort of distortion. If you
mix the output back for positive feedback, the distortion is accentuated
at lower input levels. At high feedback levels there is a sub-octave
effect that changes with the CV level. All in all, a neat module.
Thanks, Ken!
I have some extra 1J24B tubes. I am offering three pairs of these for
sale to buyers in the United States. $15 for a pair of tubes, shipping
included. Contact me off the list if interested.
Richard Brewster
http://www.pugix.com