some VCA questions

[isak]

hi Yves

i built the VCA, it works great.
i have few questions tho...

question 1:
is it normal that in high freq the sine wave will be like tri or some strange wave form?
example..







question 2:
why with AC mode i need to use log pots?
now i use lin pots and it seems to work ok.
any special reason for that?

question 3:
it seems that the vca is a bit "distorted", is it normal?
It's distorted not in aggressive way, only a bit, like a very gentle overdrive.
thanks in advance.
Isak E.

[yusynth]

It is normal that a VCA with such a simple design add some distortion but this kind of distortion is weird indeed.
What is the input level ?

[isak]

Thanks for replying.

I'll check layer on and let you know.

Cheers.

[yusynth]

I assume that the upper trace on the scope is the output and the lower trace the input ?

[isak]

Indeed.
The lower is the source the upper is the output.
As you will see the freq counter is on the upper left of the scope screen.
As I get the freq higher the wave changes the shape.

[isak]

[yusynth]

You can measure the peak to peak voltage with your scope. Since you set the voltage range of the inputs (number of volts per division)

[yusynth]

I checked on a yusynth VCA with a 19.3kHz input sinewave 20Vpp and I obtain (with the GAIN pot at maximum) a slightly rounded sinewave (normal distortion) and a perfect sinewave if I slightly reduce the GAIN pot level.

I think that may be you have a problem with your scope probe (not correctly set for high frequency). Check that there a small screw on your probe and turn it and check if that changes the shape on the scope.
Read the manual of your scope and go to the chapter dedicated to calibration of probes.

[isak]

thanks Yves.

i answered in the Arp post, i think the problem is related with both.