Very low gain on Dual VCA log mode.

[defex]

I built up a dual MFOS vca board, everything went together very well, and both sides work nicely in linear mode.

problem is, on both sides the gain on the log mode is very low.
i could increase it enough so i can see one volt p-p on the modulated signal on the output by decreasing the resistor r14/34 to 3k but obviously that is not the correct solution.
(and the waveform does look right, just very low.)

I was wondering if there are preferred 2N3905/3906 transistors to use, i just picked "Micro Commercial Co" ones on digikey (cant remember why, they are not even the cheapest ones)

i have ordered some ON semi ones.

i can not really think of anything else to try.

[forshee]

Are you running on 12 or 15v? I'm running at 15v and there are some quirks I haven't totally worked through. On mine there is a huge amount of gain on tap in the lin mode much less in log. Also the zero point of the gain knob is different in different modes (seems like thats by design since Ray mentions it).

Transistors should be fine. Check that your board version is the one your built. I put the new resistor values on the old board.

What is the range on your offset pot?

Good luck.

[defex]

FIXED

Now it is working fine. the 2N3906s were "funny" i dont know if i had heated them up when building the board or what. (i was very careful though) new ones are ON semi and the old ones were a brand i have never even heard of.

(also running at +-15v)

changed the resistor i was farting about with back to original 120k
there is a lot more "zeroable" amplitude on the log side, i can only get about 6v p-p with the linear mode, It doesnt seem to matter that much because on average it is on for longer in linear mode and sounds a lot closer in level than the scope makes it appear.

If i decide to adjust it, I expect i can probably tweak the r24 gain resistor for linear drive op amp. i have the different zero points as well, but I can live with that.