Hi
thanks for the support and suggestions.
Consider altering the Filter control pot arrangement to the SWTC arrangement (http://hammer.ampage.org/files/SWTC.gif). This will give you the same control over tone, but without the interaction that normally occurs between filter setting and volume setting.
This is an interesting idea. I found the interaction OK. I guess that is because the filter pot is only 25k and the volume pot is 100k log. Given that the output is usually large (say 3V p-p), the volume pot will usually be set only a few k from ground. I *think* this minimises the interaction. For example, if the volume pot is set at 10k from ground, there's typically 90k of resistance "upstream", so changing the filter pot by 5k, and adding 5k to the 90k will have negligible effect on volume. (ie 10/90 is almost equal to 10/95) Is that right??
The limit to this approach of using a small filter pot and a large volume pot is that the output impedance of the previous stage must be much smaller than the filter pot.
And just out of curiosity, I note that you use U1f essentially as an inverting buffer. It is not uncommon to find use of several paralleled invertor sections in other kinds of designs as a means of creating a higher-current output. Since you have 3 invertor sections just hanging around, I'm wondering if there is any tactical advantage to making use of them in that manner.
I've used parallel buffers before (in a 4049-based sine-wave generator), and was thinking of using them here. However, it messed up my layout, and I couldn't see any advantages from extra current capacity. So I just used one inverter as both a buffer for the previous stage and driver for the filter.
If someone knows a cool use for a circuit like this that could push about 100mA, then 3 or 4 parallel buffers would certainly do the job.
thanks again for the feedback
PS thanks Marty for putting up that soundclip of the Hidrosis - really nice work