M3 Mini Synthy hard/soft switch volume drop

[FootSolder]

I built one of these using this layout http://dirtboxlayouts.blogspot.com/2021/01/m3-emthree-mini-synthy.html and it sounds great but the hard/soft switch is pretty unusable in the soft setting as there is a huge volume drop. Is there any way to increase the output in that setting? This is the original schematic, kinda faint sorry:

[ElectricDruid]

The "soft" switch is that one over on the RHS that introduces an extra capacitor, right?

Assuming that it is, it's no wonder it reduces the volume. Looking at the cutoff for the first output, it drops from 480Hz to less than 10Hz. 480Hz will let most of your fundamentals through and mostly affect only harmonics, but 10Hz is going to hit everything, and will produce >30dB reduction at that 480Hz point.

There's no obvious way to increase the level with the extra cap in. It's passive filtering, so there's no gain anywhere you could use to compensate. You could tweak the value of the 47n cap that gets added so the effect is a lot less brutal - try 2n2, 4n7, 10n instead of 47n. Alternatively, it might be possible to reduce the level of the other setting to match the quieter one (this thing is *loud*, right? it looks like it if it's running three 9V logic signals direct to the output) but it's not going to be simple and would require considerable surgery.

I think the best option might be to see if there's a different value for the 47n that provides *some* extra filtering without hitting the volume too hard. It's simplest, and while it might not be a perfect solution, if the soft setting is currently useless, anything better would be an improvement, right?!?

[FootSolder]

Thanks for the response! Yes, it's that 47n cap and your explanation makes perfect sense. The vaguely-worded, original user manual implies the switch filters out the lower octaves to create a regular fuzz tone but clearly takes a lot with it! You can actually achieve that without volume loss by simply turning down the two octaves. I will socket and try a few lower values, try to go for a 'flavor' with a bearable volume drop. Gotta love these quirky old 70s circuits!

[anotherjim]

Apart from everything Tom said, I think the output cap at 1nF is small which will gut the bass, maybe this was intentional, but I'd try 10nF if you're sure it will be driving a 1M guitar-level input.
I think a better place to fit high pass filtering to reduce bass * is in the outputs of the divider chip feeding the mix pots as these can be sized to suit each octave then the output cap can be much larger at 1uF and it'll matter (slightly) less what it's driving. If the pots are wired off the board, you could try inserting some caps in the wire from the divider. Say 1nF for the straight fuzz, 10nF for the 1st octave and 100nF for the 2nd octave but those cap values are my first guess if I was breadboarding it. One caveat is the filtering really should track the note frequency as simple filters like this only work properly in a short range of around an octave outside of which they either let too much through or reduce volume noticeably.

*High pass filtering turns the fuzzy square waves into a more synthy sawtooth(ish) wave which you then low pass filter to reduce the harshest high frequencies although a guitar cab will do that for you.

[FootSolder]

Hey, thanks for those suggestions! I ended up trying a 2.2n cap on the switch which was a big improvement but I would not bother with that switch if a build another one. I do like the idea of being able to tune the filters like you suggested, definitely worth pursuing. Thanks!

[ElectricDruid]

I would not bother with that switch if a build another one.

Not surprised. It looked a bit that way to me from the schematic!