DIY mini Volca Mixer Development

Brief

Make a small mixer for use with things like the Korg Volcas and other tiny synths.  I wanted something I could just grab and use on the sofa with minimal hassle setting up.  Ideally it should contain;

Spec

A compact mixer with between 3 to 6 inputs
Fits in a 1590BB guitar pedal enclosure
Possible Effects section
Not be too expensive

Keep reading to see the prototypes...

Version 1

An early prototype of the mixer section, 4 channels;

This version is using a cd4049 hex invertor, using just one invertor per audio channel.  Audio quality was excellent, however I want the gain to distort at higher levels so a revision was needed.

Version 2

Here is a version with gain and volume pot that reaches into fuzz (using two channels of the chip and feeding it back into itself) on each of the 4 mixer channels.  I also added a delay and a basic plate reverb.  It was actually sounding pretty good but there was a couple of problems that appeared;

1. The battery ran out while testing.  The two PT2399 chips used in the delay and reverb are known to be pretty thirsty for power (not too sure the combined CD4049s?), but to continue using this version I'd probably need to add a power jack, which leads to the next problem;

2.  It'll be a struggle to fit all the into a guitar pedal enclosure.

A rethink was needed, however I may revisit this design at a later point to make some sort of multi-effect unit.  To make a smaller version I'd have to either lose a few features or input channels.  It might be possible to stack two pcbs on top of each-other, but then the cost would increase.  At this point I started to think what would be cool if it could run without batteries, just a simple passive mixer;

Version 3 

Here is a simple 3 input passive mixer.  Because it's passive the sound remains pretty much untouched as it leaves the device (what goes in comes out).  This design requires no power and leaves lots of space inside a guitar pedal, whilst also being simple to construct.  Depending what else gets added, it might be possible to fit 6 inputs into this version.

Version 3.1

When I record on the sofa, I mostly still end up putting the final track onto a pc to trim up/ compress and add reverb, so I don't really need these in a mixer (plus it'll be near impossible to compete with the quality of PC based effects) but something like a delay I use during recording as it can create rhythmic effects, which can influence the direction of the song.  If it's possible, I'd at least like to add a delay to this unit.  The current idea is that when switched off, the mixer will still function as a passive mixer, but when turned on, you'll be able to add delay to the output (possibly per channel, but this might not be possible due to space restrictions).  Also, depending if it's needed, whilst going through the delay/powered circuit, I might add a small headphone amp.