Would a simple JFET Clean Boost cicuit, with J201 after my 10K isolation resistors do the trick to boost my Volume?
JFET amplifiers are not as simple as they seem, don't give much gain (although maybe enough for this one) and are hardly ever clean.
If not that, what about a simple LM386 gain stage?...I thought that the LM386 might dirty my signals a bit...
The LM386 is pretty clean as long as it is not driven outside it's power supply limits. It is generally used at higher gains for distortions. As it comes out of the tube,
the LM386 has a gain of 20. With a 9Vdc supply, that means the biggest signal you could reasonably put into its input to have it have a clean output is something like 400mV. So you'll need to ensure the incoming signals are *small* enough if you want it to be clean. The inputs both have 50K resistors to ground inside the chip, so you could mix with, say, 220K isolation resistors. That would cut the signal by 50K/(220K +50K) = 0.185, which would then be amplified by 20 to an overall gain of 3.7 times. Might work OK. Each output would have a gain from input jack to output jack of 0 to 3.7x.
What do you think about using a simple J201 Clean Boost circuit Vs. an LM386 circuit for a gain stage....Also, my 4 inputs can be fed into the same gain stage right?
A "clean boost", capital letters or not, is rare with single JFETs. The LM386 is a better bet, I think. Yes, with the LM386, you can use four 10K pots for input pads, each wiper going through a series 220K resistor to one of the LM386 inputs, all four to the same input. The unused input should be grounded. The output will have a DC level of half the total power supply on it, so you must use a capacitor in series with the output to eliminate this DC level to avoid upsetting whatever comes after it.
So far, so good.
However - a guitar going right into this thing will be severely loaded by the 10K input level pot. Guitars don't like driving resistances below about 100K and most people don't like the treble losses from loads much below 1M. The output of a pedal driven by the guitar is probably OK, but if you are going right into this from a guitar, it really needs a buffer. The synth has an output level of -??? some synths have 10V signals out. I don't know the level of your oscillators.