Can I build a simple active mixer using just MPSA18s?

[aeiou]

Is there anyway to build an active 4 channel mixer using just MPSA18 NPN transistors? Reason being -- It's all i've got on my desk at the moment.. that or a bunch of PNP Germanium transistors, which I'd be open to using as well.

[~arph]

Depends if you have any caps or resistors laying around 

[aeiou]

yes yes, plenty of those here -- what i should have said is that those are the only transistors i have on tap.

[~arph]

 
Yeah I thought so..

I see no problems building this mixer..

Build five buffers.. four of them each have their own volume control and then a cap. 
All outputs of those buffers (after the cap) connect to the input of the fifth (no input cap needed ).
You can even add a little gain at the end.

[aeiou]

so i wouldn't encounter any issues dropping MPSA18 (bjt) into this JFET circuit?

http://www.muzique.com/schem/eagle.htm

[~arph]

I don't think that will work directly as that is a BJT for FET substitution and you might run in biasing issues. I suggest you use a simple BJT buffer instead. But it should be very similar.

[aeiou]

here's one i found that calls for a bjt --

http://www.hawestv.com/mtv_faq/amplifier_art/buffers_bjt_fet_5v.png

any good?

[~arph]

Bit low on the input impedance.. try the third one from the bottom..:

http://www.muzique.com/lab/buffers.htm

[Nasse]

http://electroschematics.com/97/simple-mixer-circuit-schematic-common-base/

old elektor application

edit...

http://electroschematics.com/2917/audio-mixer-with-one-transistor/ and another, looks like one that I built many years ago

[PRR]

Four resistors will mix.

But the sum is weaker than the sources.

If your system has gain in reserve someplace else, no "active" is needed.

If not, you want a gain of 4 or so. Easily gotten with any BJT/FET in an appropriate circuit.

[Jdansti]

Would four resistors going into a Tillman work well?

[Perrow]

If you happen to have an dual op-amp laying about...

http://generalguitargadgets.com/projects/15-boostersrouters/68-mini-mixer-project