mixing two (possibly) unbuffered audio signals

Im wanting to mix the output of two possibly unbuffered electric guitar
outputs. The two signals will be coming from 'stomp boxes' where the output
may or may not have a buffer so I don't want to drag them down.

I have found references to some simple audio mixers
(http://www.muzique.com/schem/mixer.gif) but Im not clear as to whether or
not I need to add something like an emitter follower or op-amp voltage
follower to the input lines in order to keep them from impacting the signal
source.

 

Practically all guitar boxes have buffered outputs.

No, they are very high impedance inputs, they influence very very little.

This mixer circuit is known as virtual ground mixing.
The input to the op-amp is held by feedback very close to ground.
The input signals are not influencing each other, because of that.
The signal cause some, very small, current through each input resistor,
and the op-amp sums the currents.

 

What if one of the signals is the guitar itself? Surely that would need some
buffering?

 

It is indeed advisable to use high impedance buffers for pickups, some
expect more than 100k load impedance from a tube amp, and best would be to
have these buffers as close to the guitar as possible to avoid the
capacitive loading of long cables. Otherwise a treble rolloff will occurr.
Most easy would be a dual buffer made with a TL072 opamp as a follower or
with low gain, depending on the sensitivity of your pickups. The power will
probably come from a 9V battery, so you will need capacitors on input and
output and a voltage divider on the +input consisting of two 1Mohm
resistors. With the outputs you can directly drive a passive mixer
consisting of 10k pots with 10k from the sliders to the summing point which
goes to the amp.

 

The simple answer is yes.

But you can change the input resistors on the mixer circuit we discussed,
each channel can have its own special resistor.

If you use a 500k resistor for the guitar it will work.
It needs to be high enough, 1M is normal, 500k should be no problem.

The other signals may need to be damped a little to fit together with the
un-amplified guitar signal.

If you really need to use the mixer as input stage also it is likely
that it will work.

On the other hand.. You can easily build a buffer stage for the guitar,
using only one or two fet transistors. Such a buffer can be built to give
some tube distorsion, that is it acts as a limiter, compressor.

You can easily find many schematics for such circuits on the web.
http://users.chariot.net.au/~gmarts/ampovdrv.htm
http://www.muzique.com/schem/projects.htm

There are a lot of guitar circuits at http://geofex.com/

http://www.geofex.com/Article_Folders/FXbus/fxbmod01.htm
http://geofex.com/Article_Folders/foolwfets/foolwfets.htm