This mini mixer.........

[Bernardduur]

Hey all!

Just a simple question; I was building this mini mixer from GGG for a friend and would love to know

Link

Would I go altering the potvalues (R1, R2) when the unit is fed with a guitar signal only? He wants to use it as a simple mixer to mix between his two guitar signals; one guitar is of lower output and this way he can regulate the designed sounds for both guitars

But I figured; would a value of 10k (log) work for the guitar signal or would I be better of raising it to a higher value??

[PerroGrande]

I am often guilty of over-complicating things... however...  your concern with the input pot value is appropriate.   The input impedance almost certainly will become an issue with the values chosen on the schematic.   Raising the potentiometer value will still cause a variable impedance to be presented to the guitar, depending on the position of the wiper on the pot (may be better, may be the same as before). 

Inserting a buffer stage before the summing amp (IC1a on the schematic) would eliminate the issue.  You could do something as simple as a FET source-follower buffer circuit or add another dual op-amp to act as a unity gain stage prior to the volume control.

[Bernardduur]

Why didn't I think of that

Great idea!

[Ben N]

Unless your friend is going to play both guitars at the same time, why not skip the mixer and make something like just an ordinary dual opamp like a TL072, 5532, 4558, etc, with one side configured as a non-inverting buffer (closed feedback loop) and the other with a pot in the feedback loop to control gain, a separate input jack for each. You can run the outputs to the output jack through a SPDT switch, or probably even just passively mix them with a couple of resistors. Jack Orman had something like this on his website, but now I think it is only on his CD; I also think there is a layout someone made for a dual MicroAmp using a dual oamp around somewhere. This is all you need, and nothing extra.
Ben

[Bernardduur]

Well, he wants to skip over and play his guitar in a wink of an eye...... so this designs was to my eyes the best......

And it has always worked for my other FX.

[Processaurus]

I drew this high input impedance mixer a little while ago, when the unsuitability of the GGG mixer for straight  guitar was brought up.  One could make a two channel version with a quad opamp and eliminate two of the channels.  Looking at it now, it may be desirable to make the output stage a noninverting amplifier rather than a buffer, to be able to get past unity gain.  That would add two resistors to set the gain and an electrolytic cap to de couple the gain.