Simple mixer (GGG). Question about small input pots (10kA).

[brett]

Hi
I notice that the mixer at generalguitargadgets uses very small input pots (10k), whereas other designs use JFET inputs or high value pots (I seem to remember 250k in Penfold's mixer). 
Why only 10k in the GGG mixer? 
Doesn't this unnecessarily load many circuits, and totally prevent connecting a guitar direct into the mixer?

Thanks for any suggestions

[Mark Hammer]

The working assumption is that the output impedances of whatever will be plugged in are going to be low enough that 10k meets the 10:1 impedance-ratio standard.  Many, but not all, pedals will generally do that.  If the intent is mix straight guitar, then there should probably be some sort of buffer/gain-stage inserted before the pot.

[brett]

Hi Mark
thanks.  I looked up a few schematics, and most pedals do have low output impedance if the output pot is cranked.  But few have large output caps to ensure compatibility with low Z in the next stage if the output pot is cranked.

For example, the FuzzFace needs about 200k of unused volume pot (for output impedance) to ensure that the cutoff frequency is reasonable if the next stage has low impedance (the standard 0.01uF output cap and 200k resistance give a cutoff of 80 Hz).

For the mixer, I think I'll just swap those 10k pots for 250k, just to be sure.  Hopefully it won't add much noise, but will be high enough to eliminate load worries.

Thanks again

[David]

Hi Mark
thanks.  I looked up a few schematics, and most pedals do have low output impedance if the output pot is cranked.  But few have large output caps to ensure compatibility with low Z in the next stage if the output pot is cranked.

For example, the FuzzFace needs about 200k of unused volume pot (for output impedance) to ensure that the cutoff frequency is reasonable if the next stage has low impedance (the standard 0.01uF output cap and 200k resistance give a cutoff of 80 Hz).

For the mixer, I think I'll just swap those 10k pots for 250k, just to be sure.  Hopefully it won't add much noise, but will be high enough to eliminate load worries.

Thanks again

Brett:

I have a somewhat similar situation.  I've found that I could really benefit from a line-level mixer with 4 inputs and 3 output busses -- except the beast didn't seem to exist -- so I guess I'm going to build it.  My output busses are going to be constructed with the "Mini Mixer" you discuss.  For maximum utility, I've decided that I'd better be able to cope with both line- and instrument-level signals, so I'm going to use a singleton version of R.G.'s "Adjusticator" as my input preamps.  That's the plan, anyway...

[Mark Hammer]

Given the wide range of both impedances and levels that folks like us are generally likely to use, a buffered input stage and some padding, is a pretty sensible idea.  Something as simple as allocating a noninverting op-amp unity-gain buffer to each input and having a switchable 10k resistor in series with the input lug of the existing 10k level pots would provide a lot of flexibility.