Opamp input impedance question

[gutsofgold]

I'm building a cmoy amp and it calls for a 10k pot but all I have is 500k. I've done some sleuthing on the internets and I've read that R2 (which is 100k stock) should be about 10x the value of the pot to minimize loading effects. I'm confused as to what this is referring to? I get that the pot will be the first impedance seen by whatever is feeding the cmoy amp and could definitely load down the source if too small. But why should R2 be >> than the pot?

http://tangentsoft.net/audio/cmoy-tutorial/misc/cmoy-tangent-sch.pdf

[mistahead]

Why not short the 500K with some resistors, shape the taper a little while you're at it?

Its easier to replace it later with the 10K if you don't like the outcomes, than trying to tweak the rest of the circuit to deal with one (offboard!) part.

[ashcat_lt]

Well, the series portion of the pot acts as the "top resistor" of a voltage divider with R2.  If you keep that R2/pot ratio large, you won't lose much voltage at that point.  Can't see why that's much of an issue, though.  If you're turning down the pot to put series resistance there, you're already dividing down the input voltage.  What does R2 do aside from taper that response? 

Frankly, what the hell is R2 doing at all?  In a single ended supply that resistor would be carrying the bias voltage to the positive input of the opamp.  In a bipolar supply it's not necessary.  It also helps set the cutoff point of the C2 HPF, but it's already way low, and just gets lower if you remove the resistor.  The pot dominates the input-Z here, so it ain't helping that any.

And speaking of input-Z and loading, what are we feeding this thing?  Is it meant to be a super simple speaker simulator?  That 10K input will absolutely destroy the treble response of any passive pickup connected to it, and there will also be significant broadband attenuation.  You'll still get net gain through the thing, but it'll be all mud.  Would probably be decent for most line level signals, I suppose.

[GibsonGM]

If in doubt, breadboard it, and let your ears do the deciding!  I'd want a much higher input Z, personally.   Don't know what that is so low... *shrug*

[Johan]

If you use a tl072, you could replace r2 with a 1M and use your 500k pot without problem
J