first, demonstar rules.

if you didn't understand what demonstar wrote previously, then read on and maybe this will help you. if you did understand demonstar, which i didn't, then maybe don't read on. i am writing this as a sort of penance.

i have a better handle on input impedance analysis after spending some more time on it. R.G.'s article on this is making more sense to me now. here's my understanding of how to think about the dod overdrive 250/mxr distortion plus. as already agreed, the noninverting input of the op amp has such a large input impedance that we can ignore it. so here is a redraw of the original schem with the op amp removed and everything arranged in a way that is more suggestive for input impedance analysis.

so what you see is the pulldown resistor parallel to all the stuff for biasing the op amp. except notice that i have replaced the voltage supply with a ground at the bottom of R7. for analyzing AC, all constant voltage sources look the same as ground. the quick explanation is that the capacitor C1 "blocks" DC and the constant voltage source just lifts the over all level of voltage. it doesn't affect the wiggle in the AC.

the schematic is in a form that is natural for input impedance analysis because we are basically asking what the path to ground looks like at the input of the circuit.

now for my simulations above i through out the decoupling caps in the power supply. that, as i will explain, was a mistake.

so here's effectively what i was using for my simulations in LTSpice:

if you review my previous post, you will see that i left out the pull-down resistor, and it's omitted above as well. in this schem, it's pretty easy to see that the input impedance is the sum of the divider resistors in parallel and the bias resistor R3. R7 and and R2 are small enough not to matter much in the calculation. and C1 doesn't matter much either. at the frequency f, the effective resistance of a capacitor is 1/2πfC, where π is pi, and C is the value of the capacitor. at 4KHz, that's about 4K ohms, which is also negligible. so a quick approximation to the input impedance in this schem is R3 + (R8 x R9)/(R8 + R9) ohms and that's what the simulations were giving me.

but if you back up to the first schem, and use the same calculation for the other capacitors, and note that the effective resistance of C7 is 4 ohms at f = 4KHz, then you see that the voltage divider doesn't even enter the calculations once the decoupling caps are in place. and that, after all, is what they are supposed to accomplish.

C7 is like a short to ground. so what i really should've looked at is this simplification:

i have put the pull-down resistor back in. C1 is still approximately a short. so we have that the pull-down resistor and the bias resistor in parallel set the impedance of either the dod overdrive 250 or the mxr distortion plus. which is what demonstar said previously.

given that the circuits have the same bias resistor (per tonepad), they have the same input impedance and we should expect them to sound pretty much the same. general guitar gadgets has different bias resistor values, 470K for the 250 and 1M for the dist +, so i suppose that could make a difference for a high output impedance pickup.

now i am going to crawl back under my rock and study some more. all the best, gm