this has puzzled me for a while, and I need to solve it before I get to pulling my BMP apart for some mods.
The gain stages of the BMP have a series resistor into the transistor base (e.g. R7, usually 8.2k, in the schematic at Tonepad), and a feedback resistor (R9, usually 470k, but also 390k). There is also a feedback cap, but for simplicity let's ignore it, at least for now (and the diodes, too). Other than that, it's a common emitter stage, with a medium sized drain resistor (R10, 15k, sometimes 12k) and an emitter resistor (R11, 100 ohms, but as high as 390 ohms in the second stage of the "Russian Green" BMP).
As a regular common source stage, the gain would be about R10/R11 = 15k/100 = 150. But the signals through the input resistor and feedback resistor meet and partially cancel at the base coz they're out of phase (ok, that's dead obvious
I'm wondering whether, maybe, because the same input impedance (to the BJT + "base grounding" resistor) is seen by the feedback signal (via the feedback resistor) and the input signal (via the input resistor), whether the gain stabilises at the ratio of the feedback to input resistances (ie 470k/8.2k = 57), so long as this is less than the upper limit set by Rd/Re (150). (A bit like an inverting opamp). But this is way too high for the three stages in the BMP (50 x 50 x 50 = a ridiculous number).
But then... I thought that the total output impedance between the output of the previous stage and the "mixing point" might be more relevant. That would be approximately the sum of the input resistor and the drain resistor, or 470k/(15k+8.2k) = 20. Yes, three stages of 20 would give a total for three stages of 8000, which would be out of control without the sustain pot and filtering reduce the gain to a workable level.
Does this sound right? Can anyone help me out?