Help me understand the gain stage of the Boss HM-2

[Vivek]

What's the benefit of this setup versus more common configurations we regularly see in most pedals ?

What is the role of R25 ?

[Vivek]

New...ish 9V battery.
Pedal switched on, input grounded. all knobs maxed.
Measured using a Fluke 117

Battery voltage measured between pin 2 (9V in), and 3 (0V) on the board: 8.27V

IC1 Voltages:
1: 4.14
2: 4.14
3: 4.11
4: 0
5: 3.63
6: 3.63
7: 3.62
8: 8.27

Q7 Voltages:
1 E: 7.58
2 C: 3.63
3 B: 6.98

[antonis]

Probably, R25 is incorrectly pointed at 9V.. 
(it should be pointed to 4.5V)

From signal point of view, it contributes (a little bit, due to particular Q7 configuration..) to Q7 stage gain by setting itself in parallel with R27..

[Vivek]

It's probably worth noting that the when the HM-2 was replaced by the HM-3 they did away with that 2 transistors + op-amp gain setup in favour of a single opamp whose gain was set by altering the feedback resistor which is a much more conventional setup.

It's IC1a in the diagram: http://i.imgur.com/dlIsUiW.jpg

They also removed the series clipping diodes, used transistor gyrators in the filter section, and maybe tweaked some values but otherwise the circuit topology is essentially the same.

[Fancy Lime]

Benefit? None, with a half-decent opamp. I would assume that this kind of booster before an opamp is a relic from the old days when opamps were pretty bad and had low open loop gain. So well before the 4558 or even the 741. The HM2 is not actually that old but I think the design choices were inherited from much older circuits from the time before cheap good opamps.

[PRR]

Advantage? Maybe someone just putting parts on until he ran out of parts or breadboard.

Think about cascading cutoff.

R25 keeps the opamp biased when Q2 has been driven mostly-off.