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Here is the schematic to my new Behringer UM300 guitar distortion pedal. Most of the circuitry is pretty straightforward (a lot for the $22 I paid!) but can someone explain what the emitter follower circuits are doing in the feedback paths of op amps IC2A and IC2C? I'm guessing it's some kind of frequency response modification, but?

Also I'm thinking of copying the BOSS SD-1 pedal's "asymmetric clipping" by adding a second diode in one of the clipper legs. Anyone tried this?

Edit: @G36 just answered my question but I'm including the following for completeness…

I'm asking what transistor T2 and the associated components are doing in op amp IC2A's feedback path. The situation with IC2C is similar but there are two of these emitter followers in the feedback path.

enter image description here

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  • \$\begingroup\$ Rob, it's very nice that you provided a PDF schematic. Granted. But it may help some who either don't want to examine a full schematic or necessarily go to a PDF (not everyone may wish to select the link and attempt it) if you'd provide a subcircuit here using the schematic editor. Yes, it means more of your time to do it. But it saves some time of others, too, and you may be able to redraw it better than it appears in the PDF, too. Also, you could then highlight the specific part of it towards which attention is drawn. Could you consider doing that? \$\endgroup\$
    – jonk
    Feb 12, 2022 at 16:12
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    \$\begingroup\$ I appreciate your suggestion, but TBH this is basically a "curiosity" question and not an urgent one. Also the schematic supplied is pretty clear and it's easy to locate the circuit sections I mentioned. I don't really have time to redraw it. \$\endgroup\$
    – Rob Lewis
    Feb 12, 2022 at 16:20
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    \$\begingroup\$ Please insert a screenshot here with the question and mark what components specifically you're talking about? For example, are you talking about C41 or are you talking about C16, R4 etc \$\endgroup\$
    – Ilya
    Feb 12, 2022 at 16:33
  • \$\begingroup\$ @RobLewis By the way, the IC2A stage arrangement is a piece of beauty. I'm impressed. You got a lot for $22. \$\endgroup\$
    – jonk
    Feb 12, 2022 at 16:44

1 Answer 1

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As you can see \$T_2\$ is an emitter follower. But the additional components turn this emitter follower into a simulated inductor (grounded inductor).

schematic

simulate this circuit – Schematic created using CircuitLab

Where:

$$L = R_4C_{18}(R_{14} - R_4) \approx R_4R_{14}C_{18} \approx 1H$$

The series resistance \$R_S\$: $$R_S = (r_e+ \frac{R_{14}}{\beta +1})||R_2 +R_4 \approx R_4 $$

And the parallel component is \$R_P\$:

$$R_P = R_{14} - R_4$$

All this means that in your circuit (IC2A) you have a series resonance \$LC\$ circuit.

schematic

simulate this circuit

And the resonance frequency will be equal to \$F = \frac{1}{2\pi \sqrt{LC}} \approx 1\text{kHz}\$

Thus, will well have the boost of gain (\$\frac{R_{59}}{R_S} \approx 40dB\$) around \$1\text{kHz}\$.

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