I'm currently building this distortion circuit for a guitar pedal.

blackfire overdrive pedal schematic

I didn't have any 2N5089 transistors so I decided to try using 2N3904s instead. The result has been an extremely harshly clipping output, due to the higher gain characteristics of the 2N3904. In order to tame this harshness, I would like to try and insert a low pass filter before the last stage of amplification/distortion.

So the Blackfire runs essentially:

buffer -> amplifier -> buffer -> dual amplifier -> output

And I would like it to run:

buffer -> amplifier -> buffer -> FILTER -> dual-amplifier -> output

The the stage of the Blackfire that I would like to filter into is based on a canonical multi-stage transistor amplifier circuit, but with the output of the preceding buffer biasing the input of the following transistor such that C1 and the R1-2 voltage divider aren't necessary:

enter image description here

My intuition was to put C1 and the R1-2 junction back in and then add a second capacitor such that C1 would become C1a and C1b. My thinking is that this should further decouple the segment between C1a and C1b, into which I should be able to insert the filter.

However, this has only resulted in a significant loss of signal strength, and no desirable filtering effect to speak of.

I've searched through my archive of guitar pedal schematics and various DIY pedal sites, and haven't been able to find any examples similar to what I'm trying to do. Which leads me to a few questions:

Is what I'm attempting feasible/possible?

If it is, should the filter be active or passive?

How should I go about implementing the filter?

Could someone explain why the filter I've tried to insert hasn't worked?

Thanks much in advance!

  • \$\begingroup\$ If you could, please post a schematic of the pedal with your changes. This will make it clearer, and easier for us to help. \$\endgroup\$
    – jramsay42
    Jan 8, 2018 at 0:20

1 Answer 1


Your assumptions are incorrect and thus so are your conclusions.

(edit) It seems I was too hasty to answer.

The 2N3904 has far lower hFE (>80) than 2N5089 (>400) by a wide ratio

The result bypassed CE amplifier is that positive parts of the audio wave have more current and also voltage gain than the negative peaks so that the CE amplifier tends to distort towards saturation. This makes it rich in both even and odd harmonics as Vce goes towards saturation, which seems to be the purpose of this guitar fuzz box.

When lower hFE transistors are used, the 1st CE amplifier could actually be operating with a much smaller signal and thus more linear and not distorting until the 2nd CE amplifier where it could be more likely to be clipping on both polarities with less attenuation perhaps on the gain pot and producing harsher odd harmonics.

Since the range of notes in the guitar is much wider than simply 3:1 or the 3rd harmonic of a square wave, filtering does not seem like the right approach.

Increasing the hFE of the transistors in the 1st stages seems to be the strange yet desired distortion desired for this box.

  • \$\begingroup\$ "to achieve the linearity of the high gain bypassed common emitter." -> I think I get what you're going for, but i'm missing the meaning of "linearity" in this context. And regardless of my having read the wrong datasheet for the transistors, the question was about the filter... \$\endgroup\$
    – compyouter
    Jan 8, 2018 at 3:12
  • \$\begingroup\$ Let me rethink what I said and correct it/ clarify. \$\endgroup\$ Jan 8, 2018 at 4:15

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