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I'm working on building this circuit which is based on a "Fuzz" distortion guitar effect, and am wondering what the purpose of C3 is, connecting the two collectors of the transistors. Is it for some type of DC blocking between stages? Most other design schemes I've seen for this type of circuit don't have any connection at this point. Does the actual capacitance value matter much at this junction? Any advise or reference to a source where I could learn more would be greatly appreciated.

Circuit designed by Mad Bean Pedals

circuit schematic

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    \$\begingroup\$ Welcome to EE.SE. If that's not your drawing or design then you need to credit it. (This is site policy.) Hit the edit link ... \$\endgroup\$ – Transistor Apr 4 at 17:21
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If you build it and look at the output on an oscilloscope, without C3 the output is going to be a rectangular pulse at each low-going spot in the input. With C3 (because of the Miller effect that @KevinWhite mentions in his answer) the output is going to be a sawtooth.

It is going to change the sound -- I'm guessing, but I'd be astonished if it doesn't soften the high tones, and give it a deeper, raspier* tone.

The value is definitely going to have an impact on the tone. If it were me, I'd build the thing with a socket for C3, and try a few values around 47pF -- probably anything from about 10pF to 100 or even 220pF will have an impact on the sound. Then I'd keep what I liked. If I really liked what I heard, I'd see if surplus variable caps are still available, and bring that out as an adjustment knob, or I'd put a switch in (being very mindful of layout; 47pF isn't much at all) to select between two or three values.

* Or whatever adjective fits with "more low frequencies, less high".

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  • \$\begingroup\$ Nice - I just found a document for the OP's schematic on the original website, which completely agrees with your "try it and see" approach: "Some versions of the Hipster have included the C3 cap and some haven’t. It can help reduce noise, but it also depends on what transistors you use. A helpful tip: build the Hipster without a cap there. Once you have it working, simply plug in a 47pF in the C3 spot loose to see if you like it. Or, try lower or higher values depending on the result you want." \$\endgroup\$ – SamGibson Apr 4 at 19:43
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C3 just controls the slope on the signal to reduce the high-frequency signals at the output to give the desired sound.

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