Series Capacitors Cut Low End

08 Jan

Just a few years into playing guitar, and simultaneously learning about electronics, I was experimenting with all sorts of ways to get different sounds from my electric guitar. I had a tiny transistorized practice amp that had a great fuzz sound when cranked all the way up. Probably less than a watt. But I knew that a capacitor in series with a signal would likely cut the low end and leave only highs, so I started soldering. (The word “hacking” had not been invented.)

I soldered a few different small capacitors in series with my signal using a spare phone plug and jack, and it was apparent that there were some interesting sounds to be had. Tearing apart old televisions was a source of parts for my ham radio and audio addictions, so I had a box full of 365 picofarad variable capacitors, commonly used in AM radios, like this one:

Variable Capacitor

Soldered into my test lash-up, I had a variable frequency low-cut filter. Did it scream! Without all the low end overdriving that lame little pair of transistors, the high end could get in there and sizzle. And it was totally variable, at the turn of a knob! Mom hated it, so it must have been good.

So what’s the diff between that capacitor and cable capacitance? Well, that capacitor was in series with my signal, and all the signal had to flow through it. Cable capacitance is in parallel with the guitar signal, and acts to short out the high frequencies, resulting in a dead, lifeless tone.

Try it! If you know how to solder and have some junk jacks laying around, order one of those capacitors and see how it sounds with your Marshall stack. (We don’t pay for burned up speakers, amps, or houses.)

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2 Responses to “Series Capacitors Cut Low End”

  1. Brad December 17, 2016 at 12:20 am #

    Hank, I think your deep dive into the topic of tone capacitors was very informative. One way to investigate this on my own I have used push-pull switches (built into the guitar potentiometers) to swap caps on the fly. A noticeable difference I have found is in the transient response (pick attack) when using different types of materials (eg: ceramic vs mylar) but same cap value and tolerance. I see you used magnitude and phase plots in your analysis, which is steady-state analysis, correct? Do you have access to an Audio Precision unit, or other audio analysis toolset, for measuring other performance aspects such as THD and Impulse Response? Some capacitor dielectrics are not precisely a constant and thus have a voltage-dependent derating since the capacitance is a function of the voltage applied (DC and AC components).

    For the record, I’m an EE too and this is fun to play devil’s advocate against your study, but in sincerity I have used AP units and spent some time in the audio engineering to justify this suggestion. AP units can get expensive, but there is a freeware tool called RightMark Audio Analyzer that might cut the mustard for this kind of test case.

    Jam On!

    • Hank Wallace December 19, 2016 at 4:46 pm #

      Brad — Good suggestion. I have noticed that ceramic caps can be very microphonic, which may explain some of the difference. If I have some time I’ll measure the impulse response of some of the caps I tested to see what the diff is. Could be fun! Thanks for your comment.

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