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I'm investigating how tone potentiometer works on my guitar. It's 500k tone pot with 22nF capacitor and passive humbucker pickups.

As described in this answer, I expected tone control to act as low-pass filter.

With tone control at 0%, I expected it to be:

tone

While in practice it makes a wide cut around 3k, there's still a lot of high-frequency content:

tone

This EQ curve was consistent between measurements and neck/bridge pickups, I did them by playing over the entire fretboard and averaging the spectrum. I was unable to bypass tone control and compared with the control set to 100%.

Here are the spectrums of signals with tone at 0% (red), 100% (gray) and the difference between them (white) that averages to the curve shown above:

spectrums

Intermediate control positions also resulted to a similar curve with narrower cut.

Is there a good explanation of what's going on there? How tone controls may work in other guitars in practice?

I'm asking here and not at Electrical Engineering because I'm more interested in musical effects rather than technical details.

  • 2
    What’s the make and model of the guitar? How did you measure the frequency response? What does the response curve look like with the control set to max? How was the pickup selector set? What kinds of pickups does the guitar have? – Todd Wilcox Aug 20 at 12:57
  • It's Schecter SGR with Schecter Diamond passive humbucker pickups. – ordo Aug 20 at 20:47
  • I couldn't find specs, all I know is that they have pretty much high output. Neck/bridge choise didn't affect the results at all, so I didn't mention them. I also updated the question with relevant information. – ordo Aug 20 at 20:49
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    The last diagram seems to answer your question; the tone control doesn't bother to filter anything above 5k because there's not much signal there anyway. – Your Uncle Bob Aug 20 at 20:56
  • Electrical Engineering might really be the right place to ask about this. – piiperi Aug 20 at 21:32
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Taking the difference (subtracting) between the two signals is giving you a confusing graph.

Notice the raw output of the pickup falls off dramatically above about 3 kHz. That means the filter (tone control) has less to do above that frequency. So calculating the difference between with and without the tone control above 3 kHz is thrown off because the source signal is not showing the effects of the tone control.

The difference calculation is not valid because the source signal is not evenly representing all frequencies. If you passed white noise through the filter and then calculated the difference with and without the filter, then you would see the curve you expect.

Notice that the high frequencies of the actual guitar signal are attenuated by the tone control just how we would expect them to be. Don't let the flawed difference calculation distract you from the actual before and after curves, which look correct.

In case the flaw in the calculating the difference isn't clear, imagine the following: If you passed zero signal through the tone control, and then plotted the responses with tone at 0 and tone at 10, then you would see zero at all frequencies with the tone at 0, and zero at all frequencies with the tone at 10. The difference between zero and zero is zero, so your difference curve would be completely flat. You might then think the tone control isn't working at all! That's because if you don't pass a completely even and positive signal through a device or component when you're measuring it, you throw off the measurement.

  • 1
    I think you're right in saying that the difference calculation idea is flawed, but my technical explanation is a bit different. The levels on the measured spectrum don't really go all the way to minus infinity, they are "clipped" somewhere because of physical noise and A/D conversion limitations. At 10kHz the expected attenuation is -45dB, and the measured unfiltered level at 10kHz is … I don’t know how low, because the graph bottoms out, but let’s say -80dBFS. So the expected filtered level at 10kHz would be -125dBFS or lower... The OP’s gear and audio interface probably cannot deliver. :) – piiperi Aug 20 at 21:31
  • @piiperi Agreed. I supposed, when writing my answer, that the asker was seeking a less technical response based on their own interpretation of their results and their decision not to ask on the Electronics Stack. – Todd Wilcox Aug 20 at 21:34
  • @piiperi There was no clipping. High frequency content in 0% signal was near to noise floor but it didn't hit it. As I mentioned, the curve was the same with higher pot positions where. – ordo Aug 21 at 10:27
  • @ToddWilcox Thanks. I'd gladly try to solve this with testing signals (white noise) but this would take pickups out of the equation, while they are important parts of it. That means the filter (tone control) has less to do above that frequency - this reasoning seems wrong to me, a filter is linear circuit, the amount of some frequency content shouldn't affect the way it works. Any way, it seems I figured out what's going on, see my comment to original post. – ordo Aug 21 at 10:30

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