Cable Confidential - How Guitar Cable Capacitance Affects Your Tone
- Feb 28, 2019
- in MDM Tutorials
Hello everyone, and welcome to episode one of Cable Confidential, the video series where we explore everything about cables for music and stage.
I’m Jeff with Metro DC Music and today we’re looking at how and why guitar cable capacitance changes the tone of your guitar. You often see comments in forums and videos about tone-sucking cables but little on how and why this can happen, so today we’ll explore that topic.
We’ll start where the guitar signal starts, at the pickup. We’ll remove this pickup from the guitar and remove the tape wrapping that holds everything in place so that you can see that basic guitar pickup is a coil of wire with some magnets in close proximity. When the guitar string is plucked it vibrates within the magnetic field, and causes a voltage to be induced in the pickup coil.
Let’s go to the whiteboard now and see that we can represent the pickup electrically as a voltage generator with a series resistor and inductor, and a capacitor in parallel.
The resistor comes from the long length of very thin wire in the pickup. The inductor comes from the coiling of the wire, which is how you make an inductor. And the capacitor comes from stray capacitance that results from the turns of wire being in close proximity to one another. That capacitance is real but it doesn’t exist as a separate element that we can see. Instead it’s distributed across the turns of wire in the pickup.
The way these elements are arranged creates what is called a low pass filter. This means that it will leave low frequencies alone but reduce the level of higher frequencies. Here’s a curve that shows what a typical low pass filter looks like, and you can see that the level rolls off as the frequency increases.
But there is a special characteristic of this filter if the values of resistance, inductance and capacitance are in the right range and this is called resonance. Resonance happens when the impedance of the inductance equals the impedance of the capacitance, and they effectively cancel each other out. This causes a sharp rise at a point called the resonant frequency and it looks like this.
So low frequencies are still left alone as before, but there is a middle range of frequencies that gets increased, before the high frequency roll-off begins. This is used in guitar pickups to accentuate certain frequencies, and give the pickup its characteristic sound.
Guitar pickups have a resonant frequency that is typically in the 2000 to 5000 Hz range. The sound of the pickup will be affected by not just the resonant frequency but also by the shape and height of the peak. To give you an idea on the sound, and this is subjective so your opinion and specific situation may vary, a pickup with a resonant frequency of 2000 Hz may sound warm and mellow, while with a peak of 3000 Hz it may sound brilliant and present. A peak at 4000 Hz may sound piercing and at 5000 Hz even brittle or thin. The other characteristics of the guitar and strings will also affect this of course.
Now let’s put our pickup model in a guitar and add the other basic components typically used. First we’ll add the tone potentiometer and tone capacitor. For the rest of this video we’ll assume that the tone pot is at its maximum so the tone cap is removed from the circuit as much as possible.
Next we’ll add the volume pot, which we’ll also assume is at maximum volume so it’s also essentially out of the circuit. And finally the output jack to complete our model of a basic electric guitar.
I’m going to remove the guitar because I need the space on the board and I don’t want the diagram to get too confusing, and then we’ll plug in our guitar cable.
Now you may know that a guitar cable, or any cable because of they way that they’re made, will have capacitance between its conductors. For a given cable every foot of cable will have the same capacitance so the longer the cable the greater the capacitance. The plugs at each end will also contribute a little capacitance as well.
Let’s measure a typical 20 ft guitar cable here to get a real world number and you can see that this cable has about 856 pf.
Back to our drawing, we’ve replaced the picture of the cable with a capacitor symbol across the output jack of the guitar. And this brings us to the important point of today’s video because as you can see the cable capacitance adds to the stray capacitance across the pickup, and capacitors in parallel add together. This means that the capacitance across the pickup has increased from just the built in amount of stray capacitance by the amount in the cable. And this additional capacitance causes the resonant peak to shift to to a lower frequency, which will change the overall tone.
Let’s look at some real world examples:
First we want to look at the range of frequencies that are produced by a guitar. We know that the open E string is about 82Hz and the highest note on some guitars, the first string, 24th fret has an E at about 1300 Hz. Those are the note fundamentals but what about the overtones or harmonics? We’ll use an audio spectrum analyzer to see how far up the spectrum the guitar overtones extend. I’m using a Fender 52 Telecaster re-issue on the bridge pickup, volume and tone at maximum, capo’d at the 12th fret and strumming across the strings. This particular capo only covers 5 strings so the low E string won’t be played. OK here goes
While there are some overtones that reach up to 8 or even 10kHz the amplitude of those signals is well below the fundamentals. Most of the energy is below 6kHz. Maybe your style of playing would have more or less overtone energy than this test, keep that in mind in evaluating the results.
OK now let’s look at a pickup frequency response. We’re looking at the same telecaster and using an impedance analyzer to see the resonant peak. Here it is first with a guitar cable that has about 330pf of capacitance. The analyzer starts at 20Hz and you can see that the impedance is around 7000 ohms, which is basically the pure resistance of the pickup, and the same that we would measure if we used an ohmmeter. But as the frequency rises you see that the impedance rises, due primarily to the inductance of the pickup. As we get to the peak of the curve, the point where the impedance of the capacitance matches that of the inductance we get to our resonant frequency, in this case about 3.5 kHz. Then the response rolls off as it is dominated by the capacitance.
Now we’ll run it again only this time with the 850 pf guitar cable we tested earlier.
And now let’s compare the two curves.
As you can see with the additional 500 or so pf of capacitance on the second run the resonant frequency shifted lower, from 3.5Khz to 2.4kHz. The curves are pretty similar on the low side of resonance. But as expected the higher capacitance cable rolls off earlier, so more overtones will be attenuated.
OK I think we’ve explained how cable capacitance affects the technical measurements of the guitar electronics, let’s see if we can actually hear it.
We recorded the same 52 telecaster with a very short 85 pf cable and with the 20 foot, 850 pf cable using the same capo setup on the 12th fret. We’ll play them back in alternating fashion, A, B, A, B and so on,5 times, so you can easily compare them. I’m not going to tell you which is which, let’s see if you can hear a difference. Oh and by the way, consider the speakers and audio system you’re using. You don’t want your attempts to hear tonal differences to be obscured by low quality speakers. OK here we go.
Ok what did you hear? Did you hear a difference and do you have a preference? Which sound is the short cable - is it A or B? I’m not going to tell you the answer in this video. Instead I’m going to ask you for a favor, which is to help our new YouTube channel and consider giving this video a like, sharing the video with friends, or even subscribing to our channel. Please also leave us comments, on how the listening test went for you, or questions on this video, or suggestions for future videos. We welcome all of your feedback. When we get to 50 comments we’ll post a comment with the answer on the which sound was from which cable.
Thanks for watching today. This is Jeff from Cable11 and I hope you’ll join us again on our next episode of Cable Confidential.