Copyright © 2011-2017
Hardman Guitar Amplifiers
The graphs to the left show what happens to the pickup frequency response with differing values of resistance across its output. In a practical situation this means the value of the guitar volume control in parallel with the amplifier input. Reduction in resistance not only reduces the HF but flattens the response.
The Guitar Tone Control
The graphs show the effect of the standard guitar tone control of 500K potentiometer in series with a 22 nF capacitance directly across the pickup. For most of the rotation the effect is similar to reducing the loading resistance. Its only below 30% rotation that a second resonance comes into play, that of the 22 nF capacitance and the 8H pickup inductance, to give a peak at 380 Hz (around F# on the top string).
Pickups: Loading - Tone Controls - Covers
Fitting a cover
A bare pickup does have slightly more HF than one that is covered, but it is pretty small, about 2 dB at the resonant peak.
This article has a look at what happens to the frequency response of a humbucker pickup when we change the loading, alter the tone control, change the length or quality of the cable, or even change whether the pickup is bare or covered. The measurements were made on a Seymour Duncan SH4 humbucker bridge pickup, using a one turn loop of wire to couple an input signal into the pickup. The SH4 had an inductance of 8H with a self capacitance of 105pF, these values together with the capacitance of the measurement equipment account for the fundamental HF resonance around 4.5 Khz. In practise the loading on a pickup is a complex mixture of resistance and capacitance, with both the volume and tone controls having an effect on the frequency response. If there is one thing to be learnt about the use of high output pickups like the SH4, unless you want particularly dark muffled tones, use cable with the lowest possible capacitance, that is, use the best quality and as short as is practical.
The effect of cable capacitance
Cable capacitance has quite an effect on the output of high inductance pickups. 1000 pF of capacitance causes a drop of more than 10dB at 3 Khz. The resonant peak remains but it has been moved well down into the upper mid tones. Trying to compensate for high cable capacitance by turning up the amplifier treble is unlikely to be satisfactory because such action will also lift the lowered resonant response peak
Volume control and cable capacitance
The graph shows the effect of the volume control, when the output is loaded with 470 pF of cable capacitance. Reducing the volume actually extends the HF response, changing quite considerably the overall tone. This explains why a more balanced tone can often be obtained by operating the volume control a little below maximum.
|Features & Benefits|
|Loudspeaker Efficiency and Loudness|
|Guitar Tone and Distortion|
|Hum and Noise|
|Removing Control Knobs|