Push/pull knobs made of metal presents a potential problem in that static can be transferred and be part of the signal. In other words, when touching the knob you won’t hear any static noises, but when turning, you may hear a static scratching noise.
The first reaction by many when they hear this scratching noise is that the potentiometer must be dirty. With a push/pull knob, that’s not always the case. Improper grounding or the fact the push/pull is made of metal can be the source of the static issue.
There are basically two ways to cure a static-scratchy problem with a push/pull knob.
1. Don’t use metal knobs.
Sounds farily obvious, but it does work. If you elect to use a wood or plastic knob, this kills the static transfer through the knob easily.
2. Use a metal push/pull with an additional weak spring
On some push/pull knobs by some manufacturers, there is an additional purposely-weak spring installed under the knob that puts a mechanical connection between the bottom of the metal knob and the body of the potentiometer, which of course is already connected to the ground.
The spring’s only purpose is to connect the ground direct to the knob to cure static transference issues. It’s purposely a weak spring to make the push/pull act as it normally would and so that the spring lasts a very long time.
One manufacturer that does build the push/pull knob with the added weak spring is John East (example: this product), so if you like the look and feel of a metal push/pull but want to also prevent static transference issues, you have options.
