Long-term stability of trimmers in (head) amplifier circuits

[rock soderstrom]

I have recently been using trimmers in my test microphones to determine the bias points. Normally I would replace them with fixed resistors when set, but now I'm wondering if it's really necessary.

What do you think? Good practice or unnecessary?

 

[tk@halmi]

It depends.

For one or two builds of DIY leaving a good quality trimmer in the circuit is really fine.

For production of hundreds or thousands of units a fixed resistor is a smart approach for ultimate reliability, and also to lower costs.

If you use a high quality multi turn trimmer like the Bourns parts we see here in many builds you should be safe. I have not had those fail or even drift in my home made audio gear.

These open frame style trimmers may drift or even fail eventually as shown below. They don’t like anything “moist”.

 

[VanguardAudioLabs]

I use Bourns 3266 series sealed multi-turn trimmers, although i believe I've also used some TT electronics ones in prototypes.

We leave them in rather than using a fixed resistor on the products we use them in (V14, V24, and V1).

In order to make sure they don't move once we bias, we'll do a dab of 1401 adhesive (we use the red "C" but there's also blue "B" and clear "A) against the side of the thread which helps lock it in place.

For the V14/V24 it tunes the capsule polarization voltage, and for V1 it gets the JFET biased properly.

I guess if there's no filtering in place it could cause more noise than a high-grade metal film resistor, but good filtering ought to be in place for those types of spots anyways.