Why Trimpots rather than fixed Value Resistors?

[jazzyfingers04]

Hey, this is once again, probably a real newb question, but I really want a good answer to it...

Why does everybody use trimpots instead of fixed value resistors in stuff like the little gem or the other ROG schems? I mean, couldn't we just find a fixed value and stick it in there? Is this because of differing voltages for wall warts? and finally, Especially if you are using a well regulated current, why do we not just find the correct value of resistor and then plug it into the schem?

Sorry if Im missing something REALLY obvious... I am just wondering about it all.

[R.G.]

The reason is that the semiconductors themselves vary - a lot. Ordinary transistors may have a 5:1 range of gains within the same type. Older germaniums may be even worse. The trimpots let you tune it to the things you have.

Note that "classical" electrical engineering design principles get around this for production stuff by designing the circuits to NOT depend on variable transistor characteristics. In effects, the transistor characteristics are often what is being shown off, hence the tuning to bring out the particular flavors, not designing them away.

[jazzyfingers04]

ok, thanks for clearing that up for me.

[niftydog]

I like to call it "wiggle room".

Often it's not just the transistors that vary, resistors, caps and inductors all have a tolerance spec.

Also, say you're building a filter; you might like the sound it gives, while someone else might want to tweak it a bit so it sounds more pleasing to them.

Regulated wall warts are most often VOLTAGE regulated, not current regulated.  Regulating the current flow won't result in a constant voltage.  Since a lot of the semiconductors we use are 'voltage controlled' devices, we need this solid supply voltage.

Also, the impedance (or dynamic resistance) of a circuit varies, so regulating the current provided to a varying impedance results in a voltage that varies too!