Trimpot in 1978 Phase-90

[dumbassbob]

What does the little trimpot do?

[Mark Hammer]

It divides down an already-reduced (but stabilized) version of the supply voltage to provide a suitable bias voltage for the FETs that do the actual sweeping. Think of that bias as being like "waking up" the FETs. If someone comes up to you while you are sleeping and whispers "booooooob, booooob" very quietly, you may just keep on sleeping. If they come up to you while you're asleep and scream in your ear with a megaphone "BBOOOOOOBBB, you frickin' moron. WAKE UP!!", there is a good chance that you may not awaken fully functional, but rather fall off the bed in a panic and injure yourself. The "waking up" of the FETs has to be somewhere between extremes.

The range of "reasonable wake ups" is not especially specific (i.e., the circuit and FETs can tolerate a little error in biasing voltage), and what you'll find is that a little adjustment in one direction or the other will shift the range of phase sweep so that it can be deeper and more gurgly, or higher and more airy. It was, however, intended to adjust the pedals for a consistent sound across multiple units, and not as a "hidden control".

[dumbassbob]

my P-90 seems to add a little distortion,is that common on "vintage"90's

[Robert M. Martinelli]

....If they come up to you while you're asleep and scream in your ear with a megaphone "BBOOOOOOBBB, you frickin' moron. WAKE UP!!"

Hey Mark,
that's EXACTLY the way my wife wakes me up every day....how the h&ll did you know about that?
Cheers
Bob

[Mark Hammer]

How did I know? Because I can hear it from my house, thousands of miles away!

Is it common for FET-based phasers to distort a little? Yes. There IS a workaround for that, that is implemented on the Phase 45, but NOT the Phase 90. Opinions are divided on whether it is ideal. You will note that the Phase 45 followed the P-90, historically.

Often, the simplest fix is to attenuate the signal going into the phase-shift network to minimize risk, and then add a little bit of gain on the output stage to bring the level up to a nice balance between effect and bypass. The trouble with that is that it risks an increase in hiss, particularly if the unit has any feedback/resonance introduced.

[Ronsonic]

It divides down an already-reduced (but stabilized) version of the supply voltage to provide a suitable bias voltage for the FETs that do the actual sweeping. Think of that bias as being like "waking up" the FETs. If someone comes up to you while you are sleeping and whispers "booooooob, booooob" very quietly, you may just keep on sleeping. If they come up to you while you're asleep and scream in your ear with a megaphone "BBOOOOOOBBB, you frickin' moron. WAKE UP!!", there is a good chance that you may not awaken fully functional, but rather fall off the bed in a panic and injure yourself.

Mark, can I steal that metaphor to explain amp biasing? Nobody I talk to nowadays understands that there were carburettors or that they had idle settings.

[Mark Hammer]

Be my guest. I never worry about running out of metaphors. Got a million of 'em.

[Ronsonic]

Be my guest. I never worry about running out of metaphors. Got a million of 'em.

Cool. Enough people still ride or remember riding bicycles so that it is still possible to explain output impedance settings and why they're important. But the idle settings are down the memory hole.