center-tapped pots

[Eddododo]

Knee deep in potentiometer datasheets on Mouser a few weeks back, I came across some pots with a center tap. I got a couple just for giggles, but haven't done anything with them.

They're an odd thing to 'visualize' a use for, working off of schematics and heritage from decades of *not* designing around such a pot... but it does give something of a 'on the tip of my tongue' feeling, like there are a thousand little things it would be convenient for. So with that, I'm curious what ideas come to mind for you guys

Right offhand, it could make a neat tone control.. signal at the wiper, one cap to ground at the center tap, and a bigger cap at the CW (or CCW) lug.. functions like a guitar tone control to the center tap, but then you sweep between the two cap values for a deeper rolloff freq. Obviously we've all seen the pot-based 'variable cap' snippets, and its neat to apply that with a control that can start at no-rolloff, instead of the sort-of always-on panning action. using a scaling resistor across the tap/lug can dial in the right resistance to make the cap-sweep usable feeling.

One could also picture a few interesting approaches to an eq.. something baxandall-ish, or something tilty. Maybe also an interesting panning utility.. who knows.

They're obviously not ubiquitous and abundant.. I'm not even sure what values I got offhand, let alone what handful of values would be stocked.

I'll dig up the part # when I get a chance, if anyone's curious

[elk bird]

I think the Peavey T-40 uses those for its controls, but I can't really recall how it's supposed to be set up. I always wondered what kind of pots they used to get different behavior once you're past noon.

[ElectricDruid]

Ooh, I can think of loads of uses for them!

The classic application in my head is for synthesizer pitch bend wheels, where the centre tap can be used to ensure that there's a genuine "no bend" part in the centre. You sometimes see diodes used to give a similar deadband.

Another option would be to apply two different LFO waveforms to the ends of the pot. Then you could have a depth knob with a centre-zero, where turning one way introduces triangle modulation, and turning the other introduces square modulation. You could do a similar thing on an oscillator to allow selection and level control of two waveforms with only one knob. Anything where you might want to offer an either/or choice with less controls could be done like this. Say you have an octave pedal with a -1 Oct signal and a -2 Oct signal. You could put those on the pot, and then you'd be able to mix in any amount of one or the other (but not both at the same time).

It might make a simple way to create control voltages for one of those "DJ Filter" effects, where the frequency control acts as a lowpass if you go left of centre and as a highpass if you go right of centre, with no effect in the middle. Generating the required voltages for the two filters to make that happen from a single pot can be done using rectifiers and offsets, but a centre-tapped pot would make life easier.

[amptramp]

Philco made a number of radio record players that used a pot with a centre tap to ground and audio taper on either side of the centre tap.  Turn the volume control one way and you got radio, turn it the other way and you got the record player.  Check here:

http://www.nostalgiaair.org/PagesByModel/584/M0013584.pdf

to see the schematic where there is a centre-tapped 1 megohm pot with 500 K on each side of centre.  At the centre position, the ON/OFF switch would go off.  This unit is model 46-1201, a unit put out in 1946 that was quite popular due to its styling and ease of use.

[duck_arse]

are your parts "centre-tapped", or are they "with tap"? they were used with some small percentage tap in tone controls on stereo amps to compensate for loudness, I believe.

[PRR]

Bogen and other school PA mixers gave line-in twice (say phono and tape) on one pot with grounded center tap.

Electric centering is useful but you may need mechanical center-bump to find the center by hand quickly.