Unity gain at 50% pot rotation?

[onboard]

I would like to fine tune my volume controls to get unity gain when the knob is halfway. There's a lot of good info here about customizing pot tapers, but I'm wondering if a simple increase in pot value will do.

A specific example would be an inverting opamp set for a gain 4.7 and a 100kA level pot. I'll try increasing the pot value to 500k or 1M - right now only about 20% of the pot rotation is useful as boost, that's not too good.

Does anyone else try to get their volume controls to be at unity gain at 50% rotation?

[Joe Kramer]

Hi!

I'm not aware of any hard and fast standard, but a lot of audio gear like mixers and so forth have 75% of the pot rotation as unity gain.  I try to go for that on the level control of my pedals unless I really want a lot more gain than can be got on the last 25% of the pot.  

I don't think increasing the pot value will do what you want, but it depends on how the pot functions in the overall circuit.  If the pot is hanging off the circuit output to ground, with the wiper as your final output, changing the pot will only change your output impedance.  In that case, if you're getting less than unity gain at the 50% position, you would have to increase the gain of the circuit.  Or, if there is a series resistor on the output, you can make this smaller.  Or, . . .

As before, it really depends on where in the circuit the pot is situated.

Joe

[petemoore]

When you turn the pot up, the wiper goes closer to the signal path input side [if it's hanging off the end of the circuit like that].
 Take a nice round number like 100k pot, and some resitors between say 33k and 100k, Set the DMM to test resistances in that range...
 Then test the pot while trying out different value resistors between wiper and an outside lug...probably easiest to try to figure out with a linear pot...
  Then figure you want uh...less resistance change toward the upper range [toward full on] of the pot so...add a resistor wiper to that lug [that parallels the wiper and signal path so AlOt of division equations would ensue...I just worked with it 'till it did close to what I wanted it to do]...
 Or whatever...I've done some pot tapering..mostly I get the pot the schem calls for and work with it...
 If you're starting with a log pot things are even wierder...see GEO "Secret life of Pots"

[onboard]

Thank you for the replies! I'm surprised to learn that audio level controls go for unity at 75% - it makes more sense to me have equal cut and boost. As per that guideline though, I guess the controls were working right on.

I usually grab whatever a schem calls for too, but in this case I drew my own. The level control is the usual pot hanging off the circuit output to ground, wiper out.

When I increased the gain, the only result was huge pop from the bypass switch which was previously nice and quiet. I'll go with the original gain and just be happy with the control response, unless I get nuts and decide to go with custom tapers.

Pete, Joe Davisson's calculator for getting R values to alter pot taper is exactly what you're talking about. RG's "Secret Life of Pots" has all the long math, too.

[scottosan]

A linear 100k in place of the 100k log may do the trick also

[onboard]

I wouldn't have even thought to try that since I automatically chose audio taper because the control is for output volume. Thanks for the suggestion!

[Khas Evets]

If you measure the current pot when it's at unity, we could give you more precise advice. It's hard to recomend something, not knowing the ohms required to make it unity.

Personally, on boosters I like unity to be closer to the bottom, since I'm usually using the pedal for going above unity.

[onboard]

As best I can tell unity gain by ear, the pot's at about 2 o'clock & measuring 72k / 26k - not bad for a 100kA I s'pose. Maybe I'm being too picky.

Another one I just measured was more like 62k / 31k at the same clock postion.

I'll do some more reading on adjusting pot taper. Unity closer to bottom on these circuits might not make for appreciable adjustment through the whole pot travel since I'm only in the +/-12dB neigborhood. 12 o'clock would give me equal cut and boost, I think I can do that with tapering resistors.

Thank you for the help!