Micro-Amp Clone - Potentiometer problem

[JPGraphX]

Hi,

I built a microamp clone, and it is working perfectly, but I have a problem with my gain potentiometer. My gain works from 0 to 3/4 of its range, but the last quarter, I lose all the sound.

What can it be?

Thanks,
JPGraphX

[XXISouthpaw]

Are you using a 500K rev log pot?

[JPGraphX]

Yes, I used a kit from GGG. My gain work well except for the last quarter of its range.

[JPGraphX]

Anyone got an idea?

[Mark Hammer]

yeah, clean your pot.

the gain pot on a Micro-amp is in the ground leg, between the inverting pin and ground.  The gain of the op-amp is set by the ratio of [feedback R + ground R] / ground R.  If there is a discontinuity in the pot, such that the wiper is not making contact with anything, then that pot represents an open circuit, or infinite R.  And clearly, if the denominator in that ratio is infinity, then the result is very very small.

However....that would simply turn the circuit into a unity-gain buffer, which should not result in no sound at all.  Which leads us to the next possibility.  Perhaps your pot is installed such that when you rotate too much in one direction, you accidentally short out one of the pot lugs against something.  Consider exploring that option.

[JPGraphX]

By cleaning my pot, you mean, resolder ?

[Mark Hammer]

No, I mean either spraying it inside with contact cleaner, or going the extra step of prying up the tabs that hold the back on, temporarily removing the back, and wiping down the resistive strip with a cotton applicator to make sure there aren't any particles on it.  While you're there, you can also use the extra access that provides to give the little rivets that hold the solder lugs up agaist the resistive strip a gentle pinch with your needlenose pliers.  While not a common problem, neither is it a one-in-a-million thing for one of the rivets to be juuuuuuuuussssstt a little loose and not make pefect contact between lug and resistive strip all the time.  I've had it happen to me a handful of time in several hundred pots.

[PRR]

If it's not just dirty inside, or made bad, measure DC voltage at that opamp output pin, at half and at full.