stompbox project

Discussion in 'The Projects Forum' started by docz, Jan 31, 2010.

  1. docz

    Thread Starter New Member

    Jan 31, 2010
    Hello I'm trying to build a simple overdrive pedal using a LM368 amplifier. I came across this schematic: [​IMG] and I tried to build it. I have one minor issue, I can't get the drive pot to function. It is either off or on, with nothing in between. When I look at the data sheet for the LM386, I see that the example of an amplifier with a gain of 200 uses a different way of connecting pin 1 and 8, in the datasheet they are wired with the cap in between them, in the schematic for the pedal they are both wired to pin 1 and 2 of the pot and then pin 3 of the pot is wired through the cap to ground. Can anybody please explain the differences of the two wirings? And shed some light as to why my pot is not working properly?

  2. Darren Holdstock

    Active Member

    Feb 10, 2009
    I'd imagine the original designer would have similar problems if they tried to manufacture this circuit in quantity. It might work for a few ICs in the same batch, but it's not guaranteed for all LM386s.

    It's possible the DC biasing levels are being screwed up - measure the DC voltage at the output pin, it should be about half the supply voltage, give or take a bit. If it's way out, then there's the problem, the wacky circuit has interfered with the ICs self-biasing.

    If you want to abuse op-amps for fun, try overloading the output. Design a simple op-amp amplifier (with a conventional standard pinout device like a 741, TL071 etc) and load the output hard so that the current limit kicks in. Invariably the op-amp output current limit is asymmetric with respect to positive and negative output currents, and this should lead to some pleasing even-order harmonic distortion. Try different flavours, different op-amps will have different characteristics here. But don't publish the circuit or go into mass production, the results won't be repeatable...
  3. Audioguru


    Dec 20, 2007
    Many of these guitar distortion pedals are designed by people who know nothing about electronics. Their first one might have worked due to pure luck. Any other one doesn't work because the design is completely wrong.