I'm building a big muff pi, here it is my schematic :


I also made the PCB with smd components, and all the connections seem to be correct. I've plugged in the guitar but nothing happened, no sound at all. So, since I don't have an oscilloscope I decided to measure some voltages across the PCB, just to see if I can figure out something (not a good idea I know). I noticed that the voltage across R21 is around 5V, and across C13 around 4V ( the board is powered by a 9V battery) so no voltage across J3 (potentiometer). It happens also across J1, no voltage at all.

Just to understand, Vab is equal to Vc13, as if C13 had a shortcut to gnd. I checked all the connections and they are correct. This is the circuit that I have copied:

Any suggestions?

Thank you all! I figured out that one connection beetwen a resistor and the base of a transistor was interrupted! The method that @Transistor suggested to me worked perfectly!

  • \$\begingroup\$ are these HDR1X3 jumpers J3, J2, J1 terminals to potentiometers? \$\endgroup\$ Aug 15, 2017 at 21:03
  • \$\begingroup\$ Your DC voltage measurements so far look correct, thanks to the decoupling caps. I'd expect somewhere around 1V or less (0.3V) across all the emitter resistors R7,R14 etc ... if you find 0V or 9V on any of them, check that stage. \$\endgroup\$
    – user16324
    Aug 15, 2017 at 21:06
  • \$\begingroup\$ @TomKuschel Yes they are. \$\endgroup\$ Aug 15, 2017 at 21:15
  • \$\begingroup\$ @BrianDrummond Yes I have 1V only across R14 and R22. The other 2 voltages are 0V. \$\endgroup\$ Aug 15, 2017 at 21:24
  • \$\begingroup\$ Well the bias chains for the other stages are identical to R10-R14 and Q3 so compare voltages and work out why they are different. \$\endgroup\$
    – user16324
    Aug 15, 2017 at 21:37

1 Answer 1


Use your guitar lead and amplifier as a "stethoscope".

  • Turn your amplifier down but loud enough to hear the buzz when you touch the jack tip.
  • Connect the Big Muff ground to your amplifier ground.
  • Plug the guitar lead into the amp.
  • Use the other end as a touch probe.
  • Put an audio signal into the input. A steady oscillator signal is ideal. You should be able to find a suitable signal generator app for your phone.
  • Probe at the collector of each transistor starting at the left side of the circuit. Note that there will be a DC "thump" as you connect.

This should narrow down the loss of signal to one area of the board.

Proceed from there.

  • \$\begingroup\$ Thanks for the advice, but I have a question ( probably a dumb one). How can I connect the ground of a 9V battery to the ground of my amplifier? \$\endgroup\$ Aug 15, 2017 at 21:12
  • \$\begingroup\$ With a wire! A piece of bell wire, telephone or CAT5 - anything. Solder a piece onto the ground of the PCB and twist it around the jack sleeve or else around a screw on the amplifier. \$\endgroup\$
    – Transistor
    Aug 15, 2017 at 21:14

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