I'm trying to build a guitar amplifier and I've run into some problems that I can't understand.

The input has an op amp to buffer and amplify the guitar signal which looks like this:


simulate this circuit – Schematic created using CircuitLab

Open wires on the left are where the guitar is supposed to be connected (I'm measuring it with no input) and out will go to a tube, but I've taken it out for testing now.

My problem is that without any input I'm expecting (and the simulator agrees) the circuit to output almost exactly the voltage of the virtual ground (2.04V, from what I've measured), but I'm seeing 1.45V -- 0.6 V lower than expected. The values measured are DC; when measuring AC everything shows 0 exactly as I would expect. Also my amplifier has two identical channels and the second one shows almost identical voltages -- I think that it decreases the probabilty of the problems being caused by my soldering.

I've checked all the voltages around and tested continuity to see if there aren't any unexpected shorts, but I didn't find any other problems than this, but I've run out of ideas. Are there any obvious problems with my design? What else can I try to find the problem?


1 Answer 1


If you're really using a TL071, you're way outside the input common mode range.

It's only guaranteed to work down to Vcc- + 4V. Even typically, Vcc- +3V @ 25°C. You're at Vcc- + 2.04V.

enter image description here

  • 1
    \$\begingroup\$ @cube: This means you need to find a better op-amp that can go closer to the supply rails. You should set your virtual ground to half-way between the max and min op-amp output voltage so that you can run the maximum signal without clipping. \$\endgroup\$
    – Transistor
    Feb 28, 2016 at 16:38
  • \$\begingroup\$ Thanks for the answer, this looks as a very probable cause -- unfortunately I won't be able to test it soon. Btw isn't it weird that the output voltage is lower than the input? \$\endgroup\$
    – cube
    Feb 28, 2016 at 17:44
  • \$\begingroup\$ @cube Not really- the input stage is not being biased properly so you can't expect it to behave sensibly. It's possible another sample would behave somewhat differently. You can try increasing the virtual ground to about 6V if the rest of the circuit can live with that. \$\endgroup\$ Feb 28, 2016 at 17:51

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