I found a guitar preamp on the web after trying to build one myself and having no luck. This one works for a bit, but the offset part of the OP-amp slowly depletes. It's like C1 is charging up really quickly then just discharging over time. enter image description here

The only differences from this circuit is that I'm using a gain of 10K/4.7K and a 100 uF electrolytic for C3, bypass capacitor. I'm also using a MCP6002 at 5V which isn't a FET Op-amp. Can someone please explain to me why it's not working?

  • \$\begingroup\$ R1 and R2 are a bit on the large side ... 100k for each should be fine. \$\endgroup\$
    – user16324
    Dec 7, 2020 at 18:41
  • 4
    \$\begingroup\$ For a guitar input, you need Mohm input impedance to 10 Mohm. If you don't do that you lose the top end treble response. I expect that the 100 uF is going a bit leaky and busting the DC levels. \$\endgroup\$
    – Andy aka
    Dec 7, 2020 at 18:44
  • \$\begingroup\$ A fault in C2, or a 9V battery venting gas, could also cause this effect. \$\endgroup\$
    – Whit3rd
    Dec 7, 2020 at 20:29

1 Answer 1


The MCP6001-1R-1U-2-4 datasheet specifies a few key points:

  • Section 1.0 Electrical Characteristics: Input bias current is 1pA, so the opamp is essentially a JFET-input class. Maximum output voltage swing is within 25mV of power rails, so device is also rail-to-rail output, despite it not highlighting either on the front page.
  • Section 4.3 Capacitive Loads: If C4 is really 10µF (and whatever after it is a fairly low impedance), a resistor in front of C4 may help. Temporarily replace the electrolytic caps with solid ceramic ones (surface mount, value not critical) - just to see if they make a difference.
  • Section 4.4 Bypass capacitors: do you have these installed?
  • Section 4.5 Unused Opamps: has this been followed?
  • Section 4.6 PCB surface leakage: is the board design causing leakage? Layout is important for the input. Even residual flux can cause leakage at this high impedance.

Edit: since adding the resistance before C4 helped, the issue must be that whatever is after this stage is too low of an impedance for this opamp. A solution may be to use the other amp in this package as a "unity-gain follower:"


simulate this circuit – Schematic created using CircuitLab

There is still an output capacitance limitation in the "buffer" stage, but the effect may be minimal as the gain is just 1:1.

Note that C1,R1,R2 form a high-pass filter on the input; 0.1µF likely would give very little low-frequency response, so it was bumped to 1µF. Some experimentation may be needed. Added C5 per datasheet.

  • \$\begingroup\$ Adding the resistor before C4 worked but now the gain doesn't seem to be non-existent. Changing the capacitors to ceramic seemed to have no effect. I believe the 50 uF capacitor is effectively a bypass capacitor. \$\endgroup\$
    – Jacob Lara
    Dec 8, 2020 at 1:31
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    \$\begingroup\$ I’d think twice about using hi-k ceramics in the audio chain since they tend to be piezoelectric. \$\endgroup\$
    – Kartman
    Dec 8, 2020 at 10:32
  • \$\begingroup\$ True Kartman, that was just for testing - do not leave them in there. Yes, C3 is a bypass capacitor. Section 4.4 reads "the power supply pin (VDD for single supply) should have a local bypass capacitor (i.e., 0.01 µF to 0.1 µF) within 2 mm for good high-frequency performance. It also needs a bulk capacitor (i.e., 1 µF or larger) within 100 mm to provide large, slow currents." \$\endgroup\$
    – rdtsc
    Dec 9, 2020 at 13:06

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