1
\$\begingroup\$

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?

\$\endgroup\$
3
  • \$\begingroup\$ R1 and R2 are a bit on the large side ... 100k for each should be fine. \$\endgroup\$ 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

2
\$\begingroup\$

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:"

schematic

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.

\$\endgroup\$
3
  • \$\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
  • 1
    \$\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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.