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I have built a distortion pedal with 3 variable controls: gain, tone and volume. R8 is my gain control (it will be a potentiometer with 50k to 500k resistance.) R5 is the tone control, (low pass filter.) It is a potentiometer with 1k to 10k resistance. R6 is volume, again a potentiometer with 10k to 100k control.

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The output at the op-amp seems perfect, it shows a distorted sine wave oscillating about my biased voltage (7.5V.)

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When I probe beyond C5 (at the C4, R5 and R6 node,) the voltage becomes very small and even negative.

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Does anyone have any idea as to why this happens? How can I go about fixing it?

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3 Answers 3

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This is exactly what you should expect to see. When you probe the output of the opamp you see a roughly 1.5 V signal centered around 7.5 V.

When you probe at C4, C5 is blocking the DC component, so the waveform will be centered near zero, while R5 and R6 form a voltage divider reducing the signal level by half, so you get a roughly 0.75 V signal.

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  • \$\begingroup\$ Ah, so if I understand correctly, the 1.5V is the AC component, and the biassed 7.5V is the DC component. Once C4 blocks out the DC, we only have that 1.5V to play around with. the R5 and R6 voltage divider halves that, leaving us with only 750mV of an AC signal. If the voltage is negative, does it mean you won't be able to hear the sound? Is this voltage level too quiet to be heard? \$\endgroup\$
    – Marko Jurisic
    May 21, 2022 at 22:55
  • \$\begingroup\$ @MarkoJurisic You're misunderstanding how this works; the sound is more about the rate of change of the voltage than the actual value. 750 mV sounds about right for a line-level output. \$\endgroup\$
    – Hearth
    May 21, 2022 at 23:08
  • \$\begingroup\$ @MarkoJurisic No, the voltage being negative does not mean you won't hear it. Think of it this way, you have a speaker, one terminal is grounded. When the signal at the other terminal goes more positive the speaker cone moves one way, when it goes more negative the cone moves in the other direction. So at 0V the cone is at it's resting center point, and moves back and forth from there with an AC signal. On it's own, a 0,75V signal isn't going to be enough to drive a speaker, but your distortion pedal will be connected to an amplifier and that will drive the speaker. \$\endgroup\$
    – GodJihyo
    May 21, 2022 at 23:35
  • \$\begingroup\$ IT'S Normal. But what range of amplitudes for input and output would you like? 1Vp or 1Vrms are pretty standard outputs. \$\endgroup\$
    – Tony Stewart EE75
    May 21, 2022 at 23:41
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You can double the output with 2 series diodes in each direction or back to back Red LED's for about +/- 1.8Vp (very dim) with 1.8mA from 1k.

Then Reduce R5 from 10k to 2k and increase the shunt cap 5x for the same LPF effect to boost it from 1/2 to 5/6 of the Op Amp output or a 33% increase which should average about 1Vrms to 2Vrms.

Conclusion

You could just change R5 to 1K and C4 to 220nF to go from final attenuation of 50% to 10/(10+1) K resistors *100% to almost double your output to +/-1.3V peak which is pretty darn close to 1Vrms. That also lowers your output impedance to shunt stray hum with the reduction in R out, if you had any.

Also you should reduce your 15V supply from 5 to 10V. Check datasheet. You don't need 15V.

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Your input signal is 350Hz at 300mV peak. The output signal level is not very small, it is normal with the diodes. Your capacitor C6 has a value too low and with R7 reduces the level of 727Hz and lower frequencies. C6 causes the slanted squarewaves. Your resistor R5 has a resistance that is too high and with C4 they reduce the level of 727Hz and higher frequencies. R5 and R6 are a divider that reduce the output signal to half. distortion

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