See this blog entry for additional information on debugging instruments used (multimeter, signal generator and oscilloscope)
The first stage is the verification of DC voltages with a multimeter, +9V DC input is doubled by U1 charge pump regulator ICL7660S. The voltage at the output of the dual diode D2 is actually closer to +16V due to the diodes drop.
We will compare the results of the real pedal with the LTSpice simulation.
|Fuzz-Wah pedal schematics|
|Fig 1. 2.5 kHz 0.3 V peak-to-peak sinewave at the input (TP1)|
The signal at TP2 after a two transistors (Q1, Q2) amplifier is amplified by 15.4 with a voltage amplitude of 4.63V peak-to-peak
|Fig 2. 2.5 kHz 4.63 V peak-to-peak sinewave after Q1-Q2 amplifier (TP2)|
|Fig 2.1. LTSpice simulation at TP2|
|Fig 3. 2.5 kHz 4.19 Vpp clipped sinewave after Q3 emitter at Q5 base (TP3)|
|Fig 3.1. LTSpice simulation at TP3|
|Fig 4. 2.5 kHz 5.66 Vpp clipped and distorted sinewave after Q3 collector and Q4 base (TP4)|
|Fig 4.1. LTSpice simulation at TP4|
|Fig 5. 5kHz 0.77 Vpp frequency doubled and clipped signal at Q4-Q5 collector (TP5)|
|Fig 5.1. LTSPice simulation at TP5|
|Fig 6. 5kHz 0.78 Vpp frequency doubled and clipped signal at R22 + D1 clipping diode (TP9)|
|Fig 6.1. LTSpice simulation at TP9|
Signal at TP6 is TP9 clipped and frequency doubled signal after the tone cut filter. It has an amplitude of 0.61 Vpp.
|Fig 7. Signal after Tone cut filter 0.61Vpp (TP6)|
|Fig 7.1. LTSpice simulation at TP6|
|Fig 8. Signal after voltage divider 0.124 Vpp (TP8)|
|Fig 8.1 LTSPice simulation at TP8|
|Fig 9. Signal at the output of Fuzz effect with Tone Cut filter bypassed (TP12)|
|Fig 9.1. LTSpice simulation at TP12|
|Fig 10. Signal at the output of Fuzz effect with Tone Cut filter on (TP12)|
|Fig 10.1 LTSpice simulation at TP12|
Q7, Q8, Q9 transistors implement the Wah effect. R43 is the Wah potentiometer that tunes the Wah band-pass filter frequency.
Signal at Q7 collector (TP13) with Tone cut filter off and wah at potentiometer at 0 has an amplitude of 2 Vpp.
|Fig 11. Signal at Q7 collector with Tone cut filter off and Wah pot at 0 (TP13)|
|Fig 11.1. LTSpice simulation at TP13|
Signal at Q7 collector (TP13) with Tone cut filter off and wah at potentiometer at 10 has an amplitude of 99 mVpp.
|Fig 12. Signal at Q7 collector with Tone cut filter off and Wah pot at 10 (TP13)|
|Fig 12.1. LTSpice simulation at TP13|
|Fig 13. Signal at Wah output with Tone cut filter on and Wah pot at 0 (TP15)|