Guitar Effects - Distortion Box
OP_DISTORTION_1.CIR Download the SPICE file
Whether you're a guitar player or a music appreciator,
there's something satisfying about hearing a crunching
chord or cutting riff played through a distortion box.
But, what does a distorted signal look like? Its
basically the original wave shape with the negative and
positive peaks clipped. How is this accomplished? Back in the
early days of rock music, you clipped the
waveform by cranking up the amplifier volume until the signal peaks
would hit the rails of the tube amplifiers.
Unfortunately, this required the maximum volume to
achieve this awesome effect. Ultimately, circuit
designers found ways to create the distorted sound at a
very low signal level, allowing the guitarist to achieve their crunch at
The amplifier above (based on the Ibanez Tube Screamer) has three main functions
where R2 represents a fixed resistor and a
potentiometer for a total resistance of R2=R2+R3.
The pot is really the drive knob on the box.
If R3 is fully CCW, then R2=51k+0=51k. Alternately,
if R3 is fully CW, then R2 = 51k+500k=551k.
The components C1 and R2 create a zero fz (high-pass filter) and R1, C1 create a pole fp (low-pass filter) defined by
C2 and R2 create another pole at
At the adjustment extremes of the drive pot you get
As you can see, more drive (R3 fully CW) means a gain that's higher and a mid boost that's shifted down in frequency (a lower zero fz1 and pole fp2). The gain versus frequency can be divided into four sections.
MID FREQUENCY BOOST
Now set R3=500k (max setting) and rerun the simulation. How does this setting change the response? Did the response begin to rise at a lower frequency (~6 Hz) as expected and did it reach a higher gain (120 or 42dB)?
The first time I ran this I said hold on! Shouldn’t the diodes
have clipped the signal at some point? Turns out, SPICE replaces the diode
with an AC equivalent circuit - high impedance for an OFF diode at the
operating point. This high impedance has little effect on the AC response.
And, regardless of the signal’s magnitude at VS, SPICE computes the analysis
using the same small signal AC equivalent model.
Rerun the analysis with VS at 200 Hz and 400 Hz. At what frequency does the high-pass filter boost the signal sufficiently to start the diodes clipping?
Now crank up the drive knob (R3=500k) and repeat the analysis for 100 Hz,
200Hz and 400 Hz? At what frequency does the clipping begin? How does the
severity of the clip change at 400 Hz?
Here's a sample of a clean and processed guitar sound you can download.
Download the file or copy this netlist into a text file with the *.cir extention.
* op_distortion_1.cir * VS 1 0 AC 1 SIN(0V 100MV 100Hz) *VS 1 0 AC 1 wavefile=.\guitar2a.wav chan=0 RS 1 3 1K * R1 2 6 4.7K C1 6 0 0.047Uf R2 2 4 51K R3 4 5 5k C2 2 5 51PF D1 2 5 D1N4148 D2 5 2 D1N4148 XOP1 3 2 5 OPAMP1 * ATTENUATOR RL1 5 7 5k RL2 7 0 5k * *.wave .\guitar2a_output.wav 16 44100 V(7) * Save node V(7) as a *.wav file, 16 bit resolution, 44100 samples per second * * DIODE .model D1N4148 D(Is=0.1p Rs=16 CJO=2p Tt=12n Bv=100 Ibv=0.1p) * * OPAMP MACRO MODEL, SINGLE-POLE * connections: non-inverting input * | inverting input * | | output * | | | .SUBCKT OPAMP1 1 2 6 * INPUT IMPEDANCE RIN 1 2 10MEG * GAIN BW PRODUCT = 10MHZ * DC GAIN (1MEG) AND POLE 1 (10HZ) EGAIN 3 0 1 2 1000K RP1 3 4 1K CP1 4 0 15.9UF * OUTPUT BUFFER AND RESISTANCE EBUFFER 5 0 4 0 1 ROUT 5 6 10 .ENDS * * ANALYSIS ************************************* .TRAN 0.1MS 30MS *.TRAN 10MS 5S *.ac dec 40 1 1000k .PROBE .END
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