Shin-Ei Fuzz-Wah - Univox/Unicord Super Fuzz (not a clone!) - Schematics, PCB layout, simulations (1/4)

(Update 23/08/2015: Source files on Github)
I am a big fan of shoegaze / noise rock bands and one of my favourites bands is Jesus & Mary Chain and their album Psychocandy. That album is well known for its unique and characteristic dense wall of sound with that white noise (switch-on vacuum cleaner, washing machine and all appliances and let them couple to the guitar amp) that served as inspiration for a coming shoegaze generation of bands: Ride, My Bloody Valentine, Cocteau Twins, Lush, Spacemen 3, Loop... and even today shoegaze bands like A Place To Bury Strangers, Skywave, Ceremony, Screen Vinyl Image or 93MillionMilesFromTheSun, The KVB, The Lost Rivers, The Soft Moon...
It seems that one of the keys of that sound is the use of a Shin Ei Fuzz-Wah (8 transistors) pedal. This is a two/three effects into one pedal, a 6 transistor fuzz with octaver (high octave) plus a 2 transistor Wah effect at the end. JAMC did not make lots of use of the Wah pedal as it is normally used, but they used it to reinforce the gain of a band in the mid tones to couple the sound, so they set the pedal at one position and didn't move it.
These are the schematics of the original Shin Ei Fuzz-Wah pedal:

The fuzz with octaver is also used on the Univox/Unicord Super Fuzz (6 transistors) pedal. See schematics here:

Find here a demo video of the Shin Ei Fuzz-Wah pedal:

With all that in mind I made a first version of the Super Fuzz with Octaver pedal with scrapped through-hole conventional components on a Vero board using 2N2219A transistors 

 The first version on the Super Fuzz pedal can be seen here on the left of the picture:

The result was quite deceiving, the circuit has lots of gain and it was too noisy, too much, even for playing a PsychoCandy cover, specially when powered at 9V. A higher DC power of 15V reduced a bit the noise but it was still an undesirable result.

Schematics

(Update 23/08/2015: Source files on Github)

I decided to restart the work using surface mount devices (SMD), a professional PCB and include the Wah sections as well as several improvements and modifications to the original circuit.

These are the schematics of the Super Fuzz-Wah design:

I used BC847C high gain (hfe = 520typ) SMD SOT-23 NPN transistors.

The improvements on the circuit are the following:

1. Replacing the two clipping germanium diodes by one dual BAT54S Schottky diode SOT-23 in series with 100 ohms (D1, R22).
2. Replacing the Wah bulky and expensive inductor by a gyrator circuit based on a transistor (Q9) plus capacitor and biasing resistors.
3. Doubling the 9V DC power supply, with a charge pump circuit based on Intersil ICL7660SCBA

Check my post on replacing germanium diodes with Schottky diodes plus series resistor.

The charge pump regulator ICL7660SCBA allows doubling the power supply from 9V to 18V, however this charge pump regulator uses an internal oscillator with a quite low and audible switching frequency of 10 kHz. If it is used in its default oscillation mode this frequency is seen as a ripple frequency on the power supply, since this pedal has lots of gain in its transistor circuits, this ripple noise can be amplified and heard as a very nasty and unpleasant high pitch noise. For that reason, it is absolutely required to short pin 1 (boost frequency pin) with pin 8 (V+) in order to get a higher switching frequency of 35 kHz, out of the audible spectrum.

Wah circuit simulation with LTSpice

(Update 23/08/2015: Source files on Github)
The following schematics shows both Wah circuits: to the right the original wah circuit based on an inductor, and to the left the circuit based on a gyrator circuit, where the inductor has been replaced by a transistor plus capacitor and biasing resistors. Basically the circuit framed by the square on the right side is replaced by the circuit framed by the square on the left.

This is the frequency response of the original circuit when varying the wah potentiometer:

It is a tuned band-pass filter with its peak moving from 240 Hz to 1.28 kHz, increasing the gain from 12.5 to 23.5 dB.
And this is the frequency response of the gyrator circuit when varying the potentiometer:

Again it is a tuned band-pass filter with its peak moving from 260Hz to 1.28 kHz, slightly increasing the gain from 16.5 dB to 19.2dB.
So we obtain a quite similar result with even a more stable gain peak.

PCB Layout

(Update 23/08/2015: Source files on Github)
The PCB was made on two layers with dimensions of 80 mm x 50 mm:

Schematics and PCB layout was designed using Eagle CAD. The PCB layout can be uploaded into Eurocircuits website, and a quote can be obtained immediately, I paid 70 € for 2 boards prototypes in a 7 day turnaround, the quality is really good.

I purchased the components at Mouser website. You can find here the BoM, total cost of components for one prototype was 26 €.

To solder the components I used a low temperature (138°C ) CR11 solder paste Sn42Bi58 and a hot air soldering station, the results are very professional and clean. This is the finished PCB with all components (except external switches, jacks and potentiometers) mounted.

All the cables, jacks, connectors, switches and potentiometers soldered and ready for debug and verification

Germanium diodes vs Schottky diodes for audio distortion

Germanium diodes are a preferred choice for use in distortion guitar pedals for their unique sounding characteristics when clipping an audio signal. Germanium clipping is softer than that of regular rectifier silicon diodes but clipping starts at lower levels. A softer clipping generates less high frequency harmonics and hence it produces a warmer sound. But the main characteristics that provide a more agreeable sound to the ears, like that found on valves, come from even harmonics, and that cannot be achieved with symmetric clipping but asymmetric clippping.

Germanium diodes also have a lower forward voltage than silicon diodes and hence they are able to clip signals at lower levels than silicon diodes.

But unfortunately, germanium diodes are scarce an expensive these days since their use as rectifiers in electronics is quite reduced and they have been displaced by a variety of different types of diodes made out of silicon. Regular silicon diode rectifiers have an abrupt I-V curve more adequate for rectification purposes, which translates in a hard clipping of the signal at approximately 600mV. On the other hand, Germanium diodes have a much less abrupt I-V curve which means that they provide a much softer clipping that starts at approximately 300mV.

Germanium diodes (like 1N34A) are hard to find in usual large distributors such as Digikey, Mouser, Newark, Farnell or RS, they are mostly found in specialised audio and guitar pedal boutiques since their use is more and more reduced to audio distortion in fuzz pedals. They are only found in conventional through-hole mounting (glass DO-7 package) but not in SMD packages.
The 1N34A can be found from 0.4€ to 1.71$ in a glass DO-7 package.

Schottky diodes (like BAT54) are a special but quite common type of silicon diodes used as rectifiers with very low forward voltage 200mV, similar to that of germanium diodes, but they also show a very abrupt I-V curve that generates hard clipping at a much lower voltage than regular silicon diodes. They can be found in many different packages including small SMD packages with the advantage that two diodes can be included in the same package, reducing BoM costs additionally. The BAT54S can be found as cheap as 0.022$ in SOT-23 SMD package.

In order to be able to replace Germanium diodes by Schottky diodes and reduce the steep ramp it only requires adding a resistor in series with the diode so that an increasing voltage drop is added with an increasing current. Using a variable resistor o potentiometer can additionally provide control on the clipping softness, higher resistor value means more clipping softness.

The figure below shows the schematics used with LTSpice simulator in order to compare the I-V and V-I curves of the most common germanium diode (1N34A) with a quite common and cheap Schottky diode (BAT54).

LTSpice schematic file

The schematics with diodes in series allows comparing voltage drop in both cases: Germanium vs Schottky + series resistor for a given range of current. Simulation is repeated for Schottky diode with different resistor values. Series resistor is entered as parameter Rx from 5 to 500 ohms.

The schematics with diodes in parallel allows comparing current through the diodes in both cases for a given range of input voltage and for different resistor values.

The figure below shows the resulting I-V curves with V ranging from 0V to 1V and current from 0mA to 90mA.

The red curve shows the I-V curve of the Germanium diode while green curves show the I-V curve for Schottky diode with different values of series resistor from 5 ohms to 500 ohms, the higher the resistor value the flatter the curve.
As it can be seen, there is no way to exactly replicate the germanium diode curve with a Schottky diode, but the Schottky diode may actually provide softer curves than the germanium diode which is the main purpose of using germanium diodes. For low values of current, the resistor must be higher in order to overlap the germanium curve, when current increases, the resistor must be lower, for 5 ohms both curves run parallel showing an asymptote or convergence in the infinity.

The figure below shows the V-I curves with current ranging from 0mA to 1mA and voltages ranging from 0 to 600mV.

At very low current values the voltage curves overlap for highest resistor value (500 ohms), but at higher current values the voltage curves overlap for 67 to 80 ohms.

Let's see the effects of both diodes in a real simulation using soft clipping and hard clipping configurations.
The schematics below show a circuit with both types of diodes using a soft clipping section followed by a hard clipping section.

LTSpice schematic file
Opamp PSpice model library

The soft clipping section consists of an opamp amplifying ten times the input signal. Soft clipping diodes are added in both directions (for positive and negative clipping) in the opamp feedback circuit between the negative input and the output of the opamp. The chosen reference resistor (Rin) between negative opamp input and ground is 10 kohms. The feedback resistor is 100 kohm for a x10 gain of the opamp. Varying this 10k value may require adjusting the series resistor of the Schottky diodes to match germanium diode response. The series resistor value required to match germanium diode response in this case is 250 ohms.

The hard clipping section consists of two diodes in both directions between the output of the opamp and ground after a decoupling capacitor of 4.7uF. The series resistor value required to match germanium diode response in this case is 14 ohms.

Figure below shows soft clipping signal comparison for germanium (green) and Schottky (red) for an input sinusoidal signal of 440Hz and 600mV and series resistor values of 5, 25, 100, 250 and 500 ohms. Soft clipping signal in this simulation is probed between the opamp negative input and the opamp output. As it can be seen, Schottky signal (in red) is in general lower than germanium signal (in green) for most resistor values except for 500 ohms, but the waveform shape seems closer to germanium waveform for 250 ohms.

Figure below shows hard clipping signal comparison for germanium (green) and Schottky (red) for an input sinusoidal signal of 440Hz and 600mV and series resistor values of 5, 10, 15 and 20 ohms. Hard clipping in this simulation is probed between the signal connecting output decoupling cap and diodes, and GND. Schottky signal (in red) is closer to germanium signal (in green) for a series resitor value of 15 ohms.

The figure below shows the time response signal in mV for both soft clipping (top plot) and hard clipping (bottom plot) respectively. The input is a sinusoidal signal of 440 Hz that starts at 1V and exponentially decreases with a time constant of 30 Hz.

The use of a exponentially decreasing signal is chosen to recreate the response to a real audio signal source with different voltage levels from 1V to several tens of mV. Clipping is higher for higher voltage levels. The choice of Schottky and Germanium diodes also allows clipping starting at lower voltage levels (200 to 300 mV) compared to silicon (500 to 600mV) or LED (>1200mV) diodes.

The figure below shows the FFT signal (frequency domain spectrum) in dB between 300 Hz and 30kHz for soft clipping (top plot) and hard clipping (bottom plot) signals respectively using a Blackman window to soften the signals and enhance harmonics visibility.

As it can be seen, frequency spectrum for both types of diodes (germanium and Schottky) are very similar and almost overlap perfectly.
Soft clipping shows less high frequency harmonics (warmer sound) which is more suitable for overdrive distortion while hard clipping shows high frequency harmonics (harsher sound) more suitable for fuzz distortion.

Conclusion

Spice simulations show that germanium diodes can be replaced by Schottky diodes plus series resistor for distortion applications with almost no difference in the waveform signals obtained.
But these simulations must be completed with real life experiments to confirm simulation results.

I plan to implement a version of the famous and noisy Shin-Ei Fuzz-Wah with the option of germanium or Schottky diodes for comparison. Stay tuned to my blog for Spice simulations of this pedal and real implementation of it.

Notes

The figure below shows the I-V curve at near zero current for Germanium diode (red curve) and Schottky + Rseries (magenta curve). Zero bias resistance is the value of the slope of the previous curve (Rbias=dV/dI) at 0A. Using a function dependent current source (BI in LTSpice) I could simulate a current whose value is the derivative of voltage respect current for the Germanium diode (green curve) and the Schottky + Rseries (cyan curve). The value at 0A is the zero bias resistance, which is 167 Kohm for germanium diode and 257 Kohm for Schottky +Rseries.

Zero bias impedance and I-V curve at near zero current