Rezz-Fuzz 3 in 1: RAT + βr + Octaver

For this guitar pedal project I wanted to mix some of my most iconic distortion effects into a single one:

1. Turbo-RAT is the distortion that was used by one of my favourite 90s bands: Teenage Fanclub
2. βr (Beta or hFE reverse) is a technique used on the Fuzz War pedal by DeathByAudio, designed by Oliver Ackermann, singer and guitarrist of one of my favourite 2000s bands: A Place to Bury Strangers
3. Octave Up was kind of a wink to noisy fuzz pedals like the Shin-Ei Fuzz used by Jesus & Mary Chain, one of my favourite bands of 80s, 90s that adds a doubler to increase high frecuency harmonics.

Gerard Love, TFC bassist, showing a Turbo RAT pedal (picture from Bandwagonesque album vinyl jacket)

Fuzz War pedal by Death by Audio

Shin-Ei Fuzz Wah pedal

William Reid "pedalboard" (Fuzz-Wah on the right)

So the Rezz-Fuzz 3 in 1 pedal basically had to be a:

RAT + βr + Octave Up

Apart from that, I wanted to experiment with different type of diodes for the hard clipping section of the RAT distortion part.

The main issue was that mixing both pedals into one would require too many knobs: 3 for the RAT, 3 for the Fuzz War, plus one for the octave up, plus at least 2 switch buttons: one for clipping diode selection and at least one for effect switching, that is far too many controls for a single pedal to be packed in a 1590BB box.

Some simplification had to be made, the first one was to remove tone control on the RAT and replace it by octave up control. But since only one effect is used at a time, it would be good to reuse the same knobs for both effects. The solution was to use analog SPDT switches for two potentiometers on each of the 3 potentiometer contacts, plus another 2 SPDT switches for effect input and output selection. A 4053 device includes 3 SPDT switches, 3 of them were required.

Schematics

These are the schematics of the Rezz-Fuzz 3 in 1 effect in Eagle CAD:

On the top it's the βr or reverse hfe part of the circuit consisting of 7 transistor stages. At first sight there seems to be a big mistake on this circuit: Those NPN transistors seem to be inverted, but they are actually not, collector and emitter are inverted on purpose, actually base-emitter and base-collector both consist of a PN junction and they could be inverted, except that the gain of the resultant inverted transistor is what is called the βr (or hfe reverse) which usually is much lower than the direct gain. Here is an article from AMZ website that explains the characteristics of  reverse beta amplifiers, apparently the clipping on a reversed transistor is quite different from a regular transistor generating different harmonics. The reason for having 7 transistors is because the gain on reverse beta transistors is much lower.

The problem with this is that actually the reverse hfe on most transistors is not specified, is not part of the manufacturing control process, and hence it may have wide variations. So the only choice here is manual selection after gain measurement with a multimeter able to measure hfe. On the other hand, this add a uniqueness to each pedal, and maybe the need to manually adjust resistors for each manufactured effect. The other issue is that I am not sure that Spice models can actually simulate the real behavior of transistor in reverse configuration. For this reason I will not include LTSpice simulations of the circuit.

I had to manually adjust the resistor values to make this circuit work, the final resistor values were very different from schematics published on internet. The recommended transistor base pull-ups were 430K on the first 5 stages and 910K on the last two stages, I had to change the value to 820K on the first 5 stages to properly bias the circuit and make it work. The recommended transistor emitter pull-downs were 390 ohms, I changed the values to 270 ohms. The recommended transistor collector pull-ups were 100K on the first 6 stages and 180K on the last 2 stages, I had to change the values to 1.2K in all stages in order to make it work.

In the middle of the schematics page, there are three 4053D 3-SPDT analog switch devices, the one on the left only uses two SPDT switches to select the input and the output of the selected effect (RAT or βr), the other two switch devices connect the three potentiomenter terminals to the desired effect. The switch device at the center is used to select the gain (RAT or βr) and the switch device on the right side of the page is used to select distortion/octave up mix for the RAT or tone for the βr effect.

At the bottom of the page it's the RAT effect with input and output buffer, soft clipping amplifier (that uses unbalanced diode clipping, R42 resistor is not installed) on top and octave-up on the bottom. Octave-up amplifier uses the feedback voltages between the feedback diodes and resistors to build a fully rectified version of the distorted signal.

The mix of soft-clipped signal and octave-up goes through a hard-clipping section where three options can be chosen: hard clipping with germanium diodes, no clipping or hard clipping with LED diodes.

On the bottom left side of the page it's the +9V DC input jack connector, battery terminals, an EMI filter, resistor divider to generate mid point voltage of +4.5V, decoupling capacitors and inversion protection diode.

On the top right side there is a 6-pin 0.1'' header footprint to solder a flat cable between the effect PCB and the 3PDT push-button foot switch PCB. A small PCB was designed in order to directly connect the foot switch and the PCB with a 0.1'' pitch flat cable.

PCB layout

The PCB was made on two layers with dimensions of 85 mm x 75 mm with chamfered corners to leave place for box screw holes. Three footswitch PCBs were also added.

PCB top layer

PCB bottom layer

The finished pedal with a Turbo RAT look:

Rezz Fuzz 3 in 1: RAT + βr + Octaver

Source files

If you want to build your own RezzFuzz 3-in-1 pedal find Eagle 6.3.0 files (schematics, PCB, gerbers, BoM) on this github repository.

DMFX-1: Open Source Digital Multi-Effects guitar pedal (1)

Rezzonics© presents the most compact digital multi-effects guitar pedal, completely open source, giving you the opportunity to create your own stereo effects or use a huge range of pre-programmed effects.

Every analog or digital effect your guitar needs in one compact format. The best of both worlds: ANALOG for overdrive, distortion, fuzz, octave-up, DIGITAL for echo, delay, chorus, tremolo, phaser, flanger... and many others you can imagine: looper, pitch shift, reverb, leslie...

DMFX-1 main features:

• TI C55x 16-bit fixed point DSP, providing quality, low power and low cost
• Dual DSP for low latency, real time signal processing and fully independent Audio and Control processing.
• Up to two SD/MMC cards for Software and audio data storage
• One graphical LCD 128x32 pixels Blue with White LED backlight for better visibility
• Up to 5 configurable LEDs
• One 5-button display navigator for seamless effect configuration
• Up to five digitally controlled and configurable potentiometers
• Analog configurable input distortion (overdrive, crunch, vintage, fuzz...)
• Analog octave-up effect, mixable with distortion
• Dual Mono L/R Line Output
• Stereo Headphones output
• Guitar Mono Line input
• Analog input / output buffers and filters
• Standard +9V DC or USB +5V DC power input
• Up to 2x USB 2.0 ports (Audio, Control) for PC connectivity
• Optional USB debugging port XDS100v2, TI Code Composer Studio compatible
• Foot switch input
• Compact size: 130 mm x 110 mm x 32 mm (W x D x H)

A stand-alone unit: just plug your guitar, your headphones and configure your desired effect easily via the navigator and LCD menus.

Stereo effects like reverb or speaker rotating effects like leslie can be emulated thanks to the dual mono and stereo outputs.

Analog feel can be emulated thanks to five configurable potentiomenters digitally controlled.

JFET buffer input, low THD distortion, low-voltage, rail-to-rail audio opamps and filters to better adapt guitar signal input and reduce input noise. Exclusive low-cost Schottky diode distortion circuits that emulate germanium clipping diodes.

Analog support for Tuner and Noise compression.

One graphical LCD 128x32 pixels Blue with White LED backlight for better visibility in dark places. A 5-buttons navigator (up-down-left-right-OK) allows easy navigation and effect configuration through menus.

One dedicated audio DSP and one dedicated control DSP provide low latency and real time, while reducing the bill of materials. Control DSP is used for status / commands operations: LCD, buttons and potentiometer control, while Audio DSP is fully dedicated to real time audio effects.

Up to two USB 2.0 (type Mini-B) interfaces allow pedal connection to a PC, for command / status or audio exchange.

Up to two SD/MMC cards can be connected to audio DSP for huge program and audio data storage.

One additional USB (Type-B) interface allows JTAG-Boundary Scan DSP debugging using TI XDS100v2 and Code Composer Studio for implementing your DSP effects.

Open source libraries will offer free digital effect exchange and improvement.

Several assembly options for low cost and high performance scalability.

DMFX-1 is low power, it can be powered by batteries, external +9V DC-DC converter or USB port.

DMFX-1 is compact: Main board + mezzanine board can be packed on a 148 mm x 100 mm x 24 mm enclosure (W x D x H)

Compact size: 148 mm x 100 mm x 24 mm (W x D x H)

3D view of the 2 PCB DMFX-1 guitar pedal (image created with FreeCAD)

DMFX-1 front view showing connectors

DMFX-1 rear view showing USB connectors

DMFX-1 block diagram

COMING SOON!!
STAY TUNED!!

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