Thursday, 23 July 2015

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)
Oliver Ackermann from A Place to Bury Strangers showing a Fuzz War pedal
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.


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.

Wednesday, 1 July 2015

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