The Fuzz Bucket is an 8HP eurorack fuzz and delay module made up of a two transistor fuzz circuit and an MN3005 bucket brigade delay (BBD). Unlike most delay implementations which aim to mask sampling and reconstruction error, the Fuzz Bucket provides direct unfiltered access to delay line inputs and outputs as well as override of the BBD clock. All of the aliasing, distortion, and clock noise peculiar to bucket brigade delay is celebrated as a primary effect instead of being hidden away as an unwanted problem. In addition, a silicon "fuzz face" is grafted onto one of the audio inputs, to add some extra colour.
Two audio inputs are scaled and mixed before being fed into a 4096 stage delay line. Input channel A passes through a two transistor fuzz circuit, while input channel B is passed through unmodified. When nothing is patched into an input, it is fed with a copy of the module output.
A two-phase clock drives the bucket brigade delay line from an input signal at one half the frequency input. When no external clock signal is patched in, an internally generated clock signal is used.
For a brief demo of some simple connection examples, see the Fuzz Bucket Demo Video.
|3||1k||R2 R4 R20||R_0805||ERJ-S06F1001V|
|9||100k||R8 R10 R11 R12 R13 R14 R21 R24 R25||R_0805||1%||ERJ-U06F1003V|
|2||100pF||C8 C17||C_0805||COG ≥25V||08053A101FAT2A|
|6||100nF||C1 C3 C4 C14 C16 C18||C_0805||≥25V||GCE21BR71H104KA01L|
|5||10uF||C10 C11 C12 C13 C15||C_0805||≥25V||GRM21BC71E106KE11L|
|2||22uF||C5 C6||CP_2312||≥20V Tantalum||F971D226MCCHT3|
|1||C1M||RV2||Alpha9mmSG||B or C ok||RD901F-40-15R1-C1M|
|6||J2 J3 J4 J5 J6 J7||Thonkiconn||WQP-PJ398SM|
An HEF4093 quad CMOS NAND IC was chosen for schmitt trigger U1 for a low typical hysteresis voltage of about 1V. This selection allows for a wide range of clock sources without the need for amplification. Any 4093 equivalent should work, but may require a larger clock input to reliably switch.
The overall gain of the delay block is set by resistors R23 and R22. Resistor R23 reduces the level of the input so that the BBD will clip when the input signal at R21 and R25 exceeds approximately 10V peak to peak. Output resistor R22 reverses this attenuation and adds a small amount of gain to allow self-oscillation.
Capacitor C17 is required to reduce high frequency ringing on the output which is triggered by spikes that appear at the BBD output as it switches between buckets. The value of 100pF provides a compromise between suppression and reduced bandwidth.
Fuzz transistors Q1 and Q2 were selected arbitrarily for availability, low gain and poor noise performance. Capacitor C8 helps to suppress unpleasant oscillation and AM radio pickup at high values of fuzz.
Unfiltered outputs from the BBD carry a small amount of the clock signal, and at near ultra-sonic rates this clock noise may be perceived as a loud hiss. Passing the output of the module through a low pass filter can help to suppress the clock, and eliminate the hiss sound. When driving the BBD from a VCO, a low pass VCF with cutoff set to track the VCO roughly three octaves below the oscillator rate will reduce most of the clock noise. Note that while this will suppress clock noise and some of the output error, aliasing will still occur if the input signal contains frequencies greater than half the BBD clock rate (one quarter of the external clock input).
Since the output and inputs are shorted together when nothing is patched, insertion of a patch lead will temporarily connect the input signal directly to the output, and connecting to the output may mute an incoming signal briefly. In order to keep the circuit design as simple as possible, these transient patching issues have not been addressed.
While the fuzz circuit was derived from a fuzz face layout, adaptation to eurorack levels and power has changed the way it clips, particularly at low input levels. Instead of asymmetrically clipping the positive and negative signal, the circuit will clip both equally. This behaviour is quite unlike that of the fuzz face, but it works adequately in this context.
In the design presented above, the MN3005 operates between 0V and -12V, requiring AC coupled input and output. By using a pair of linear regulators and trimming the output offset, it should be possible to run the MN3005 on a balanced local power supply, and DC couple both input and output. This change would allow use as a control voltage delay, which when used with feedback could be quite interesting.
Based on tests of the prototype module with other instruments, many of the most satisfying patches involve overriding the BBD clock with a sequenced VCO. By providing a direct clock override, the work of generating an interesting clock has been avoided and the circuit kept as simple as possible. It might be worthwhile to extend the clock circuit either with a precision VCO like the CEM3340 or a PLL like the 4046. This could then be paired with an output filter designed to track and suitably suppress clock noise automatically.
The fuzz block applied to input A is a crude adaptation of the classic two transistor fuzz face circuit as published on ElectroSmash utilising notes on the use of silicon transistors in The Technology of the Fuzz Face by R. G. Keen.
The delay core and the two-phase clock that drives it is based on a circuit published in Barry Klein's Electronic Music Circuits.
PCB footprints and schematic symbols are derived from the Kicad official repositories, or created from scratch - these have been included in full within the Kicad project.
Many thanks to Adam Cole for his help with design and testing, and for listening patiently to dumb ideas. Thanks also to Morgan McWaters, Lewis Boyes and the team at Found Sound for their support and encouragement.
Nathan Fraser, August 2018