Embedded Files
Introduction
Introduction
The Fuzzball is our take on the classic Fuzz-face design. The Fuzz-face was a pioneering design that played a large part in the introduction of guitar pedals as a whole. Our version provides more versatility than this base circuit. The gain switch provides an option between two gain settings, while the tone knob allows a wide variation in tone. In this page, we are going to break down this circuit and explain how it works.
(If you are reading on a phone, when we refer to an image to the right, it will be below the text, and to the left will be above)
Circuit Overview
Circuit Overview
The Fuzzball's schematic can be broken up into four simple parts: power supply, LED indicator, fuzz stage, and tone control.
There are two reasons why none of the components in this schematic have values. Firstly, for the time being, we wish to keep the exact values to ourselves for the sake of having something different. Secondly, there isn't really any point, as every circuit will need to be figured out individually, particularly the transistor biasing resistors. However, we will explain how to figure out the values needed for each individual circuit.
Although the Fuzz-face pedal this circuit is based on is relatively simple in terms of parts, it takes a bit of knowledge in order to tune it properly, and it took us many months of development.
Power Block
The power block for this circuit is extremely simple, due to this pedal not requiring a virtual ground. However, it still performs a couple of important functions. This pedal runs off a standard 9V centre negative adapter. These are usually fairly quiet, but the 100µF Capacitor C1 serves to filter out any remaining noise. D1 is a 10V zener diode that serves two purposes. Firstly, it protects against reverse polarity connection by allowing current to flow through the diode rather than through the rest of the circuit. Secondly it prevents against higher voltage connections as when the reverse voltage on the diode exceeds the zener's 10V rating, it will conduct between the power and ground rail, protecting the rest of the circuit from excess current. R1 is a low value resistor limiting current when the diode is conducting.
Fuzz Stage
The fuzz stage is the most important part of the Fuzzball's circuit. R2 is a large-value (1M+) resistor to prevent popping when switching between the effect and bypass. C2 is the first spot where modifications can be made. Its primary purpose is to block any DC offset going between the effect and the guitar. However, it also acts as a high-pass filter, along with R7, RV1, and the impedance to ground of the two transistors. The -3 dB frequency of a high-pass filter is: 1/2πRC. This is a bit difficult to calculate due to the transistors; however, in the case of the fuzzball, this frequency is about 17.6 Hz in the low-gain setting, below the audible range (We cheated and used a simulator). This means that this capacitor doesn't really affect the guitar's tone, but if you wanted to remove some bass, you could use a small capacitor here. Other websites will go into far more detail about input impedance, but using a simulator, it can be seen that in this configuration, the Fuzzball has a very low input impedance, as low as 3.12k with the gain completely up on the high gain setting. This is typical of Fuzz-face circuits, and simply means the high-end frequencies coming into the circuit are attenuated heavily, giving a unique sound. If you wish to change this, simply add a common collector transistor buffer to the front of the circuit. However, the circuit does amplify higher frequencies further on, so the high-end roll-off isn't necessarily a bad thing. At maximum gain on the low-gain setting, the input impedance is about 75.2k, still quite low, but high enough that the Fuzzball could be placed after another pedal with a very low output impedance. On the high-gain setting, the Fuzzball should be the first pedal in the signal path.
Q1 is where the signal starts to become distorted. The collector is biased to 1V in the low-gain setting (R3, R4, and R6). This serves to asymmetrically clip the signal, as can be seen to the right. In the high-gain setting, the collector of Q1 is biased to 2.1V, clipping the signal less. Q2 serves two different purposes depending on the gain setting. In the low-gain setting, the collector is biased at 4.5V, meaning the transistor amplifies the signal without changing the amount of clipping much. In the high-gain setting, the collector is biased to 2.6V, clipping the other side of the waveform and resulting in less asymmetric clipping. On top of this, it also amplifies the signal. R7 is a 100k resistor serving two purposes as well, providing DC bias to the base of Q1 and negative feedback on the input signal. RV1 (Note that pins 1 and 3 should be the other way around in this drawing) is the fuzz control and essentially sets the gain of Q2. It does this by changing how much of the potentiometer is bypassed by C2. This lowers the AC resistance to ground, increasing gain without disturbing the DC bias of the system.
Now, this is where things become difficult. Because there are no coupling capacitors between the transistors, they interact with each other. This means that when biasing the collector of Q1, you need to be mindful that it is biasing the base of Q2 as well. As a general rule of thumb, the base of a transistor used as an amplifier should be biased about 0.6 to 0.7V above whatever voltage is at the emitter. This is how the low-gain setting of the Fuzzball is set up. The high-gain stage has the base of Q2 set slightly higher at about 1V above the emitter, but the incoming signal still stays within the linear range of the transistor (~2V). However, the size of the signal entering Q2 means that the base does go out of the linear region, creating more clipping. It should also be noted that taking the output from Q2's collector rather than between R8 and SW2B makes the circuit louder, but gives less clipping on one side of the waveform. When designing your own Fuzz-face circuit, there are two ways to go about it. Either set up the circuit with potentiometers on the collectors and adjust them until either your multimeter or your ear is happy (warning: you can sometimes get very loud amplification within a small tweak of the potentiometer, so set your amp volume down very low), or use an electronic simulator such as KiCAD or LTSpice to tweak the values first. We used the first method for this pedal as we didn't know how to do simulations at the time, but looking back, the best method would be to use the simulator to get close, then adjust it until it sounds good afterward.
Output Waveforms
Output Waveforms
The chart above shows the waveforms for the Fuzzball when the volume and tone knobs are set to 50%. As you can see, even at 0% fuzz, the output is very asymmetrically clipped, although rather quiet. As the fuzz setting is increased, the clipping gets more pronounced, squaring off the bottom of the waveform more. Note that this simulation uses a 200mVpp signal, but the sound demos on our Instagram were recorded with a Strat-replica, which has an output of about 100mVpp. This doesn't affect things too much; it just means that the waveform will be slightly less clipped with the smaller input signal. Note that between 0% and 50%, the waveform doesn't change much, but there is a big change between 50% and 100%. This means that it is probably better to use a C1k reverse audio taper potentiometer rather than the B1k linear taper potentiometer used in the Fuzzball. This is an example of where it is useful to use a simulator.
These are the sound demos for the low-gain setting. It also includes a tone-sweep that is explained further down
As you can see here, the high-gain setting significantly increases output given that the volume knob is still at 50%. Compared to the low-gain setting, the bottom of the wave is slightly more squared off, but the top is also clipped, due to the second transistor being biased at 2.6 volts. These waveforms are a lot more evenly spread along the fuzz sweep, which is why the B1k potentiometer was used in the Fuzzball. Notice that the phase of the output slightly shifts as the fuzz is swept. This is because the time constant of the RC circuit formed by the fuzz knob and bypass capacitor changes. The outputs are sloped rather than square and slightly phase-shifted due to various tone-control elements. This isn't necessarily better or worse than a perfectly squared waveform, just different.
These are the sound demos for the high-gain setting.
Tone Stage
Tone Stage
The Fuzzball uses a tone stage similar to the EHX Big Muff. It simply splits the signal into a high-pass path and a low-pass path and then mixes them via the potentiometer. Equalization is the part that really makes a tone pedal sound unique. The low-pass filter is made up of R12 and C5, while the high-pass filter is made up of C4 or R13. The formula to find the -3dB point of the filters is 1/2πRC, the only difference being which side of that point is attenuated. With this knowledge, it is possible to give the pedal a mid-hump by overlapping the high-passed and low-passed frequencies, or a mid-scoop by putting a gap between the two points (assuming the potentiometer is centred).
This graph shows the mid-hump scenario, where the low-pass filter (green) and high-pass filter (blue) combine to give an output (purple) with attenuated highs and lows, but a relatively unaffected mid-range. Note that the potentiometer averages the voltage from the two filters, rather than summing them, meaning that the output sits between them (more or less).
Meanwhile, this graph shows the mid scoop scenario, where the two filters attenuate some of the same frequencies, creating a scoop centred where they cross.
The two graphs show the pedal's frequency response in both gain settings with the volume and gain set to 50%. It can be seen that the high gain circuit has a much more treble focused frequency response compared to the low-gain circuit, which has a wider range of frequency responses.
Finally, the signal has its DC voltage removed by C6. Note that it is probably better to have this capacitor before the tone stage, as in this setup, RV2 and R13 have an effect on the biasing of Q2. RV3 is set up as a volume control, bleeding some of the signal to ground before it goes to the output. This setup of having a passive volume right before the output with no output buffer tends to lead to a high output impedance at maximum volumes. However, due to the tone control before it, it isn't too bad. Rough calculations give an output impedance of 20.4k at maximum volume for the high-gain setting. At 25% volume, this drops to 12.7k. The general rule of thumb for guitar pedals is to have an output impedance of 10k or lower to prevent tone sucking (loss of high frequencies). 20.4k is still low enough that, combined with a high input impedance on whatever follows it, the signal should be relatively well preserved. Note that these values are for high frequencies, as I was lazy and treated the capacitors as short circuits while doing calculations. For lower frequencies, the output impedance will be a bit higher.
The Fuzzball is a pedal based on the tried and tested Fuzz-face topology, yet provides a unique sound. Head over to the Fuzzball product page if you want one for yourself.
Google Sites
Report abuse