Fuzzbox, Distortion, Phaser, Chorus, Echo and other Guitar Effects Info

Just got a new package from OSHpark with some prototype pc boards. Besides the Squeezer compressor, there is a new op amp fuzz and 3 sets of mini-boards, one of which is a tiny mini-booster. More info soon.

While looking through some boxes of old material, I came across this completed pcb for the ADV-Snare project that I designed with Thomas Henry. The article on this project was published in Nuts-and-Volts magazine over 20 years ago!

Low input impedance is not really the issue making a fuzz incompatible with a buffer.

A fuzz face has a lot of gain…. VERY high. It is basically a crude inverting op amp with a 100k resistor in the negative feedback loop. There is no resistor in series with the audio input that would otherwise reduce the gain. The simplified gain equation is Rf/Ri where Rf is the feedback resistance (100k) and Ri is the input resistance. When connected directly to a guitar, the combined resistance and impedance of the pickups are in series with the input (acting as Ri) and will reduce the gain, thereby preventing the fuzz from going bonkers.

If you drive the fuzz with a buffer, the output resistance/impedance of the buffer is very low so the result is that the fuzz circuit runs at max gain with oscillations and other mayhem often as the result. Buffered input = max gain = bonkers, and guitar pickup = lower gain = fuzz heaven 🙂

The Rangemaster does not have this problem because the available gain from the Ge transistor is much lower than that available from a FF.

The Big Muff does not have this problem because each gain stage has a series resistor to reduce their gain… 39k on the input stage, for example.

Charge pumps are fairly simple devices. They involve charging a capacitor to the voltage of the supply, and then stacking it on top of that power supply so that the voltage potentials are added together, effectively doubling the output.

It looks like the pcb on this Daphon E20CM compressor was supposed to have a square cutout in the top left corner to match the one on the right side, in order to fit around the mounting post. When cutting the pcb, the board fractured and chipped off a small piece including a portion of a copper trace. Instead of trashing the damaged board, they added a bodge wire to replace the missing trace.

I ordered a selection of prototype mini-boards from Oshpark, including tiny boosters, fuzz, and more.

I asked the manufacturer of my audio analyzer instrument to modify one of their plugins to be able to measure compression. They did so, and it works really great, so I am planning on doing some tests on various guitar compressors.

In my first trial run of the compressor test setup, I learned 3 things:

1. My Empress Compressor seems to be broken;
2. The LM3080 based compressor is not as noisy as I anticipated;
3. Opto-coupler compressors do not react linearly.

I’ll have an article about the compression results in the near future.

Did you know that DC power jacks have a voltage and current rating, and they may not be as high as you would think?

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A popular article on my web site is the one about pickup simulation, and questions come up several times every year about the transformers recommended for this project. One technique that works well is to get a cheap guitar pickup and use it as the input inductor.

The black wire of the pickup goes to the input jack while the red wire connects to the input of the electronic circuit. If there is also a braided shield, it can be connected to ground. The 10k trimmer and the 330pF capacitor are simulating the cable connecting the guitar to the pedal.

Cheap pickups are readily available for $5 or $6 on Ebay, or from US suppliers of parts for cigar box guitars (cbg). The 3-pole pickups for the cbg projects will be physically smaller and there are even versions with a metal shield over the coil.

The pickup has the electrical characteristics of a guitar pickup (because it is one), and in this circuit it works quite well to modify the tone and response of a fuzz circuit or other project.

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