I’m taking apart a distortion pedal that I own. I bought it at a garage sale in Minneapolis for $20, no tax. The case is bright yellow (hence the “Yello”) and it adds scratchy, high-frequency distortion to anything that passes through it (hence the “Mello” - nobody in our 5-piece folk rock band was a fan).
The innards are hand-soldered and mostly intact, and there’s probably fewer than two dozen wires altogether. It seems like a great place to start learning how circuits work.
Who knows how circuits work? Not me! I watched some videos to get my head on straight:
I took apart the pedal, and got a sense of how everything was connected. I spent a day trying to draw a good circuit diagram of the thing. It took a few tries, and I’m still not sure what some of the components are, but at least I’ve got things sketched out on paper. Take a look here.
At the moment, I’m working through the Khan Academy videos to better understand how to work out the total resistance in the circuit. I’ll need to do that, and identify a couple of mystery components (which look like resistors, but that doesn’t make electrical sense) before I’m able to figure out the whole circuit.
Then, I’ll be able to:
Aha! Those switches are 2P2T and 3P2T switches. It doesn’t change much, but at least I can draw them correctly in the schematic now.
I bought a digital voltmeter, and learned a few things about the circuit:
Next steps for the Mello Yello project:
I set up audio in the workshop, so I can fiddle with the pedal, and understand what’s going on. I’m lucky enough to have the Make Noise 0-coast, which I can use to generate pure tones.
Here are my observations for the day:
I set up the 0-coast to make a “ping” noise - a sudden start, with a long tail. I had these additional observations:
Voltage seems to correlate to the loudness of the noise produced by the 0-coast (not necessary the loudness of the nosie that makes it to the speaker).
Maybe there is a component that doesn’t work at these low voltages, or we’re routing around a resistor at these low voltages?
I’ll have to read up more on how an electrical current is turned in to an audio signal. I still don’t understand what the distortion circuit is doing to change the quality of the noise.
Further, I think getting everything into Eagle and performing a more thorough circuit analysis will help me understand the unusual behavior between ~0.050V and ~0.027V.
But, “voltage == loudness” seems like a pretty sensible conclusion to have drawn - the more voltage pouring into the speaker, the stronger the vibrations, the louder the noise. Progress!
I didn’t know where to go next, so I took every measurement my volmeter was capable of taking. I learned some things about the circuit, and I learned some things about the voltmeter!
| Measurement | w/o distortion | w/ distortion |
|---|---|---|
| Voltage | .515V -> 0.027V | .044V -> 0V |
| Current | 36mA | No reading, audio interference |
| Resistance | 0Ω | 94.8kΩ |
| Frequency/Duty Cycle | 70.75Hz | 141.5Hz |
What I learned about the voltmeter:
What I learned about the distortion circuit:
Open questions:
Took some more measurements today. Figured out how to use the alligator clips on the voltmeter (turns out, you need to remove the tips that are on the leads before putting the clips on!), so now I’m able to take measurements as I’m manipulating the current. Very useful!
I did some research on how an octave pedal might work - one that raises the input signal by one octave. Some of them work like this - say a wave goes from +5V to -5V. For the bit that’s -5V, reverse it, so you have two peaks at +5V instead of one. This effectively doubles the frequency. Apparently some pedals do this “with diodes”, but it’s not clear to me what the electrical mechanism is that actually makes this happen.
I also discovered that the 0-coast uses a triangle wave (or square wave), instead of a sine wave. This complicates my experiments - these waves produce overtones, so what I’m hearing through the speaker isn’t just the fundamental frequency, but sympathetic frequencies as well. I don’t have access to a sine wave generator, so I’ll have to spend some time to understand how overtones work.
I also found that the 0-coast has a knob that allows you to filter between the base frequency, and the overtones - you can get all of one, or all of the other, or a mix of both. You can also amplify certain overtones! The voltmeter registers the frequency of whichever overtone is the loudest - so with a base frequency of 100Hz, you can see 100Hz, 300Hz, 500Hz, etc. If you’re very careful, you can get a 200Hz reading by straddling the base frequency and the first overtone, and the voltmeter averages the two out. There’s no overtone at one octave above the base frequency (I don’t think?), so this is just a neat measurement artifact.
This is getting in to audio engineering, in addition to electrical engineering. But, that’s the gear that I have, so I’ll just have to sit at that intersection.
Anyway - Mello Yello doesn’t just raise the input signal an octave, it also distorts it. This might be a function of the aforementioned octave pedal function - maybe having no negative signal distorts the sound as well.
Open questions: