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Distortion pedal project Answered

I was planning on making a motion detector with my roommates but that proved a difficult first project so now we have the plans for a distortion pedal for guitar.
Here is the design for it and here is where we are going to buy the equipment (except for the audio jacks which we bought at Radio Shack already).

Here are a couple of things we need to know first:
1) The plan calls for two 10uf electrolytic capacitors, and the store has those but of 5 varieties with different voltages, which ones should we get?

2) What kind of capacitors do we need? Ceramic disk?

3) The plans call for a 0.0056uF capacitor yet I couldn't find one like that, can I just substitute one similar? If so, which one?

4) If anyone wouldn't mind, could someone explain how this circuit works? Preferably answer the other questions, but I would like to learn how it works also.

Thanks a lot to all who answer!



9 years ago

1) 25V is probably fine for the 10uF caps.

2) Ceramic disks aren't preferred for the audio signal path; better to use mylar, polypro, polyester, etc. Although if "bees-in-a-can" distortion is the goal, it doesn't matter too much....

3) The 0.0056uF capacitor looks like a low-pass filter (rolls off the hi freq), and is probably there to quiet the circuit and help maintain stability. A larger value (toward 0.01) will reduce the high end and make it quieter (but duller), and a smaller value (toward 0.001) will let more highs pass through (but could result in oscillation, uncontrollable noise, etc.)

Sorry, I don't have time to explain the circuit in detail...but it probably is very important to use a germanium transistor as indicated. Also, you could substitute a 1M POT for the 1M resistor above the opamp. That's the opamp feedback loop, and larger the resistor here, the higher the gain/noise. Substituting a POT would make the gain adjustable... It's just a thought--transistors distort rather abruptly, might work, might not...

Ok, so I have everything set up to what I THINK is correct, yet it obviously doesn't work (what does on your first try really?). I think something is wrong with the way I am connecting the audio jacks, so I took a picture of the diagram on the package and of what I did. Something just seems wrong with it, perhaps because I bought stereo phone jacks not mono? Here are some pictures I took of it:


The input plug is wired so that the power is automatically killed when you unplug the guitar cable. A stereo jack is required for this scheme. You can ignore that setup for the breadboard testing, tho. Otherwise, just be sure the shaft of the plug is GND, and the tip is the signal. Troubleshooting: -- Check and recheck your wiring. -- Test the circuit in stages. Attach the output plug (tip) before the transistor to see if the first stage (opamp) is functioning. If there's no signal here, that narrows down the problem to the opamp part (assuming it's a single problem.) If that works, recheck the transistor wiring and pins. -- Be sure the opamp can work with only 9V. }{itch explained that this circuit doesn't use a bipolar power supply. Some opamps want +9V and -9V.

So does the way I have the jack plugged in work? I have nothing plugged in to one of the out put things (number 1 if you can see on that diagram at all). On the schematics, what does the little trident beneath the audio in and out mean? Should there really only be one wire going the the audio out jack?

The trident thingie is also a ground, and should be connected to the other ground symbols. Technically, it's an "earth ground," it's marked like that since the jack collar will be grounded to the chassis or metal stompbox. I wouldn't sweat the distinction, just tie all the grounds together... I cannot read the diagram, but wire the jacks so the tip of the plug is the signal, and the shaft is the ground. When a mono plug is inserted into a stereo jack, both the collar and the first prong should contact the plug shaft. Use either one for the ground; if the other is unconnected, don't sweat it... (you'll still have two connection points--tip and shaft.)

And here is another more close up, those others are hard to see:


Oh, and what wattage of resistor? They supply anywhere from 1/4 to 4 watt ones, and then also DIP and SIP ones whatever those are.

The 1/4 watt resistors are fine.

RE: transistors subs-- you can always try a silicon transistor instead, and it will very likely work, but in this case the germanium transistor is probably an important part of the sound...

There are simpler projects, as }{itch wrote...

Thank you very much!! Your response was very useful. So there is one thing more. I noticed that they don't carry the 2N44 transistor, would it be ok to replace it with a 2N3906 (Transistor, PNP, GP Amplifier)? I know the instructions say I can, but I just want to make sure. Or would this be better: 2N4249 (Transistor, PNP, Low Noise Amplifier), or any of the other ones on the website?


9 years ago

heres a brief overview of how (i reckon) the circuit works: There are 2 main sections to the circuit, spereated by the 10uf capacitor in the middle of the schematic. The right hand side provides distortion by "overdriving" the germanium transistor. The left hand side amplifies the signal and (i think) provides optional high pass filtering using the normal/high switch. There are a couple of sections to the left hand side.First there is the 0.01uf cap on the input, this is a dc blocking capactior, it filters out the continuos dc component of the input signal but allows the ac component to pass (the ac component is the sound), this input signal now varies between some negative voltage and some positive voltage (from usually between -0.1v and +0.1v to -1v and +1v depending on how "hot" the guitar picukups are) The power supply for this circuit is a 9v battery and cannot provide a negative voltage, so the negative portion of the input signal cannot be amplified, this is where the 2x 20k resistors and the 1M resistor come in. These provide what is called biasing, it takes the ac signal and adds a known dc component to it (set by the potential divider formed by the 20k resistors), using ohms law we can find the dc value between the 20k resistors is 4.5volts (half the power supply voltage), this means the input signal will now vary between around 4.3v to 4.6v (again this could be as high 4v to 6v depending on pickups) wich is within the range of the power suplly so can be amplified, (the 1M resistor stops excessive current from th ac component being drawn to the power rails.) The singal is now sent to the op-amp which is set up in a non inverting configuration where the signal is amplified and optionally (with the high/normal switch) filtered before being sent through another dc blocking capactor (the central 10uf cap) to the distortion section of the circuit, (which i probably won't be able explain coherently, allthough i have no doubt someone else here will).

Thank you! So I have almost everything now (the person I bought it from forgot the two .22uF capacitors), and I am starting to put it all together. How to I know which input is which for the transistor though?? It has three prongs but I don't have any way of identifying them.

type in the name of the transistor + datasheet into google and you should be able to find a pdf that will show you what each of the pins are.

As a general rule the pins will be something like one of these: transistor pinouts

good luck!

ignore that last link if your using a 2n3906, it has the collector and emmiter switched around. this is better:
2n3906 pinouts

p.s if your looking for something a bit simpler for a first project this vox treble booster is pretty straight forward:
vox treble booster
and can be modded to act as a distrotion unit. I've made a few and they're ok.