Basic guitar pedal - do I need to limit the output signal's amplitude?

Thread Starter

YurkshireLad

Joined Nov 12, 2021
3
I'm slowly building a very basic distortion pedal from the ground up, trying to learn some basic electronics as I go. I'm a programmer by trade, so my electronics skill level compared to learning how to bake, is like knowing where the kitchen is.

I have a basic gain stage as shown in the diagram, with a gain of 11 and a power supply of +9V DC. I verified the input signal from my guitar and the output signal from the op amp (TL082) using my oscilloscope. The input from my guitar is about 250mV p2p and the output signal is boosted as expected. I didn't configure enough gain to distort the signal but I will do that later (and add transistors to clip the output) once I'm happy with a basic boost stage.

I plugged the output into my small Peavey practice amp's clean channel and set the volume to around 2. When I played my guitar, I could hear the signal through the Peavey, but the signal broke up when the guitar volume knob was set turned up. I guess it was overloading the amp and it sounded terrible, but not distorted. Perhaps the bass frequencies are too strong. I will learn how to add tone control in a bit.

Do I need a resistor on the output, after the capacitor to lower the amplitude? Is there a max voltage the output should be, say less than 1V? I only have a 10k potentiometer (need to order some) so I could put that at the output to control the volume. Is this the correct way to go or is there something fundamental I'm missing? I'm not even sure I'm asking the right questions here.

I've looked at some schematics for existing pedals and they have a volume pot at the end of the tone stage or output buffer. This is definitely something I can add. Any help is appreciated. Thanks

(PS I didn't create the attached diagram, but I somehow managed to build the same circuit. I'm using this diagram as I'm having trouble creating a schematic that correctly shows the +ve/-ve inputs to the op amp).
 

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Audioguru again

Joined Oct 21, 2019
4,524
An electric guitar is usually loaded with 1 Million ohms at the input of a vacuum tubes amplifier. Your opamp has an input that is only 56k/2= 28 thousand ohms that mutes high frequencies from the pickup making low frequencies sound too strong.
The TL081 opamp works fine when the 56k resistors in your circuit are 2M, then the input will be 1M what you want. The 1uF input capacitor will take a long time to charge when the opamp gets its supply turned on so the 1uF capacitor can be changed to 0.01uF (use a film capacitor marked 10nF or 103).

The opamp gain can be reduced by reducing the value of R4. 47k will reduce the gain a little, 22k will reduce it a little more and 10k will reduce the gain to 1 like a piece of wire.
 

MisterBill2

Joined Jan 23, 2018
11,582
The input to the Peavey amplifier is intended to handle low level signals, and so the range of the input is limited to lower levels. So overdriving it will cause distortion, but not the kind that your circuit is supposed to create.
A very simple way to create a bit of distortion and prevent an overload will be to put two diodes in parallel from the output of your circuit to the common of your circuit. The diodes need to be opposite polarity from each other. Different diodes will provide somewhat different sounds.
To see a lot of guitar sound changing circuits go to the "schematice for free" website and look under "musical" they have lots of interesting circuits.
 

Ian0

Joined Aug 7, 2020
4,843
Most pedals have a volume control pot on the output, so you can avoid overloading the amp's input.
I would suggest connecting the diodes from the op-amp output to its inverting input (in parallel with R4)
My favourite distortion circuit uses two 2.4V zeners in series (cathode to cathode) in parallel with the gain resistor (R4).
The 2.4V zener is particularly useless as a zener, but does seem to make distortion that guitarists like.
You could also try a single 2.4V zener, because guitarists also seem to like asymmetric distortion.
 

Ian0

Joined Aug 7, 2020
4,843
The Fuzz Face circuit made asymmetric clipping. It used transistors biased wrongly (no diodes) to make the asymmetry.
As well as "biassed wrongly", I might suggest "biassed randomly" because every fuzzface is different, due to the varying Vbe of the germanium transistors, and they also vary with temperature. I built five using different germanium transistors and they all sounded different. Then I built another using the same type of transistor the the first one, and it also sounded different.
By far the weirdest used some germanium transistors in a 4-pin can, with a pin to screen the case which I removed.
 

Audioguru again

Joined Oct 21, 2019
4,524
As well as "biassed wrongly", I might suggest "biassed randomly" because every fuzzface is different, due to the varying Vbe of the germanium transistors, and they also vary with temperature. I built five using different germanium transistors and they all sounded different. Then I built another using the same type of transistor the the first one, and it also sounded different.
By far the weirdest used some germanium transistors in a 4-pin can, with a pin to screen the case which I removed.
The fuzz face circuit has 100% negative feedback for DC and some for AC so it should be repeatable:
 

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Thread Starter

YurkshireLad

Joined Nov 12, 2021
3
Most pedals have a volume control pot on the output, so you can avoid overloading the amp's input.
I would suggest connecting the diodes from the op-amp output to its inverting input (in parallel with R4)
My favourite distortion circuit uses two 2.4V zeners in series (cathode to cathode) in parallel with the gain resistor (R4).
The 2.4V zener is particularly useless as a zener, but does seem to make distortion that guitarists like.
You could also try a single 2.4V zener, because guitarists also seem to like asymmetric distortion.
Thanks - I want to put a couple of diodes in parallel once I have this basic circuit working. Just to see what the difference is.
 
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