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Distortion Pedal

  • Thread starter Deleted member 139372
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Deleted member 139372

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Hello all. I found this website in a search of the web to help answer a question I have. Just from the bit of reading I've done I can tell this is already a new favorite. Nice to meet you all.

I was wondering if someone can help me figure out how this circuit operates. The schematic is that of a distortion pedal for a guitar. Now, I know that distortion is caused by clipping of the input AC signal and that the worse (or better if you're a guitar player looking for this effect hehe) the sound is, is caused by heavier and sharper clipping.

With this circuit I understand that the first transistor is used as an initial gain booster and the second is what is causing the distortion. The collector of the second transistor leads into a low pass filter I understand as well.

Can someone explain what actually causes the signal to clip in this circuit please? Also, the capacitors I'm not sure of what they are doing in this circuit really. I'm not too concerned with them as I don't believe that they are responsible to the clipping, I could very well be wrong though.

Any help in explanation would be greatly appreciated!

Thank you in advance for help with my question and also for hosting such a great website!

Chris

 
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Deleted member 139372

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Also I just thought that if anyone is interested, this is for a research project that I am writing. If you care to see what I have so far you can download it from the link below. The question asked above will help me complete it.

Any criticism is certainly welcome but I'm not asking for any :).

Free File Hosting Made Simple - MediaFire
 

audioguru

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The circuit is so simple and is so poorly designed that both transistors clip the signal.

The first transistor is almost saturated when the gain is maximum and is completely saturated when the gain is low. The second transistor is almost saturated but the LED blocks the signal. A transistor clips the signal when the signal causes it to become saturated or cutoff.

The circuit should be designed so that it does not need transistors that are tested and sorted into certain hFE numbers.
 
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D

Deleted member 139372

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Thanks for the reply!

Here is my understanding so far.

The first 100Kohm pot is the gain control. The first transistor clips the signal a little or a lot dependent on the gain. The output (collector) of the first transistor feeds the clipped signal into the second transistor which clips the signal even harder. Again the signal leaves the transistor through the collector and is fed through a low pass filter, the cutoff frequency dependent on the 50kohm pots wiper. After passing through the filter the signal is routed through to the output, passing over a potentiometer which controls over all volume/amplitude of the signal by adjusting the 100kohm pot wiper.

Questions:
1. What is the upper part of the circuit for? It doesn't look like a voltage divider to me. Is the second transistor using negative feedback, or does that only work for op and diff amps?

2. What is the purpose of the first .1uF capacitor (right by input) and the 10uF capacitor?

Thank you again! I give my presentation tomorrow. I was hospitalized for 2 weeks and missed a large part of my research opportunities. I was offered an extension but I'm shooting for tomorrow since this is the last part of my research.
 

audioguru

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Most Helpful Member
Here is my understanding so far.

The first 100Kohm pot is the gain control. The first transistor clips the signal a little or a lot dependent on the gain. The output (collector) of the first transistor feeds the clipped signal into the second transistor which clips the signal even harder. Again the signal leaves the transistor through the collector and is fed through a low pass filter, the cutoff frequency dependent on the 50kohm pots wiper. After passing through the filter the signal is routed through to the output, passing over a potentiometer which controls over all volume/amplitude of the signal by adjusting the 100kohm pot wiper.
Yes.

Questions:
1. What is the upper part of the circuit for? It doesn't look like a voltage divider to me.
Each transistor has a 10k collector load resistor.

Is the second transistor using negative feedback, or does that only work for op and diff amps?
The first transistor uses the left pot for negative feedback. The second transistor uses the 100k resistor and LED for negative feedback.

2. What is the purpose of the first .1uF capacitor (right by input)?
It is a coupling capacitor. It passes the AC signal but not the DC voltage.
It keeps the guitar's pickup coil from shorting the DC voltage at the base of the transistor to ground.

What is the purpose of the 10uF capacitor?
It is a coupling capacitor. It passes the AC signal but not the DC voltage.
It keeps the DC voltage at the collector of the second transistor from going into the volume control that would make crackling sounds if it happened.
The 0.1uF capacitor is also a coupling capacitor.

Thank you again! I give my presentation tomorrow. I was hospitalized for 2 weeks and missed a large part of my research opportunities. I was offered an extension but I'm shooting for tomorrow since this is the last part of my research.[/QUOTE]
 

Hero999

Banned
The circuit is so simple and is so poorly designed that both transistors clip the signal.
Isn't that the point of a a distortion pedal?
 

audioguru

Well-Known Member
Most Helpful Member
I think the designer of the distortion pedal was trying to make an amplifier with low distortion.
 
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