Simple Guitar Distortion Pedal

[TrevorP]

Hey,

This winter break I'd like to finally site down and make a distortion pedal for my guitar, mainly because I would like one and the knowledge to make them.

So over the past few months I've been looking at using transistors on and off from a number of sources. But none of them have really helped me for some reason.

The idea is to make one similar to the old fuzz-face pedals, but with NPN Silicon transistors (or MOSFETs???).

Attached is a basic schematic that I have drawn up (it's almost identical to the fuzz-face pedals.)

Now what I actually want to know from you guys is:
-How should I go about choosing transistors?
-How do I go about choosing resistor values for the transistors?
-What is the input voltage from a Guitar (I've got Humbuckers so they are pretty high gain). I've heard that 500mV is a reasonable figure to use?
-What kind of gain should I be going for?
-Should I even be biasing the first transistor to get the sound I want? (aka Fuzz-Face)
-Is my coupling capacitor pointing in the right direction?
-Is 100uF a reasonable value for the supply capacitor? If I were to use a battery.
-How do I find out what kind of values I should use for the coupling capacitors? My guess is that I use 1/(2(pi)f) where f is the lowest frequency the guitar could produce. And should they both be the same value?
-Do I need to add any capacitors in parallel around the emitter resistors?
-Do I need a resistor before the first transistor?
-If I were to add a pot along the negative feedback path would kind of value should I go for? Does this control gain?
-Would 50k be a reasonable value for P1?

And please don't just tell me the answer I would really like to know why it is I am doing something so that I can have an understanding of the subject.

Thanks for any help or suggestions in advance,

Trevor

(PS: I'm not doing this for homework I assure you, I'd just like to learn a bit before second year engineering in which I'll actually start this kind of stuff.)

 

[TrevorP]

Ok so I'm trying to figure some of this stuff out and am I right in assuming that the total current drawn from the battery will be equal to the sum of the currents going through R1, R3 and R6? NOT C2 because C2 will charge up and only deal with slight oscillations.

 

[Nigel Goodwin]

As those are the only possible current paths you are correct, the slight leakage of C2 can be ignored.

 

[TrevorP]

Ok that makes sense then.

Ok so would the current through R1 be equal to 9v/(R1 + R2) OR 9v/(R1 + (R2+R4)/(R2*R4)).

I'm basically asking if I consider R4 to be in parallel with R2.

 

[Nigel Goodwin]

No, R4 isn't in parallel with R2.

 

[TrevorP]

So base current would be (9v/(R1 + R2)) then?

 

[audioguru]

Your circuit doesn't have any solder-joint "dots" so it is difficult to see if lines cross or connect.
Look in Google at Fuzz Face Circuits to see many.
The best link in Google is this one: http://www.geofex.com/Article_Folders/fuzzface/fftech.htm

Here is a modern version that uses silicon transistors:

 

[TrevorP]

Yeah sorry about that the program I am using has hard to see dots...for my diagram the only thing that is crossing and not connected is the R3 wire and the negative feedback wire.

 

[audioguru]

Your circuit doesn't work.
I added some solder joint dots and it is clear that it has positive feedback instead of negative feedback.
Look again at the circuit I posted before.

 

[TrevorP]

Ohh **** your right... that should be going from the emitter of the second transistor to the base of the first...not collector to emitter.

 

[audioguru]

The negative feedback is supposed to be through a resistor so the transistors are not shorted together.

 

[TrevorP]

Hey ok so I made some changes to it and here is what I have now.

I decided to go with a 100uF supply bypass capacitor it was recommended for the same purpose before.

 

[audioguru]

My Fuzz face circuit doesn't have your R1, R2 and R4. Its negative feedback adjusts the AC, not the DC like yours does.

 

[TrevorP]

Wait can you possibly explain that to me? How does my negative feedback adjust the DC? and what is the difference?

I understand that mine has initial DC biasing...is it because mine has no capacitors going to ground from the negative feedback loop?

 

[audioguru]

The DC of your circuit changes when the gain control is changed. Mine doesn't.
Your circuit is not a fuzz-Face. It is different.

 

[TrevorP]

Wow thanks. Yeah I see what you mean there. That seems really clever to me for some reason.

As for mine not being a fuzz face the only difference I can see right now is that mine has initial biasing...(Which I'd actually like to make variable with some sort of potentiometer, but that can be for later).

Now I see that there is a voltage divider on the output...I assume this is to step down the output power drastically? So that the output has similar power as the input?

 

[audioguru]

If your circuit is biased properly then it is a linear amplifier. It doesn't need the voltage divider resistors at its input. If its gain is low and it is not overdriven then it won't have distortion.

The Fuzz-Face circuit is designed to make lots of distortion. It has an attenuator at its output to reduce the level to be about the same as a guitar by itself.

 

[TrevorP]

Ok so my circuit is pretty close now to the design you posted.

And since I want a high gain system I saw that BC108's can have a Hfe of around 400...so if I was to go with that how would I go about doing the math to find out the resistor and capacitor values.

(Also the volume and gain pots should both be logarithmic right?)

 

[audioguru]

I remember the old BC108. I think I still have some that are 40 years old.
It has a wide range of current gain from 110 to 800. Selected ones had a suffix letter for less range.

The current gain of a transistor doesn't have much to do with voltage gain.
The voltage gain is the ratio of the collector rresistor (and its parallel load) divided by the emitter resistor (in series with the tiny internal emitter resitance). Negative feedback reduces voltage gain.

The Fuzz-Face circuit already has resistor and capacitor values.

The volume control should be logarithmic. The gain control should be a reverse log which is no longer available so use a linear pot. It is set only one time.

 

[TrevorP]

Yeah but I don't want to just copy this design from there...I'd at the least like to know the math behind it so that I can mess around with maybe adding a tone adjustment in the future.