Trouble With Basic Distortion Circuit

[Leverd]

I recently decided to give building guitar effect circutis a shot. Distortion circuits didn't seem

all that bad so I figured I'd try them first. Turns out I've run into some trouble.

The circuit can schematic be found via the link below. The BJT I used for the is a MPS2222A (hfe min is 50).

I biased the BJT to achieve a gain of 3 and made sure the input (base) could take a swing of 1v. I wanted

a clean gain and all of the clipping to be caused by the diodes.

http://www.imagehosting.com/show.php/240042_dist.GIF.html

(click on the image to make it bigger)

I can get sound out of the circuit but it is quite and is not distorted, which means the signal at the output is too small

to hit the .6V rails of the diodes. I thought it might be due to the following problems.

Input Resistance is too small: the ressitance is greater than 100k

Output Resistance is too karge: the resistance is around 10k

Center Frequency is too high: the lower 3db point is a little over 100Hz

Ground Connection might be wrong:

Right now I've connected the guitar and amplifier ground (through the input jacks) to one rail and connected

the negative battery terminal to that rail as well treating it as my ground. Seems right but I may be missing somthing.

Any ideas? On a side note although the circuit doesn't distort very well it does pick up on a short wave christan gospel radio station.....WEEEEEE!

Thanks for any help!

[unklmickey]

hi Leverd,

the first change i would make, would be to get rid of R6. you don't need it, and it will attenuate the input signal.

i would make R3 larger (maybe 33k) and increase the value of C2 (.1 uF?).

then adjust the value of R1 to get about 5v at the collector of Q1 (smaller value to decrease, larger value to increase.)

this should put you in a region that will provide a reasonable amount of gain.

finally, i would bypass R1 with a cap (25uF ?) and the gain will skyrocket, since you will no longer be getting degeneration at the emitter.

in fact you might be able to get by with the circuit as-is, and just bypass R1, but i won't guarantee it.

cheers,

unk

[Leverd]

hey unklmickey,

Thanks for the advice, I'll try making the adjustments. I have one nagging question however.

I read in a few places that guitar pickups can output around 1V. Is this a realistic value? When

I designed the circuit the 1V input was my biggest issue. I wanted to make sure the BJT

was forward active at all times with all the signal clipping done by the diodes. This also limited my

gain on the output considerably. If I had an oscilloscope I could check this out but does anyone know a

rough estimate for guitar pickups output amplitude?

Thanks!

[unklmickey]

1V ? seems too high. i would guess about 1/10 V.

[thegarehanman]

why not just break out a multimeter, open up your guitars, and find out what the average output voltage on all of your pickups is?

[Leverd]

But if the output is sinusodial the average would be zero. My multimeter doesn't measure AC magnitudes unfortunately but

maybe I can find a cheap on that will. I have seen 1/10V in a few places. I'll try desiging the circuit around that and see where it gets me.

[thegarehanman]

Granted you're going with the 1/10v, which is probably not a bad estimate. anyhow, that sin wave problem would be easily remedied with some diodes, in a pinch.

[unklmickey]

i think it will take more than diodes to be able to measure AC at such low voltages.

[thegarehanman]

your area of expertise, not mine.

[Mr Alex]

You seem to know your stuff, alot more theory than me at least, but I should just point out, the mps2222 isn't exactly known for its fidelity(you said you wanted a clean gain). Hell, I've seen its chosen because of its lo-fi, high noise sound. Now BJT's aren't my thing, but I'm sure a bc547 will do a much better job of what you want it to do. I've used them in a few designs I've done before, they are pretty good. There is probably an even better choice though, I just have hundreds of them.

I think you'd be better off with jfets though, diode clipping was never my thing.

[suicidecustom]

"i think it will take more than diodes to be able to measure AC at such low voltages. "

a couple of diodes and measure the voltage as DC. use the peak/hold function on the meter, might be able to help to determine the peak voltage of the AC signal.

[Leverd]

Hey all,

Thanks for the replies! I've been messing around with the design. I removed the 2Meg as unklmickey suggested

and cranked up the gain. It is a lot louder but the distortion is extremely weak. I've read that diode clipping

is suppost to sound fairly harsh but what I'm getting is barely noticeable. I think I may opt for a better BJT as

Mr Alex suggested. The MPS2222A might not be cutting it. I'm using 1N4001 diodes. Does anyone have experience

with the quality of distortion you can achieve via parallel diodes? When I simulate the circuit it looks more

like soft clipping than hard clipping.

Thanks for the help again!

Notes:

1) Changing the output capacitor doesn't seem to have much of an effect, it just shifts the corner frequency.

2) Bypassing the emitter resistor with a Cap also doesn't have a noticeable effect

[unklmickey]

...I've read that diode clipping

is suppost to sound fairly harsh but what I'm getting is barely noticeable....

... Bypassing the emitter resistor with a Cap also doesn't have a noticeable effect

the signal needs to be considerably larger than the voltage drop across the diodes.

since each diode will have about 0.7 volts across it, you will want the signal to be capable of much more than 1.4V peak-to-peak.

you might want to try some germanium diodes. or some Schottky diodes instead.

i'm amazed that bypassing the emitter resistor didn't result in a huge increase in gain. what size is cap did you use?