Need help on op amp guitar distortion effect

Thread Starter

EarlAnderson

Joined Nov 13, 2011
166
i am building a guitar distortion effect with a single LM308 op amp, and i for the life of me can not figure out how to get enough distortion from it. i set the gain to about 200 and i still get very little distortion, and it sounds like crap. does anyone have any suggestions on how i can get more, better sounding distortion.:confused:
 

Audioguru

Joined Dec 20, 2007
11,249
The LM308 is so old that it is not made anymore.
There are thousands of "guitar fuzz" distortion circuits on the internet and most use diodes to clip the signal instead of amplifying too much and letting the output of an opamp clip the signal.

The LM308 drops frequencies higher than 2kHz when its gain is as high as 200 so it can't produce the high frequency harmonics of distortion.

If you use a better and newer opamp that has good high frequency response then it can produce lots of distortion.
 

bountyhunter

Joined Sep 7, 2009
2,512
i am building a guitar distortion effect with a single LM308 op amp, and i for the life of me can not figure out how to get enough distortion from it. i set the gain to about 200 and i still get very little distortion, and it sounds like crap. does anyone have any suggestions on how i can get more, better sounding distortion.:confused:
I remember building a "fuzz box" for the guitars in my band back around 1969. It had two controls: one was "volume" and the other was "attack" which boosted the amount of distortion. It was a two transistor circuit no op amps at all.. I think it ran off a 9V battery.


I'll bet there are schematics on the net.

I might even have a copy somewhere of mine.
 

Thread Starter

EarlAnderson

Joined Nov 13, 2011
166
yeah at first i thought it had something to do with the power supply (i was using a single rail power supply), but your point does make sense.
 

Thread Starter

EarlAnderson

Joined Nov 13, 2011
166
Alright i tried everything and for some reason, still no distortion. I have attached a schematic of my design. if anyone sees any problems or flaws with my design, please tell me so i can fix them.


P.S. sorry about the image quality, i was gonna scan it, but my scanner isn't compatible with a mac, so i had to take a picture of it instead;)
 

Attachments

Last edited:

Audioguru

Joined Dec 20, 2007
11,249
With your 100k gain control at max (100k) then the gain is 101 times the input level.
The diodes will clip the output when it tries to exceed about 0.65V peak.
The tone control will do nothing until its resistance is almost zero (the very low output impedance of the opamp) when its muffled sound will become a more distorted and a lower level sound.
 

rogs

Joined Aug 28, 2009
279
Your circuit looks very similar to the famous MXR distortion + circuit of 30 years ago. That worked really well.
See that schematic here: http://www.freeinfosociety.com/electronics/schemview.php?id=562

Two suggestions on your circuit:

- Forget the tone control. Let the amplifier provide that.

- Try using germanium diodes. That's what the original used. Means the unit will 'clip' more easily, and will probably sound less harsh.
Not essential, but worth experimenting with.
 

Thread Starter

EarlAnderson

Joined Nov 13, 2011
166
Your circuit looks very similar to the famous MXR distortion + circuit of 30 years ago. That worked really well.
See that schematic here: http://www.freeinfosociety.com/electronics/schemview.php?id=562

Two suggestions on your circuit:

- Forget the tone control. Let the amplifier provide that.

- Try using germanium diodes. That's what the original used. Means the unit will 'clip' more easily, and will probably sound less harsh.
Not essential, but worth experimenting with.
yeah my circuit is pretty similar to that isn't it. both designs are very simple. but for some reason on my design, im not getting any clipping at all, yet rather a more harsh buzzy sound. i think it's coming from the 1M resistor connecting the power supply to the noninverting input on the op amp
 

Thread Starter

EarlAnderson

Joined Nov 13, 2011
166
With your 100k gain control at max (100k) then the gain is 101 times the input level.
The diodes will clip the output when it tries to exceed about 0.65V peak.
The tone control will do nothing until its resistance is almost zero (the very low output impedance of the opamp) when its muffled sound will become a more distorted and a lower level sound.
okay i can see what you mean there, but for some reason, i don't get any smooth clipping at all, but rather a harsh "buzzy" sound that just doesn't sound anything like a fuzz or distortion. i think it's coming from the 1M resistor connecting the power supply to the noninverting input on the op amp, because when i take the 1M resistor out, the buzzing stops, but i still don't get any clipping. i might try replacing that 1M resistor with a 500k and add a preamp stage before the main op amp so maybe ill get a higher voltage on the output, and therefore more clipping. if you suggest anything different please tell me.
 

Thread Starter

EarlAnderson

Joined Nov 13, 2011
166
the attached is a schematic of my new design. it is the same as my old one, except i have added a preamp before the main circuit.i did this because i found out that if a guitar pickup has, lets say, 100mV output p/p, than with my original design with a gain of 100, i only got a 1V output, which is a little low for driving silicon diodes. i added the preamp with a gain of 3 to turn the 100mV into 300mV so i get an output of about 3V p/p. i haven't tried building this yet, but hopefully it will bring some results (good ones)
 

Attachments

Audioguru

Joined Dec 20, 2007
11,249
The original circuit produced a "buzz" probably because its unshielded wiring picked up mains hum. Its input impedance was high at 500k ohms to avoid loading down the pickup's level and keeping its high frequencies.

Your new preamp has a very low input impedance of only 5k ohms which shorts and reduces the level from a high impedance guitar pickup a lot and it also cuts the shrill high frequencies that an electric guitar needs. Its input is properly biased at 4.5V but it has a DC gain of 3 so its output will be saturated as high as it can go and it will not work.
 

Audioguru

Joined Dec 20, 2007
11,249
i found out that if a guitar pickup has, lets say, 100mV output p/p, than with my original design with a gain of 100, i only got a 1V output, which is a little low for driving silicon diodes.
Your math is wrong. 0.1V x 100= 10V, not 1V. Maybe your gain-setting resistor values were wrong or the tone control was shorting its output.
 

Thread Starter

EarlAnderson

Joined Nov 13, 2011
166
But this just doesn't make sense. If I'm getting an output voltage of more than a volt, than I just don't know why I'm not getting any diode clipping on the output. I have double checked, even triple checked my original schematic after I made all the corrections, and I just don't know what the **** is wrong. If I'm getting a gain of 100, than I shOuld be getting, more than enough to clip the signal. This is just proving to be a huge ******* pain in the ass so far
 

Thread Starter

EarlAnderson

Joined Nov 13, 2011
166
The circuit is not designed properly. It cuts low frequencies:
oh okay i get what your sayin. with the 47nf capacitor and the 1k resistor, that gives me a rolloff frequency of about 3.3 kHz, so as the frequency drops, so does the gain. so if i replace that 47nf capacitor with a 4.7uf capacitor, it gives me a rolloff frequency of about 40Hz, so the gain should stay the same until it goes below 40Hz, so that should filter out a lot of the "noise" from the feedback loop, which is just what i'm looking for:D
 

Audioguru

Joined Dec 20, 2007
11,249
oh okay i get what your sayin. with the 47nf capacitor and the 1k resistor, that gives me a rolloff frequency of about 3.3 kHz, so as the frequency drops, so does the gain. so if i replace that 47nf capacitor with a 4.7uf capacitor, it gives me a rolloff frequency of about 40Hz, so the gain should stay the same until it goes below 40Hz, so that should filter out a lot of the "noise" from the feedback loop, which is just what i'm looking for:D
The capacitor in the feedback loop does not "filter noise", instead it cuts the gain at frequencies below its calculated value.
 

Thread Starter

EarlAnderson

Joined Nov 13, 2011
166
The capacitor in the feedback loop does not "filter noise", instead it cuts the gain at frequencies below its calculated value.
Right. well i built this circuit with all of the modifications done to it, and FINALLY i get some distortion from it, although not much. i think the diodes aren't really "responding" that well to the low output impedance, however im not quite sure.
 
Top