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I was breadboarding a pedal for my guitar using LM386. I read that as long as the gain is too large, the output signal is clipped to produce a distortion effect.

So I simply tune up gain pot between pin 1 and 8. But the output is as dirty as a cheap amp with overamplification. What did I do wrong?

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    \$\begingroup\$ You did say you wanted distortion didn't you? \$\endgroup\$ – Andy aka May 6 '15 at 13:31
  • \$\begingroup\$ I wish to have clean sounds. But either there is no distortion, or it become heavy metal \$\endgroup\$ – user2513881 May 6 '15 at 13:31
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    \$\begingroup\$ Try using a pot to mix the dirty sound with the clean sound to give the balance you like. \$\endgroup\$ – Andy aka May 6 '15 at 13:45
  • \$\begingroup\$ Great. I'll try it out. Just curious why the famous Ruby Amp is free from this dirtiness. \$\endgroup\$ – user2513881 May 6 '15 at 13:48
  • \$\begingroup\$ Do you have a circuit diagram of the ruby amp? \$\endgroup\$ – Andy aka May 6 '15 at 13:48
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The "distortion" the LM386 is producing is "clipped" distortion, and probably looks something like this if the output were viewed:

Signal Clipping

That is a "hard" clipping at the tops and bottoms, and we perceive that as a "metal" type of tone.

When you overdrive a vacuum-tube amplifier, it also distorts the waveform, but the edges are much more rounded, and thus, pleasant to listen to.

Here is some more information on musical distortion, including a blurb about the legendary Ibanez TS9 "tube screamer."

The bottom line is, hard clipping will always sound "metal", and soft clipping will sound more pleasant. It is possible to achieve soft-clipping with solid-state electronics, but it's a little more involved than simply overdriving an input. DIYstomboxes have many schematics available for various types of distortions and other effects.

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  • \$\begingroup\$ Does it help if I reduce the gain? Does that sound a bit softer? It would be nice if you have a sample clip but I know it's too much to ask for \$\endgroup\$ – user2513881 May 6 '15 at 14:48
  • \$\begingroup\$ Here is an example of a tone being generated, hard clipped, soft-clipped, and more. :) youtube.com/watch?v=5aniQ_TmOE4 \$\endgroup\$ – rdtsc May 6 '15 at 15:00
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    \$\begingroup\$ @user2513881: What matters is that the signal is clipped rigidly. Reducing the gain will change where the signal gets clipped, but rigidly clipping even a tiny portion of a signal will have a surprisingly noticeable (and generally not desirable) effect. If you look at the signal shown above, you'll notice it has very angular "corners". To produce musical-sounding distortion, it's necessary to more gently squash the signal in such a way that the whole thing keeps a somewhat-rounded shape. \$\endgroup\$ – supercat Nov 14 '15 at 0:09
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For a decent overdrive/distortion sound you do not want the LM386 to clip, but you want to use diodes to do the clipping. The diodes are either used in the feedback loop of the opamp, or they are connected between the opamp's output and ground. Some examples and explanations can be found here.

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  • \$\begingroup\$ nice to see you over here, Matt. also the link was nice, especially all of those circuits. for my money, it doesn't matter much how the hard-clipping is done (with the op-amp saturating or with diodes in the op-amp loop), hard-clipping will sound pretty bad. there are so many variations of soft-clipping (like even/odd symmetry, pre and post-filtering, even filtering in the middle of the distortion element like what happens with tube emulation) that a simple diode circuit usually sounds crappy. good-sounding distortion is no easy feat. \$\endgroup\$ – robert bristow-johnson Nov 13 '15 at 22:40
  • \$\begingroup\$ @robertbristow-johnson: Hi over here. Just in support of diode clipping: the clipping of the famous Ibanez Tubescreamer (guitar pedal effect), and of many other popular pedals, is entirely based on diode clipping (two anti-parallel diodes in the feedback-loop of an OpAmp), and I wouldn't say that it sounds crappy or similar to a clipping OpAmp. \$\endgroup\$ – Matt L. Nov 14 '15 at 20:41
  • \$\begingroup\$ what i critique is hard clipping and i mentioned two incarnations of it: op-amp saturating and diode in the loop (i meant like en.wikipedia.org/wiki/Precision_rectifier#/media/… ). anything with hard edges. there are plenty of decent soft-clipping circuits with rounded corners that have op-amps (or a more fundamental active device), resistors, and diodes, that are designed to follow a curve. it's when the diode corner (from 0 to 0.6v) is divided by the op-amp gain that things get sharp and sorta "crappy" sounding. i dunno, square waves sound nice. \$\endgroup\$ – robert bristow-johnson Nov 15 '15 at 1:15
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Lm386 does not like diodes after its signal, particularly because it will clip on INPUT before any amplification will occur. This is why gain is achieved via a simple capacitor between pins 1 and 8 and a pot controlling the INPUT level as opposed to controlling resistance against that capacitor. With the pot at input the signal has a chance to be weak enough not to clip. Otherwise even with a gain of only 20 it will clip on input with even the weakest pickups. Personally I think of classic rock when I think of 386 distortion, not so much a metal sound. It really has the same type of sound as a tube power section being overloaded. If your really want it as clean as the ruby amp then wire it exactly like the ruby amp and replace the speaker with a 10ohm resistor and a 100k linear pot. Take signal in one side, ground the other, and the middle lug is your output lead. You can get away with losing the 10 ohm resistor but you'll be loading the pot.

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To have your LM386 amp clean, don't put anything between pins 1 and 8, nothing, zero. To get distortion switch in a 1000K or a direct wire between 1 and 8, then drive the distortion using the vol/gain control at input of chip - between FET and chip.

If you're only going into a high-impedance input like an amp or other pedal, you can put a volume at the output, about a 50K, and probably use a smaller capacitor than the 220mf at the output, like a .1mf.

Don't worry too much about it's harshness of distortion. An over-driven tube amp, or even a Tube Screamer or compressor will smooth out harsh clipping from all that goes before it.

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