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I built a guitar pedal which looks like this (the schematic is a clone of a Tube Screamer) and I have a question regarding something unusual I discovered.

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There are two diodes, D1 and D2, which provide soft clipping. It turns out that removing one diode improves the sound significantly, making it less compressed and still very nicely overdriven! There's suddenly much more bass and details.

However, removing both diodes makes it too fuzzy and uncontrollable. Obviously, the removal of just one diode means that only one half of the signal gets soft clipped, and the other half is clipped through the op-amp.

Are there any other distortion pedals that utilize this fact? Am I doing something wrong? Did I discover something new? How would it sound if you could mix three kinds of signals - one clipped with diodes, one completely without and the clean signal and vary the proportions? Does any guitar pedal utilize this?

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    \$\begingroup\$ I designed a guitar pedal which looks similar to this - sorry but in engineering we deal with exact circuits. As a fellow guitarist I've seen designs that use single sided distortion using one diode. \$\endgroup\$
    – Andy aka
    Feb 11 at 8:28
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    \$\begingroup\$ "Beauty in in the eye of the beholder"... Or, in this specific situation, "Euphony is in the ear of the listener",... Or whatever. In any case, you could alter anything in this circuit without being "wrong", as long as you like the sound it makes (and you don't make components smoke). \$\endgroup\$
    – dim
    Feb 11 at 12:13

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You found better sound with just one diode. When you play this to your associates you will probably find that most of them agree. The reason for this is that your asymmetrical clip achieved with one diode gives lots of even harmonics that are pleasing to the ear. Old single ended tube amplifiers would do this when overdriven. You could experiment with germanium diodes for a softer clip. You could place a pot in series with each diode to optimise your effects.

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If you take out one diode you are clipping only in one direction. You will still have some distortion but the clipping and, as you say, compression (an artifact of clipping) is less severe.

Of course, "better" is highly subjective, but you can find many different ways to produce clipping. For instance, if you use a low value zener diode, it tends to have a softer knee than s signal diode like a 1N4148 (because you are back-biassing it into the zener region, not forward biassing it. I designed a pedal which used this principle, people that bought it liked the soft distortion sound - sorry for the self plug but it seems relevant). Changing the configuration and values of resistors around the circuit also produces many subtle variations. That might be why there are so many overdrive/fuzz pedals on the market.

If you are seriously into this stuff, buying a second hand analogue scope (if you don't have one) so you can see what the circuit is doing to a signal is highly instructive. It's a great way to learn analogue electronics too.

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  • \$\begingroup\$ This is great! Thank you for your input! If I end up using a Zener diode here, what voltage should I try? Also, should it be placed in the opposite direction to the 1n4148 and alongside with it? I actually have an oscilloscope, but it's digital and thankfully it suffices for my needs for now :) \$\endgroup\$ Feb 11 at 8:42
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    \$\begingroup\$ trial and error! but remember that a zener is just a normal diode in the forward direction, in the backward direction it will clip at more or less its rated voltage. So if you took out both diodes in this circuit and used a 2v2 zener you would have hard clipping one side, at 0.7V and 2V2 the other way. To make it clip more softly, you need some series resistance, to stop the hard forward biassed clipping, put an ordinary diode in series the opposite direction. The possibilities are endless, but you have to know the components. There is no "should", it's about the sound you want to achieve. \$\endgroup\$
    – danmcb
    Feb 11 at 8:47
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You're clipping one half of the signal and not the other half. Logically, this is basically the same as summing distorted and clean paths.

This is a well-recognised variation of overdrive known as clean blend, and as per that link you'll find a bunch of pedals which do it. As that link says, it's an effective way of reducing the "wasp in a bottle" effect that overdrive can have on your tone. The difference with your overdrive is that you don't have control over how much you use - one half is always distorted and the other half is always clean, so you're basically stuck at 50:50 blend.

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