Guitar Effects Explained

I picked the Jack Orman ebook Guitar Effects Explained from Amazon today. So far I am finding it very readable. It avoids too much math, and reads like a plain English description of how effects work at the circuit level.

I highly recommend this to anyone interested in building pedals, and also to anyone who just wants to know more about the magic.

There is some assumed knowledge required to understand parts of it, but nothing so far that a quick Google search can't fix. For example, the early section on boosters says "our signal is AC coupled into the booster so the clipping takes place symmetrically". Jack doesn't explain what coupling is though, or how it leads to symmetric clipping in the gain stage. Hopefully he explains the related role of the coupling capacitor after the gain stage.

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Sounds great, exactly some of the things I am currently learning with building a few fuzz circuits and playing with assym and symmetrical clipping. I was actually troubleshooting that decoupling cap at the end for the last few days, but seemingly the problem was elsewhere. Would be great knowledge have .. thanks for the tip

Small Signal Audio design by Douglas Self is a great book on audio electronics. Now in an updated (2020) third edition. https://www.amazon.co.uk/Small-Signa...s=books&sr=1-1

But like most technical books, it's not cheap. But it's one of those books that will help you expand your understanding of audio circuits in general. I doubt it's light reading (I haven't got it), but if you want to do more designing rather than just copying circuits or making basic modifications, it could be a very useful book.

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Originally Posted by Simon Barden

Small Signal Audio design by Douglas Self is a great book on audio electronics. Now in an updated (2020) third edition. https://www.amazon.co.uk/Small-Signa...s=books&sr=1-1

But like most technical books, it's not cheap. But it's one of those books that will help you expand your understanding of audio circuits in general. I doubt it's light reading (I haven't got it), but if you want to do more designing rather than just copying circuits or making basic modifications, it could be a very useful book.

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Yes, it does look good, and definitely won't gloss over details like Jack's book does. But at nearly 120AUD I might give it a miss for now. The Orman book does a good job of explaining how things work, and what the design options are in general terms, so it makes a good starting point.

Might have to get a copy

AC coupling itself doesn't determine the type of booster clipping, though you'd normally only use a coupling capacitor before and after the boost circuit in an asymmetric clipping circuit that added a small a DC offset to achieve the asymmetric clipping. No point in adding components in the audio path that aren't required.

The capacitor in the audio signal path before the boost stops any DC voltage getting back upstream of the boost/clipping section, and the capacitor in the audio signal path after clipping section removes the applied DC offset so that you just have the AC component of the (now asymmetrically clipped) audio signal as the output. The 'booster' part of the boost/clipping section is the pedal's gain control, and will set the level of the signal passed to the clipping section. A higher signal level before the clipping section will give more signal clipping, so more overdrive/distortion.

Symmetrically clipped signals use the same type and number of diodes to clip the positive and negative halves of the audio signal. So the positive and negative halves of the waveform have the same basic shape. (In reality the initial audio waveforms themselves are rarely truly symmetrical, but assume you are passing a sine wave through the circuit).

Adding a small DC offset is one way of achieving an asymmetrically clipped signal. The DC offset shifts the whole audio signal up (or down), so by adding a +0.1Vdc offset, instead of both the positive and negative peaks reading say +2v and -2v, the positive peak now peaks at +2.1v and the negative peak is now at -1.9v. At this point, the audio signal waveform is the same basic shape, but its centre line now rests at +1v, not 0v. If the signal then passes to the diode clipping circuit, then the signal peaking at +2.1v will clip its half of the signal a lot more than the other half peaking at -1.9v. So the resulting waveform is no longer symmetrical. Diodes will typically start to clip the signal once it exceeds 0.7v, so the resulting signal is now clipped and peaking at ±0.7v. Passing the signal through another capacitor removes the DC offset, so the signal now peaking at +0.6v and -0.8v, with the shape of the top and bottom halves of the waveform looking very different.

As the maximum output level of the audio waveform is fixed by the diode forward voltage value, then it gets passed to a volume boost circuit, which can raise or lower the output level to your requirements (but without affecting the wave shape).

Another way of asymmetrically clipping a signal is to use different types of diode for the positive and negative halves of the signal, that have different forward voltage values, and so will clip at different starting signal levels. So one may clip at 0.65v and the other at 0.75v. There is no DC voltage introduced in this case, so there is no need to isolate the clipping section with capacitors either side of it.

Or for even more flavours, you could combine a DC offset with different diode types.

I believe you can also use different numbers of diodes in series for the positive and negative halves. You'll need a bigger signal level to do this, so the gain section will need to be beefier, but you can probably get more asymmetry this way (if you want to).

Note that I may have got some details of that wrong, as it's a very long time since I did any of this stuff in anger!

Maybe, once I get back to Darwin, I could breadboard a diode clipper circuit and do a screenshot of the display of my oscilloscope so we can see what diode-clipping actually does to a guitar signal, has anyone checked out my Oscilloscope tutorial thread?

I mostly know what the coupling is doing in the circuits. Just pointing it out as an example of things that the book glosses over. The passage "our signal is AC coupled into the booster so the clipping takes place symmetrically" is talking specifically about an op amp booster with no clipping diodes anywhere. The DC offset is applied to get the input signal centered in the middle of the voltage range handled by the op amp so that when it does clip, it will tend to clip symmetrically. Pretty sure this is one reason for the standard voltage divider in op amp drive pedals to get Vr of around 1/2 of the supply voltage. The largish capacitor after the booster gain stage then removes the DC offset, getting the signal back to centered around 0 volts (more or less).

In my pedals I have done asymmetric clipping by various methods including uneven numbers of diodes in each direction (multiple diodes on one side in series to raise the voltage at which they clip), by having no diodes on one side, or by using different types of diodes with different forward voltages. They all sound different, and there is an interaction with the amount of gain. On my TS clone, some diode combinations sound the same as the diode lift position because the voltages aren't getting high enough to cause audible clipping. On this circuit I found single diodes of different types in each direction gave the best asymmetric clipping. Although it can be worked out in theory what they are doing to the signal, it would be interesting to visualise that.

Dr Nomis: yes, I have been reading your oscilliscope tutorial. It's also bookmarked for when I finally get one of my own.

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Originally Posted by Simon Barden

I believe you can also use different numbers of diodes in series for the positive and negative halves. You'll need a bigger signal level to do this, so the gain section will need to be beefier, but you can probably get more asymmetry this way (if you want to).

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That's exactly what I noticed when experimenting with my TS clone. Multiple diodes in series didn't clip much. Blue LEDs (any colour except red actually) didn't clip noticeably unless I maxed the gain, the guitar volume, and stacked a booster to really push the pedal gain stage. I think I ended up using a 1N4001 (measured Vf 0.6v). It sounded a bit more open, less compressed and had higher volume than the stock 1N4148/1N914. All of which mean it clipped a bit less. I just left 1 diode in so it's asymmetric - 1 side clipped and the other isn't.

But of course, as you point out if I had higher voltages coming from the gain stage the result would be different.

If you don't have an oscilloscope, but you have a DAW, you can simply use that to show you the waveform. Not quite as immediate as an oscilloscope, and you may miss some of the finer detail due to the limited bandwidth, but good enough for rock 'n' roll. All the important stuff for your ears is in the audio frequency range anyway, especially as your guitar/bass amp speaker will filter out most of the frequencies above 6kHz or (depending on speaker size). So you'll actually hear a slightly different waveform to that your oscilloscope or DAW shows you.