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 Post subject: The Nature of Diodes
PostPosted: Tue May 03, 2022 11:17 am 
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Putting together the Mouse pedal and paying a lot of attention to the schematic and what it's actually doing (especially with the different types of diodes) has been huge for me in my understanding of hard clipping.

I have one nagging question about how diodes work.

My understanding (which may be wrong) is that in a hard clipping circuit, there's an amplifier of some sort that's pushing the guitar signal up into significant voltages, then there are a pair of diodes after that can shunt signal to ground. They don't start working immediately though, and it takes a certain amount of voltage for them to "kick in," and this is the forward voltage spec that pedal people are talking about when they compare Ge to Si to LED or whatever. This has the practical effect of clipping the tops off of the waves, which is what we hear as distortion.

So in this crappy drawing, the blue line is what be what the amplifier would be doing if the diodes weren't there, and the red line is what happens when there are clipping diodes in there:

Attachment:
clippin.png
clippin.png [ 15.46 KiB | Viewed 2331 times ]

*not to scale

If this is correct, my question is: in the time between t1 and t2, why is the signal-flat lining and not going to zero? Like once the threshold has been met for the diode to engage, why doesn't ALL the signal go to ground?


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 Post subject: Re: The Nature of Diodes
PostPosted: Tue May 03, 2022 12:00 pm 
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Yes, your interpretation is basically correct, though there is a time factor in the turn-on that rounds off the corners of that plateau somewhat. But the diode, once the voltage threshold is reached, is acting like a dam, not an open door--only the voltage above the threshold is shunted to ground, not all of it.

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 Post subject: Re: The Nature of Diodes
PostPosted: Tue May 03, 2022 1:11 pm 
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Awesome, thanks so much.

I'm not an engineer, but when I was in college (25 years ago) I hung out with mostly engineers. One of them was a guitar player, and we would occasionally go into his lab and fire up an oscilloscope and zoom in on that part of the wave you're talking about, trying to get a solid state thing to sound more like a tube thing by softening that knee.

Is that "speed to engage" a spec that's typically in a diode's data sheet? It would be cool to be able to compare them when diode shopping. I've tried googling for it, but I don't think I know enough of the correct jargon to get there.


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 Post subject: Re: The Nature of Diodes
PostPosted: Tue May 03, 2022 1:20 pm 
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I've googled all of this, by the way. I know way more about P-N junctions than I ought to just make a guitar chug, and if anything, I think it was making it worse. :lol:

I really appreciate you entertaining these questions.


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 Post subject: Re: The Nature of Diodes
PostPosted: Tue May 03, 2022 3:24 pm 
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chip wrote:
...when I was in college (25 years ago) I hung out with mostly engineers.

When I was in college (49 years ago), I hung out with mostly co-eds.

chip wrote:
Is that "speed to engage" a spec that's typically in a diode's data sheet? It would be cool to be able to compare them when diode shopping. I've tried googling for it, but I don't think I know enough of the correct jargon to get there.

Hmm, I think I'd search using the terms you mentioned. Not sure if there's a way to find that out on a datasheet, but it's certainly possible. There's some good info included in this classic R.G. Keen article on musical distortion (along with a lot of other useful stuff): http://www.geofex.com/effxfaq/distn101.htm

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 Post subject: Re: The Nature of Diodes
PostPosted: Tue May 03, 2022 5:49 pm 
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There is in fact an amount of time that it takes for the innards of the diode to slosh around into its new configuration. I've encouraged my students to study the difference between rectifier diodes (1N4001, say) and signal diodes (1N914, say) by plotting their I vs. V in real time, as driven by a function generator, and cranking up the frequency until hysteresis loops appear. Sadly, or happily, I am at home at the moment and don't have my notebooks at hand so I can't quote you any frequencies, but the differences are easily seen in the lab. But yeah, never mind my notebooks, what do the data sheets say?

I didn't find anything useful on the first diode data sheet I scared up (1N400x) so instead I went to try to learn about how they are modeled in LTSpice.

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/spice-models/

There's a follow-up here:

https://electronics.stackexchange.com/questions/566706/what-is-exactly-modeled-by-transit-time-in-spice-diode-model

I'd read somewhere that LTSpice simulations show the "onset time" effect but that must be when you change some of the default parameters from zero. Perhaps the "transit time" and "junction capacitance" are the ones to look at, but even so I haven't yet found much in the way of actual numbers, much less what they translate into concerning whether they contribute to corner rounding. But there are some--if you scroll down in that All About Circuits page, you'll see a table that shows significant differences between the 1N4148 (signal diode, small capacitance, small "transit time") and the 1N4004 (rectifier diode, larger capacitance, much larger "transit time"). Even so, if the 4.32 microsec "transit time" for the slow one (1N4004) is the order of magnitude time over which something happens, that still corresponds to > 200 kHz, way out of anyone's hearing.

I always thought you could round off the distortion corners by putting a small resistance in series with the clipping diode, but I suspect that for that to work the way you want it to, you need to know a lot about the rest of the design of that stage of the circuit so you'll know roughly about how much current will be flowing.


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 Post subject: Re: The Nature of Diodes
PostPosted: Wed May 04, 2022 4:52 pm 
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I had a chance to check my notebooks and I discovered that even at 100-150 Hz there is discernible evidence of hysteresis in the 1N4001 I-V characteristic. So it definitely seems that there could be some audio-band consequences of the sloshiness of diodes when used as clippers.


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 Post subject: Re: The Nature of Diodes
PostPosted: Wed May 04, 2022 8:31 pm 
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Cool, thanks. It sounds like the exact curve of when they kick on or off is dependent on a lot of variables, and wouldn't necessarily lend itself to a simple one-liner in a data sheet, but good to know that there is a difference in the audible range.


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