What is LED distortion?

[boobtoob]

Hello:

I read alot of stuff on the internet about amps that I sometimes just don't quite understand. One thing is LED distortion. Are they talking about the same LEDs that are used to light up a stomp box to let you know when it is on? Do you just put these LEDs into the signal path to get added distortion? Why do they distort?

Thanks for any explanations,

[mavrick10_2000]

I've never heard of that done in amps, but many fuzz and distortion pedals use LED's to create the distortion in part of the circuit. Yes they're the same LED's used to indicate your pedal is on, or they can be a little larger or smaller, nothing special really. They're cheap and they work. MI Audio Crunchbox 2 - Red Leds, Red Fuzz LED's and if you want it switchable add an SPDT switch with 2 Si Diodes on the other pole to get more hairy splatty fuzz.

Go to DIYStompboxes forum there's a lot of info there.

[VaughnC]

Many OD pedals (like a TS9 Tube Screamer) use a clipping stage to clip off a portion of a guitar's input signal to create distortion. Part of the clipping stage are components called diodes....and it's the diodes themselves that actually do the clipping. LED's are just another form of a diode so they can also be used in a
clipping stage to create distortion....but they have slightly different clipping characteristics than regular diodes.

[Rod]

The Marshall Jubilee series of amps used a bunch of these LED clippers...If you doubt it, pull the chassis half way out and play it...It looks like the 4th of July...If you think they were flat sounding with non complex tones, the clippers were part of the problem....They needed more distortion without adding any more 12AX7's....I'll probably receive Jubilee hate mail for this...I don't care..If you have one and love it..That's wonderful

[Old Tele man]

...DIODES "clip" signals at about 0.3V (Germanium) or 0.6-0.7V (Silicon)

...LEDS "clip" signals at about +/-1.2-to-2.2V, depending upon "color" of LED emission, ie: RED ~ 1.2-1.5V, BLUE ~ 2.2V"

[ChickenLover]

As OTM mentioned...regular diodes clip at a much lower threshold. LEDs clip at different thresholds based on their color and brightness (and probably other stuff but color and brightness are good enough)...I think some super bright whites and blues can go way higher.

Lower clipping threshold means more distortion/clipping and less output...higher clipping threshold means less distortion and more output. Put a Fulldrive II in 'Comp Cut' mode and listen to the output go up...you just removed the clipping diodes from the circuit.

Rod is correct that the Jubilees use LEDs (but there are only two LEDs and they are part of a little 'clipper network' that uses 2 LEDs and 3 regular diodes) but the LEDs are only for the Lead channel. The 'Rhythm Clip' mode is basically the clean channel with two regular diode clippers added. I think Jubilees sound pretty good but not everyone does...especially when they find out there's dreaded SS stuff in there. Probably 90% of the guys I see using old 4-hole Marshalls sound way worse (which probably shouldn't be the case...but it is). If you think you're in that top 10%...that's wonderful.

[vibroverbus]

As OTM mentioned...regular diodes clip at a much lower threshold. LEDs clip at different thresholds based on their color and brightness (and probably other stuff but color and brightness are good enough)...I think some super bright whites and blues can go way higher.


The other thing is that I think LED's clip 'softer' as they begin to turn on and conduct more and more current. You could bump up the clipping point of regular diodes easily in your circuit to match the same point but LED's will still sound different.

I have a couple in a tube amp with a trimpot between stages as a switchable mild fuzz/distortion effect and really like it actually. Ironically lots of tubers frown on diode clipping inside the amp but then slap down 4 different distortion boxes in front of it that are just diodes being driven by (often cheap) opamps...

[John Phillips]

Yes, LEDs clip more softly than silicon diodes, and have much higher clip voltages (germaniums are softer too, but lower voltage).

This can be useful... a great example is the Boss MT-2 Metal Zone pedal. Replace the two silicon clipping diodes with LEDs (miniature red ones like normal Boss pedal indicators work fine), and you get more headroom, slightly less distortion, and a less harsh tone... a fairly big improvement for about $1 and five minutes work.

[Affectronix]

LEDs are just Light Emitting Diodes. They are used in distortion pedals while Germanium diodes are used in overdrives for smoother clipping. LEDs usually wont emit light in a signal path. LEDs break up more crunchy than silicon or germanium.

[scopeboy]

John, you mentioned replacing the diodes in a Metal Zone with LEDs. What happens if you just replace one of them? If I understand right, it should give asymmetrical clipping, which should make it more touch sensitive and interesting to play.

One interesting clipper circuit, that I invented as far as I know :P uses a four diode bridge with a MOSFET in the middle, with its gate and drain connected together. I get very soft clipping indeed with this, it just rounds the tops and bottoms of the waveform off. It's not really aggressive enough for dirty guitar, but I found it makes a good smooth sounding limiter for bass.

http://scopeboy.com/overdrive.html

[Trout]

A Bit of a sidebar here,

Leds can also be used in a bias setup. Over at DIY audio there is a guy that has been building some incedible sounding amps that utilize an LED array for output tube bias.
Its Called the "Red Light District"

http://www.diyaudio.com/forums/showthread.php?s=&threadid=81492

If that link does not work,

Here is an online magazine that has the build article (5meg DL) I think following this stuff opens up a lot of possibilites for design ideas.
http://basaudio.net/pubs/DIYMAG_2006_2.pdf

Trout

[John Phillips]

John, you mentioned replacing the diodes in a Metal Zone with LEDs. What happens if you just replace one of them? If I understand right, it should give asymmetrical clipping, which should make it more touch sensitive and interesting to play.I didn't try it, but you're going to be right :). Personally, I tend to prefer symetrical distortion usually, especially for 'heavier' sounds. I don't think you'd want even a 'slightly tamed' Metal Zone for anything subtle ;).

One interesting clipper circuit, that I invented as far as I know :P uses a four diode bridge with a MOSFET in the middle, with its gate and drain connected together. I get very soft clipping indeed with this, it just rounds the tops and bottoms of the waveform off. It's not really aggressive enough for dirty guitar, but I found it makes a good smooth sounding limiter for bass.

http://scopeboy.com/overdrive.htmlI'm not 100% sure, but I think I have seen something like that before... I can't remember where.

It doesn't make it any less original really though, if you came up with it independently (I have with a few things, too).

[Zero Point]

Using an LED diode in a clipping section of a pedal will actually sound really nice. THe diode reacts with difference values as the input dynamics vary. In other words, when you hit a power chord, the LED will get really bright and be at max clip. When you pick lightly it will barely come on :) This is a neat way of varying the clip and adding some interesting compression to things.


I've never heard of one used in an amplifier tho. The diodes in an amp is a lot more beefy than any LED could possibly be. Unless it is used in some kind of solid state preamp.

-Z

[scopeboy]

Yeah, I guess, most amps designed for heavier styles are made to clip symmetrically. Asymmetrical clipping combined with high gain can start sounding kind of faulty, as if the amp was about to die on you. The most famous example being Marshall's newer amps with the bridge rectifier clipper. I forget which ones they are, apart from the JMP-1.

Having said that, I had good fun with one of the old black Marshall Shredmaster stomp boxes, by pulling a capacitor from the tone stack to get a treble boost, and adding a schottky diode somewhere (I've forgotten exactly where...) to make it slightly asymmetrical. I ended up using it like a Tube Screamer, to drive lower gain amps.

I don't know if I did invent that MOSFET and diode clipper circuit, but if you can find any other mention of it before I published it online in 1999, I'd be very interested, and maybe a bit disappointed too :( R.G. Keen mentions "MOSFET diodes" but I think his article came after mine.

Zero Point: All diodes react like you describe. The only difference is that the LED gives you a cool light show while it's doing it.

[John Phillips]

I've never heard of one used in an amplifier tho. The diodes in an amp is a lot more beefy than any LED could possibly be. Unless it is used in some kind of solid state preamp.They're used in a lot of Marshalls, both in tube-driven and IC-driven circuits (in the preamp).

And they certainly do light up when you play, too :).

[ChickenLover]

FWIW, I have a Blues Pearl Blue Balls overdrive and I didn't like it very much...as the note decayed there was some fizz/crackle that would abruptly stop at some point. This pedal has a boatload of gain, a lot of available distortion, and used two regular silicon diodes as clippers. I first tried subbing two white LEDs for the regular diodes and that helped a lot...then I took it a step further and put in the same 'clipper network' you see in the Jubilee lead channel (except I used those same two white LEDs instead of red) and that made the pedal even better. Even with just the two white LEDs that crappy/crackly decay was gone.

If you look at that clipper network from a Jube, it's slightly assymetrical. In any case, if you have a pedal that has enough gain inside you might try that clipper network...also note that in the Jube the clipper network is bypassed with a small cap to shave off some fizzies in the high-end.

[vibroverbus]

FWIW, I have a Blues Pearl Blue Balls overdrive and I didn't like it very much...as the note decayed there was some fizz/crackle that would abruptly stop at some point. This pedal has a boatload of gain, a lot of available distortion, and used two regular silicon diodes as clippers. I first tried subbing two white LEDs for the regular diodes and that helped a lot...then I took it a step further and put in the same 'clipper network' you see in the Jubilee lead channel (except I used those same two white LEDs instead of red) and that made the pedal even better. Even with just the two white LEDs that crappy/crackly decay was gone.

If you look at that clipper network from a Jube, it's slightly assymetrical. In any case, if you have a pedal that has enough gain inside you might try that clipper network...also note that in the Jube the clipper network is bypassed with a small cap to shave off some fizzies in the high-end.

my bet is that the fizz/crackle was a cold joint, probably 'south' of the OEM diodes so as you dropped below the forward voltage threshold it wasn't in circuit anymore, then when you put the new LED's in you fixed that up. i like the LED effect myself, too bad you can't see them light up inside my amp...

[JJman]

...DIODES "clip" signals at about 0.3V (Germanium) or 0.6-0.7V (Silicon)

...LEDS "clip" signals at about +/-1.2-to-2.2V, depending upon "color" of LED emission, ie: RED ~ 1.2-1.5V, BLUE ~ 2.2V"


Ahhhh. That explains why my DS-1 pedal is louder, at the same settings, after I changed to led clippers. I recorded it both ways and saw that change to the amplitude of the wave file. Not a huge tone change for me in my DS-1 but I kept them in.

Imagine the buzzzzzzz if I put germanium diodes in there?

[John Phillips]

Ahhhh. That explains why my DS-1 pedal is louder, at the same settings, after I changed to led clippers. I recorded it both ways and saw that change to the amplitude of the wave file. Not a huge tone change for me in my DS-1 but I kept them in.

Imagine the buzzzzzzz if I put germanium diodes in there?Probably less buzzy than with silicon diodes, but even more squashed and with lower headroom. Silicons have the hardest, buzziest tone because they clip the most 'perfectly' (from a technical point of view).

[Jef Bardsley]

Yeah, I guess, most amps designed for heavier styles are made to clip symmetrically. Asymmetrical clipping combined with high gain can start sounding kind of faulty, as if the amp was about to die on you. The most famous example being Marshall's newer amps with the bridge rectifier clipper. I forget which ones they are, apart from the JMP-1.

The JCM 900 Dual Reverbs have two LEDs in the clean channel, and a full bridge in the lead channel. They have 15 opamps and one tube in the preamp section, making them true hybrids. They also go to 20, doing Nigel 9 better. :)

True story: When I got sick of poor quality of the Russian tubes at the end of the '80s, I borrowed my brother's Jazz Chorus for a while and loved it. However, I didn't care for the overdrive, so I went shopping for a stompbox. The clerk laid about 6 on the floor, and plugged me in to a handy Marshall. After trying them all with poor results, I plugged straight into the Marshall. "There!" I announced, "That's what I'm looking for." I came home with a 50 watt DR combo, matching 1x12 cab, and an SE100 attenuator. Granted, even compared to a boutique fuzzbox it's a bit pricey, but hey, it's got a built-in 50 watt amp and Celestion speaker! And it does sound more like a Marshall than any other overdrive pedal I've tried. ;)


As far as MOSFET's go, I don't know who was first, but Jon's been making this pedal for quite a while now. I thought of posting a link to this in the discussion on even/odd harmonics, as he has a nice explanation:

http://www.blackstoneappliances.com/

[scopeboy]

Wow, two LEDs, four diodes and 15 op-amps :eek: The amp I mostly play now has a 2-band parametric EQ that uses 10 op-amps and a tube. I wanted scooped mids like a Boogie, but didn't want to use a graphic EQ (you have to cut funky shaped slots for the sliders, and besides, Mesa have done it already) so I came up with this. http://www.scopeboy.com/toaster.html And I don't feel so bad about it now :-) Ironically, when I finally ended up playing it in a heavy band, I ended up cutting bass and boosting mids instead. That scooped sound seems to be best saved for shredding at home :(

The Blackstone overdrive pedal looks great, but I don't think it is related to my clipper circuit. He doesn't say how it works, but the harmonic plots on the Blackstone site suggest that the thing is made of single-ended FET stages. (If he were using mine, he would have almost all odd harmonics.) I'd guess it's probably a similar circuit to the preamp section of a classic tube amp, with the tubes replaced by FETs, and the appropriate tweeks and impedance scaling.

[Old Tele man]

...all these SS-components in attempts to "make" a decent "tube sound"! ha,ha!

[scopeboy]

Well, I started experimenting with hybrids because plain tube amps weren't that interesting. Adding solid-state components lets you do a whole lot more. Whether it helps or hurts the sound is a different matter altogether. Personally I believe that solid-state parts in the signal path, if used correctly (a big if) are transparent and don't degrade the tone at all.

[Jef Bardsley]

...all these SS-components in attempts to "make" a decent "tube sound"! ha,ha!
Here, let me post an MP3 of real tube sound... :rolleyes:

I think the goal is "decent tube" sound. I wish the goal was decent tubes, but we're lucky to have tubes at all.

Well, I started experimenting with hybrids because plain tube amps weren't that interesting. Adding solid-state components lets you do a whole lot more. Whether it helps or hurts the sound is a different matter altogether. Personally I believe that solid-state parts in the signal path, if used correctly (a big if) are transparent and don't degrade the tone at all.
Ha! I knew we could agree on some thing! :dude