MXR - Custom Badass '78 Distortion  [schematic]

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Re: MXR Custom Badass '78 Distortion

Postby FakingJeffrey » 11 Jan 2015, 19:52

Nocentelli wrote: I think the lower forward voltage of 1N4148s compared to LEDs means that this second 1N4148 pair will conduct instead of the LEDs, which are effectively removed from the circuit as they will no-longer conduct to ground,


Are you sure about this part? I'm reading more and I would think the antiparallel set of 1N4148s and the antiparallel set of LEDs, when paired together in parallel, would behave as if they're are single components. So basically the same as having two diodes with different forward voltage in an antiparallel pairing, creating Asymmetrical distortion. Although I admit that I don't fully understand how the resistor affects this.
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Re: MXR Custom Badass '78 Distortion

Postby Nocentelli » 11 Jan 2015, 20:42

I'd like to read about that - I understood that if you put two different diodes or LEDs in parallel facing the same direction (not anti parallel) the one with lower forward voltage will conduct first, and the other won't conduct. In the crunch setting, you have one 1N4148 in parallel with one LED facing ground - The 1N4148 will conduct first. You also have a 1N4148 in parallel with an LED both facing away from ground, and the 1N4148 will conduct first.
mmolteratx wrote:absolutely zero commercial use allowed. If I find anyone selling these, I'll fly to your house and kick you in the nads. And you may or may not find yourself in trouble.

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Re: MXR Custom Badass '78 Distortion

Postby FakingJeffrey » 11 Jan 2015, 21:02

Well damn... I could've sworn before I starting looking into the circuit of this thing that the reason I liked it more than the other distortion pedals I own was because of LED clipping, and I only use it with the crunch mode on. For some reason, from my ear, I thought it was CrunchOFF-Silicon, CrunchON-LED. I'm a little sad now to be honest. I mean, I still love this pedal, but now I feel like I know less about what makes up the sound I love. Still, Thanks for the info!
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Re: MXR Custom Badass '78 Distortion

Postby Nocentelli » 11 Jan 2015, 21:16

Well, I'm only going off the position of the switch in the diagram, it might be that crunch is the LEDs: Does it get louder or quieter when switched to crunch?
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Re: MXR Custom Badass '78 Distortion

Postby Nocentelli » 11 Jan 2015, 21:45

Reading some reviews and mxr's description, it suggests that engaging the crunch switch gives a "more open, less compressed" sound, which does suggest LEDs, especially if it gets a touch louder.
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Re: MXR Custom Badass '78 Distortion

Postby FakingJeffrey » 12 Jan 2015, 00:24

Nocentelli wrote:Well, I'm only going off the position of the switch in the diagram, it might be that crunch is the LEDs: Does it get louder or quieter when switched to crunch?


It might get a touch louder, yeah, but not by a lot. The most noticeable difference is when CRUNCH is engaged it gets more gritty, lively. Sharper and and brighter, like maybe there's more harmonics at work. A less smooth texture. Then when you switch it off it sounds duller, maybe more compressed... definitely less sparkle.
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Re: MXR Custom Badass '78 Distortion

Postby Nocentelli » 12 Jan 2015, 00:42

Well, it sounds like the crunch switch disengages the lower diode pair and allows the LED pair to clip instead: The 470r resistor may well be there to slightly raise the clipping threshold and reduce the volume drop.
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Re: MXR Custom Badass '78 Distortion

Postby FakingJeffrey » 12 Jan 2015, 00:49

So that means that the schematic above is showing the switch OFF. That makes much much more sense to what I thought I was hearing! That makes me feel a lot better. So to recreate that is easy, just two antiparallel silicon diodes in series with two antiparallel LEDs. I don't have to worry about the lower pair or the resistor at all. Beautiful. Thank you. I love talking about this kinda stuff with people who are smarter than I am. Why haven't I joined a forum before now!? Haha.
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Re: MXR Custom Badass '78 Distortion

Postby FakingJeffrey » 01 May 2015, 14:51

betoknigth wrote:Hi I'm from Brasil, Really thanks for the schematic of MXR '78 Distortion


Hey betoknigth! That looks awesome! How does it sound? Have you compared it to the original?
Did you print more than one of those boards? I'd buy one off you!
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Re: MXR Custom Badass '78 Distortion

Postby skylark44 » 13 Jul 2015, 02:11

I'd buy one as well. ;-)
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Re: MXR Custom Badass '78 Distortion

Postby bordonbert » 13 Jan 2016, 12:38

Just come across this post and I know it's an old one but the MXR use of a more complex diode setup is really interesting. There are way too many back to back diode designs with just 2 or maybe 4 diodes. They all generate basically the same distortion until the tone shaping circuitry does its job. Forget about which diode gives the best sound, they are all pretty much the same at the distortion generating level, spend more time investigating the shaping circuitry both before and afterwards as that is what gives a pedal using just a couple of plain diodes its character. A couple of observations from questions raised in previous posts:

Firstly, any pair of back to back basic silicon diodes (1N914, 1N4148 types), simply chops the signal down starting at about 0.5V in both directions. When each diode is reverse biased it effectively does absolutely nothing, the other one takes over.

If two diodes are put in parallel in the same direction and they have significantly different characteristics, one will start to conduct before the other. This starts the process of signal compression as the voltage across them then rises more slowly. But assuming their forward voltages are close enough, the voltage can continue to rise until the other eventually kicks in and they are then both on and in parallel. This makes the voltage rise even more slowly so you get a double dose of clipping ending up in a sharper final clip.

The 470R resistor between the pairs of diodes is a magic addition. With only 2 diodes and the LEDs in circuit they will show a good gradual clip starting around 1.5-2V mainly due to the LEDs bending the signal more gradually. With all 4 plain diodes in circuit the bending will start earlier because the small signal diodes U2A and U2B turn on a lot earlier than the LEDs do. But the 470R resistor now limits their clipping action and makes sure the signal can still continue to rise until the LEDs then kick in. This gives the same final degree of clipping but an earlier start and a more gradual bend. Now that's what makes a good pedal, clever use of basic generic parts not mystical magic components!

I'm hoping this will be a good model for experimenting myself. The other circuitry around that is pretty standard and can be messed around with very easily but that diode setup is really well thought out.
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Re: MXR Custom Badass '78 Distortion

Postby betoknigth » 13 Jan 2016, 14:57

FakingJeffrey wrote:
betoknigth wrote:Hi I'm from Brasil, Really thanks for the schematic of MXR '78 Distortion


Hey betoknigth! That looks awesome! How does it sound? Have you compared it to the original?
Did you print more than one of those boards? I'd buy one off you!



Hello guys,

The sound of the pedal is good, lacks bass, but I think this is the proposal of the pedal.

I can not compare with the original or with the DS-1 (base to the diagram).

I found it strange the gain potenciomentro acting, just short circuiting terminals 1 and 2 and putting a 10k resistor between the potentiometer and pin 1 of the opamp.

I made some modifications to my taste. I can not provide PCBs unfortunately, but I can provide eagleCAD files to those who need.

It took me to answer because my profile was deleted :shock:
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Re: MXR Custom Badass '78 Distortion

Postby deltafred » 26 Oct 2017, 20:03

I got one of these in to fix today (cracked PCB track somewhere not visible), the schematic was a huge help in fixing it.

Thanks go to all who contributed towards tracing the schematic. :applause:
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Re: MXR - Custom Badass '78 Distortion

Postby roseblood11 » 28 Oct 2017, 18:03

I'd like to see a schematic of the Super Badass.
A combined version with the TMB tone stack and the clipping diode switch would be really interesting. Aion? Madbean?
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