MXR - Custom Badass '78 Distortion  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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FakingJeffrey
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Post by FakingJeffrey »

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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Nocentelli
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Post by Nocentelli »

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.
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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Post by FakingJeffrey »

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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Post by Nocentelli »

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?
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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Post by Nocentelli »

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.
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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Post by FakingJeffrey »

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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Post by Nocentelli »

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.
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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Post by FakingJeffrey »

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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Post by FakingJeffrey »

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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Post by skylark44 »

I'd buy one as well. ;-)

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Post by bordonbert »

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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Post by betoknigth »

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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Post by deltafred »

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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Post by roseblood11 »

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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Post by george giblet »

7line wrote:
MXR badass.jpg
Schematic with values

I think some strange values, you can take from the DS1
FWIW, the schematic shows a 20kB tone pot but the pics at the top of the thread clearly show a 10kB tone pot.
Given that, making assumptions about the cap values around the tone control is a bit unwise. A few people have said they prefer the MXR over the Boss, so maybe the tone control has been revoiced a tad.

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Post by Ichabod_Crane »

Indeed, the first schematic hand drawn show a 10k Tone pot.

But, some values are still doubt?

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Post by george giblet »

Indeed, the first schematic hand drawn show a 10k Tone pot.

But, some values are still doubt?
Since the capacitor were measured in circuit you cannot be 100% sure of the value.
Sometime you can guess the value using the DS-1 values as a guide but
it's still a guess.

For example, imagine I tweak a circuit by changing 10n to 12n. If the measured cap
values are not accurate you use the DS-1 schematic to guess 10n will never know the
cap is really 12n.

With the current values in the schematic I have simulated the tone control and got
roughly what the documentation show of +/- 4dB @ 100Hz and +/-5dB @ 2kHz.
However, you have to trust those specs, which is unwise and this specs alone
are enough to pin-down the part values *exactly* they only tell you you are in the
ball-park.

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Post by george giblet »

Here's what I've done. I've attached a new schematic.
It's in two sheets just so I don't have to re-draw what's already been done.
---------------------------------------------------------------------------------
MXR Badass 78 distortion V1.1 notes
George Giblet
2019/08/12

Re-traced board from pics


Using MXR badass (7line 2013_11_29).jpg as a reference,

VR1 Tone pot should be 10kB
VR1 pin 1 (ccw) and pin 3 (cw) reversed, pin 1 should connect to R5
VR2 pin 1 (ccw) and pin 3 (cw) reversed
Q1 designator should be Q7; gain stage
C2 100uF not 100nF
C4 should be 47n not 47uF
C14 designator should be C13; tone control cap to gnd
C13 designator should be C12; gain transistor Q7 and collector goes to R27, C12
C17 100nF? perhaps across DC input of IC U4
C18 47nF or 100nF? ; schematic OK, located near U1 & R7. R7/VR to ground
All caps are non-polarized except for electrolytic caps C1, C2, C3.
VS1, VS2 should be bidirectional types.
Semiconductor part numbers added.

R22 = "220" = 22R
R28 = 0R ; in series with C5 as per Beto Crazy
R29 = 0R ; goes in series with CW terminal of tone pot

Crunch LED added:
R30 3k3
D7 3mm Blue LED

PSU:
U4 still not 100% determined. 1.23V LDO Regulator
C17 47n or 100n?
R30 240k
R32 38k3

Switching Circuit
R23 470k
R24 3k3
R25 2M2
C14 47n?
Q1, Q2, Q3 SMD J177, P-channel JFET
D1 Red LED 5mm

No D4 or D5 on PCB these might have been U2 and U3 in
a development version of the effect.
---------------------------------------------------------------------------------
SW2 = crunch
Switch in = Crunch, LED comes on.
Crunch LED on/off with crunch switch regardless of effect/bypass.
Crunch mode is a little louder ie. switch opens.
It's percieved as being more open and having more bass.
---------------------------------------------------------------------------------
Switching ckt:
The switching circuit only appears on Beto Crazy, it's not 100% correct but pretty close.

PSU input and Regulator
The power supply chip doesn't appear on any circuits.
Possibly reference voltage is 1.23V, as per 123 marking.
240k (R30) +38.3k (R32) calculates to give +V = 8.94V.
---------------------------------------------------------------------------------
See parts for semiconductors
2xLED = 3mm red
---------------------------------------------------------------------------------
Semiconductors:

U1 = 2262C TLC2262C, CMOS rail to rail o/p opamp, SOIC-8
U2 = JTW 07 BAS28, dual Si diode, SOT-143; 1=D1.k, 2=D2.k, 3=D2.a, 4=D1.a
U3 = JTW 07 BAS28, dual Si diode, SOT-143; 1=D1.k, 2=D2.k, 3=D2.a, 4=D1.a
U4 = IG123 Maybe SOT-23-5L, adjustable LDO, ~1.25V
IG 123, 1.23V (as per 123 marking), maybe Richtek
Could be Richtek RT9167/A wired abnormally
but this regulator is not adjustable.
Calculate: 1.23V + 240k (R30) +38.3k (R32) give +V = 8.94V.
Q1 = 6Y K MMBFJ177LT1G, p-channel JFET, SOT-23; eg. ON
Q2 = 6Y K MMBFJ177LT1G, p-channel JFET, SOT-23
Q3 = 6Y K MMBFJ177LT1G, p-channel JFET, SOT-23
Q4 = 1R? MMBT5089, NPN, SOT-23; eg. ON
Q5 = 1N K SMD Marking ambiguous:
FMMTA14 or SMBTA14, NPN darlington, SOT-23 ;eg. Zetez
MMBT3904, NPN, SOT-23 ; brand?
Q6 = 1R? MMBT5089, NPN, SOT-23
Q7 = 1R L MMBT5089, NPN, SOT-23

VS1 = ZK P SMAJ60CA, 60V bidirectional Tranzorb
VS2 = ZK P SMAJ60CA, 60V bidirectional Tranzorb

D1 = footsw led, 5mm RED, opaque white plastic
D2 = small 3-pin sot-23 diode, under distn' pot
marking KA2, MMBD4148, SOT-23 3pin, pin 1=A, pin 3=K, pin 2=nc
D3, D6 = 3mm red LED, red plastic
D7 = crunch LED, 3mm Blue, clear plastic

D4 and D5 not on PCB?

SOT-23: pins 1=b, 2=e, 3=c ; number like IC with single pin on RHS
---------------------------------------------------------------------------------
Attachments
MXR Badass 78 distortion V1.1 2of2.png
MXR Badass 78 distortion V1.1 2of2.png (9.04 KiB) Viewed 3917 times
MXR Badass 78 distortion V1.1 1of2.png

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george giblet
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Post by george giblet »

Small bug on V1.1:
The battery jack ground should go to the input socket to switch the battery out when the 6.5mm jack isn't present.

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Post by Wes_M_L »

Thanks for the info schem and pics :applause: , started building one today, bought a TLC2262 for this project.

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