MXR - Custom Badass '78 Distortion  [schematic]

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Re: MXR Custom Badass '78 Distortion

Postby Ice-9 » 31 Jul 2011, 21:08

Beto Crazy wrote:I also found, but looking at the circuit now, I see that it is basically the structure of a DS1, of course with modifications.

:applause:

Looking at just the images, it is difficult to make the circuit layout. My layout "was almost" like yours. :mrgreen:

One more time: Thanks man!!!

I never imagined that U2 and U3 were diodes, connections were strangers. The impression he gave me were to be switches, but for U2, this function did not fit.


Yes it is just a DS1 with just a different arrangement of clipping diodes. The diode switching is done entirely with the little dpdt push switch and all it does is remove two of the diodes as can be seen in the schematic. The only other differeces are a few component values altered and the use of the second opamp (i'm not sure for the diode from pin5 down to gnd)
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?
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Re: MXR Custom Badass '78 Distortion

Postby clapointe » 02 Aug 2011, 02:45

I just compared my Badass 78 to my Keeley Ultra Boss DS-1 and there almost identical but the Badass 78 has more clarity ! The MXR Badass 78 is a winner !
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Re: MXR Custom Badass '78 Distortion

Postby RnFR » 02 Aug 2011, 07:30

Ice-9 wrote:I am a bit dissapoineted that MXR have not made good use of the first Opamp, rather than just bung it in as a unity gain buffer it could have been put to better use.
Ice-9 wrote:I am a bit dissapoineted that MXR have not made good use of the first Opamp, rather than just bung it in as a unity gain buffer it could have been put to better use.


yup. they could've probably put it on the end, and done away with the BJT buffer. also, I'm surprised they didn't bootstrap or simply use a JFET on the input for higher impedance. I do like the LED bypass clip switch. btw- I think it's most likely that the diodes are tied together like the Timmy simply to make switching less complicated.
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Re: MXR Custom Badass '78 Distortion

Postby roseblood11 » 11 Dec 2011, 20:10

Ice-9, can you please post a bigger version of your schematic?
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Re: MXR Custom Badass '78 Distortion

Postby Ice-9 » 13 Dec 2011, 14:36

roseblood11 wrote:Ice-9, can you please post a bigger version of your schematic?



Here ya go, this should be a better res. Right click and save and open up in picture viewer and zoom in if needed.

Image
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?

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Re: MXR Custom Badass '78 Distortion

Postby Ice-9 » 13 Dec 2011, 14:39

Sorry , WTF is it with photobucket that makes my picture crap. The picture I uploaded was fine then when i get it back from this link its rubbish.

give me a little time and i will sort it out.
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?
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Re: MXR Custom Badass '78 Distortion

Postby Ice-9 » 13 Dec 2011, 14:54

I will try and upload it as an attachment.
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It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?

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Re: MXR Custom Badass '78 Distortion

Postby Audiowonderland » 30 Dec 2011, 16:30

I just got this pedal as a Christmas gift. I like the idea that the crunch button switches in a pair of LED's in place of the silicon diodes. I had bought a used DS1 a while back and the first mod O wanted to try was replacing diodes with LED's

Looking forward to giving this a thorough test drive
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Re: MXR Custom Badass '78 Distortion

Postby pdodge77 » 16 Nov 2012, 20:23

Quick question - how hard would it be to add an input trim to the Badass '78? I love the sound of it with the volume on my guitars rolled off. It would be great to have that sound at the click of a switch.
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Re: MXR Custom Badass '78 Distortion

Postby Ice-9 » 17 Nov 2012, 15:27

It should be quite simple to add an extra input pot, the only difficulty would be that the input jack socket is pcb mounted. The easiest option would be to remove the capacitor on the input and use the pads to wire up the pot and remount the cap on the pot.
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

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Re: MXR Custom Badass '78 Distortion

Postby mirosol » 08 Sep 2013, 19:56

Currently unverified.
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Re: MXR Custom Badass '78 Distortion

Postby drd » 17 Sep 2013, 21:00

Thanks a lot for the schematic and the layout both. I've built the effect based on the layout provided by mirosol, but it does not work. Actually the signal passes through, but there is no distortion whatsoever. The volume pot works (no wonder), the tone contol has a subtle effect, but the drive knob does nothing. At tagboardeffects a the folks who tried this had the same issue. I compared miro's layout to the schematic from Ice-9, and found a few little differences:

1) The Re resistor on transistor Q2 is 22Ohm on the schematic and 220Ohm on the miro's layout

2) There is an 1M resistor from input to ground on the layout which is not on the schematic, I think this is a pulldown resistor, so it makes no difference

3) The final output cap C11 is 1uF on the schematic but is only 47nF on the layout

4) In the Vref part C18 is drawn, but in the BOM no value exists for that. Since it is only a power filtering cap I don't think it makes any difference

6) In the BOM there are a few resistors that are not on the schematic, might be because the schematic ommited the switching circuit?

7) There are two diodes that are not on the layout: one from ground to input and one paralel to the 100k resistor that is at the output (VS1 and VS2)

Also I compared the schematic to its close relative the DS1, and found some differences too:

1) The 290nF cap in the feedback loop of the second half of the opamp seems to be way too much. DS1 has 100pF in there, checking other designs the typical value is between 100pF-1nF

2) The 12nF cap in the feedback loop of the transistor Q2 seems to be too big, DS1 has 250pF in there

Now I'm just a little noob, so I'm not sure if these make any changes, at a point when I get the time I'll swap these parts in my build and see if some changes help. Until then I thought the good people at FSB are maybe interested too and would check too...

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Re: MXR Custom Badass '78 Distortion

Postby drd » 18 Sep 2013, 20:08

Had the time to fiddle around with it today, and now it is ok. The following changes were necessary:

1) Changed R2 to 22Ohm as on the schematic
2) Changed both C5 and C6 to 100pF. Actually anything below 220pF will work, I hear some difference in the amount of distortion between 220pF and 100pF, but not below that.

As a mod I also added 55k in series with the Distortion pot's 3rd lug to get even more distortion, alternatively using a 200k pot could be an option here.

Thanks again for the schematic and the layout, with the changes it is quite a useable distortion pedal. Nothing exceptional, but decent enough.

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Re: MXR Custom Badass '78 Distortion

Postby mirosol » 29 Sep 2013, 18:25

Here's the corrected layout. Thanks to drd.
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Re: MXR Custom Badass '78 Distortion  [schematic]

Postby 7line » 29 Nov 2013, 00:25

MXR badass.jpg


Schematic with values

I think some strange values, you can take from the DS1
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Re: MXR Custom Badass '78 Distortion

Postby Ice-9 » 29 Nov 2013, 00:59

From memory of doing this schematic I recall

C18=47uf
C5 = 220n
The rest of the values on the above picture are correct. Except I can't remember the R2 value . I originally wrote it s 22ohm but I suspect it was 220ohm (typo probs)
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?
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Re: MXR Custom Badass '78 Distortion

Postby 7line » 29 Nov 2013, 13:28

on pics R2 marked as 220, it is 22 ohm. on the 220 ohm writes 221
c5-220n is very big capacitance, probably 200-250p will enough
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Re: MXR Custom Badass '78 Distortion

Postby Ice-9 » 29 Nov 2013, 20:18

7line wrote:on pics R2 marked as 220, it is 22 ohm. on the 220 ohm writes 221
c5-220n is very big capacitance, probably 200-250p will enough


I will see if I can find the original photos I took as I can't read it on the forum pics.
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?
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Re: MXR Custom Badass '78 Distortion

Postby FakingJeffrey » 11 Jan 2015, 00:22

HELLO! First time poster here.
I have an MXR '78 and I absolutely love it. I'm a beginner pedal builder, just building kits from places like BYOC. I'm not that great at reading schematics yet, and I don't full understand circuits, but I'm learning.
My question is about the clipping section. Dumb it down a little for me if you would. When the crunch buttons is ENGAGED, what clipping diodes are in play?
I'm building a distortion pedal kit now and I want to just use the same clipping diode arrangement that is in the M78 when Crunch is on.
THANKS!
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Re: MXR Custom Badass '78 Distortion

Postby Nocentelli » 11 Jan 2015, 15:15

The crunch switch seems to connect a second pair of 1N4148 anti-parallel diodes in parallel with the LED pair to ground. 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, giving only the part of the clipping circuit ringed in the diagram below.

This would mean you could just use two anti-parallel pairs connected via a 470 ohm resistor to ground, shown on the right in the rectangle to replicate the crunch setting:

MXR badass clipping.png


I think that the lower forward voltage of 1N4148 means a lower clipping threshold, so the same loudness of guitar signal will sound more clipped than with LEDs.
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