Sunday 8 September 2013

MXR Custom Badass '78 Distortion

The first in a line from the MXR Custom Badass design team, the MXR Custom Badass ’78 Distortion is a factory-modded pedal that roars with huge amp stack tones and old school tube amp-like distortion.
We took a classic distortion circuit and hot-rodded it to the next level for over-the-top soaring leads and rich, saturated rhythms. With only three knobs, dialing in Badass tone is a cinch. The CRUNCH button allows you to choose between two different modes of diode and LED clipping, boosting the harmonic content of the distortion.

The '78 Distortion's wide open amp-like sound sounds great whether it's in front of a clean amp, a slightly dirty amp, or a full on overdriven amp. Under the hood, this high performance machine features top notch circuitry and hardware meant for a lifetime of use.


Some of the cap values may seem a bit off. These are the measured values of the SMD components on the actual board. So i suggest you try on the values that are closest to the ones measured. Same thing with semiconductors.

Now this layout is also verified, thanks to Csaba and John for their footwork.


43 comments:

  1. I owned the original one, a lovely distortion pedal. The volume is a bit too much for my taste, but I'll try this one and change the volume pot if necessary. Thanks for the vero, mirosol. Btw: do you have any suggestions for substituting the MPSA14 darlingtons? I can't find them where I live, neither at Tayda. I have BC517 and BC618. I think BC517 might fit...

    ReplyDelete
    Replies
    1. I have no idea why they have darlingtons in there. You should be fine with any NPN Si. I would personally try standard 2N5088 for those first.
      +m

      Delete
    2. Thanks, I'll try 2n5088s then, I have them at hand anyway.

      Delete
  2. Ok, I finished it today, but no good :( Sounds like the signal would just pass the circuit, but it does not do too much to it. Output pot works, Tone pot alters the sound slightly, Distortion does a very little something, but really just a very subtle change in tone, no distortion at all. The switch doesn't do anything. The clipping LEDs don't blink at all (which they normally do I think). I used some different values for caps:

    290nF -> 220nF
    12nF -> 10nF
    38nF -> 33nF (which actually measures about 38nF)
    72nF -> 68nF (which actually measures about 72nF)
    890nF -> 1uF (which actually measures about 980nF)

    IC is a 4558, for the transistors I tried both BC517 (Darlington) and 2n5089s as well. Transistor orientation was checked. Here are the voltages on the trannies and the IC (seem to be not ok on the IC I think):

    Top left transistor:
    C: 4.32V
    B: 0.7V
    E: 0.1V

    Bottom left transistor:
    C: 3.94V
    B: 4.14V
    E: 9V

    Right transistor:
    C: 3.36V
    B: 3.66V
    E: 9V

    IC:
    1: 4.5V
    2: 4.5V
    3: 4.5V
    4: 0V
    5: 9V
    6: 4.5V
    7: 4.5V
    8: 4.5V

    Of course I doublechecked the parts and also checked for solder bridges - nothing found. Any ideas?

    ReplyDelete
  3. Just saw what I wrote yesterday: the IC voltages are not ok, mixed a bit the legs... Here is the correct sequence:

    1: 4.5V
    2: 4.5V
    3: 4.5V
    4: 0V
    5: 4.5V
    6: 4.5V
    7: 4.5V
    8: 9V

    ReplyDelete
    Replies
    1. Voltages seem correct. I'll triple check the layout tonight. You all are welcome to check this against the schematic as well. Schem can be found at FSB.
      +m

      Delete
    2. I checked the schem, for the first sight I could only see that C11 at the end should be 1uF according to the BOM, but is 47nF in your layout. Don't think that would make a big difference. Also I think there are a couple caps missing from the Vcc part (C18 and C2), but those are probably only for noise filtering, so again nothing that would make any difference here.

      Delete
  4. I have the exact same issue with mine.

    ReplyDelete
  5. Yeah +1 for it passing sound but with no distortion. I only had 2 MPSA14s left so I used a MPSA13 for the other transistor, I'm wondering if that's the problem as the hfe for the MPSA14s was ~64000 whereas the MPSA13s I had were ~24000. It could be that this thing needs a lot of gain in the transistors, not sure why though as by all accounts it's just a souped up DS-1. Anyone else use all MPSA14s?

    ReplyDelete
    Replies
    1. I tried it with mpsa14 and had the same issue madferret. Figured I would try to help for once instead of just asking for help all the time. I think some home the signal isn't passing through all the stages of the circuit, but not sure what's wrong or why.

      Delete
  6. I have some on order, but for now I'm using 2n5088s

    ReplyDelete
  7. Ok, so it seems that either the layout or the schematic is wrong somehow. I tried to compare the schematic with the layout and I think it is ok, but a doublecheck would not harm I think. The schematic can be found on FSB or I can also send it to any of you if you wish. Once I have the time for that I'll try to audioprobe the build, but I have to build a probe for that too (I've never done it myself so far...)

    ReplyDelete
    Replies
    1. Doublechecked myself, compared with the BOM and the schematic I've found following:

      1) The Re resistor on the upper left transistor is 22Ohm in the BOM and 220Ohm on the schematic (R2 on the schematic)

      2) There is an 1M resistor from input to ground which is not on the schematic, I think this is a pulldown resistor

      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 from FSB there are a few resistors that are not on the schematic, might be because the schematic ommited the switching circuit?

      Other than this I don't see any differences between the schematic and this layout.

      Delete
  8. Ok, I did some reading and also compared the schematic with mainly the DS1 schematic, but also with some others. I have some suspects:

    1) The Re resistor on the upper left transistor really should be 22Ohm instead of 220Ohm

    2) 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

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

    I'll try to socket these components and play around. Any further ideas are welcome ;)

    ReplyDelete
    Replies
    1. I don't think 220R -> 22R will change anything. As for the caps - i too thought that those values can't work at all, but there are similar values in those position on the Badass OD. To my surprise that does work...

      I just triple checked the layout against the traced schematic and i can't see anything wrong.. Maybe i should tag this "on hold" and some of you could ask questions on the FSB thread. Surely it seems that there's something not quite right on the schematic.
      +m

      Delete
    2. Just entered my comment there too, let's see if anyone has any ideas there. Anyway I'll try to fiddle around with these values once I'll have some time and of course report back if I find something interesting.

      Delete
    3. Also out of curiosity I checked the Badass OD, and there are way smaller caps in the feedback loops: 10nF and 250pF. I'll definitely start with changing those, hopefully I'll have some time tomorrow.

      Delete
  9. Alright folks, it's kinda verified now. Swapped the two caps in the feedback loops (12nF and 290nF) both to 100pF and also changed the 220Ohm resistor to 22Ohm and then it works fine. For the caps I've found that below 220pF actually anything works, between 100pF and 220pF I still hear some difference in gain, but below that I don't hear any differences. You could leave the 220Ohm resistor in place too, the effect is already useable that way, but changing it to 22Ohm adds even more gain. Not too much more, but the difference is audible. On mine I also soldered a 55k resistor in series with Distortion 3 to achieve even more distortion, alternatively you could use a 200k pot there. Tried the effect with both the BC517 darlingtons and normal 2n5089 trannies. With the 2n5089s there is more distortion, but the sound gets loose, sort of misbiased feeling, but (to me) not in a good way. With the darlingtons the sound is nice crisp and tight, somehow more focused.

    ReplyDelete
    Replies
    1. Thanks Csaba.

      I used a 250k pot for distortion and it seems to work well, in case anyone was wondering.

      I ordered some MPSA14 transistors just for this pedal, but also tried several others. While the MPSA14s sound decent, I tried BC550Cs and that's what I'm sticking with. LOTS of balls.

      I'd say that built this way it's a pretty damn good distortion pedal, but there are several others on here that are just better. This will probably end up as a "give away" pedal. All the same, if I didn't have better I would definitely use this one.

      I'm wondering now what kind of bizarre transistor swap could make this a bit more unique. Any ideas?

      Delete
    2. I might be wrong, but the first and last transistors probably don't make much difference, as they are only used as buffers. The second transistor is already doing some amplification, so I'd start with that. If you want drastic changes I'd try something with low hfe, 2n2222 for example.

      I think it would be possible to convert this stage to a FET stage too, but then you would need to change the 10k resistor from collector to +9V to a 100k trimmer, change the 22Ohm resistor to something between 1k5-15k and possibly add a cap in paralel to this resistor with the value between 1uF-47uF (though on this layout it is only possibly in an ugly way, by crossing over other components, or in the same holes as the resistor). With the trimmer then you could bias the drain voltage of the FET to 4.5V.

      You can also experiment with the clipping section, different diodes or diode arrangements can change the texture of the distortion quite a bit. I'm thinking to remove the switch, and only use some asymmetric arrangement, probably a Ge and a LED or a Ge and two Si in series.

      You could play with the 4k7 resistor and the 890nF capacitor in series. These form a high-pass filter, if you increase the capacitor value you will get more bass content, lowering the resistor will give you more gain in the opamp stage, but also you will filter out more bass (if you keep the 890nF the same).

      Optionally you could play with the tone stack and the 72nF cap after the opamp to change the tonality, but I think it is quite useable as it is, so I wouldn't change that.

      If I was to redesign the whoel circuit, I'd probably try to use the second opamp in the IC better, at this moment it is only a buffer. I'd either try to use it as a first gain stage or use the first opamp for gain stage and the second for an active tone control. But this can't be done on this vero layout.

      Delete
    3. Oh, and if you want to change the second transistor to a FET, then you need to remove the 470k resistor and the 12nF (or actually 100pF in the revised layout) going from the base to the collector, the FET does not need that.

      Delete
  10. I just built it and it sounds great. you can verify it if you change a couple of things. reverse 1 & 3 on the tone and distortion pots, change the 47n output cap to a 1u, and change both the 12n and 290 caps to 100p's. the original pedal also uses a TLC2262 op amp and that's what i'm using in mine.

    here's my 'corrected' layout:

    http://johnkvintageguitars.homestead.com/Effects/Fuzz-ODs/MXRCB78/MXR-Custom-Badass-78-Distortion.png

    ReplyDelete
    Replies
    1. John, on mine I left the output cap as on miro's layout. Does this make a big difference, did you try that? I'm quite happy with how it sounds now, so if the difference is not too much, then I won't bother desoldering it... Thanks!

      Delete
    2. I didn't try it with a 47n, but since I use my pedals mostly with a bass, I just went with the stock 1u. I also ended up with a 220p for the 12n and a 150p for the 290n

      Delete
    3. oh, and I also went with a C250K for the distortion pot on mine for even more gain.

      Delete
    4. Hi,
      In case this has any impact: I see mirosol's layout has the capacitor that comes before the last transistor as 1µF, and the output capacitor as 47nF, when it's the other way around in John Kalls' layout (and in the latest schematics posted at FSB).
      Hope this helps...

      Delete
  11. Mirosol, just realized that you updated the layout, thanks for that. However the emitter resistor of the upper left transistor is still 220Ohm, the schematic at FSB says 22Ohm there. I changed it too on mine and indeed it makes a difference, so I'd suggest to change this in your layout too if you have a little time.

    ReplyDelete
    Replies
    1. Forgot. I'll replace it once i get to my desktop.
      +m

      Delete
    2. Fixed. So the layout should now be ok for anyone to build.

      Delete
  12. Guys, how different is this from the Super Badass Distortion, in terms of tone? Also, any leads for the schematic for that?
    Thanks.
    J.

    ReplyDelete
    Replies
    1. Well, it's been quite a while since I had the original one, but as much as I can remember it is not too far from it. It actually is a nice ballsy distortion, the clipping options add some variety to the sound. I did not box it, but only because currently the Crunch Box is my favourite and don't need another higher (for me at least) distortion at this moment.

      Delete
  13. Hi all! Sorry for the (noob) question... How do I connect sw2 to output1 ? I mean, I'm using a SPDT switch... How should I do the connections?... please help me!
    thanks

    ReplyDelete
    Replies
    1. That's the ground rail so all it means is that both SW2 and Output 1 need to be grounded. Take a wire from the board to SW2 and then another wire daisy chained out of SW2 to Output 1 lug. That's it.

      Delete
  14. Thank you Ivlark, so (if I'm not wrong) I have to:
    a) connect the central lug of the SPDT switch to lug n.1 of output potentiometer AND to "switch 2"
    b) connect one of the (other two) lugs of the SPDT to "switch 1", and leave the other lug empty

    Is this correct?
    Thanks!

    ReplyDelete
  15. :( it doesn't work... just a lot of noise... the pots seem to work properly anyway; I measured the voltages around the IC, and I found these:

    pin1 = 4.5V
    pin2= 4.5V
    pin3= 4.5V
    pin4 = 9V
    pin5 = 4.5V
    pin6 = 4.5V
    pin7 = 4.5V
    pin8 = 0V

    It seem there's something wrong with pin 4 and 8, which have one the voltage of the other...

    The transistors I used are MPSA18, and I used 68nF instead of 72 nF, 39nF instead of 38nF, and 880nF instead of 890nF (putting 4 capacitors of 220nF each, in parallel)...

    :/ could anyone help me?

    ReplyDelete
    Replies
    1. Your ground and 9V wires must be crossed (maybe positive centre pin power supply instead of negative?). As you say, pin 4 should be 0V, pin 8 should be 9V so you've got a build error somewhere that you're going to need to identify.

      Delete
    2. Mmmm, things get worse... I've crosse 9v wire with the ground one... I stopped when I became to smell something strange... and I realized that the IC warmed up very much... Now, if I return to the previous configuration, the voltages that I measure around the IC pins are all 0, except for the 4 and 8 (which still remain 9 and 0)....
      I tried different transistor, but nothing changes... What else could have been broken? an electrolitic capacitor?

      Delete
    3. Yes you could have damaged any polarised components so the electrolytics could potentially be damaged. Plus I don't know whether the IC will have liked 9V on pin 4 or if that could have caused any internal damage.

      So I think you may have a few things to swap and re-measure. If you're still not getting any sound through then you're going to have to look at sorting yourself out with an DIY audio probe so you can identify where the signal is dropping.

      Delete
  16. Ok so... I've changed 3 electrolitic caps, the IC and one led that were all damaged... Now I have all the correct voltages around the IC, but still no sound come out...
    uff....
    Seems that I have to use a sound probe... I know how to build it, but not how to use it properly...

    ReplyDelete
  17. Just thinking of this: "If all the voltages on the IC pins are correct, then the circuit itself should be correct"
    Maybe reason why I don't get any sound out of it could be due to the transistors choose? I use three MPSA18, and the IC is the TL072... :/

    ReplyDelete
    Replies
    1. The voltages being correct doesn't mean it will work. Everything else could be perfect but one bad connection on the output wire and you won't get anything out of it.

      The MPSA14's are darlingtons and so much higher gain than the MPSA18's. That could be an issue, I'm sure darlingtons were selected for a reason. I wouldn't expect it to be silent with 18's though even if they aren't high enough gain. But it is certainly a difference from the intended parts and so something worth trying.

      Delete
  18. Amateur question: I have the M78 and I love how it sounds with the CRUNCH button engaged. I'm not very good at reading schematics but I'd just like to know what clipping diodes are engaged when the CRUNCH is on. I assumed it was just 2 LEDs, but the schematics look more complicated. Is it a combination? Thanks...

    ReplyDelete
  19. Hola a todos

    me interesa saber si funciona 100% de una y sin problemas.....

    Hello everyone

    I want to know if 100 % of one and runs smoothly .....

    ReplyDelete