MXR Distortion III/Classic Distortion Mods?

[ZiggyZipgun]

Around Christmas I picked up one of the M-86 Classic Distortion pedals for $29 at GC, but as it basically sounded like the first half of the gain knob of the Rat model on my PODxt Live, I don't think I've plugged it in since.  I did open it up, and after seeing pics of the Distortion III's guts, they're definitely the same thing, mine's just sort of gift-wrapped in black. 

Has anyone tried any mods on either of these?  They have a Motorola MC33178 IC and a tantalum cap, though I don't know what value and I haven't seen a schematic yet.

[CynicalMan]

Here's the schematic:

http://analogguru.an.ohost.de/001/schematics/Mxr_Distortion-III.gif

Removing C4 seems to be a popular mod.

[ZiggyZipgun]

Anyone know of different op-amps to try? 

[kurtlives]

If its a dual op-amp with standard pin configuration the usual suspect will work...
JRC4558
RC4558
JRC4559
TL072
TL082
TL2272
NE5532
Burr Brown fancy ICs

[stompbox steve]

Hello
Can anyone comment about what audible difference removing the C4 will do?  I had picked up one but it is more of a overdrive than a distortion, to me.  I have the drive pegged 100% and still looking for more.  Thinking about selling it (un-touched) unless the c4 mod will help or another (easy) hot rodding mod otherwise, off it goes.
Any insight is appreciated, S

[aron]

I wish I had picked one up. I love stuff like this.

[CynicalMan]

Hello
Can anyone comment about what audible difference removing the C4 will do?  I had picked up one but it is more of a overdrive than a distortion, to me.  I have the drive pegged 100% and still looking for more.  Thinking about selling it (un-touched) unless the c4 mod will help or another (easy) hot rodding mod otherwise, off it goes.
Any insight is appreciated, S

It is definitely more of an overdrive. Removing C4 will increase the highs (which are way too low for my taste) and the noise.

[Mark Hammer]

Assuming the linked-to schematic is accurate, C4 rolls off treble frequencies above 72hz.  Kinda low, dontcha think?  Personally, I wouldn't stick anything in there higher than .01uf.  That will roll off starting at just under 1.6khz.  If you need mellow, then I suppose I'd accept .033uf for a 480hz rolloff.  That will still provide some bite because the rolloff is a shallow 6db/octave.

Don't waste your breath or time on the op-amp choice.  VERY little about this unit, besides maybe noise or battery-consumption, will depend on op-amp choice.  This is principally because the clipping diodes are NOT in the feedback loop, like the Tube Screamer, but are after the output, going to ground.

If you want to invest time with some chance of reaping tonal rewards, I suggest reading the document on TS-9 tone control mods, over at Jack Orman's site: http://www.muzique.com/lab/tstone.htm

[stompbox steve]

I love in this forum that people actually answer with useful info, not like other places.  Everyone's input is always appreciated.
I will crack it open and see how I feel.  The tone mod might be more than what I am looking to do.  At this point I am leaning to sell it, but deep in my heart I can't let any pedal go...
my name is Steve and I am a pedal-holic. 

[Gus]

1/ 2 Pi RC,   1/ 6.28 X (10 x10^3) x  (.022 X10^-6)

723hz

[Mark Hammer]

Bah!!  You're right.  it's 22n, not 0.22u.  So yeah, sleepy-head  here totally agrees with Gus's math  ; bump the rolloff freq up by 10x from what I said earlier. 

However, the recommendation remains the same to both check out Jack's article and consider making the value of that cap a little smaller (something between 22n and 10n, like 15n or 18n) to permit just a bit more bite.

[CynicalMan]

However, the recommendation remains the same to both check out Jack's article and consider making the value of that cap a little smaller (something between 22n and 10n, like 15n or 18n) to permit just a bit more bite.

Or even set up the rolloff at something like 10kHz so that it only rolls off noise. Something like 2n2 or 1n would work for that.

[chi_boy]

Is the Classic Distortion being sold any more??

My local CG sold out immediately and Christmas time and never had them back in since.

[nighthawk]

I put in a .01uF in mine yesterday. Now it has enough highs available when needed without being too bright. Works for me.  Thanks for the tips!

[Jurbo]

Hi! This is an old thread, I know, but still seems like the right place to post this, and it might even benefit someone... 

I have a Distortion III that has been subject to all kinds of modding experiences, here are some of my findings:

To get more gain and a juicier bottom end, place:
R4 1,2 k
C2 4,7 uF

(I also installed a "boost" switch to change between these and original values)

The above mod and a red/yellow led combo at clipping stage will get quite a nice Marshall type distortion.

My other mods:
op-amp 4558P (the original MC33178P is not bad at all, but I prefer this)
C4 reduced to 4,7 nF (more openness and treble control)
C6 raised to 100 nF (more bass)
Switch to disable clipping diodes
Switch to select original diode clipping or red/yellow led clipping

My DIII is already quite versatile (3 switches, each making a big change in sound) - but it still has a hole empty for a miniswitch to add something selectable... Any ideas?

[CynicalMan]

Adding in diodes in the first opamp's feedback loop?
Maybe something from one of these:
www.muzique.com/lab/tstone.htm
www.muzique.com/lab/zenmos.htm

How does the LED clipping sound?

[Jurbo]

I tried to add clipping in the first opamp's feedback loop, but I didn't really find anything usable. It may be because I only tried 1N4148's (singles, doubles, triples) and some LEDs. Germaniums etc. might get me somewhere (generally, I'm not a fan of the germanium sound). Should the achievable effect be really prominent or really subtle? Maybe I should pop the pedal open and really try to dig in deep with more diodes. I was a bit tired when experimenting, so maybe I wasn't putting my all into it.

LED clipping is really cool in my opinion. It gives a louder sound than (the usual) silicon diodes, and the distortion has a bit more flexibility and airyness than silicon, which is more squashed. Different colour LEDs have different operating voltages, and mixing them gives a softer, asymmetrical clipping (tubey sound) with more harmonics. My favourite LED clipping sound is the red / yellow combination. I'd say it's very much a high gain Marshall-sound.

For metal lead work, the pedal sound really good with clipping disabled, too.

By the way, my ultimate goal with this pedal is to have one single distortion / overdrive unit that does pretty much everything from mellow blues sounds (the original setup) to high gain heavy rock, and I think I'm getting all that. I think the basic sound structure of the DIII is fantastic - a big, ballsy sound that can be tailored to the taste. 

[soupbone]

I bought one of the MXR Classic Overdrive pedals.I found out that the builder of the pedal put a secret black switch inside.I unsoldered the switch and put in a SPDT(on-on switch).Know you can switch from the stock mode to another mode that has less volume,and less of a mid hump with softer clipping.

[zombiwoof]

I bought one of the MXR Classic Overdrive pedals.I found out that the builder of the pedal put a secret black switch inside.I unsoldered the switch and put in a SPDT(on-on switch).Know you can switch from the stock mode to another mode that has less volume,and less of a mid hump with softer clipping.

I know about the switch inside (it changes it from the ZW-44 circuit to the Custom Shop OD circuit), but I don't understand what you did.  Are you saying you put an EXTERNAL switch on the pedal to switch between the modes?.  It's a little unclear.

Al

[soupbone]

I bought one of the MXR Classic Overdrive pedals.I found out that the builder of the pedal put a secret black switch inside.I unsoldered the switch and put in a SPDT(on-on switch).Know you can switch from the stock mode to another mode that has less volume,and less of a mid hump with softer clipping.

I know about the switch inside (it changes it from the ZW-44 circuit to the Custom Shop OD circuit), but I don't understand what you did.  Are you saying you put an EXTERNAL switch on the pedal to switch between the modes?.  It's a little unclear.

Al

You just take out the switch.It has 3 holes that it is connected to.Cut off 3 pieces of wire solder them to the blank holes,then solder the 3 wires to the 3 lugs of a spdt switch(on-on).You're done!!(you do have to drill a hole in the enclosure though for the switch).