Maxon OD-880: an odd one or schematic errors?

[Fancy Lime]

Hi all,

diving deeper into the rabbit hole of my recent obsession with classic "natural overdrives", I came across the Maxon OD-880. This seems to be essentially a MXR Distortion+ type of circuit with extra bells and whistles and should not be confused with the Maxon OD808, which belongs to the tube screamer family. Anyway, all schematics I could find are a bit odd but all essentially identical:




Needless to say, I have questions. I will be referring to the part numbering from the first of these schematics.

1: R5 is utterly redundant isn't it? It drops the bias resistance for IC2 from 20kΩ through R3+R4 to 19kΩ and forms a voltage divider with R4 that shaves 2% off the signal. Both effects are well below parts tolerances, especially considering the parts of the late 70's and carbon film reristors. So why bother? Design error? Error in the schematics? The schematics may well not be independent but be copies of one-another.

2: The minimum gain seems high compared to how this thing sounds on Youtube. In the schematics, IC2 has a gain of 21-128 (26-42db) and IC3 has a fixed gain of 5 (14db), only for the 10kΩ Volume pot together with the 68kΩ of R13 to dump 17db to ground again (not so in schem 2, which has a 100k Volume pot). Is that again a very odd design choice or an error in the schematics?

3: The buffer IC1 seems pointless the way the schematic is drawn unless the fork between clean and effect signal going to a SPDT switch on the output is after this buffer instead of before (as schematic 3 implies). The C1-R2 high pass is below audio, so only the 340Hz high pass of C2-R3 can be of importance to the sound, and that could have been achieved by choosing a smaller cap and keeping the 470kΩ input impedance.

Is all of this important? No. I'm just curious and try to understand how other people design circuits.

Cheers,
Andy

[j_flanders]

1: R5 is utterly redundant isn't it?

R3 forms a high pass/low cut filter together with C2.
-3db around 360Hz
-13db around 80Hz

Edit: sorry, misread R5 for R3, ignore the above

[Mark Hammer]

R3 does that.  R5 is for re-biasing after the DC-blocking action of C2.  Small typo.  No worries.

If anything, I would think that C5 is redundant.

Although C2/R3 increases the bass cut beyond what C3/R6 do (identical values in the TS-808/9 and SD-1), C6/R12 provides substantial treble cut beyond what occurs in the TS-9 and SD-1.  So the circuit should sound smoother.

I built myself a few OD-1 clones, using a 3403 quad op-amp.  The OD-1 is in many respects the same as an SD-1, except that instead of a variable tone control, it also uses a fixed treble-cut filter as the OD-880 does here.  Personally, I like it.

Although yes portions of this circuit behave like a Distortion+ or DOD250, the inclusion of IC3 provides some much-needed gain-recovery.  The Dist+ suffered level-loss when the gain was less than dimed, largely because it used germanium diodes for clipping.  The DOD250 improved matters by using silicon diodes.  The 880 design allows for use of any diode types you want, because IC3 brings the level back up again.  Decent output levels can be achieved without having to max out the Gain control.

[iainpunk]

yeah, maxon pedals seem to have a few redundant parts here and there, those engineers were definitely smoking something that ain't kosher. haha

i think they are just design errors, not that it makes a big change in the end product or the bottom line.

cheers,
Iain