MXR Commande Overdrive

[drummer4gc]

Anyone ever played this pedal before?



It's got a reputation as one of the worst sounding overdrive pedals ever, but I've been playing with one and there are a couple nice sounds to be coaxed out of it.

Anyways, I'm asking about it because it's got a strange power scheme that I've never come across before. Check the schematic:



The opamp gets power through the LED, and the voltages seem real bizarre. With a power supply giving 9.5v, pin 8 is only receiving about 7.5v when the effect is switched on (although it gets closer to 8v when off). What the hell is this about? No idea why the power is set up this way, and it seems to be causing a sort of lag effect - when switched on, the pedal loses volume for a split second, then kicks in, like the IC is starving momentarily.

Can anyone help explain why the power section of the pedal is built like this?

Thanks!

[Mark Hammer]

There are aspects that seem to be drawn, or at least labelled, wrong.

For starters, C10 (.005) makes no sense.  At one level, one would think "Okay, a bit of a treble pre-emphasis...seen that one before".  But then C10 makes no sense in the face of R10/C6, which roll off treble below where one might expect any treble pre-emphasis to start.

As well, the Distortion pot seems to be connected in a manner that also doesn't make sense.  As a non-inverting stage, the traditional connection of a pot like that would be to Vbias or ground, and it goes to neither.

Confusing.

[PRR]

J1 is drawn wrong, puts 9V on your guitar.

I'd say the battery is drawn backward but many draftpersons don't know the long-short polarity of a battery symbol.

The distortion pot is not totally bogus. In concept, B+ is AC ground just like Vref or GND. And note the polarity on C3-- it is aimed at a positive DC level.

I think someone went totally bonkers reducing the switching to DPDT, quite cleverly, and wasn't too concerned about the low (and jumpy) chip voltage.

The jumpy voltage might be lessened with change of C12 or R15.

[drummer4gc]

Looking at the board, that section actually looks quite accurate. Other than doubling up on pin numbers in the schematic, the only difference I see is that there is a resistor in series with c10 before it runs into all of those parallel components in the feedback loop. I haven't checked resistor values, but the .005 is correct (.0047 on the board).

PRR, are you suggesting reducing the values of c12 and/or r15? And any idea why MXR wouldn't just wire up one side of the switch led -> ground and the other side like a standard spdt? I don't see any benefit from this bizarre switching.

[PRR]

> why MXR wouldn't just wire up one side of the switch led -> ground

The LED uses more power than the 4558 and all.

Their series trick saves battery life.

The low voltage on 4558 may make little difference (output is 1.2V p-p max, so 6V ought to be ample), or may add "flavor" (4558 may run a bit lean at 6V-7V).

If you don't care about "authentic vintage", or battery consumption, you can try it a "normal" way.

[Mark Hammer]

Looking at the board, that section actually looks quite accurate. Other than doubling up on pin numbers in the schematic, the only difference I see is that there is a resistor in series with c10 before it runs into all of those parallel components in the feedback loop. I haven't checked resistor values, but the .005 is correct (.0047 on the board).

PRR, are you suggesting reducing the values of c12 and/or r15? And any idea why MXR wouldn't just wire up one side of the switch led -> ground and the other side like a standard spdt? I don't see any benefit from this bizarre switching.

A resistor in series with C10 would make it far more plausible.