I try so hard to not believe in MOJO but...

[Bill Mountain]

I try so hard to not believe in MOJO but I used some no-name Ge diodes in an JFET overdrive last night and they sounded amazing!  I can't remember where I bought them.  I plan on measuring the forward voltage and taking measurements on the FETs but it bothers me to be using parts with such inconsistency.

I prefer to build with easily obtainable and marginally consistent components to prove to myself that tone is in the engineering and NOT the MOJO components.  Repeatability of design is always my goal.  I usually end up giving away my builds and I like to be able to design it once and build it the same way when I need another one for myself.

[allesz]

So true.
Btw, I will pm you to buy some mojo

[EATyourGuitar]

I have experienced this with 1N4001 sounding different from batch to batch or brand or whatever. there is that moment after you used all your old diodes when you realize that you didn't save any and the new ones are different.

[Thecomedian]

I think OP understands MOJO is just a way of saying that the circuit behaved just right   

The fuzz face is the most obvious circuit proving mojo is all about how each component contributes to a good or bad sound based on it's most important properties. Farads for caps, ohms for resistors, gain for transistors, and voltage drop for diodes. That's why people use different colors of LEDs to try to get a sound they want. If a circuit puts out 1 volt, a diode that turns on at 0.9v will sound different to a diode that turns on at 0.7v.

[amptramp]

I prefer to build with easily obtainable and marginally consistent components to prove to myself that tone is in the engineering and NOT the MOJO components.  Repeatability of design is always my goal.  I usually end up giving away my builds and I like to be able to design it once and build it the same way when I need another one for myself.

I have the same design philosophy - you should be able to use any component that meets the specs of its particular part number without a difference in behaviour.  I would also add that the circuit should be free of temperature effects, a non-issue with tubes but critical for transistors.  I like to use op amps because the large amount of feedback and the use of differential inputs tends to solve consistency and temperature problems.

[Mark Hammer]

I think there is a price-point knee at which something comes to be considered as "mojo".  Below that price-point it is merely a "preference".

Had you shopped around for those diodes and paid $50@, then it definitely falls into the mojo category. 

[Bill Mountain]

I prefer to build with easily obtainable and marginally consistent components to prove to myself that tone is in the engineering and NOT the MOJO components.  Repeatability of design is always my goal.  I usually end up giving away my builds and I like to be able to design it once and build it the same way when I need another one for myself.

I have the same design philosophy - you should be able to use any component that meets the specs of its particular part number without a difference in behaviour.  I would also add that the circuit should be free of temperature effects, a non-issue with tubes but critical for transistors.  I like to use op amps because the large amount of feedback and the use of differential inputs tends to solve consistency and temperature problems.

Except opamps are cold and sterile sounding.

[Thecomedian]

I have experienced this with 1N4001 sounding different from batch to batch or brand or whatever. there is that moment after you used all your old diodes when you realize that you didn't save any and the new ones are different.

I could imagine developing some sort of voltage difference sampling circuit that actively detects the diode drop and then adds or subtracts the extra or lack of voltage drop as compared to some intended level the designer wants, such that any diode you drop in there is always within 0.2v of the +/- of an input signal. That'd standardize any LED/diode so you never get a difference. Sounds a bit complicated to me though, I have no idea how someone would implement that, although it's clearly possible.

[samhay]

I have no idea how someone would implement that, although it's clearly possible.

You use a couple of those cold and sterile op-amps to set up antiparallel super diodes with a user defined threshold voltage (the intened clipping level).

[teemuk]

There can be great differences in diodes made out of material X. The forward voltage usually falls within certain range but the sloping of the characteristic curves can be all over the place.

Generally we just think of an approximate similar to this:


...While in reality the situation with real devices is more like this:


Now, do note that AA112 and 1N34A are both Germanium diodes but their characteristic curves and even forward voltage levels are quite different. Expect to find similar differences from ordinary silicon, Schottky, LEDs of different colours and so on.

Just being "germanium" or "silicon" doesn't neccessarily indicate anything concrete in terms of how the diode will react to forward bias voltage and current. A designer of a generic diode rectifier circuit will not neccessarily care for other aspects than average forward voltage, power handling and reverse voltage limit of the diode. For him all germanium diodes and all silicon diodes can in practice be pretty "consistent" in terms of forward voltage characteristics.

...But when we design audio signal processors that rely on diode clipping the sloping characteristics suddenly begin to have a ton more importance. Suddenly we discover that such application actually portrays large incosistency within performance of devices that were supposed to be "equal".Now that you are aware of it I fear the "mojo" aspect of selecting diodes has reduced...

Luckily our ears aren't too good on subtle nuances. We hear large enough differences in clipping thresholds very prominently but different slope's effects on overdrive are much, much harder to hear and identify. But where these characteristics begin to have an utmost importance are applications that rely on diodes biased on that very narrow range of operation between cutoff and full forward bias. "Ring modulation" with diodes springs into my mind as first example of such application.

Oh, the nice thing is that most of these differences can be compensated very easily by altering both "input" signal level in relation to diode forward voltage (for average Vf of clipping) or the forward current (for sloping characteristics). Latter adjustment practically just calls for a single series resistor.