Same Pedal, Slight Differences

[dafortier]

I may have found something of interest.  I went through all of the capacitors in the drive section and they are all the correct values and all identical.  I didn't remove them to check their actual vaules.. Actually, I don't really know how to do that anyway.  So I started checking the resistors.  They all checked out with the correct values, with the exception of a few and that could be because of their respective connected components.  I compared those ones between the two pedals and they were the same on both, so they seem fine.

However... R12.  On pedal A I can't get a reading at all.  The meter looks like it's quickly charging and discharging but never showing an actual reading.  BUT on pedal B it slowly builds up to 420k and stays there.  Attached is the schematic and circled is the resistor in question.

Could C8 or C7 on pedal B be bad?

[dafortier]

Well this is crazy. I thought I needed a better way to AB these pedals so I rounded up some spare parts and robbed another old pedal and built an A/B switcher.  Once hooked up I was able to switch quickly between the two.

I was able to dial in the exact identical tone on both. I mean indistinguishable! Even with that weird capacitor/resistor reading I had earlier.

Before this I would either try them one at a time or I'd run pedal A into Pedal B, or vice versa. Neither was ideal. Esp for weeding out such small differences. What I was hearing could have been just the compounded effect of having them back to back.

Still got allot of good info on the thread. But I think I've got this one solved! Thanks for all the help!

Sent from my VS995 using Tapatalk

[Fender3D]

Stomping on the psychoacoustic field?
I wonder if this happens to the sound gurus as well....

[teemuk]

This is something that's always stumped me until recently. How can a large pedal company manufacture so many pedals and have them all sound the same when they're using components with tolerances on the order of 20%?

Sometimes it matters, sometimes it doesn't. In my experience, when manufacturers DO need consistency they only use those poor tolerance components in places where it doesn't matter.

Not to mention, great deal of circuit design aims for minimizing inconsistency issues that are due to device or component variation.

For example, gain of a generic bipolar transistor can vary much more than 20%. In a simple common emitter amp circuit you most likely identify effects of such variation. I'm pretty sure you identify them much more easily than, say, identifying 20% tolerance difference in filter cap capacitance. ...And then we proceed to put hundreds of those transistors to an integrated circuit like an opamp, wrap it to a global feedback loop, and -drumroll- suddenly only the feeeback loop components display any prominent effects of tolerance drift. Why? Because the circuit design has eliminated slight effects of component variation, and we can expect consistent performance.

IMO, the designers used those 20% tolerance components because they could, with sufficiently consistent results. No, same designers likely wouldn't have put them to a RF tuning circuitry that needs to operate precicely, but that's the point. 20% tolerance is quite sufficient to stuff that is not precision circuitry, like guitar effects, amps and alike (even now you have to focus to hear a difference), especially because there are circuit designs that minimize "external effects", and plenty of places in the circuit where 20% tolerance doesn't matter at all (e.g. filter caps, almost all components inside global feedback loops, etc.). If something really, really, needed to be consistent then the designer would have chosen smaller tolerance for those parts. There usually aren't that many in a generic guitar effect circuit.