I bought an open box Electro-Harmonix 44 Magnum with this issue. I tried replacing the pot prior to figuring out it was only for the first 15-20 seconds after power on. The new Alpha Taiwan Pot behaves the same as the original. The pedal works fine, even when the pot is noisy. The time constant of this problem has to be in the range of 3-4 seconds and with a 10K audio taper pot, I can't think of any failure mode to induce this problem... Maybe all of them do this???
Internally the "44" watt power amp is a TI TPA3106d1 (low passive parts count class D, H-bridge output, AC coupled input). There is a quad opamp that is likely doing EQ to get it to sound guitar-amp-like. There are three large diodes, probably to protect it from reversing the 24 volt DC external supply. I suspect the opamp virtual ground is created passively with resistors and a capacitor.
E-H does not offer technical support and because I've opened it up it is no longer under warranty. They have some sort of standard repair system that requires you ship the pedal to them and is very reasonably priced at $35.
Should I ignore the problem, or send it to E-H for repair?
Thanks!
Jon
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Here are some pictures:
Internally the "44" watt power amp is a TI TPA3106d1 (low passive parts count class D, H-bridge output, AC coupled input). There is a quad opamp that is likely doing EQ to get it to sound guitar-amp-like. There are three large diodes, probably to protect it from reversing the 24 volt DC external supply. I suspect the opamp virtual ground is created passively with resistors and a capacitor.
E-H does not offer technical support and because I've opened it up it is no longer under warranty. They have some sort of standard repair system that requires you ship the pedal to them and is very reasonably priced at $35.
Should I ignore the problem, or send it to E-H for repair?
Thanks!
Jon
-------------------------
Here are some pictures:
Electro-Harmonix got back to me this morning and made an exception to their no end user technical support. They said this was normal for this pedal and offered to send me a replacement pot to restore the pedal to factory condition. I declined the pot as I kind of like how easy it would be to replace my replacement compared to the factory one.
I can only imagine that the Vdd/2 needed for the opamp virtual ground is derived from the single ended supply with a very large resistor and a very large capacitor and this is the transient behavior as "ground" finds its way to Vdd/2 (and maybe the isolation network to the opamp power rails charges up too). Pedal companies are stingy at best with schematics and I didn't bother to even ask. As most of it is surface mount there would not be a much I could do anyway.
I can only imagine that the Vdd/2 needed for the opamp virtual ground is derived from the single ended supply with a very large resistor and a very large capacitor and this is the transient behavior as "ground" finds its way to Vdd/2 (and maybe the isolation network to the opamp power rails charges up too). Pedal companies are stingy at best with schematics and I didn't bother to even ask. As most of it is surface mount there would not be a much I could do anyway.
15 to 20 seconds being the time it takes for DC to stabilize.
Dont know if it is poor design or typical behavior.
They could mute it while it stabilizes, like many other audio devices.
Then musicians would whine it takes 15 seconds before signal starts.
So you cant win really regardless.
Aint gonna redesign the whole thing so is what it is.
Dont know if it is poor design or typical behavior.
They could mute it while it stabilizes, like many other audio devices.
Then musicians would whine it takes 15 seconds before signal starts.
So you cant win really regardless.
Aint gonna redesign the whole thing so is what it is.