gswitzI have an old M-Audio interface that the pres went bad on after a while.
Which one? I was a designer there way back when, it might be one I worked on, and if it is I might be able to provide some pointers...
A couple questions:
- Do any of the front panel LEDs still illuminate?
- Does the preamplifier still work with a dynamic microphone?
- Is the output noisy, with or without a microphone connected?
gswitzA friend of mine has a Zoom device that has gone bad.
Can't really help with that one, sorry.
gswitzI'm starting to wonder whether plugging in mic cables while phantom is on might have caused this.
Phantom Power is, at best, a compromise. It probably causes as many problems as it solves! Well, almost.
For lower priced preamplifiers the 48VDC is usually generated using what's called a voltage doubler, which can in fact double the voltage, but it does limit the current - the lack of a 'real' 48V power supply is challenge #1 for several reasons, but I'll spare you. Current is not a big challenge - the maximum current allowed for phantom power is on the order of 10mA.
The 48VDC is applied to the "plus" and "minus" pins of the microphone input through a pair of 6800 ohm resistors, and matching is important, but not critical for the phantom power. It is critical for Common Mode Rejection Ratio, since an imbalance between the resistors will create an impedance imbalance at the input. That's challenge #2.
Phantom power applies the same voltage to both pins 2 and 3 so that the microphone element does not "see" a potential difference. Again matching the impedance from each signal pin to ground becomes important, since any imbalance will place a voltage across the element.
AND... this is where several myths about phantom power arise. Hot-plugging phantom powered microphones is considered a bad idea, but in fact there is little difference between hot-plugging an XLR connector and flipping a switch. (For the record, there are ways to ramp up the power to avoid thumps and other problems, but you don't find these refinements in lower cost devices.)
HOWEVER, hot plugging a 1/4" TRS connector with phantom power applied is a really bad idea. Each side of the power supply gets shorted, briefly, and that not only causes very loud thumps (think a 36 dBu transient), but it can also damage the power supply, and the microphone element.
Another popular myth is that dynamic microphone elements can be damaged by phantom power. This is a case of "it depends", but almost all modern designs place dynamic elements (even ribbons) across the pair, so that there is no potential voltage applied across the element. I still try to avoid knowingly connecting dynamic microphones to phantom power supplies, but I don't lose sleep over it.
If you are still curious about phantom power I recommend the following papers:
- The minds at THAT Corp presented a paper at the 2001 AES convention that describes the challenges facing circuit designers. They followed this up with a second paper at the 2009 convention. These are must reads for anyone designing or repairing microphone preamplifiers!
- NTI (Neutrik Instruments) published an application note that goes into excruciating detail about testing phantom power. I really wish this had been around when I was designing!
- Eddie Ciletti wrote a great article for EQ magazine way back in 1999 that remains one of the best I've come across. If you read only one read this one (but really, read them all!)
Repairing your M-Audio preamplifier may be as simple as replacing a couple of capacitors (or quite a few, depending on the extent of the damage.) Well worth the effort I think.
If the preamplifier is still working for dynamic microphones you could also consider an external phantom supply. I have a couple, and they provide better, more stable, and quieter phantom power than many of my other preamplifiers.