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I couldn't get a used condenser mic I bought to work and eventually (doh!!!) I figured out I had a bad XLR cable (the mic worked after I made one out of new balanced cable--i.e. no faulty return path for the +48V phantom supply). However (not quite sure about this), after I tried the (phantom powered) condenser with my joemeek preamp (before I figured out the bad cable and using said bad cable) it seems to have gone noisy and poor sounding. Does this sound plausible? I did do some reading and apparently there is some possibility for phantom power to cause input stage damage from (the turn on surge?). OTOH there also seem to be plenty of people who haven't experienced huge numbers of failures. Feeling wary of phantom power although I did try the mic with my old Tascam 488mkII to check the mic out, and everything seems to be working and so far (a couple of times) no discernible damage (plugging in mic before powering P48, and powering down--except one time I forgot to turn P48 before unplugging the mic).
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It is entirely possible that a spike generated by plugging into live phantom power can damage semiconductors in a mic pre. It depends on the mic pre design. It's always good to follow safe practices with plugging/unplugging phantom-powered mics and other devices. The GOOD news is that it's usually just the first semiconductors at the input, whether it is an IC or differential pair. I don't know the design of the Joe Meek pre you have, but it is most likely discrete transistors, since it is a high-end product. -
Switching phantom on and off, or connecting and disconnecting the cable can damage circuits that are not well designed. The issue is that a voltage spike can go through the DC blocking cap on the inputs. I have personally seen this spike kill caps, resistors, diodes and chips. The normal way to deal with it is to use a current limiting resistor in the audio path followed by clamping diodes. But the resistors and diodes need to be able to handle a lot of current. Often the resistor will fail by an increase of resistance, resulting in an impedance mismatch between the + and -. The net result of this is lots of noise. So size the parts to handle 3+ amps and you should be good. And using pulse or surge rated resistors is a huge plus.Comment
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Thanks guys. So it sounds like if I can replace the IC at the input (SSM2017--I actually have a spare since I bought one a while back "just in case") and anything else damaged or gone off spec (such as the resistors mentioned) it's possible to fix it.
re: protection (not 100% certain, but) there are some diodes around the input IC but whatever protection was there may not have been enough.Comment
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