Solstace Posted November 10, 2008 Share Posted November 10, 2008 A problem I'm experiencing with a particular type of microphone at work has had me talking to the manufacturer. It has transpired that their design is something akin to: Microphone element on the end of a body or thin tube of some description, connected to an in-line pre-amplifier built into an XLR connector. The length of cable between element and pre-amplifier is about 10 feet, and is unbalanced. Our experience therefore is that we have a very nice-sounding mic until we have anything nearby that gives any electromagnetic output, at which point all bets are off. I was rather surprised that a run of that length for a live sound microphone would be unbalanced, given its likelihood of being exposed to significant interference from various sources. Even if this means putting the pre-amp inside the mic body itself. My question is - is this really so unusual a design principle? Link to comment Share on other sites More sharing options...
Andrew C Posted November 10, 2008 Share Posted November 10, 2008 747 by any chance? Link to comment Share on other sites More sharing options...
mostlyharmless Posted November 10, 2008 Share Posted November 10, 2008 Well, to be a condensor you need power (phantom or otherwise) to charge the plates to be able to measure the changes in capacitance caused by the plates vibrating. So once you have power there, it seems sensible to just build a pre-amp at the same time? Link to comment Share on other sites More sharing options...
paulears Posted November 10, 2008 Share Posted November 10, 2008 Not really any different from the hordes of lav mics that take their power from the lav pack and aren't balanced either. Short lengths of unbalanced cable don't really cause that many problems, or we'd have loads of keyboards and guitars picking up interference on the cables. Guitars kick up plenty of noise, via the pickups. AKG build the amps into plenty of their mics - like the tiny drum ones. I've never had any cause problems. Link to comment Share on other sites More sharing options...
Simon Lewis Posted November 10, 2008 Share Posted November 10, 2008 The mic element has a high impedance output, so the preamp is there to present a higher impedance input, as well as providing some gain. In fact, the 747 (if indeed that's what we are discussing) isn't a condensor mic, but an electret design - so it does not need a voltage to polarise the backplate and diaphragm, but solely to power the preamp. Link to comment Share on other sites More sharing options...
Solstace Posted November 11, 2008 Author Share Posted November 11, 2008 Okay I'll put everyone out of their misery... The mic in question is in fact a Shure MX412 - the "side entry" version aimed at neater permanent lectern or pulpit installation. This issue is a continuation of previous threads I've raised about using said mic to drive our induction loop without interference and even feedback caused by said electromagnetic device. So if the unbalanced cable isn't an unusual design feature, then I wonder whether users of AKG 747's (which from what's been said earlier are of a similar design) have faced similar issues. I also wonder whether shortening the length of unbalanced cable will make much of a difference, or whether I should scrap this model of mic and go for something else? Link to comment Share on other sites More sharing options...
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