Quote:
Originally Posted by Jim Williams  Don't apply 48 volts to these. Any mismatch of the phantom distibution resistors will cause one end of the capsule's coil to be a higher voltage potential than the other, possibly frying the coil wire. Most phantom resistors are 6.81 k 1%. This could let around .25 volts difference from one end to another. If one matches these resistors to .1% that would keep the potential down to 25 mv or less.
The other effect would be to push/pull the capsule out of it's idle position, sort of like when dc offsets push/pull a speaker in a power amp.
Best to be attentive and keep that phantom switch off at all times.
Protect the Unidyne III's!
Jim Williams
Audio Upgrades |
Many many apologies for resurrecting an old thread. Wouldn't have done it except I got pointed to Jim's reply from another site here:
Shure SM57 (RecordingHacks.com) where this post is linked to to expand on the statement there that phantom power may damage a tranformerless SM57.
As a newbie to this forum I ask this question with some trepidation..but ask it I must. Where, in an electrically isolated capsule, as is the case in a balanced microphone circuit, can current flow? With no current flow, irrespective of the source resitance of each leg, the voltage (to ground) is 48v. Both sides of the capsule surely are at 48v and hence no current can flow through it, and no damage can be done.
If current flow could occur (due to mismatched resistors or whatever) then the situation with a dynamic mic WITH a transformer might also be a problem. These tiny transformers might be fully or partially saturated by the DC current flow Tim refers to above. This surely would have a substantial effect on the audio quality of the mic's output.
Quote from the Shure website "A balanced dynamic microphone is not affected by phantom power. However, an unbalanced dynamic microphone will be affected. Although the microphone will probably not be damaged, it will not work properly."
(source
Shure - Phantom Power and Bias Voltage: Is There A Difference?)
If the microphone was wired for unbalanced use, ie fitted with a 1/4 mono jack, phantom power might be an issue - assuming of course it was somehow possible to plug it in to the phantom powered source. Not something that's likey to be done.
Anyone got any thoughts?