Lt. Bob
Spread the Daf!
kinder and gentler ...... nothing bad intended.
Bobbsy
Boring Old Git
2-pin mikes are happy as long as they get a couple volts. Typically I'd set up a supply somewhere near the max voltage fed via a resistor around 2.2 K ohms (though this isn't all that critical. More voltage makes little or no difference in terms of headroom or dynamic range, it's the output level that changes (and this only slightly.
On another topic, just to expand on what ecc83 said about phantom power working on ANY 3 pole connector (but not being recommended):
This, of course is totally correct but I think it's worth a sentence or two on why it's not recommended to avoid any accidents. Phantom power feeds the same voltage on pins 2 and 3 of and XLR which would translate to the Tip and Ring on a TRS. If you think about how a balanced circuit works, this means that the phantom voltage cancels itself out once one leg has it's polarity inverted (any mic needing phantom having pulled off the voltage before the polarity inversion). This is why it's generally safe to plug a dynamic mic into a phantom power source without damage to the mic.
This all works because, on an XLR, all three pins make contact at the same time. With a TRS though, there is an instant when plugging that the tip and/or ring may be touching the wrong contacts inside the connector. For example, the tip will touch the contact for the ring as it passes through. This'll put the phantom voltage on one leg only so anything connected will get a brief jolt of power. This isn't automatically death to a dynamic of course--but it's bad procedure and why phantom tends to be restricted to XLRs only. (Anecdotal aside--I once DID see a dynamic mic--a cheaper Sennheiser from memory--damaged when somebody plugged it into a mixer channel strip with phantom turned on. Turned out the guy's XLR cable had cheap connectors, pin 3 on the male end had been pushed slightly into the body, and pin 2 was carrying a full 48V phantom. Not the most likely of accidents--more a case of the holes in a Swiss cheese all lining up at once--but it shows what can happen.)
On another topic, just to expand on what ecc83 said about phantom power working on ANY 3 pole connector (but not being recommended):
This, of course is totally correct but I think it's worth a sentence or two on why it's not recommended to avoid any accidents. Phantom power feeds the same voltage on pins 2 and 3 of and XLR which would translate to the Tip and Ring on a TRS. If you think about how a balanced circuit works, this means that the phantom voltage cancels itself out once one leg has it's polarity inverted (any mic needing phantom having pulled off the voltage before the polarity inversion). This is why it's generally safe to plug a dynamic mic into a phantom power source without damage to the mic.
This all works because, on an XLR, all three pins make contact at the same time. With a TRS though, there is an instant when plugging that the tip and/or ring may be touching the wrong contacts inside the connector. For example, the tip will touch the contact for the ring as it passes through. This'll put the phantom voltage on one leg only so anything connected will get a brief jolt of power. This isn't automatically death to a dynamic of course--but it's bad procedure and why phantom tends to be restricted to XLRs only. (Anecdotal aside--I once DID see a dynamic mic--a cheaper Sennheiser from memory--damaged when somebody plugged it into a mixer channel strip with phantom turned on. Turned out the guy's XLR cable had cheap connectors, pin 3 on the male end had been pushed slightly into the body, and pin 2 was carrying a full 48V phantom. Not the most likely of accidents--more a case of the holes in a Swiss cheese all lining up at once--but it shows what can happen.)
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E
ecc83
Well-known member
No capacitor mic gets the full 48volts. Take 5mA as a fair operating current? That leaves us with 31V DC after dropping through 3.4k (two 6k8 spook feed Rs) . That is plenty for decent amplifier output but maybe a bit low for the polarizing voltage? The better cap' mics include a DC-DC converter to boost the supply to a higher voltage for the capsule. Duggy Self shows his design in Small Signal Amplifiers where it develops 63V for the capsule (I won't rip the circuit. Buy the book! We have few enough good technical writers I won't limit their income!).
The polarizing voltage will indeed have a direct effect on headroom but then the diaphragm spacing is minute and with higher voltages goes the risk of shorting on high dispacements. Then, different poalrizing voltages will change the diaphragm tension and alter the response! Little wonder it took SO long to get really very acceptable capacitor mics at stupid prices!
(Oh. And cool Bob, cool, ta')
Dave.