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Phantom Power

Will accidentally applying 48 phantom power damage a lectrosoncs' receiver's output?

Way back with the early CR185's (VHF compact receiver), when dinosaurs roamed the earth, we lost a few output stages…

Way back with the early CR185's (VHF compact receiver), when dinosaurs roamed the earth, we lost a few output stages to cheap mixers that had a single switch for all or nothing phantom power. We decided that we couldn't expect our customers to always be 100% perfect (we do expect 99.8%) and so we protected the output stages with bridge diodes. However, if the diodes were conducting and protecting the receiver, the signal was killed until the phantom power was removed. Some customers didn't realize what was happening, and returned perfectly good units to us for repair. That's when we realized that 99.8% of the customers didn't read the manual. Near perfection of another kind. My apologies to those two people who did read the manual. I also might mention here, that I never read a manual until I've got the device operating. It's an ego thing.

So, we added a resistive series circuit to protect against capacitive discharge, resistive bleeders to ground to reduce peak voltages and non-polar capacitors for protection against mis-wiring (pin 2 to pin 1, etc.). We have had no output stage failures since we put in all the gimmickry. We have lost an output stage or two when it appeared the outputs got wired up to 110Vac in some manner. This can usually be spotted by the large quantities of charred circuit board.

None of this is mentioned in an FAQ because we don't consider phantom power a problem. I will add this note, since I haven't done an FAQ in a long time and it is a good question to ask.

Posted 10 months agoby LectroAdmin
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Does the UH400 have DIN standard phantom power? Is the 48 Volt setting actually 48 Volts?

It is a gentle statement to say that our phantom wiring is merely unconventional. The UH series has always had…

It is a gentle statement to say that our phantom wiring is merely unconventional. The UH series has always had an unbalanced input. Mics used with the plug-on units usually go right into the unit and a balanced input was unnecessary. Given that, a balanced phantom feed would be a waste of effort. So pin 3 has phantom power on it but audio is tied to ground with a series 500 Ohm resistor in series with a 20 uF capacitor. Pin 2 is audio in (hot) and is tied to the phantom voltage with a 1k resistor and goes to an amplifier with a 1k input impedance. (See FAQ Is the XLR input to the plug on UH transmitters a true balanced input?)

If that isn't enough, the 48 Volts is really a regulated 42 Volts to compensate for our lower impedance feed resistors. Forty two Volts is an approximation of what the usual 2 mA, 48 Volt mic "sees" from a 48 Volt supply due to voltage drop in the DIN specified 6.8k feed resistors. One of our goals was to be able to operate higher current pro mics. With the 1k feed resistors and 42 Volts , we can provide 7 mA with reasonable voltage drop. Just don't think we cheated you on Volts if you measure the phantom voltage with a meter without a mic load. 

There is yet another facet to our madness. The lower voltage and reduced resistor loss means less power has to be supplied by that overworked 9 Volt battery that you have to buy. By using constant current diodes instead of larger lossy resistors the supply noise is still well filtered but we don't have big power losses. This arrangement also meant we didn't have to use huge 63 Volt capacitors to filter out supply noise on a 48 Volt supply. We can use 50 Volt parts on the 42 Volt lines. The inside of the UH400 is really packed and figuring out how to get around those large caps is one of the things that kept us from doing phantom power years earlier.

Finally, by using lower value feed resistors, we can accommodate some older pro mics that want 12 to 15 Volts at about 10 mA . Doing this much current with 6.8k resistors would waste a lot of valuable battery power. In fact we make a T-power adapter that takes advantage of our capability of delivering relatively large currents at low voltage

Posted 10 months agoby LectroAdmin
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What happens if 48 Volt phantom power is accidentally applied to the balanced output of a receiver?

The 185 series has diodes to the power supply to protect the output from 48 Volt phantom power or other…

The 185 series has diodes to the power supply to protect the output from 48 Volt phantom power or other high DC voltages on the output jack. If the voltage on the output line exceeds 12 Volts, the diodes turn on and shunt the excess voltage. Since the diodes are now conducting, they also shunt (kill) the audio. The solution is to remove the 48 Volts. Some years after the first 185 design, we ran into mixing boards (poorly designed IMHO), where the 48 Volt was on all the mics or none of the mics. We redesigned the protection to either include non polar capacitors or shunt resistors to ground as well as the diodes. The newer designs prevent the audio from being shunted. This is what you will find on all receivers after the 185 series. It is possible to change the output on your 185, but I recommend just removing the 48 Phantom power when necessary.

Posted 10 months agoby LectroAdmin
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