By Ray Rayburn
Ray Rayburn tests two battery powered phantom power supplies. This article shows his procedure and the results.
In the “Standalone 48VDC power supply” thread, the Core Sound 2Phant was mentioned. Core claimed about their unit:
“Unlike all other battery-powered, two-channel phantom power supplies that can deliver only two to four milliamps per channel, 2Phant can power every microphone on the market that specifies P48 phantom power including some of the hardest-to-power microphones from Earthworks, Schoeps, AKG, CAD Equitek, Shure KSM-series and Neumann.”
Since I own two different battery powered phantom power supplies I decided to test them. My usual test for phantom power involves a dummy load of two 6.8 k ohm resistors mounted on a male XLR insert. I first measure the open circuit voltage which should be between 44 and 52 volts. Then I plug in the dummy load and the voltage should drop to half the original voltage if the feed resistor values are correct, and the internal 48 volt supply voltage does not droop. For a multiple input device I then go on to put a dummy load on each additional input, until I have a dummy load on every input. Many devices that claim to be able to supply P48 phantom power can’t deliver the 7 mA per input required by my dummy loads, not to mention the full 10 mA required to meet the IEC Standard.
The latest edition of the IEC 61938 Phantom Power Standard specifies 5 versions of Phantom power:
- P12L for low power applications
12 V +/- 1 V
8 mA max
4 mA rated
3300 ohm feed resistors
12 V +/- 1 V
15 mA max
15 mA rated
680 ohm feed resistors
24 V +/- 4 V
10 mA max
10 mA rated
1200 ohm feed resistors
- P48 – the recommended style
48 V +/- 4 V
10 mA max
7 mA rated
6800 ohm feed resistors
- SP48 – “super power” used by some high end studio microphones
48 V +/- 4 V
22 mA max
22 mA rated
2200 ohm feed resistors
The first supply I tested I will call “Brand X”. It runs from two 9 volt batteries, and powers two microphones. It was switchable from 12 volt phantom to 48 volt phantom. Set for 48 volts the open circuit voltage was 52 volts. Connecting one dummy load the voltage across the load was 21.2 volts. Since this was well under half the open circuit voltage, either the source resistors were higher in value than correct, or the 48 volt power supply voltage was drooping under load.
I then connected a second dummy load to the second channel, and now the voltage across both dummy loads went down to 15.9 volts. This was a clear indication of a drooping 48 volt power supply.
I did not have dummy loads for P12, so I just measured the open circuit voltage which was 14 volts. I was curious how the switching from P48 to P12 was handled since the value of the feed resistors would need to change so drastically. Taking the power supply apart, I found that the feed resistors were 5600 ohm and did not change when the unit was switched from 48 to 12 volt modes. The feed resistor values being lower than correct for P48 helped compensate to some degree for the drooping supply voltage, but even still the loaded voltage fell way to far. In the 12 volt setting the feed resistors were over 8 times the correct value for P12 and 1.7 times the correct value for the low power P12L. In short the 12 volt mode of this supply is unusable.
The second battery powered phantom supply I tested was a Deneke PS-2 two channel 48 volt phantom power supply that runs on a single 9 volt battery. This supply is more expensive than the Brand X supply, but is much more rugged, and smaller. Inserting a 9 volt battery the open circuit voltage was 52 volts. Connecting a dummy load to one channel produced a voltage of 25.6 volts or almost exactly half the open circuit voltage. Connecting a second dummy load to the second channel resulted in a voltage of 25.2 volts across both channels. This showed that the internal 48 volt power supply drooped very little under load, and could drive the full rated current for P48 into a pair of channels at the same time. Opening the case showed the phantom power feed resistors were indeed the 6800 ohms specified by the Standard.
This shows that Core is incorrect in their claim that “Unlike all other battery-powered, two-channel phantom power supplies that can deliver only two to four milliamps per channel, 2Phant can power every microphone on the market that specifies P48 phantom power”. The Deneke PS-2 can deliver the full rated power as well.
Ray A. Rayburn – AES Fellow
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