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Microphone Powering Schema

Filed under: history,live sound and recording — 2009-03-04 by Richard L. Hess — Last Edit 2009-03-11 by

In addition to my business of transferring tapes, I also record a few community groups for fun. Someone recently gave me an older Sennheiser MKH-804 interference tube (“shotgun”) microphone. Finding information on how to connect it proved more difficult than expected.

In the 1960s, transistorized microphones from AKG, Neumann, Schoeps, and Sennheiser became available. There are several niches of early microphone powering that continued on for many years. Perhaps the easiest way to look at it is backwards.

I have adopted the terminology “hot” and “low” to refer to the audio signals in a balanced line. Hot is defined as the voltage going positive in respect to the low lead with a positive pressure at the microphone diaphragm. I’m using “hot” in this context rather than “hi” because so many people have heard of “pin two hot” or “pin three hot” that I wanted to be consistent with that nomenclature.

PHANTOM POWER

Today, 48 V phantom powering is almost universal. In phantom powering the positive voltage is fed through a pair 6k81 ohm resistors, one to each modulation lead. The matching of these resistors is often done to 0.1% to maintain common mode rejection. The negative power runs on the mic shield. XLR: Pin 1-shield; Pin 2-audio hot, +48 V; Pin 3-audio low, +48 V. Tuchel: Pin 1-audio hot, +48 V; Pin 2-shield; Pin 3-audio low, +48 V. This was standardized in the 1960s in DIN Standard 45596.

A caveat about phantom powering voltages. There are a wide range of microphones that will work with phantom voltages from 9-52 V, but many that are rated at 48 V will not work well outside of the +/- 4 V tolerance in the standard. AKG, Audio Technica, and Schoeps, for example, make many 9-52 V powered microphones, while DPA, Neumann, and Sennheiser mics generally need 48 V. Some devices (e.g. the first version of the MicroTrak digital recorder) had an odd 30 V phantom that probably worked with a number of mics, but might have degraded their performance. M-Audio took this to heart and the MicroTrak II has real 48 V phantom power. There once was a 24 V phantom power option in the standards, but apparently it was never adopted in practice and it has since disappeared.

Prior to standardization, in 1964, Schoeps produced the CMT-20 microphone which used negative 8.5 V phantom power. The CMT-200, according to Schoeps drawing SB316, dated 1964-10-14, used the same -8.5 V phantom. Later this was broadened to negative 8-12V phantom followed by the switch to positive phantom at some later point. With vintage microphones, at least from Schoeps, be very careful as they might be negative phantom.

For more details about phantom power, please see Rick Chinn’s page here. This page on Wikipedia has some further history.

NOTE: You can use a polarity-reversal cable with phantom power, but not with any of the other schema.

CAUTION: Phantom Power can damage some battery-operated microphones like the Audio Technica AT822 stereo mic. It can also damage ribbon microphones which are not floating (i.e. those that have their centre tap grounded).

T or AB POWER

Moving backwards in time, the next most widely used microphone powering scheme is called “AB power” “T-power” or “Tonader Power” and is standardized in the 1960s in DIN standard 45595. In this design, both the power and the audio are on the same wires. The hot audio and the positive power is on one conductor while the low audio and the negative power is on the other conductor. The shield is just a shield in this scheme. This was widely used by Sennheiser and Schoeps in the film industry. Neumann made a FET-70 series that used this powering scheme and many of the mics in that series are the same as the much more widely known FET-80 series (as in U-87 and KM-84, for example). Rick Chinn has more information here.

If you have T-powered microphones, you can power them off phantom power WITH THE APPROPRIATE ADAPTER.   These adapters can be purchased or made. Rick Chinn has a transformer design and a transformerless design. I developed a similar transformerless design,   but used 680R resistors instead of the 4k7 resistors that Rick used and I placed a 180 ohm resistor between the zener and the filter capacitor to reduce the noise of the zener even more.

T-Power, as introduced by Schoeps with the CT100 series in 1965 was wired to XLR connectors with the hot/+ connected to pin 3, following the Ampex standard. This polarity practice is documented on the same Schoeps drawing referenced above. Sennheiser, to the best of my knowledge, always connected the hot/+ to pin 2, which became the international standard. Many Nagra recorders came with pin 3 hot, but I believe they could be ordered either way. Sennheiser introduced this powering scheme in late 1963 based on catalog research by Lonn Henrichsen.

It got to the point where there was the term “red dot” microphones which had been rewired for pin 3 hot/+. If in doubt, this is the one legitimate use of a polarity reversal cable with T-power. Sennheiser adopted T-Power with the MKH-105/405/805 microphones in the mid 1960s. They later provided “P48″ versions of these microphones and at that point also designated the T-powered mics with a “T”. So there could be an MKH-416T and an MKH-416P48 which differed only with the powering. This appears to have been introduced with the XX6 series of microphones.

CAUTION: T-Powering WILL damage ribbon and may damage dynamic microphones.

EARLY ANOMALIES

In addition to the Schoeps NEGATIVE phantom mentioned above, Sennheiser introduced their first line of RF condenser microphones with an unbalanced, negative power connection. The MKH-104/404/804 have odd wiring. Pin 1 of the Tuchel connector is audio output. Pin 2 is ground (audio low and power +). Pin 3 is -8 V power. This series of microphones was introduced in 1963 and discontinued between the 1968 and 1969 catalogs.

Later, Sennheiser introduced the extended-low-frequency special-purpose microphones, the MKH-110, with the same powering scheme, except Pin 3 is +8 V power!

It is trivial to power an MKH-x04 microphone from a 9 V battery, and after having noise problems with an inexpensive off-the-shelf DC-DC converter, I ended up with an alternate powering scheme, described here.

I’ve decided that this will be an application to keep in the Tuchel connector domain, so this oddball powering doesn’t get into any other microphones and possibly fry them.

CAUTION: These powering schema can damage many types of microphones.

CONCLUSIONS

While this may sound complex, in practice certain combinations of mics/recorders are used together and it’s fairly trivial to keep track of what’s what.

For example, for almost four decades, I’ve had AKG C451 microphones in my kit, and I’ve adapted almost everything to power them. Other mics that take 9-52 V phantom obviously will work there as well. I’ve had several (currently three) Sennheiser MKH-416T short shotgun mics for about a decade (I got them used). I made a P48-T12 adapter for each of them, and I keep them with the mic. When I grab the mic, I grab the adapter. Sometimes it goes right on the mic, other times it goes at the end of the first mic cable coming from the mic. Placing this at the mixer increases the risk of damaging other microphones.

When I first got the MKH-416T mics, I made a stereo powering box that had a toggle switch that could select T12 or P12 so I could use one box with a pair of short shotguns or C451s. This had unbalanced outputs (all resistor-capacitor networks, not transformers) that connected to the portable DAT Walkman recorder and used 8 AA batteries for long running time.

I recently got an AKG C460/CK63 and that’s still 9-52V.

Some of the newer mics in my collection (Neumann TLM-103, KMS-105; DPA 4006 TL) are P48 only and, in fact, most of the new equipment I have has true 48 V powering. This includes a Sound Devices 722 portable recorder, a MOTU 828MK II multichannel FireWire audio interface for my laptop, a Mackie 1402VLZ mixer, and a Shure FP410 mixer. The church I do sound for has a Mackie 1604VLZ that I previously owned, so P48 is very common in my world. People who have used Nagras in the field report adapting everything to one scheme. In one case it was the “red dot” mentioned above.

All in all, don’t be afraid of some of the oddball powering schema, just work through what is needed. Since all of these schema are low-powered, the likelihood of any significant damage to a microphone is probably low, but still, don’t take chances with expensive, excellent performing antiques.

It is my understanding that European broadcasters at least influenced the various powering schemes by requiring compliance to their specific powering standard across several manufacturers. When I started working at ABC-TV in New York City in the early 1970s, there was a system of remotely powered microphone preamplifiers built into extended length female Cannon UA cable connectors. So this is yet another odd scheme, although it was an accessory to the microphones rather than in the microphone itself. This preamplifier was used to boost the signal level at the microphone in an attempt to overcome hum and noise on the long lines.



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