Topic: 3-wire powering for Audio Technica (and other) mics FAQ & circuit

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[disclaimer] Mods, if I posted this in the wrong forum, I apologize. [/disclaimer]


I have flagrantly copied the format/layout of content from Brian Skalinder’s NJB3 FAQ and whataboutbob's iriver H1xx Tapers FAQ (thanks Brian and whataboutbob!). The data has been collected from various threads/comments here at taperssection.com.

Please note that this is very much a beta version of the FAQ. I’m posting this because it's a great way to compile all known information on this topic into a single thread.

Enormous THANKS goes out to poorlyconditioned for patiently answering a plethora of "n00b" questions from myself and others, as well as for introducing this alternative microphone powering method to this community.

For legal purposes, neither myself nor taperssection.com, can be held responsible for any damage caused to your gear by following the information included in this thread. I will make an effort to keep all the information up to date, but it may become outdated at times. Additionally, particularly during beta, it may contain errors. </legal mumbo jumbo>



[000] Intro
[010] I'm a little confused about how this works, generally, much less specifics about the 3-wire powering.  Where should I start?
[020] The circuit / schematic
[030] I've heard that 3-wire powering isn't very useful for taping. Is this true?
[040] Pros / Cons of 3-wire powering
[050] What about taping quiet shows with 3-wire powering?

[more to follow]



[000] Intro...

The 3-wire powering method is a method of powering microphones, specifically the Audio Technica mics (AT831, AT853, AT933/AT943) as well as modified AKG ck9x mics or modified Nakamichi CM300s ("Franken Naks"). I think of it as a "pseudo-phantom" powering method.

Simply put, microphones like the AT853s do not perform at their maximum in very bass heavy shows or very loud shows when they're run on "2-wire" or "normal" powering (where both microphones terminate to a single mini-plug, a la the typical termination type from Sound Professionals). I ran AT853s > 2-wire battery box (no bass rolloff) > DAT at a club taping a blues artist, and the bass was what I would call *moderate* at best, and my tape is unlistenable. The recording is distorted big time due to the bass and the moderately loud SPLs [Note: I generally don't tape heavy metal shows, to give you some idea.]

You can make fine recordings with the basic "2-wire" powering, and mask/prevent the distortion by using a bass rolloff filter. However, the Audtio Technica mics simply perform to more of their inherent potential with 3-wire powering, i.e. a 3-wire battery box or via phantom powering.


[010] I'm a little confused about how all this stuff works, generally, much less specifics about 3-wire powering.  Where should I start?

In the Where To Begin forum, of course! (http://taperssection.com/index.php?board=46.0)


[020] The circuit / schematic

Below are several circuits/schematics. They're all basically the same, just presented differently.

Courtesy of poorlyconditioned:


Courtesy of graemecogger:

Those are the capsules in the SP-CMC-8, I believe?  If so, this should do the trick:



The ideal value for R1 depends on the caps - on my stereo pair, the ideal values vary by over 20%.  Something around 30k should be fine, though.
The values for C1 and R2 are not critical - as chosen, the capacitor will give no significant bass rolloff into any reasonable load.  R2 simply prevents the output from having a DC voltage on it prior to plugging in.

Obviously this needs to be duplicated for the 2 channels.

HTH

My own color-coded drawing:



[030] I've heard that 3-wire powering isn't very useful for taping. Is this true?

It depends on what you're recording. Using a 3-wire battery box isn't as needed if you only record acoustic and/or unamplified music. Also, if you run a phantom power supply unit (Denecke PS-2, etc.) then you already have the benefit of the mics handling higher SPLs without premature distortion.

poorlyconditioned made a very nice summation of phantom powering:

"Phantom power modules do two things: They give a low voltage (3 to 9V) to the electret elements, in 3-wire configuration.  But also they provide a low impedance balanced output, using a transformer, or some transistor circuit.  If you're doing long runs and/or you need to connect to an XLR mixer input, you want the phantom power.  However, for us tapirs, we're probably running from our hat to our pocket, and we're probably running unbalanced lines anyway, so the [3-wire] batt. box is fine.

For short wire runs and high impedance mic inputs they are identical.  For longer runs, the AT/Samson adapters provide "buffering" or strengthening of the signal (via a transistor driving an output transformer).  This provides a balanced and low impedance output.  Balanced is good for long runs (>50' or so) and low impedance means you can drive any kind of input.  Notably, you could put a step up tranformer there and drive a line input!"

Thus, phantom powering is nice if you plan on using very long mic cable runs, or have a nice preamp you want to use with the mics (MP-2, UA-5, Sonosax, etc.). Two advantages of a 3-wire battery box are: 1) it's usually much smaller than any phantom power supply unit. (See pictures below, courtesy of a post by MLKLuke), and  2) this method does not require Samson PM4 or AT8533 power module adaptors (whereas phantom powering does require these modules).

Denecke PS-2 vs. 3-wire batter box - Pic 1
Denecke PS-2 vs. 3-wire batter box - Pic 2
Denecke PS-2 vs. 3-wire batter box - Pic 3


[040] Pros / Cons of 3-wire powering

Pros:
* Less distortion, particularly at high sound levels
* Generally speaking, no need to worry about bass rolloff

Cons:
* slightly lower gain, i.e. mics aren't as "hot" into preamp/recorder (you will probably have to turn up the gain on your recorder)... the mics will provide an estimated output of around 6 to 10 dB lower running on 3-wire powering
* nonstandard wiring


[050] What about taping quiet shows, like acoustic or unamplified performances, with 3-wire powering?

There are several possibilities in this situation:

1) Do nothing, i.e. just boost the levels in post.
2) Turn up the gain/recording levels on your recorder/preamp.
3) Buy/build a 3-wire preamp.
4) Use a dual miniXLR > mini-plug cable (a.k.a. "Y-cable"), and run the mics on plug-in power from the recorder, or with a standard 2-wire battery box.


[more to follow]