Is 28V Phantom Power acceptable?

pchorman

pchorman

New member
I just inherited an old Shure M267 Field Production Mixer, as it was called, and noted that its Phantom Power (strangely labeled "Simplex") is down a pint in voltage. After sticking voltmeter probes down its throat I see that it only delivers about 28 VDC. Is this ok? Most condensor mic specs I recall seeing call for 48 V, a considerable difference.

Thanks for any insight you can offer.
 
Although the mic may work as low as 15V, what type of performance can be expected from mic that claims to need 48V + or - 4V? Would there be any type of signal degradation, like increased noise, less output, or inability to handle the normal SPL it's rated at?
 
Actually any electret condenser mic will operate perfectly with 9-15 volts. There are many electret mics from the $35 Behringer ECM8000 to a $1000 AKG C414.

People often don't realize just how many mics are electrets, here are just a few.

AKG C1000 C2000 C3000 C4000 C414B-ULS and C414 TLII

Audio Technica AT3035 AT4033 AT3031 AT3032 AT4041 Pro 37R

Shure KSM32 SM81 SM94

If you have an externally polarized condenser you should have a phantom supply that puts out a proper 48v. LooneyTunez answered his own question, a mic that requires 48v will not function at it's best with less than proper voltage. More noise, less headroom, and generally lower quality sound will be the result if you have substandard phantom power with a true (non-electret) condenser mic. Most of the current Chinese condensers are not electret mics and will therefore need proper powering.
 
Thanks for the replies. I was hoping to use this "phantom power" source for a Marshall MXL603s that I ordered, but it seems that there will be serious drawbacks. Why would any manufacturer put in a lower voltage supply? strange
 
Because the mixer might well have been designed with electrets in mind. To my mind, "Field Production" means "disposable, because it's gonna get rained on"... For that application, you probably wouldn't expect to be running with super-high-quality studio mics!

Any true condenser mic that uses the phantom voltage to polarize the capsule will have significantly reduced output and headroom with the lower phantom voltage, and a lower-signal-to-noise ratio in most cases. However, as others have pointed out, a lot of the electret condensers won't care at all- and for this mixer, you'll just need to tune your mic choices accordingly. There are some fine electrets out there these days...

There are a number of products that put out substandard phantom voltage: that Behinger battery-powered portable mixer, the Midiman Audio Buddy, and many others. And people still get usable results on them even with true condensers. Let your ears decide: every mic will behave differently when run outside the published specs!
 
Exactly Skippy, phantom is a place that manufacturers can skimp to save a couple of dollars but now that there are so many cheap externally polarized (non-electret) mics around I think you will see less low cost micpreamps offering substandard phantom.

In the past most home recordists were using either dynamics or electret condensers so 48 volts just was not in issue for many people but things are certainly changing now.
 
JM350 said:
Actually any electret condenser mic will operate perfectly with 9-15 volts. There are many electret mics from the $35 Behringer ECM8000 to a $1000 AKG C414.

People often don't realize just how many mics are electrets, here are just a few.

AKG C1000 C2000 C3000 C4000 C414B-ULS and C414 TLII
Click to expand...
Are you certain that the 414s are electrets? The AKG site claims that the 414s are externally polarized condenser mics.
 
JM350 said:
People often don't realize just how many mics are electrets, here are just a few.

AKG C1000 C2000 C3000 C4000 C414B-ULS and C414 TLII

Audio Technica AT3035 AT4033 AT3031 AT3032 AT4041 Pro 37R

Shure KSM32 SM81 SM94
Click to expand...

no version of the 414 is an electret, sorry.
 
I was hoping that "Field Production" implied some ruggedness and the ability to short out everything and pee all over it without destroying the thing.

Damm Skip, you're breaking my heart. First you insult this inherited old piece of defunct junk, then you remind me that even the Audio Buddy which I ordered last week is also down a pint in supplied voltage. I just downloaded the manual on the Audio Buddy and note that it doesn't provide a spec for phantom power. Is that plain sneaky, or did I overlook something? How low is its voltage?

As for those who feel that lower voltage implies "skimping" (JM350), I don't see this as a means to cut corners. Omitting a few dozen turns around the transformer secondary to drop the output 20 volts or so doesn't equate to any meaningful savings by the manufacturer. This thing was apparently created at a time when lower voltage was more commonplace, at least for the Shure brand of condensors for which it was probably geared.

My last question on this subject: can anyone give me a feel for how bad the degradation might be using 28V for condensors (like the MXL 603s) where 48V is preferred? Of course there's no hard and fast rule and no objective way to answer, but would you guys even bother trying it, in a bind, or will it clearly result in lousy mic performance? I have no experience messing with condensor mics.


Thanks again
 
Harvey Gerst said:
Are you certain that the 414s are electrets? The AKG site claims that the 414s are externally polarized condenser mics.
Click to expand...

The 414's according to AKG run on 9-52v for the TLII and 12-52v for the B-ULS so they must not be externally polarized, the BULS does say externally polarized while the TLII seems to say nothing. Akg seems to go out of their way to be deceptive about their other mics being electrets, so it is not surprising they would also be about thier pride and joy. These guys are pretty blatent about it however.

http://www.bswonline.com/public/prod_item.asp?item=C414BULS

I guess people just can't believe a mic that expensive and as popular in studios as the 414 could be an electret but it certainly answers the question of why a 414 sounds the way it does.
 
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there have always been different standards around for phantom power, one is 48V, another is 24V. Many mics work perfectly well with lower voltages, even some externally biased condensors. AFAIK either Schoeps or MBHO offer a preamp body for their capsules that works with supplies as low as 12V.

On another forum there is a thread about a tascam mixer that puts out 64V phantom. This is far more severe, because this way you might damage the microphone.
 
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Some mics use built-in voltage doublers and triplers to get to the desired polarizing voltage. Considering the low current involved, it's not a difficult task.

Even though AKG has been sneaky in the past (along with Audio Technica) in identifying which mics are electrets and which mics are externally polarized (not to mention clearly stating actual diaphragm sizes), it's hard for me to believe that AKG would print in their specs that the 414 is an externally polarized mic when it was really an electret.
 
The 414s are true condensers, but ISTR that they have an internal capsule bias generator that steps the voltage up to on the order of 60V to bias up the capsule. If that is the case, they may well be able to handle phantom down to a low voltage. There are a number of high-end mics that do this. Not all true condensers just use the phantom power directly to bias the capsules- and at the top end of the scale, very few do (since a higher capsule bias voltage makes it easier to achieve low noise figures).

The Audio Buddy puts out a nominal 36V phantom. It runs a voltage quadrupler off its 9v input. That's what happens in a lot of low-cost products: you just make a little DC-DC converter (a charge pump, for example, good for a few tens of mA) to step up a convenient voltage you already have available. That's a key point for wall-wart powered devices, which never have more than 1 supply rail available...
 
Harvey Gerst said:
it's hard for me to believe that AKG would print in their specs that the 414 is an externally polarized mic when it was really an electret.
Click to expand...

It would actually be a scandal. I am shure that they couldn't get away with this for such a long time. The competition would immediately jump on this and try to sue them.
 
Just did some checking, to bolster my failing memory. Yes, the C414s are indeed true condensers. Many of the variants did have internal voltage converters that would allow them flexibility in phantom supplies down to 9v. The ones that didn't are so designated, like the C414EB/P48, which does indeed require the full-on 48v to work.

The P48 mics require less power to operate, since they lack the voltage converter. But they are the real deal...
 
skippy said:
Just did some checking, to bolster my failing memory.
Click to expand...

so did I...
there are 3 specifications for Phantom Power, P12, P24 and P48. The latter is the most commonly used nowadays, while P24 has disappeared (the original Neumann 170 is one of the few mics requiring 24V phantom voltage). But there are many mics that are designed for P12 (12V through two 680Ohm resistors) that work perfectly well on a P48 (48V through 2 6.81k resistors) if the supply provides up to 10mA current.
 
skippy said:
The 414s are true condensers, but ISTR that they have an internal capsule bias generator that steps the voltage up to on the order of 60V to bias up the capsule. If that is the case, they may well be able to handle phantom down to a low voltage.
Click to expand...

Interesting, so if the mic increases 48v by 25% up to 60v then if the phantom offered only 9v then the mic would bump the voltage up 25% to 10.5v.?

AKG themselves claim the 414's will work on as little as 9 or 12 volts and even bumped up 25% this will still only be either 10.5 or 15 volts which still seems awfully low for a non-electret.

If the voltage were raised two or three times by the step-up in the mic wouldn't that give you 96 or 144 volts when powered by a 48v supply? Now that seems too high to be usable?

Maybe those guys at BSW were just figuring as I was that a mic that claims to function between 9 and 52 volts must be an electret but AKG is pretty sneaky about thier other mics, it would be interesting to know what the real deal is.
 
You're assuming that the voltage scaler is a straight doubler, quadrupler, or whatever. I suspect that it is more sophisticated than that- that there is some regulation involved that allows the mic to acommodate a wide range of voltages. Even if they just used a simple zener to drop the voltage to 9V (regardless of the input voltage), and then scaled it up with an 8x charge pump, they could get (for example) a 72V supply from any arbitrary input voltage 9V or higher... There are a lot of ways to skin this cat. I just don't know which one they chose.

I'll now step out of the way and let Harvey take over- I'll bet a beer that he's digging through his documentation even as we speak, and will be back with the definitive result. But the early C414s shared their capsule with the CK12, and even the late ones with the BULS teflon capsule are still true condensers. I never had occasion to open up the pair of EBs I owned in the '80s, so I don't have the schematics- but that's the way I see it...

I don't think AKG is scamming us, on this mic anyway. These are well known items, and their voltage flexibility is not a reflection on their capsule design. Their price sures is a reflection of their complexity, though!
 
Going back to pchorman's original concern, is there anything that can be done to modify the lower voltages in these phantom powered units that are supplying less than optimum voltages? I myself am using a M-Audio Omni Studio, and have heard that it's mic pres are based on the DMP2 (which is known to have less than 48V). I haven't been able to find my multi-meter, so I can't say for sure if I have less than 48V...but if it is indeed the same as the DMP2, then my main concern is getting optimal results when using my MXL v67's and my soon to be purchased 603s. Bottom line, is there a way to modify the the supply voltages without too much hassle? I'd be very willing to modify my Omni Studio if it turns out that it's supply is lower than say 44V.

Thanks for any input you guys can provide.
 
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