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Phantom Power Source Help

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Phantom Power Source Help

Postby nsg3 » Wed Mar 27, 2019 9:56 pm

I just purchased as Behringer Xenyx802. In case you're not familiar it has two inputs for microphones and built in phantom supply for both, you can switch on or off. I'm not having any problems, but before I bought this mixer I already had 2 phantom power supplies, 1 for each mic obviously.

My question is there any benefit of me running the microphones through the stand alone phantom power supplies into the mixer and turning the phantom power off on the mixer?

Versus just plugging them both into the mixer and using the mixer's phantom power.

Hopefully that's not a stupid question, I considered that the mixer using more power could some how add noise if it starting getting hot or something.
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Re: Phantom Power Source Help

Postby John Willett » Wed Mar 27, 2019 10:43 pm

If you already have a couple of good phantom power supplies - personally, I would switch off the phantom on the Behringer and use the dedicated supplies. :thumbup:

Many cheap mixers can't supply the full ampage required and doing it as above wouykd give you the best quality. :thumbup:
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Re: Phantom Power Source Help

Postby Wonks » Wed Mar 27, 2019 10:49 pm

My mistake!
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Re: Phantom Power Source Help

Postby nsg3 » Wed Mar 27, 2019 10:51 pm

Thanks I thought they could still be useful.
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Re: Phantom Power Source Help

Postby Wonks » Wed Mar 27, 2019 11:08 pm

However phantom power doesn't put any significant extra load on the mixer. A typical capacitor mic will draw maybe 3mA. At a full 48v that's less than 0.15 W, but as the voltage on these circuits is reduced significantly by the current loading, you are probably looking at nearer 0.05W power draw per mic.

So there's no danger of phantom power causing any mixer overheating if you do use it.
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Re: Phantom Power Source Help

Postby John Willett » Wed Mar 27, 2019 11:38 pm

Wonks wrote:However phantom power doesn't put any significant extra load on the mixer. A typical capacitor mic will draw maybe 3mA. At a full 48v that's less than 0.15 W, but as the voltage on these circuits is reduced significantly by the current loading, you are probably looking at nearer 0.05W power draw per mic.

So there's no danger of phantom power causing any mixer overheating if you do use it.

The phantom specs state that phantom power should be capable of supplying 10mA per mic.

Many cheap mixers can't supply this, also some only supply a lower voltage.

Behringer do not list the specs on the mixer, so I can't check.

The mixer is unlikely to overheat, but the performane of the mics could be compromised.

I would definitely use the external supplies if I was in that position.
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Re: Phantom Power Source Help

Postby ef37a » Thu Mar 28, 2019 5:29 am

As Wonks says, the power dissipated internally by the phantom power supply is minute, not a consideration (the LED level display probably use 3 times as much!)

There is nothing to be gained IMO in using the external supplies, how do you know they are any better than those in the mixer? I have bought 48V supplies that were well down on the 10mA spec.

The single mic input 502 mixer did originally have just 12V supplies but I am pretty sure they booted that up to 48V on later models.

The 802 has always had 48V and though I don't know the current delivery I have run a Sontronics STC-2 LDC on it and a pair of AKG P150s without any problems I was aware of.

But, mA capability is but one factor in a good phantom supply. Noise is another vital area. I can say the Behringer pre amps are very low noise and spook juice make no difference to the noise floor.

When it be daylight, if it is dry AND if I feel up to it I shall load test my 802 for thee!

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Re: Phantom Power Source Help

Postby Wonks » Thu Mar 28, 2019 7:20 am

John Willett wrote:Behringer do not list the specs on the mixer, so I can't check.

If you look at the pictures of the mixer, the writing by the phantom power light says +48v, so it's not a reduced voltage supply (like the cheapest Behringer mixers have).
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Re: Phantom Power Source Help

Postby ef37a » Thu Mar 28, 2019 8:03 am

Wonks wrote:
John Willett wrote:Behringer do not list the specs on the mixer, so I can't check.

If you look at the pictures of the mixer, the writing by the phantom power light says +48v, so it's not a reduced voltage supply (like the cheapest Behringer mixers have).

I think Mr W that John meant that Behringer do not give a figure for current delivery?

They are by no means the only ones to omit that information, both for mixers and AIs.

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Re: Phantom Power Source Help

Postby Wonks » Thu Mar 28, 2019 8:06 am

There are also a lot of mics that don't give a current draw figure either, which makes it twice as hard!
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Re: Phantom Power Source Help

Postby ef37a » Thu Mar 28, 2019 8:17 am

Wonks wrote:There are also a lot of mics that don't give a current draw figure either, which makes it twice as hard!

Indeedeedoodee! I was in need of such information some months ago for an SE mic but to be fair an email elicited a swift response and the numbers.

I have just recalled (got THE most stinking cold). The crap power supply I got was an "ITA" brand and was twin unit and switchable to 12V (not sure why?) It could barely manage 2mA iirc . A while ago I modded a Blackstar pedal PCB that normally produces 300V to 48V and that gave a lovely clean, regulated output. Basically a chip, a small inductor and a power MOSFET. I shall try to find the IC type number. (how else can one spell inductor? this PC says it is wrong)

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Re: Phantom Power Source Help

Postby Hugh Robjohns » Thu Mar 28, 2019 10:27 am

nsg3 wrote:My question is there any benefit of me running the microphones through the stand alone phantom power supplies into the mixer and turning the phantom power off on the mixer?

Almost certainly, no -- but it's far from being a a stupid question.

If you're not having any problems then the implication is that the phantom power supply in the Behringer is entirely adequate -- as I'd expect it to be. And a simple substitution of the mixer's phantom supply for the external phantom unit(s) will soon reveal if there is any detectable quality or performance difference.

Assuming there isn't, why would you want the bother and hassle of carrying around extra hardware, extra audio cables to hook it all up, and extra mains cables and plug-board sockets to plug it all in? Pointlessly unnecessary! ;-)

Just to be clear, though, the kind of problems that can happen if the phantom supply is inadequate are audible hums or buzzes, transient distortion, reduced headroom, reduced output level, increased noise, increased prevalence of interference... Although there are other technical issues that can also cause all of those symptoms, too, of course.

Having said all that, the other contributors here do make valid points about issues with phantom power supplies in some other products that have failed to provide the nominal 48V or the mandated 10mA of current. However, even in these cases there still may not be any problems at all if the mics being used don't require 10mA (and very few do), and/or can cope with a lower supply voltage (which a great many can)!

If it really concerns you, there are ways of testing the conformity of the phantom supply. For example, you can buy 'phantom voltage test plugs' from a number of different suppliers which will light up some LEDs to indicate a 'go/no-go' status, or you can use an audio testing system with a phantom test function built in, or, if you're into electronics you can make some simple measurements with a multimeter and/or make your own phantom test plug. If you want more info, let me know...

...but in your situation, I'd just use the phantom supply in the mixer and, once I was convinced all was good, sell the separate phantom units on.
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Re: Phantom Power Source Help

Postby John Willett » Thu Mar 28, 2019 3:25 pm

Problems can occur when you run several mics into a mixer.

I once had an 8-channel mixer that would power up to 4 mics (drawing 4mA each) without problem, but plug in a fifth and it wouldn't work. In the end I cut the phantom buss bar after i/p 4 and used external power supplies for the other channels.
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Re: Phantom Power Source Help

Postby Hugh Robjohns » Thu Mar 28, 2019 3:59 pm

John Willett wrote:Problems can occur when you run several mics into a mixer.

Yes, of course. But how many is several? A rubbish mixer could be perfectly fine with eight low-current mics, yet struggle to power three of four thirsty mics...

However, this is all unhelpfully hypothetical as the OP's mixer only has two mic inputs, and no powering problems have been detected with the available mics anyway.

And to be fair, it's extraordinarily rare to find any modern phantom-power source that has serious failings. The last (very minor) phantom power issue I found was with the new Golden Age preamp (reviewed in the March 2019 issue) where it couldn't quite manage the full 10mA spec (currently DIN EN 61938, but previously DIN 45596). It wasn't a disaster though because it still delivered more than 8mA (which exceeds the original P48 specification's 7mA limit) and so would power most mics without trouble anyway.... and there was an easy manufacturing fix which I was told was being implemented as we went to press. And I can't remember the previous occasion where I found a problem, which shows just how rare it is, even in budget gear.

Out of interest, I ran a search on the http://www.microphone-data.com website and found that of the 780 mics listed there that require phantom power, 94% required less than 7mA of phantom power current, and 60% need less than 3mA.

Nevertheless, it's worth being aware that a few familiar and popular brands are known for making thirsty mics, including most of the CAD and Earthworks mics which tend to require 8mA or more, as does the Rode NT3 and the Oktava MC range.

My policy, though, is always: If it ain't broke... don't over-complicate things! ;-)

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Re: Phantom Power Source Help

Postby where_theres_a_will_theres_a_way » Fri Mar 29, 2019 6:15 pm

The "standard" for 48v phantom power is to use a 6K8 resistor between the +48v and each of pin 2 & 3 (inside the mixer / whatever) so even if you short both pins to ground the current is limited to 48/6800(*2) which is 14.1mA - per channel.

So from a manufacturer's perspective the test of the phantom power supply is very simple, short both pins 2 & 3 of all inputs to ground (pin 1s) and make sure the 48v is still there.

If it were me, I would short the pins 2 & 3 to ground and leave it for an hour or so - if the phantom powering stops working after that then the internal resistors aren't capable of handling the power (I've seen that) so send it back. If it's OK, which it ought to be, then you don't need external gear - a plus!

Sounds cruel? Not really, those inputs can easily be connected to a grounded output (accidentally) so what I described is easy to do in the real world, and the equipment is supposed to be designed to cope with it.
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Re: Phantom Power Source Help

Postby where_theres_a_will_theres_a_way » Fri Mar 29, 2019 6:18 pm

Forgot to add, those resistors are there regardless of using an external supply, so it can't get any better with that mixer using external supplies.
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Re: Phantom Power Source Help

Postby Hugh Robjohns » Fri Mar 29, 2019 7:02 pm

where_theres_a_will_theres_a_way wrote:So from a manufacturer's perspective the test of the phantom power supply is very simple, short both pins 2 & 3 of all inputs to ground (pin 1s) and make sure the 48v is still there.

But it won't be there (on any of the shorted mic sockets) precisely because you've shorted them all to ground! :oops: :-)

Of course, there should still be between 44 and 52V (48+/-4V) on the 48V power supply feed inside the mixer/interface/preamp, but getting to that would involve dismantling the unit while it is powered up which isn't something that can be condoned here for anyone without the appropriate technical qualifications and experience. (He says with an eye on the Health and Safety Executive!)

However, in case anyone wants a slightly more practical test, a reasonable compromise would be to leave one input socket disconnected for measurement access where the voltage between pins 2-1 and pins 3-1 can be measured with a DC voltmeter and should be identical on each leg somewhere between 44 and 52V. And for the really keen, if the meter is switched to measure the DC current between pins 2-1 and 3-1, it should show between 6.5 and 7.7mA for each leg.

Personally, I find it much quicker and easier to use the NTI MR2 Pro audio tester when evaluating phantom power in preamps etc, with a bespoke loading cable so that it measures the voltage delivered to the notional mic when drawing the specified 10mA per leg.

Testing with a full short condition is a bit pointless in my view as that delivers no volts to the mic and thus no attached mic can possibly work anyway... At the practical 10mA total maximum current test limit defined in the spec the voltage at the mic should be between 9.5 and 12.2V.

BTW, 7mA through a 6k8 resistor dissipates roughly 0.3W of power. It would be nice to think every manufacturer uses at last 0.5W resistors in their phantom feeds... but I bet some of the budget designs use 0.25W and hope for the best! So I really wouldn't want to leave a hard short on all mic channels cooking away inside a budget preamp/interface/mixer design for an hour or more... I take the point that it provides a maximum stress test, but it's not really a valid real-world test and I'm not convinced you'd win the case in court if the manufacturer sues for damage. Anyway, there are more intelligent and safe ways of evaluating the phantom power supply.

In my experience, the voltage and current capability of the vast majority of phantom power supplies are usually perfectly adequate these days. The more serious and variable practical problem I come across is caused by poor tolerance matching of the 6k8 feed resistors in budget equipment, and that has a disastrous effect on the differential input's CMRR and ability to reject interference signals. The matching should be better than 0.4%, and ideally 0.1%, and that's expensive to achieve either through purchasing 0.1% resistors or by hand testing and matching pairs of resistors during production, so is often ignored.

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Re: Phantom Power Source Help

Postby AllAudioSound » Sun Apr 14, 2019 7:58 pm

Just an addition to this discussion. I recently acquired a Behringer PS400 phantom power unit, a cheap box used as an insert to supply phantom voltage to a condenser microphone, switch selectable for nominally 12V and 48V, fed by a 12V mains adapter.
Being not at all an anti-Behringer user, I was unpleasently surprised to find this aparatus only supplied 30V (unloaded, i.e. no microphone!) in the 48V-mode. Also, in that same mode it only delivers 2 mA at a mere 12.5V output voltage, quite far off from anything like 48V and 10 mA.
Noise is OK (some broad band, with only a weak spike at 100 kHz rep rate), and the box is of the common Behringer heavy duty variety. So, in line with the comment of ef37a of Mar 28, 2019, 10:17 AM, I will use the box, substitute the electronics (now a square-wave driven N diode stack) by a regular DC-DC-boost converter with an inductor + regulator, augmented by a max. current setting circuit which will make the output voltage to drop to "low" if more than the preset max. current is overrun.
Not too complicated, but .... to be done when time comes.
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Re: Phantom Power Source Help

Postby ef37a » Mon Apr 15, 2019 12:48 am

http://www.aimtec.com/am2m-1224dh30-nz- ... wer-supply

^ Looks like a simple solution, you don't need the massive isolation voltage so further searching could find a cheaper device.

I have used two Aimtec converters (5V stacked, all Maplin had) to derive 10V from USB power for pedals. No detectable noise.

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Re: Phantom Power Source Help

Postby Mike Stranks » Mon Apr 15, 2019 9:53 am

AllAudioSound wrote:I recently acquired a Behringer PS400 phantom power unit, a cheap box used as an insert to supply phantom voltage to a condenser microphone, switch selectable for nominally 12V and 48V, fed by a 12V mains adapter.
Being not at all an anti-Behringer user, I was unpleasently surprised to find this aparatus only supplied 30V (unloaded, i.e. no microphone!) in the 48V-mode.

Trades Description Act violation surely?

... and another well-known 'own-brand' supplier was selling 48v boxes that could only manage 36v a few years ago. Even after I protested they still advertised them as 48v for some time.
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