Phantom current

I am hoping to start to build a new solid state circuit for a microphone idea I have(If it stops raining and the basement dries out).

My question how much current can I pull from Phantom supplies in the REAL WORLD or when does it get to be a problem?

I know what the spec is(10ma) but I want to make sure what the upper limit is on real world installed systems.

10 milliamperes is probably the max "real world" current that you should draw from a P48 phantom supply per channel.

A dead short draws 14 milliamperes.

Per the specification, phantom power is a 48V voltage source supplied common mode through two 6.81K build out resistors common mode to pins two and three.

You can draw as much current as you can get, but the voltage will drop per Ohms law across the 6.81Ks limiting working voltage in any simple supply.

I have powered dual opamps with reasonable signal swing but you are limited to only a few mA.

Practical considerations are that some cheap gear uses less than 48V phantom, and large mixers may assume that every mic input will not max out phantom current draw.

JR

Thanks I understand what has been posted. I know the max power is a 3.4K load etc...

The question is kind of simple, what current do old and newer Real world mixing boards and preamps supply without problems?.

Can I design for say 6 ma and be somewhat sure it will work most of the time?

I have a design that uses 4 - 5 milliamperes with no problems.
Older phantom powered equipment might be a problem.

A good article on old and new phantom powered microphones is:

http://www.rycote.com/products/pdf/The%20Feeble%20Phantom.pdf

You probably need to research specific models. I find lots of install (background music) products use 12V or 15V phantom. Some entry level powered mixers will likewise use 12V phantom. When I was working at Peavey we were putting real 48V phantom in some modest priced gear, like XR600 top boxes or small series mixers, but again check the specs.

Regarding how many channels in a mixer can you effectively pull full current, I don't think anybody engineers for all inputs shorted because that's just not realistic, that said a series of consoles may share a common power supply so a smaller frame or short load in a given series could have extra PS capacity.

JR

I agree that 10 mA is likely as high as you will ever encounter from a "real" mic...and it makes the math easy! <g> So, a 48 channel desk can be safely powered from a 1/2 Amp "Power-One" open-frame linear module, the smallest that they sell with a 48VDC output.

Sidebar topic....I've always seen (and personally used) 1/4 Watt 6k8 phantom powering resistors. However, I recently "ran the math" in case of an "OOPS!", (aka, short from XLR pin 2 or 3 to pin 1), and the poor little resistor(s) will be dissipating 0.34 Watts under that worst-case "fault" condition.

I tend to believe that that sort of fault will cause a 1/4 Watt resistor to quickly "go out of tolerance", if it doesn't just go completely open-circuit.

Hence, 1/2 Watt phantom R's in all of my new gizmos from now on....

Bri

I've had preamps that crapped out at less than 5 mA. But that's the preamp's fault, I'd say. When P48 was introduced, the max current was specified at 2 mA. Later the max curent was upped to 10 mA, so that's what a modern preamp should be capable of. Unfortunately some manufactureres never heard of the newer specification. At least they say so. A couple of years ago I had a conversation with the designer of a well known British brand who was dead sure, P48 was still specified at 2 mA max current.

When I receive a new preamp or microphone I usually measure the voltages and currents. A fully capable preamp supplies something like 7 mA when you short either pin 2 or pin3 and pin1 with your multimeter (or 14 mA for both legs). With microphones I usually measure the voltage drop, which is more convenient than measuring the current. Usually the voltage drops to no less than about 35 V (so slightly less than 4 mA current draw). I think that's a figure most mic manufacturers go for. Higher current requirements is asking for trouble, even though its not your fault that a lot of preamp manufacurers are P48 scrooges.

Good point, Brian. Sounds like a wise idea.

So there are still some pres that might not supply more than 2ma?

I am not going to care about under 48V phantom power

I quess I will use 5ma as the upper level at 44VDC to 52VDC supply and just not care if the pre can not supply it correctly.

1/4 watt is bad design IMO. I always do the math for power when I make something.

[quote author="Gus"]I always do the math for power when I make something.[/quote]
I wish I did!!!

Careful, Gus... -That quote might come back to haunt you if you ever overlook something in haste! :twisted: :wink: :twisted:

Keef

I guess they are counting on things not shorting.

With overheated what drifts more(after it is cooled down) metal or carbon film over time? Any good writeup on this on the web I don't remember reading anything in books about this.

Yeh maybe I should add "when I remember to"

And with tube circuits you need to think about the voltage rating of the resistor.

I think it more that Brian is the only one (perhaps in the entire world) that wasn't to lazy to actually calculate it.

Click to expand...

I know somebody else. :wink:

Anyway, the resistor power ratings are for 70 °C ambient temperature, so using 1/4 W is not really a problem unless you forgot the ventilation slots as well. Of course if you plan to leave a short for half a year the resistor matching will be better with 1/2 W.

Samuel

Gus,
I second what John Roberts said. Think of max voltage drop you can allow, then think of a current you can draw for such a loss, then develop your output stage according to this max current. I've found that unfortunately it is not possible to avoid matching transformers if you want a decent output. Otherwise, use an internal battery to power whatever you want, and use 48V to power the capsule and some switch that will switch on your internal battery when the phantom power presents.

While I like to view myself as a conservative engineer, I did the math years ago and decided to stay with 1/4 W.. In my experience it has been an OK call (not aware of one problems with many channels in use) but I don't suggest it is fully optimized for worst case.

I also did some unintentional research into resistor degradation due to overheating years ago and was perhaps spoiled by ROHM carbon films. I could burn the paint off those suckers and they'd pretty much hold their 5% tolerance, but their 5% parts ran closer to 2% in use. Make them glow and you might have to replace.. I usually replaced them when I couldn't read the colors any more :grin: Forced me to spec in dedicated flameproof resistors for specific use as cheap fuses on console strips and such, since the ROHMs wouldn't easily fail.

True a phantom dropping resistor will exceed 250mW at more than 40V drop across it. Also true there are some cheap phantom powered mics with internal shunts of less than 10V so you could encounter an actual valid circuit scenario dissipating rated power. I am fairly comfortable with good quality through hole parts to handle that. With tight SMT designs I might be inclined to look ar 1/2 W parts.

YMMV.

JR

[quote author="Samuel Groner"]

I think it more that Brian is the only one (perhaps in the entire world) that wasn't to lazy to actually calculate it.

Click to expand...

I know somebody else. :wink:

Anyway, the resistor power ratings are for 70 °C ambient temperature, so using 1/4 W is not really a problem unless you forgot the ventilation slots as well. Of course if you plan to leave a short for half a year the resistor matching will be better with 1/2 W.

Samuel[/quote]

Just for grins, I looked at the spec sheet for the Xicons that Mouser sells (an easy brand name to recall/look up). While they are indeed rated for full power at 70C, I see no indication that you are allowed to "cheat" the power rating below 70C. The derating graph shows "100%" (ie, 0.25 Watts or whatever) at or below 70C.

That makes me tend to believe that 0.34 W is not legit for a part rated at 0.25W, although the environment can obviously make a difference (as in dipping the R into oil, like RF guys do with "Cantenna" dummy loads).

Obviously I was looking at worst-case conditions, but I believe it is possible for a sustained short circuit to exist in a real-world situation...ie, a faulty mic cable.

I guess I should wire some 6k8, 1/4 W R's across 48 VDC and walk away for awhile to see exactly what damage can be expected. A second test would be to repeatedly apply the 48 VDC across the resistor, in case repeatedly applied overcurrents casue a differeent failure/degradation.

Bri

If you do perform some power tests on 1/4W resistors, besides logging before and after resistance, perhaps a visual characterization of the solder joints. If the over power condition allows the lead temperatures to rise, that could degrade the solder connections over time. You could make the test more difficult while still valid by pushing the phantom voltage to the high end of it's tolerance range. In some resistors the resistance may vary with temperature so take that into consideration when comparing before and after test results.

Let us know what you find.

JR

> unfortunately it is not possible to avoid matching transformers if you want a decent output.

I must agree, if "decent" is pretty big.

The maximum DC power available is 24V and 3.4K.

The audio load can be 10K, or it can be under 200 ohms. (Don't forget line capacitance.)

Right away we have mismatch. We have ~3K power supply and 0.2K load. Now, for a single-ended totem-pole amplifier and Sine wave, the average supply drain is like 6 times the load. A 8 ohm speaker-amp at full roar acts like around 50 ohms to the supply. A 0.2K load is like 1K to the supply. This means we can't use even 1/3rd of that 24V, only 8V. And the biggest steady sine we can put in 200 ohms is 2.8V.

OK, that's pretty decent.

But we like balanced output. And so we have two 100 ohm loads. We have two 500 ohm DC drains. We have 250 ohm DC load on 48V and 3.4K, or 3.2V to work with. We'll get 1.1Vrms per side, 2.2V differential.

That's above the maximum level I ever hope to see off my hottest mikes. But I no longer do "loud". A full orchestra in the third row is nothing like close-mike rock-n-roll, or even some singers in the near field.

The above leaves nothing for the head-amp. The headamp or driver may need substantial current, which directly hurts the maximum output.

The now-older AKG 414 (non-TL) uses a single-ended output coupled through a 1:1 transformer, and I think is rated for 100 ohm load. I did the math once, and thought that it could hold its rated Max Output at that load. It has a good cap, it would surely pass the rated max level as a transient; or with a higher (and more typical) load above 300 ohms or so.

There is a -10dB pad, and with that engaged it seems to be ample up to unmusical sound levels.

So it is barely possible to get "decent" output. But there is a LOT of engineering in there. More stuff inside the mike than the preamps I use.

Interestingly, a simple 800:200 or 600:150 ohm 2:1 transformer gives a near ideal power match between 3.4K Phantom and 200 ohm loads.

Ah..... the "6X rule" assumes class B/AB output. For pure Class A push-pull the factor is 4, and you can't cheat with transient peaks. Class A SE really sucks.

Rossi is right. Old-spec 2mA, new-spec 10mA, all jacks; real-world does not know specs. If your mike is fussy, you WILL find situations that make it unhappy. The AKG is un-fussy: internally it runs on 9V and idles 2mA. The regulator dropout is below 12V, and it is rated to run full-spec on P12 which IIRC is 12V through two 680 ohm feeders.

I find "big" nominal 1% metal film resistors (~1/4 W) in this value range from reputable sources are usually very tightly toleranced these days. I think the process control of spiral cutting machines and maybe the advances in the machines' design have gotten better. Also it's easier to control the spiral on a 1/4 W cylinder than an 1/8 W or smaller.

I have some war stories about resistors that will have to wait, but out of which the best advice is Know Thy Supplier.