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[tvanderbrook]

Hi everyone. I have a pair of old U87s - mid seventies I believe, that have the battery compartments. A friend of mine recently told me to always use the batteries, because the battery voltage PLUS the phantom power resulted in better performance.

Have you ever heard of this- and would there be a simple way to verify this?

[klaus]

On a U87 it is technically not possible to switch the mic's red powering switch to 'dual-operation'. It's either battery or phantom ("INT" or "EXT").

Some background: DC phantom powering for the DC needs of a condenser mic rides into the mic on the same two mic cable wires the sound output of the mic (AC) rides back into the mic pre. So, there needs to be a way to block the DC voltage from entering the mic pre for otehr types of mics, otherwise a terrible hum would be audible. That blocking is done through blocking capacitors sitting as 'gate keepers' in the mic pre where the audio enters the mic pre.

It is true, however, and, that may be where your friend got the idea, that, if you had a hard-wired input on your mic preamp/mixing board which does not contain decoupling capacitors normally necessary when operating phantom power mics, a slight increase in fidelity would be achieved by running the mic on battery setting, as any component between mic and mic pre will dilute audio fidelity, even a high quality decoupling capacitor (and they are normally not very high quality).

But in 99% of all applications the difference between standard phantom powering with decoupling caps and battery operation into a hard-wired mic pre input without decoupling caps is so minute that it practically does not matter. Besides: few consoles these days have an option to turn phantom off AND bypass the decoupling capacitors too.

There is one scenario where you can use phantom and do not need decoupling caps, especially if you are fond of transformer-coupled mic press like Neves, Focusrites and the like: when you use a mic pre with transformer input, phantom power present at the transformer's primary is elegantly decoupled by the transformer's secondary windings: DC phantom voltage is not picked up from the primary side, and no additional filtering out of DC is needed.

It could also be that your friend was not even thinking about audio purity but supply current: phantom power standards developed in the mid 1960s and since then only marginally updated, were envisioning single FET operation with relatively modest current draw. It is true that the current capacity of many traditional phantom power supplies limits the headroom of the mic to a degree. So maybe he thought, adding teh extra juice from the battery may help. But I found that the mic then also needs quite some modification (starting with the Zener) to make use of the extra current potentially available through paralleling pahntom with batteries.

I'll leave it here and solicit ideas from others to explore that aspect further.

[Jim Williams]

Older U-87's use the phantom voltage for capsule polarization. Newer models use a DC converter like AKG and others. I have seen older 87's and high draw mics like Schoeps have powering problems with many phantom power designs.

These mics need an honest 48 volts to operate. Many phantom schemes use a series resistor before the 6.81k phantom distribution resistors, sometimes as large as 3k ohms. This is where power up problems reside. Swap those series resistors with 100 ohms and all phantom mics will operate successfully.

[Kai]

I wouldn't vote for battery power on the U87.
The batteries can leak and damage the electronics if installed and forgotten for a longer time.
I even suspect that those 22.5V batteries aren't easily available in new condition.

These older U87 with battery compartment have a very nice tone, even with phantom power.

Regards
Kai

[Kai]

Many phantom schemes use a series resistor before the 6.81k phantom distribution resistors, sometimes as large as 3k ohms. This is where power up problems reside. Swap those series resistors with 100 ohms and all phantom mics will operate successfully.

This series resistor, if not combined with a cap to ground as flter, is completely senseless.
Wire-bridge them or add a cap.
100 Ohms + 100uF gives good filtering if the P48 isn't clean enough.

How can you decide if your P48 is clean enough?
Short cut ONE SIDE of your mic pre's input, e.g. PIN1 to PIN2.
Bring the mic pre's gain to about 50dB and listen to the noise.
If the noise difference between P48 on and off is remarkble, extra filtering is needed.
This is a course test, but P48 doesn't need to be hyper-clean anyway.

Be careful - switching P48 under this condition causes a huge Blop in your speakers, that can kill them.
Don't listen too loud and mute speakers before switching.

Regards
Kai

[Uwe]

As Klaus pointed out correctly, without modification it is not possible to operate the U87 from internal batteries and phantom power. It is either INT or EXT selected by an internal switch. However, I don't get the coupling capacitor thread for pantom powered microphone amplifier inputs at all... On of the major reasons for success of the phantom scheme is the absence of any DC-potential difference between the signal carrying conductors, thus making such (de-)coupling capacitors superfluous.

Back to the original question, disregard and forget your friend's advise! There is absolutely no benefit in attempting to run battery and phantom power simultaneously, and aside from a potentially minor 0.6 dB difference in sensitivity * due to the possible 3 V difference in capsule bias, there is no difference in any of the microphone parameters, whether it is powered by the internal batteries of from 48 V phantom power. Then consider the difficulties in finding these obsolete 22.5 V photo flash batteries and the risk of damage from leaks.

*) 20 log (48/45) = 0.56 dB

[klaus]

(…) However, I don't get the coupling capacitor thread for phantom powered microphone amplifier inputs at all... On of the major reasons for success of the phantom scheme is the absence of any DC-potential difference between the signal carrying conductors, thus making such (de-)coupling capacitors superfluous.

On most phantom powered mixers there there is a "phantom off" switch, to cut phantom from that channel (when connecting unbalanced mics, for example). That is usually accompanied by blocking caps, which is a fixed installation, regardless whether phantom is used or not- to keep the audio from the mic into the input of the mixer's pre-amp (downstream from the phantom insertion point) DC-free, under any circumstances.

[Kai]

... a potentially minor 0.6 dB difference in sensitivity due to the possible 3 V difference in capsule bias, ...  20 log (48/45) = 0.56 dB

There is no significant difference in sensitivity, as you need to take into account the voltage drop over the P48 powering resistors (6.8 kOhms) and the fact, that a fresh battery delivers slightly more voltage then it's nominal value.
Looking at the schematic, the P48 is feed over 4.5 kOhms (two parallel branches with 6K8 + 2K2 each).
The current is 0.4 mA.
So the voltage drop is 0.0004 A x 4500 Ohms = 1.8 V.
48 V - 1.8 V = 46.2 V
Two fresh batteries with 22.5 V in series are 45 V plus a bit more if really fresh.
Furthermore P48 is defined as 48 V +/- 4 V, so it may vary from 44 - 52 V !

Not that this all really matters for recording music, but I find it interesting from a technical point of view.

In the schematic below you can even see that the battery switch allows either P48 or battery power, but not both at once.

See although the clever way Neumann switches off the battery power if the cable is removed:
For the Tuchel connector version there's a (not drawn) cable link between Pin 3 (screen) and Pin 7 (0 V) in the cables female plug or at the preamp side.
For XLR there´s a switch (S 2) built into the mic's connector.

Regards
Kai

[Jim Williams]

This series resistor, if not combined with a cap to ground as flter, is completely senseless.
Regards
Kai

Most do add a 100 uf cap to ground, but by then the current is reduced by that series resistor. Sadly, one must be able to read and interpret schematics in order to determine if your phantom supply will provide enough current. Many a session has been interupted by these recurring problems. That's why I've always brought a full tool kits and spares to any session outside my domain. Can't tell you how many times I've had to use it.

[tvanderbrook]

Hi, thank you for the replies and the discussion. FYI, I was able to easily find the 22.5V batteries online, when I got the first of this pair a few years ago - and have used them on location for nice mono recordings, feeding a U87 to a Sony portable flash recorder - a quick and dirty setup for demos, etc.

I am doing almost exclusively location classical recording these days, and the friend who advised me on this is an excellent engineer with a great mind for details, whom I've learned a lot of useful and effective mixing techniques from over the years. His father was a classical on location recordist for many decades, with a very nice arsenal of gear, so I'm guessing this tidbit about the U87 could be a hand-me down from people in the trenches.

Klaus, I think you touched on it when you mentioned supply current increasing the headroom of the amp circuit. This is, I'm sure, what he was referring to.

I trust your expertise when you say this is not possible without modification, however given the source, I'm wanting to investigate, and understand this a little better. Is this what is being discussed in the technical posts here? Apologies for not quite following some of the ideas, as I'm not as versed in electronics as I'd like to be.
 

[klaus]

Yes, that is by and large what is being discussed here: that there are additional restrictions to optimal current flow in some phantom powered mixing boards.

But that may not be an issue in your case, as you are use a location mixer (or is yours supplying phantom? In that case, you could research whether it has that extra buffering resistor installed).

If using your U87 exclusively with batteries, (by the way: Radio Shack stocks these still!) the question becomes, and I admit, I have never looked into it: does battery supply in the U87- and never mind all the shortcomings for this type of powering system already mentioned- supply more current to the mic than standard phantom?

[tvanderbrook]

Yes, that is precisely the question!

That should be easy to verify - what would be the way to do that?

I am generally supplying 48v via Millennia HV3's. The advice my friend gave me was: keep the batteries in there and turned on, when you supply the phantom you'll have better headroom with both together than either individually.

[klaus]

The advice my friend gave me was: keep the batteries in there and turned on, when you supply the phantom you'll have better headroom with both together than either individually.

That advice assumed that it is possible to have dual-powering mode on a stock U87, which it is not.

[tvanderbrook]

Oh, I see I misunderstood the question.

Let me rephrase to make sure I'm understanding correctly. The only question left is whether the battery supply gives the mic more power than the phantom power supply - because, by design, the mic uses one OR the other - no exceptions (unless it's been modified outside of the factory).

[klaus]

Correct.