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Author Topic: how do preamps generate 48V phantom power?  (Read 10376 times)

dennis1127

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how do preamps generate 48V phantom power?
« on: December 03, 2011, 05:54:29 pm »

I'm interested to know how the Apogee Mini-Me preamp gets 48V phantom power from its 12 V DC input. The main thing is I'm concerned it's some kind of switching circuit that would introduce RFI into the rest of the preamp. Anyone know how it works or how it is likely to work?
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David Satz

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Re: how do preamps generate 48V phantom power?
« Reply #1 on: December 04, 2011, 12:21:05 am »

In DC-powered portable preamps the 48 VDC for phantom powering is generally obtained from an internal oscillator (DC/DC converter)--but that doesn't imply any problem like what you're concerned about. If that problem exists, it should be easy enough to observe. It only takes an XLR connector with a pair of well-matched resistors in it to simulate the output impedance of a balanced professional microphone. Connect that to the preamp input, observe the output noise (by listening, measurement or both), then turn on the phantom powering and compare.

If the noise levels are the same either way, then it doesn't matter how the circuit works. And if not, then it still doesn't matter how the circuit works ... because what really matters is the quality of the implementation.

--best regards
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dennis1127

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Re: how do preamps generate 48V phantom power?
« Reply #2 on: December 04, 2011, 02:14:07 am »

I appreciate your explanation. There may be some controversy here, but in my experience as an audiophile I have found that clean power dramatically improves circuit performance, and not just reducing the noise floor. Better dynamics, extension, resolution, and so on.

I modified my DAC to run off a battery rather than its linear regulated supply, and nice improvements. (Actually I don't have that much technical know-how.. I paid a guy to modify it.)

I worded my email slightly wrong.. the circuit I am concerned about is the amplifier in the microphone which has all its power coming from the phantom source, not so much the Mini-Me circuitry.

So it seems like a relatively simple improvement to provide the 48V via a battery. Do you know if it's complex to wire up a system that introduces the phantom power from a battery rather than from the preamp?

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klaus

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Re: how do preamps generate 48V phantom power?
« Reply #3 on: December 04, 2011, 02:52:14 am »

For a start, check out the simple circuit that Neumann employed or decades in the U87, which could be powered from two 22.5 V batteries, eliminating phantom all together.





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John Roberts {JR}

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Re: how do preamps generate 48V phantom power?
« Reply #4 on: December 04, 2011, 10:38:05 am »

Another vote for don't fix something that isn't broken.

Phantom power is not a low impedance voltage source but actually delivered common mode through two 6.81k resistors in series (one in each mic leg). In theory, with a good preamp, since it is applied common mode, it doesn't even have to be perfectly pristine, because its noise will cancel out. It is relatively easy to filter the low current phantom supply, so that is the general design practice.

If you do modify your unit, please use precision matched (6.81k) build-out resistors for the phantom, since matching of these resistors is actually important to maintain good CMRR, which could make an audible difference if degraded.

JR
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BobSchwenkler

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Re: how do preamps generate 48V phantom power?
« Reply #5 on: December 11, 2011, 07:51:07 pm »

There are +48V phantom standalone phantom supply units available out there as well. Plenty of options to choose from.

dennis1127

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Re: how do preamps generate 48V phantom power?
« Reply #6 on: December 11, 2011, 08:13:21 pm »

Thanks, guys.

Regardless of whether noise in the phantom power should in *theory* be cancelled out or whatever, a lot of my experience in the audiophile (playback) world tells me that one can't go too far in providing clean power to a device like the preamp in the microphone. I know a guy who does a lot of custom modifications of audio equipment and nearly everything I own has been modified by him. Clean power is close to the top of his priorities.

So I think I'll have him build me a box to introduce phantom power from a battery bank, with capacitors in parallel for transient current supply. The other option would be a good AC power supply, a *linear* one (not switching), but batteries/capacitors is probably the best, and these mics don't use much current.
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BobSchwenkler

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Re: how do preamps generate 48V phantom power?
« Reply #7 on: December 11, 2011, 08:32:20 pm »

I'll reiterate that I think it'd be worth at least listening to see if you have an actual issue or not.

As far as batteries go, it would probably be worth testing vs. a well designed power supply as well. I'd give a high chance that most all of the highly regarded audiophile albums one might be listening to did not use batteries for any required 48V supplies, let alone the supplies used for tube mics.

dennis1127

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Re: how do preamps generate 48V phantom power?
« Reply #8 on: December 12, 2011, 12:31:57 am »

As far as I can tell from talking to people (I'm not an insider myself) most audiophile recordings use some good techniques and some not-so-good techniques at the same time. I think that music is so powerful that you can do only 50% of the job in a superlative way, the rest "adequate," and still get a great recording.

It's not that it has to be batteries, but I want to avoid the DC-DC switching convertor in the Mini-Me that I own, and it's a lot simpler to chain five 9V batteries than to build a 48V linear supply (not to mention batteries keeps me mobile).
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Kai

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Re: how do preamps generate 48V phantom power?
« Reply #9 on: December 12, 2011, 04:56:49 am »

chain five 9V batteries than to build a 48V linear supply
Some mic's don't even need the full 48V, so building a battery supply can be easier.

AKG's e.g. can use down to 9V without compromising anything.
You have to lower the value of the 6K8 feeding resistors when using less then 48V.

The latest Schoeps do switch internally if you supply 12V via 2x 680R.
http://www.schoeps.de/en/products/ccm4/specs

This does NOT work with older Neumanns which use the P48 for polarisation of the capsule.

Have a look into your mic's spec sheet for details.

If you build a supply based on line power I suppose to make it a standard P48, this would be universal.

BTW: it's not that hard to do this if you have some experience with electronics.
Lately I completely redesigned one in a 32CH mixer that had a construction flaw in the original design (it couldn't deliver enough current for all channels, although the transformer used could).
It's just a handfull of parts.

Regards
Kai
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Kai

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Re: how do preamps generate 48V phantom power?
« Reply #10 on: December 12, 2011, 05:18:20 am »

other option would be a good AC power supply, a *linear* one (not switching), but batteries/capacitors is probably the best, and these mics don't use much current.
Not too much - 2-4mA, this means a usual 9V block array gives 20-40h of supply for ONE mic (10-20h for two)

I'm not a great fan of switching supplies either, mainly because in my experience their long term reliability is bad.
It's even hard to keep the switching frequencies out of the audio with those supplies.
But - this is even true for your preamp's audio section. So just changing the P48 won't take you far, as it's very likely that the whole supply of your preamp is a switching type.

There's a test you can make:

Get yourself 4 200R  (the exact value isn't important) and 4 6K8 metal film resistors (paired within 0.1%).
Solder the 200R beween pin 2-3 of 4 XLR plugs.
Solder the 4 6K8 between pin 1-2, 1-3 of two of the above.
Get yourself 2 10K and 2 8K2 resistors.
Solder the 10K across pin 1-2, the 8K2 across pin 1-3 on the other two XLR plugs.

Now you're ready to test: Use the 1st pair of XLRs, bring the mic gain fully up.
Record and listen to the signal with and without P48 on.
It should be straight noise, no descrete frequencies you hear.

Same for the other two XLRs, the "unbalanced" ones.
There might be a LITTLE more noise due to the unbalance.
With P48 on you can hear how clean (or not) the build in PSU works.

Regards
Kai
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dennis1127

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Re: how do preamps generate 48V phantom power?
« Reply #11 on: December 12, 2011, 08:52:00 am »

Thanks for the details, Kai.

I'm medium-experienced with electronics (built a lot of kits when I was a kid and did a minor in electrical engineering for my bachelor's degree) but I prefer not to get into that... currently I'm a full-time student in music composition and don't have a heck of a lot of time. It occurs to me that I could chain together four standard 12V linear supplies. I know of a good one that MCM Electronics sells.

My Mini-Me is powered by a 12V sealed lead-acid battery with parallel capacitors for current transients-- a custom design by this guy I know, which I use for my DAC normally, but I can take it into the field for recording.

EDIT: I don't own a really good mic, but I think I can depend on renting microphones, because I won't make a critical recording all that often. That means I don't know ahead of time what the mic will be, so I need to plan to provide 48 volts.

By the way, is 45V close enough? That's five 9V batteries-- maybe the exact voltage depends on the type of battery..
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klaus

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Re: how do preamps generate 48V phantom power?
« Reply #12 on: December 12, 2011, 02:49:38 pm »

As a previous poster had alluded: It seems somewhat out of scale to pay that much attention to 48V phantom power alternatives, when dealing with low-cost microphones whose intrinsic limitations set in much sooner than any difference between battery and phantom powering could ever be detected.

The more audible difference between battery and phantom-powering may be heard in the omission of decoupling caps at the mic pre's input when using an external powering system, not in the quality of most of the phantom supplies used today.

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dennis1127

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Re: how do preamps generate 48V phantom power?
« Reply #13 on: December 12, 2011, 02:58:35 pm »

I'll probably be using expensive mics because my plan is to rent them when needed, which won't be very often.
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klaus

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Re: how do preamps generate 48V phantom power?
« Reply #14 on: December 12, 2011, 03:27:23 pm »

...and I feel it as my duty, as an experienced individual in a very specialized field, to point out that this obsession with battery power for condenser mics will at best make your recordings microscopically better-sounding.

Why not start on the macro level: select good songs, good players, a good take, a good room, properly maintained microphones, well-designed mic pres, high quality A/D converters, good cables.....?
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dennis1127

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Re: how do preamps generate 48V phantom power?
« Reply #15 on: December 12, 2011, 04:16:58 pm »

I need to correct you.. it's not an obsession with battery power, it's an obsession with not using a switching power supply. A linear power supply would also be acceptable. It's just easier and cheaper to chain five 9V batteries (if 45V is an acceptable voltage for most mics).

Anyway, I am working on the other stuff you mention. I think I may be able to rent good microphones and preamps. Not sure about the A/D.

This Thursday I'm getting a chance to play with rental microphones, using my Apogee Mini-Me as the "all in one" preamp/ADC. After that I'm shipping the Mini-Me to this guy who does mods for me, we'll evaluate its potential, and go from there.
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Kai

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Re: how do preamps generate 48V phantom power?
« Reply #16 on: December 13, 2011, 09:43:13 am »

My Mini-Me is powered by a 12V sealed lead-acid battery with parallel capacitors for current transients-- a custom design by this guy I know, which I use for my DAC normally, but I can take it into the field for recording.
This does not eliminate the Mini-Me's internal switching supply generating +/-15V for the audio electronics.

Of course - having the option to do field-recordings is a nice thing to have, so this battery is still useful.

Regards
Kai
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dennis1127

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Re: how do preamps generate 48V phantom power?
« Reply #17 on: December 13, 2011, 01:05:03 pm »

I didn't realize that. What voltages does the internal circuitry need besides +15 and -15? I guess the guy modding my Mini-Me will have to provide a way to use a linear supply for all the internal voltages. Does it need +5 for the digital chips?

By the way, I acknowledge that I'm kind of fixated on this idea of avoiding a switching supply -- I know I'm making it close to my top priority and I know I'm not looking at it in context of all the other problems (choosing microphone, not to mention room and mic placement, and finding players).

I'll sort it out eventually.

The reason I'm fixated on it, is that my home audio system has experienced vast improvements by cleaning up the power. I run my DAC entirely on a battery, and the other equipment has power conditioning and power supply upgrades. Cleaning up power has done more than anything else to improve the system--- but there is a caveat. The guy who did these mods for me has his own formulas with components chosen carefully for synergy. Before I heard his stuff, my use of power products only went so far.

It looks like I can rent EVERYTHING. My plan is to own the ADC and make it nice with the help of this guy. But maybe I should look into renting the ADC also. I can rent an Apogee PSX-100 for $70.
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Kai

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Re: how do preamps generate 48V phantom power?
« Reply #18 on: December 14, 2011, 01:53:47 am »

What voltages does the internal circuitry need ... will have to provide a way to use a linear supply for all the internal voltages.
One should have a look into the circuit diagram for the answer.
A coarse Google search didn't show up one, so contacting Apogee seems to be the way to get it. Post it here and I will have a look.

Before starting the cure I would examine the disease. It might even be likely that Apogee did a good job in screening, filtering and designing the GND topologie for the PSU. Replacing it with a linear one isn't that easy, to say the least.

The simple test described above makes that audible and, if you use an FFT analyser (e.g. software) with high resolution setting, even visible.

Using the battery in the current configuration has an advantage not yet mentioned: There is no spill of AC line noise and the grounding is completely decoupled from AC line.
This could be tested in the same way, by comparison.

Regards
Kai
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BobSchwenkler

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Re: how do preamps generate 48V phantom power?
« Reply #19 on: December 15, 2011, 07:09:05 pm »

AKG's e.g. can use down to 9V without compromising anything.

And my next thought is how is this done? I'd assume with a switching DC-DC converter.

Kai

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Re: how do preamps generate 48V phantom power?
« Reply #20 on: December 16, 2011, 08:51:37 am »

And my next thought is how is this done? I'd assume with a switching DC-DC converter.
No, they don't need a higher volatage for the amplifier.
The feeding resistor (6K8 @ 48V) needs to be adapted to the lower voltage according to the diagram below (to almost zero if you really go down to 9V).

In the circuit you can see the oszillator around T3 that generates the capsule polarisation voltage, not really a "switching supply" but something close to that.
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BobSchwenkler

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Re: how do preamps generate 48V phantom power?
« Reply #21 on: December 16, 2011, 01:41:16 pm »

Oh, interesting. I haven't looked at these mics before. U26 must be a step up? And then it's re-rectified and filtered.

klaus

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Re: how do preamps generate 48V phantom power?
« Reply #22 on: December 16, 2011, 02:56:35 pm »

The AKG 414EB has a jumper bridge on its circuit which shorts an 8.2kΩ resistor when powering the mic with less than 48VDC. I find that with some cases the current draw with the remaining 330Ω resistor in this configuration is so high that the phantom supplies cannot produce full phantom, causing hum.

This is borne out by the current consumption curves in the drawing above. That is why AKG recommends to clip the jumper, activating the 8.2kΩ series resistor, to bring down the current consumption to manageable levels when using the mic with phantom 48VDC.

P.S.: A customer of mine reported increased output when substituting the 8.2kΩ resistor with a 6.8kΩ. Which still keeps the current consumption in a manageable range.

P.P.S.: Current consumption, even as high as 10mA or more, is normally manageable by most current-production phantom supplies. However, using older designs, one can run into current starvation and voltage back-down, especially when using more than one mic with current consumption in excess of 10mA on the same supply.
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Kai

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Re: how do preamps generate 48V phantom power?
« Reply #23 on: December 17, 2011, 07:12:23 am »

the current draw with the remaining 330Ω resistor in this configuration is so high

P.S.: A customer of mine reported increased output when substituting the 8.2kΩ resistor with a 6.8kΩ.
I have some variations of the circuit diagram.
The position of the resistor R4 (330R value) varies either like above or directly from the transformer center tap to the V+ of the whole mic.

This might explain why changing this resistor can sometimes have influence on the output level, because V+  for the FET T1 is not Zener shunt regulated in the version above.

The regulation itself is changed to 1 FET + 1 bipolar transistor in another version.
So AKG made several versions, non of which are the way I would do it.

BTW: feeding the mic with only 9V phantom power would need a center tapped input transformer to feed in the current.
Otherwise the extremly small feeding resistors would shunt out the audio and cause distortion.

The current would be 3mA, so a 9V block batterie can last about 50h - the idea has some beauty for outside recordings.

Regards
Kai
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