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Author Topic: how do preamps generate 48V phantom power?  (Read 10804 times)

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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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

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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