Adapting a microphone to remote phantom power

[Franc Zabkar]

http://www.mediacollege.com/audio/phantom-power/

That's not the best of explanations. Phantom power is distributed from a
central PS to the mixer inputs via standoff resistors. Which limit the
amount of current available to any single mic. Necessary to prevent a line
or mic fault bringing down the entire supply.

Maybe these two examples (Method 1 and Method 2, also reproduced at
the epanorama site) explain it better:

http://www.uneeda-audio.com/pzm/pzm_ch.htm

The same DC current flows from hot to ground, and cold to ground.
Therefore the current flowing between hot and cold is zero.

- Franc Zabkar

 

[David Nebenzahl]

The supply for each of the L and R microphone inputs is derived from a
buffered, zener regulated 2.6V source. The series resistor is 7.5K.
For comparison purposes, the condenser elements within Sony's ECM-737
mike are each powered from a 4.8V supply via 10K resistors.

I thought that the OP may be measuring the voltage while the mike was
connected, but it appears that 2.8V was the O/C voltage of the
Walkman.

This is the relevant section of the circuit:

2.6V |-- 7K5 --> L
IC201 | |
5.9V o--- R ---+---------+---- buffer -+- 1K ----+---|
| | | |
ZD --- 220uF 47uF --- |-- 7K5 --> R
HZ3ALL | --- ---
_|_ _|_ _|_
= = =

Franc--thanks for taking the trouble to draw this out. I've been looking
all over the schematic, and so far I can't find anything to the left of
IC201 in your "drawing" (the 220 uf cap, zener, etc.) Can't see D301 at
all on the schematic. The only zener I can find is one on the DC-DC
board, but nothing there is labled in the diagram. Help!

(I'm not considering modifying the recorder, just satisfying my
curiosity as to how the "plug-in power" works.)

 

[Eeyore]

[someone else] wrote:

In fact the Wikipedia article recognises this:

=====================================================================
The low-current 3 to 5 V supply provided at the microphone jack of
some consumer equipment, such as portable recorders and computer sound
cards, is sometimes called "phantom power."
=====================================================================

Yes. Wonder who added that to the original article?

You can easily find out: just check the "history" tab and trace it back.
(But it'll probably turn out to be an "anonymous IP", meaning that it
could be anybody, qualified or not.)

And there's really no such thing as an "original article" on Wikipedia,
as all articles are constantly undergoing rewriting for better or worse
(often the latter).

Well you can take it from me the current explanation is pretty decent and
rather better than many others you'll find.

I've amended the entry slightly to make it clearer.

Aha! So much for relying on Wikipedia as a reliable source of information.

Go and take a look. I am a registered contributor.

Graham

 

[Eeyore]

Maybe these two examples (Method 1 and Method 2, also reproduced at
the epanorama site) explain it better:

http://www.uneeda-audio.com/pzm/pzm_ch.htm

The same DC current flows from hot to ground, and cold to ground.
Therefore the current flowing between hot and cold is zero.

Correct. Hence minimising any noise contribution from the phantom PSU. Having
said which they are usually very well regulated anyway.

Graham

 

[David Nebenzahl]

On 4/14/2009 4:58 AM Eeyore spake thus:

:
[someone else] wrote:

In fact the Wikipedia article recognises this:

=====================================================================
The low-current 3 to 5 V supply provided at the microphone jack of
some consumer equipment, such as portable recorders and computer sound
cards, is sometimes called "phantom power."
=====================================================================

Yes. Wonder who added that to the original article?

You can easily find out: just check the "history" tab and trace it back.
(But it'll probably turn out to be an "anonymous IP", meaning that it
could be anybody, qualified or not.)

And there's really no such thing as an "original article" on Wikipedia,
as all articles are constantly undergoing rewriting for better or worse
(often the latter).

Well you can take it from me the current explanation is pretty decent and
rather better than many others you'll find.

I've amended the entry slightly to make it clearer.

Aha! So much for relying on Wikipedia as a reliable source of information.

Go and take a look. I am a registered contributor.

Yes, I see you seem to be the entity known as "Pasoundman" there. And
looking through your "contributions", I can see you tried to muck up the
article on total harmonic distortion, resulting in a page that would
have completely baffled anyone going there looking for information on
the subject:

http://en.wikipedia.org/w/index.php?title=Total_harmonic_distortion&diff=prev&oldid=89170973

However, that edit, which the subsequent editor termed "vandalism", was
quickly reverted (in less than an hour), and your content is gone to
this day:

http://en.wikipedia.org/w/index.php?title=Total_harmonic_distortion&diff=89176930&oldid=89170973

 

[Franc Zabkar]

Franc--thanks for taking the trouble to draw this out. I've been looking
all over the schematic, and so far I can't find anything to the left of
IC201 in your "drawing" (the 220 uf cap, zener, etc.) Can't see D301 at
all on the schematic. The only zener I can find is one on the DC-DC
board, but nothing there is labled in the diagram. Help!

(I'm not considering modifying the recorder, just satisfying my
curiosity as to how the "plug-in power" works.)

Look at pins 20, 21, and 22 of IC101 in the other channel. Pin 20 goes
to the zener. Pin 20 of IC201 would also go to the same zener,
although it is not shown explicitly because the two channels are
mostly identical (see the note "R-CH is the same as L-CH"). Pin 22 of
IC101 terminates in a capacitor, and is not used (?), whereas pin 22
of IC201 goes to the mike's power circuit and to the LED board.

- Franc Zabkar

 

[David Nebenzahl]

Look at pins 20, 21, and 22 of IC101 in the other channel. Pin 20 goes
to the zener. Pin 20 of IC201 would also go to the same zener,
although it is not shown explicitly because the two channels are
mostly identical (see the note "R-CH is the same as L-CH"). Pin 22 of
IC101 terminates in a capacitor, and is not used (?), whereas pin 22
of IC201 goes to the mike's power circuit and to the LED board.

Thanks again for the quick reply.

I'm stumped. I must be making some really dumb mistake. Looking at the
schematic, pin 20 of IC101 (left channel) goes across and down to an
unspecified pin in IC201, connecting to C138 and C305 on the way. I
don't see any zener nor any connection to one. I do see the capacitor on
pin 22 (C132) and how pin 22 of IC201 goes to the mike's power circuit.

Can you tell me where (grid coordinates) D301 is? I can't find it at all.

I'm viewing the schematics in the service manual and its supplement that
you gave us a link to. Am I missing something really obvious here? Or do
you have another diagram?

 

[Franc Zabkar]

Thanks again for the quick reply.

I'm stumped. I must be making some really dumb mistake. Looking at the
schematic, pin 20 of IC101 (left channel) goes across and down to an
unspecified pin in IC201, connecting to C138 and C305 on the way. I
don't see any zener nor any connection to one. I do see the capacitor on
pin 22 (C132) and how pin 22 of IC201 goes to the mike's power circuit.

AFAICT, there is only one zener. It regulates the power for both
channels.

Can you tell me where (grid coordinates) D301 is? I can't find it at all.

D301 is adjacent to C305, at coordinates E15. Right above all three
components is a label, "D301 HZ3ALL".

I'm viewing the schematics in the service manual and its supplement that
you gave us a link to. Am I missing something really obvious here? Or do
you have another diagram?

I'm looking at page 19 of the service manual.

- Franc Zabkar

 

[David Nebenzahl]

D301 is adjacent to C305, at coordinates E15. Right above all three
components is a label, "D301 HZ3ALL".


I'm looking at page 19 of the service manual.

Ack! No wonder: I was looking at the manual supplement, which omits the
zener. Like I said, a dumb mistake. Thanks!

 

[Franc Zabkar]

Ack! No wonder: I was looking at the manual supplement, which omits the
zener. Like I said, a dumb mistake. Thanks!

As you say, the zener has been deleted from the parts list in the
supplement, yet pin 20 of the IC is still sitting at 2.6V. Maybe the
potential divider inside the IC sets the voltage.

- Franc Zabkar

 

[Bob Larter]

I guess you never saw any of the battery powered tube hearing aids?
They used less power than a mic preamp, due to their directly heated
filaments.

I actually pulled apart a tube hearing aid as a kid. The whole hearing
aid was around the size of a packet of cigarettes, & the tubes were
maybe half the size of a cigarette. I have no idea whether or not they
did anything unusual to heat the filaments.

 

[Bob Larter]

Directly heated filaments don't have a separate cathode, and require
a lot less current to operate. That hearing aid would be worth a lot of
money to a collector these days.

It's long gone, I'm afraid.

Motorola built portable two way radios with direct heated filament
tubes to extend battery life, plus the fact that they heat n a fraction
of as second, you can shut down the entire transmit section while
monitoring the base station. The US military was still using them for
the MPs into the '70s because they were rugged, and heavy. They would
survive if the MP had to subdue someone, and it hid the ground.

That's pretty impressive for a gizmo with tubes in it. ;^)