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Adapting a microphone to remote phantom power

D

David Nebenzahl

Jan 1, 1970
0
OK, another potential project: re-jiggering the mike for my Sony Walkman
Professional cassette recorder to use phantom power.

I have the WM-D6C Walkman "Professional" recorder and a Sony ECM-737
stereo microphone. The rig still makes very nice recordings. (No, I
don't usually use it for music, and I'm not pushing cassette as the
ultimate recording medium.)

The annoying thing about it is that while the microphone requires a
battery (a single AA cell), the recorder is apparently perfectly capable
of supplying power to the mike. The mike jack is marked "PLUG IN POWER",
which I suppose is Sony's way of identifying phantom power. (This may
not be "phantom power" in the sense of standard supply voltage, but
instead a proprietary Sony level.) I can't tell you how many times I've
had problems recording which turned out to be a low or dead battery in
the microphone. Probably my bad for not buying the next-better mike,
which I think did run on the recorder's power.

So I'm thinking I could modify the microphone to accept power from the
recorder, without modifying the recorder in any way. There are a few
things I don't know, however:

o The voltage of the power supplied by the recorder, which runs on 4 AA
cells. (48 volts? I'm guessing not, but who knows?)

o The configuration of the microphone jack. My mike uses a 3-conductor
plug, so what happens to the power contact in the jack?

o The wiring inside the mike and the difficulty of rewiring it (haven't
opened it up yet to look).

Anyone familiar at all with this equipment? Care to give some advice on
how to proceed?

Thanks in advance.
 
D

David Nebenzahl

Jan 1, 1970
0
So I'm thinking I could modify the microphone to accept power from the
recorder, without modifying the recorder in any way. There are a few
things I don't know, however:

o The voltage of the power supplied by the recorder, which runs on 4 AA
cells. (48 volts? I'm guessing not, but who knows?)

Further research reveals that the recorder supplies 2.8 volts on both
channels of the microphone input.
o The configuration of the microphone jack. My mike uses a 3-conductor
plug, so what happens to the power contact in the jack?

See above; power carried on the inputs.
o The wiring inside the mike and the difficulty of rewiring it (haven't
opened it up yet to look).

There's a small circuit board inside (surface-mount, surprisingly
complicated for a microphone preamp). Battery power comes in on two
separate wires. If battery - goes to ground, then I might be able just
to connect battery + to one of the inputs, perhaps with a 1.5 volt zener
diode to limit voltage. If not, I'd be afraid of messing it up. Time to
look for a schematic?
 
D

David Nebenzahl

Jan 1, 1970
0
There's a small circuit board inside (surface-mount, surprisingly
complicated for a microphone preamp). Battery power comes in on two
separate wires. If battery - goes to ground, then I might be able just
to connect battery + to one of the inputs, perhaps with a 1.5 volt zener
diode to limit voltage. If not, I'd be afraid of messing it up. Time to
look for a schematic?

Whaddya know? Found a schematic, in this copy of the ECM-737 service
manual: http://www.coutant.org/ecm737/ecm737serv.pdf (from this page:
http://www.coutant.org/ecm737/index.html).

So looking at the schematic (page 4 of the manual), I'm wondering if a
guy couldn't just connect the battery power input to one of the outputs
through a 1.5v voltage regulator. While that would provide power to run
the mike, I'm wondering what detrimental effects that would have on the
channel it's connected to. Any way to isolate the signal from the DC power?
 
E

Eeyore

Jan 1, 1970
0
David said:
OK, another potential project: re-jiggering the mike for my Sony Walkman
Professional cassette recorder to use phantom power.

From a quick glance it uses a lot ( relatively for a mic ) of 1.5V power. So
the powering scheme from the walkman is unlikely to be compatible and even
if it was, you'd run the battery(ies) dry in that in no time.

Graham
 
E

Eeyore

Jan 1, 1970
0
David said:
Whaddya know? Found a schematic, in this copy of the ECM-737 service
manual: http://www.coutant.org/ecm737/ecm737serv.pdf (from this page:
http://www.coutant.org/ecm737/index.html).

So looking at the schematic (page 4 of the manual), I'm wondering if a
guy couldn't just connect the battery power input to one of the outputs
through a 1.5v voltage regulator.

No, the 2.8V supply is likely VERY current limited. Sounds more like a sound
card powering method that's good for about 1 mA.

Graham
 
E

Eeyore

Jan 1, 1970
0
Eeyore said:
No, the 2.8V supply is likely VERY current limited. Sounds more like a sound
card powering method that's good for about 1 mA.

Note that the schematic shows 16mA current draw. I'd invest in some good AA
batteries. Wikipedia says you can get up to 3Ah which would be nearly 200 hours
of use.
http://en.wikipedia.org/wiki/AA_battery

" Alkaline batteries from 1700 mAh to almost 3000 mAh cost a little more, but
last proportionally longer. "

Graham
 
D

David Nebenzahl

Jan 1, 1970
0
Phantom power - as regards mics - is only applicable to balanced mics and
inputs. A stereo balanced mic has five connections. Mono balanced
connectors are usually a 3 pin XLR, stereo 5 pin.

The DC is between both balanced audios and screen - the idea being there
is no potential difference across the audio allowing a non phantom powered
balance mic to be used on a phantom powered input.

You can feed DC to an unbalanced mic - but my guess is the mic has to be
designed for this.

Well, as I pointed out in a follow-up post above (don't know if you saw
it), the Sony Walkman in this case applies power (2.8V) across both
inputs at the microphone jack. The mike input uses just 3 connections.
They call it "Plug-in power", which may be Sony's own version of phantom
power. And yes, the voltage is present even if one uses a
non-phantom-powered mike; presumably, the voltage is low enough not to
damage any unpowered microphones. (In my case, the phantom power is just
wasted.)
 
D

David Nebenzahl

Jan 1, 1970
0
No, the 2.8V supply is likely VERY current limited. Sounds more like a sound
card powering method that's good for about 1 mA.

Hmmm; I could run it through a dummy load (resistor) and measure the
current draw and find out. You may well be right. Now I need to get a
service manual for the Walkman (WM-D5C); ugh. Finding the microphone
manual was easy; I struck out looking for one for the recorder.
 
D

David Nebenzahl

Jan 1, 1970
0
But both inputs in this case are the 'hot' connections of a stereo input?

Yes. The 2.8V DC is carried on both mike inputs (+2.8V = L/R inputs,
with respect to common ground).
There are no 'versions' of phantom power - it's a universal standard.
Anything else would be a nonsense.

Apparently this disproves that theory. (I thought the standard for
phantom power was 48 volts.) Talk to Sony. (And remember that this is
"Plug-in power"(R)(TM), whatever that is.)
 
D

David Nebenzahl

Jan 1, 1970
0
From a quick glance it uses a lot ( relatively for a mic ) of 1.5V power. So
the powering scheme from the walkman is unlikely to be compatible and even
if it was, you'd run the battery(ies) dry in that in no time.

Another unrelated question, since you've obviously looked at the
schematic: what do you think the function is of the LEDs in each buffer
output stage (D1 & D2)? I'm puzzled since I saw these inside the mike;
they're both clear LEDs which are completely hidden once the cover is in
place. The one visible LED is the power-good indicator, D5.

Are these used for some kind of voltage regulation or current
restriction? Maybe I could just clip them out and reduce that 16mA power
draw ...
 
W

William R. Walsh

Jan 1, 1970
0
Hi!
Further research reveals that the recorder supplies 2.8 volts on both
channels of the microphone input.

I was just going to say...

I have what must be a very similar Sony microphone that takes a CR2032
cell in its underside. The battery cover is also the on-off switch.

If I didn't loan it to my brother (which probably means it is now
permanently "his"...) I will get a model #.

I've used it with a battletank of a late 70s Panasonic tape recorder
and a computer to produce recordings of an acoustic guitar. It works
fine, and the audio quality is decent.

William
 
F

Franc Zabkar

Jan 1, 1970
0
Another unrelated question, since you've obviously looked at the
schematic: what do you think the function is of the LEDs in each buffer
output stage (D1 & D2)? I'm puzzled since I saw these inside the mike;
they're both clear LEDs which are completely hidden once the cover is in
place. The one visible LED is the power-good indicator, D5.

Are these used for some kind of voltage regulation or current
restriction? Maybe I could just clip them out and reduce that 16mA power
draw ...

They set the bias current for the output transistors, approximately
0.45mA.

- Franc Zabkar
 
D

David Nebenzahl

Jan 1, 1970
0
Yes. The 2.8V DC is carried on both mike inputs (+2.8V = L/R inputs,
with respect to common ground).
Apparently this disproves that theory. (I thought the standard for
phantom power was 48 volts.) Talk to Sony. (And remember that this is
"Plug-in power"(R)(TM), whatever that is.)

Seems to be a problem here with what is phantom power. Perhaps you'd read
my earlier post on the subject. What you have is a form of line powering -
not the same as phantom powering.[/QUOTE]

Aaaaargh.

I've read all the posts here on the subject, as well as other material
out there in the greater world, and most people seem to agree that
phantom powering = line powering, always. (But not necessarily the
reverse, or obverse.)

To use another analogy, if phantom powering is a square and line
powering is a rectangle, one can say that all squares are rectangles,
but not all rectangles are squares.

Or something like that.
 
F

Franc Zabkar

Jan 1, 1970
0
Hmmm; I could run it through a dummy load (resistor) and measure the
current draw and find out. You may well be right. Now I need to get a
service manual for the Walkman (WM-D5C); ugh. Finding the microphone
manual was easy; I struck out looking for one for the recorder.

Each channel of the mike amp PCB has a 2K2 load resistor. The current
draw is therefore ...

2.8V / 2K2 = 1.3mA

I suspect that the Walkman has an internal series resistor of around
1.8K and an O/C supply voltage of 5.0V.

If you could bring the Walkman's 5V (?) supply out to a separate
connector, and bypass the preamp's DC-DC converter by connecting the
Walkman's 5V supply directly to the preamp's B+, then you would have a
mike that drew around 5mA or less from the Walkman, plus the 2.6mA of
wasted line power via R12 and R32. The latter could possibly be
recouped by removing R12, R32, and the two 1.8K series resistors
alluded to above.

FWIW, I notice that in the present configuration both C8 and C23 are
slightly reverse biased by 0.4V (= 2.8 - 4.8 / 2).

- Franc Zabkar
 
E

Eeyore

Jan 1, 1970
0
Dave Plowman (News) said:
Phantom power - as regards mics - is only applicable to balanced mics and
inputs. A stereo balanced mic has five connections. Mono balanced
connectors are usually a 3 pin XLR, stereo 5 pin.

The DC is between both balanced audios and screen - the idea being there
is no potential difference across the audio allowing a non phantom powered
balance mic to be used on a phantom powered input.

You can feed DC to an unbalanced mic - but my guess is the mic has to be
designed for this.

It's the standard 'computer mike' way. Not very elegant. And don't twist the
connector in use because it's carrying DC and will crackle nicely.

Graham
 
E

Eeyore

Jan 1, 1970
0
David said:
Well, as I pointed out in a follow-up post above (don't know if you saw
it), the Sony Walkman in this case applies power (2.8V) across both
inputs at the microphone jack. The mike input uses just 3 connections.
They call it "Plug-in power", which may be Sony's own version of phantom
power. And yes, the voltage is present even if one uses a
non-phantom-powered mike; presumably, the voltage is low enough not to
damage any unpowered microphones. (In my case, the phantom power is just
wasted.)

Google computer mike / mic power and you'll see how it works. Very different
from your ECM.

Graham
 
E

Eeyore

Jan 1, 1970
0
Dave Plowman (News) said:
But both inputs in this case are the 'hot' connections of a stereo input?

There are no 'versions' of phantom power - it's a universal standard.
Anything else would be a nonsense.

The current versions of professional phantom power are covered by IEC
standards. Your Sony mic does not comply to any of them.

Graham
 
E

Eeyore

Jan 1, 1970
0
David said:
Yes. The 2.8V DC is carried on both mike inputs (+2.8V = L/R inputs,
with respect to common ground).


Apparently this disproves that theory. (I thought the standard for
phantom power was 48 volts.) Talk to Sony. (And remember that this is
"Plug-in power"(R)(TM), whatever that is.)

It's 'proprietary' not a standard.

Graham
 
D

David Nebenzahl

Jan 1, 1970
0
The current versions of professional phantom power are covered by IEC
standards. Your Sony mic does not comply to any of them.

To reply to you mis-attributed post (the person you responded to is not
the owner of the Sony mike): Well, duh. Who ever claimed that Sony's
"plug-in power" scheme was standard? In fact, I've been taking
particular pains to point out just the opposite.
 
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