Using old microphone on PC

Tim Wescott <tim_at_seemywebsite.com> wrote:

On Tue, 20 Sep 2011 10:29:05 +0100, Terry Pinnell wrote:

In my shed I just came across a very old but apparently high quality
mic, a Sony 'F-99B', 'One point stereo dynamic microphone IMP.LOW' It
proved useless when I tried it on my PC, in the usual mic socket.

My multimeter shows its left and right connections as about 170 ohms.

What's the simplest way of getting it to work on my PC please?

What _are_ the specs for a typical PC microphone input port? And are
there even specs, or does the circuit just happen to be what the
manufacturer felt like using at the time?

I gather that it's designed for electret microphones, then -- that makes
lots of sense. In theory that means that you could make a preamp that
would be powered by the PC, but that's only "simplest" if you have a well-
stocked junkbox, some knowledge, and some time.

Thanks Tim. I'm thinking about experimenting with that F-99B (seems a
shame to waste a fairly expensive mic) but it means cleaning up my shed
and workbench first - something I've been putting off for a couple of
years!

I have the first two of those requirements, but time is in short supply.

The circuits in the article referenced by bw up-thread are one starting
point, if I do make the initial effort.
http://www.hobby-hour.com/electronics/computer_microphone.php

I could even avoid the spring clean by doing some bread-boarding here at
my PC.

On 2011-09-21, Rich Grise <richg_at_example.net.invalid> wrote:
Terry Pinnell wrote:

In my shed I just came across a very old but apparently high quality
mic, a Sony 'F-99B', 'One point stereo dynamic microphone IMP.LOW' It
proved useless when I tried it on my PC, in the usual mic socket.

My multimeter shows its left and right connections as about 170 ohms.

What's the simplest way of getting it to work on my PC please?

I once had a mic like that so I slapped together a bog-basic common
emitter amplifier as a preamp.

For this you can do common base, too!

On Wed, 21 Sep 2011 20:51:13 +0100, Terry Pinnell
terrypingm_at_DELETEgmail.com> wrote:

Tim Wescott <tim_at_seemywebsite.com> wrote:

On Tue, 20 Sep 2011 10:29:05 +0100, Terry Pinnell wrote:

In my shed I just came across a very old but apparently high quality
mic, a Sony 'F-99B', 'One point stereo dynamic microphone IMP.LOW' It
proved useless when I tried it on my PC, in the usual mic socket.

My multimeter shows its left and right connections as about 170 ohms.

What's the simplest way of getting it to work on my PC please?

What _are_ the specs for a typical PC microphone input port? And are
there even specs, or does the circuit just happen to be what the
manufacturer felt like using at the time?

I gather that it's designed for electret microphones, then -- that makes
lots of sense. In theory that means that you could make a preamp that
would be powered by the PC, but that's only "simplest" if you have a well-
stocked junkbox, some knowledge, and some time.

Thanks Tim. I'm thinking about experimenting with that F-99B (seems a
shame to waste a fairly expensive mic) but it means cleaning up my shed
and workbench first - something I've been putting off for a couple of
years!

I have the first two of those requirements, but time is in short supply.

The circuits in the article referenced by bw up-thread are one starting
point, if I do make the initial effort.
http://www.hobby-hour.com/electronics/computer_microphone.php

I could even avoid the spring clean by doing some bread-boarding here at
my PC. ;-)

If you are going to build a preamp anyway, you may want to
connect it to Line In instead of Mic In. Although Mic In is
about 10 times more sensitive (say, 100 mV vs 1V
full-scale), it may be more than 10 times noisier. It also
has (in the sound cards I've tested) a reduced frequency
response of about 9 kHz or so, instead of the full 20 kHz.
(That's not a problem for the originally-intended use with
voice, probably even a good thing.)

I've also seen some sound chipsets that have strange
behavior on Mic In, which might be related to the reduced
frequency range. Looks like it is sampling at a reduced
rate (maybe 18 kHz instead of 48 kHz) and then using sample
rate conversion to get back to 48 kHz equivalent. But there
seems to be minimal anti-aliasing on the original
conversion, so high frequencies (above 9 kHz) wrap back down
with hardly any attenuation. This would be a Very Bad Thing
for music recording.

Mic In is always mono-only, with the Left (white) line
carrying the signal. The Right (red) line is only used for
phantom power for the electret's built-in FET preamp, which
I seem to recall is typically 5V through a 4.7K resistor (or
something like that). But I think some (most?) setups may
apply that directly to the Left. That bias doesn't cause a
problem since the sound card input is AC coupled.

Best regards,


Bob Masta

N0Spam_at_daqarta.com (Bob Masta) wrote:

On Wed, 21 Sep 2011 20:51:13 +0100, Terry Pinnell
terrypingm_at_DELETEgmail.com> wrote:

Tim Wescott <tim_at_seemywebsite.com> wrote:

On Tue, 20 Sep 2011 10:29:05 +0100, Terry Pinnell wrote:

In my shed I just came across a very old but apparently high quality
mic, a Sony 'F-99B', 'One point stereo dynamic microphone IMP.LOW' It
proved useless when I tried it on my PC, in the usual mic socket.

My multimeter shows its left and right connections as about 170 ohms.

What's the simplest way of getting it to work on my PC please?

What _are_ the specs for a typical PC microphone input port? And are
there even specs, or does the circuit just happen to be what the
manufacturer felt like using at the time?

I gather that it's designed for electret microphones, then -- that makes
lots of sense. In theory that means that you could make a preamp that
would be powered by the PC, but that's only "simplest" if you have a well-
stocked junkbox, some knowledge, and some time.

Thanks Tim. I'm thinking about experimenting with that F-99B (seems a
shame to waste a fairly expensive mic) but it means cleaning up my shed
and workbench first - something I've been putting off for a couple of
years!

I have the first two of those requirements, but time is in short supply.

The circuits in the article referenced by bw up-thread are one starting
point, if I do make the initial effort.
http://www.hobby-hour.com/electronics/computer_microphone.php

I could even avoid the spring clean by doing some bread-boarding here at
my PC. ;-)

If you are going to build a preamp anyway, you may want to
connect it to Line In instead of Mic In. Although Mic In is
about 10 times more sensitive (say, 100 mV vs 1V
full-scale), it may be more than 10 times noisier. It also
has (in the sound cards I've tested) a reduced frequency
response of about 9 kHz or so, instead of the full 20 kHz.
(That's not a problem for the originally-intended use with
voice, probably even a good thing.)

I've also seen some sound chipsets that have strange
behavior on Mic In, which might be related to the reduced
frequency range. Looks like it is sampling at a reduced
rate (maybe 18 kHz instead of 48 kHz) and then using sample
rate conversion to get back to 48 kHz equivalent. But there
seems to be minimal anti-aliasing on the original
conversion, so high frequencies (above 9 kHz) wrap back down
with hardly any attenuation. This would be a Very Bad Thing
for music recording.

Mic In is always mono-only, with the Left (white) line
carrying the signal. The Right (red) line is only used for
phantom power for the electret's built-in FET preamp, which
I seem to recall is typically 5V through a 4.7K resistor (or
something like that). But I think some (most?) setups may
apply that directly to the Left. That bias doesn't cause a
problem since the sound card input is AC coupled.

Best regards,


Bob Masta

Thanks Bob. That seems to square with the circuits above, yes?