If you bought an SM-81 in 1976 and kept it in extremely humid environments,
it will have lost less than 1% of its charge.

On the other hand, if you bought a Sony ECM-series mike in 1976 and it works
at all today, it's something of a miracle.


Well then my place is due to become something like Lourdes. Both of my
ECM377's and my ECM23F still work, despite being stored in a relatively
humid cellar. I admit that one of the 377 shows pretty low out put.

Which brings me to the next couple of questions. Is there a way to
restore charge to these thing? Failing that, would it be possible to
convert them to externally polarized? I'm not worried about the 23F.
It wasn't much of an investment, and I've seldom used it for years. I
do like the 377's and paid more for them. The capsules do not look
significantly different than capsules in extrenally polarized mics. The
cases are probably roomy enough to accept some extra circuitry, and they
already take phantom for their amps. Is there possibly a simple
daughterboad that could be installed?

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Patrick Covert wrote:

Is there a way to
restore charge to these thing? Failing that, would it be possible to
convert them to externally polarized?

No, and no. An electret is sort of the functional equivalent of a
permanent magnet, and it's formed in a similar manner, but since it's
an integral part of the capsule, (unlike the magnet on a loudspeaker)
it doesn't lend itself well to re-charging. The charge is (or maybe
this the old method) applied when the plastic material is still soft,
and you don't want to do that once it's built into the microphone. When
they're dead, they're dead.

I was in a TV studio in Japan in 1970, and they had a bunch of electret
mics with the capsules stored in laboratory dessicator jars.

Just for kicks, I unearthed my two ECM-21 mics from 1969. They both
work, probably near as well as they did originally.

Patrick Covert wrote:
Well then my place is due to become something like Lourdes. Both of my
ECM377's and my ECM23F still work, despite being stored in a relatively
humid cellar. I admit that one of the 377 shows pretty low out put.

Hey, I wouldn't complain.

Which brings me to the next couple of questions. Is there a way to
restore charge to these thing?

No. In theory, you could take the capsule apart and physically replace
the electret with a new one cut to shape. But, these capsules are small
and not intended to be repaired. It would be more difficult and more
touchy work than rebuilding a U87 capsule, and you wouldn't be able to
charge anywhere near as much for it.

Failing that, would it be possible to
convert them to externally polarized?

Maybe. I tried to do this on some Japanese microphones from the early
seventies and I never had any luck with it, but in theory you might be
able to make something like this work. It won't sound the same, though,
because you won't necessarily have the same polarization that the original
electret had.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article ,
(Scott Dorsey) wrote:

Failing that, would it be possible to
convert them to externally polarized?

Maybe. I tried to do this on some Japanese microphones from the early
seventies and I never had any luck with it, but in theory you might be
able to make something like this work. It won't sound the same, though,
because you won't necessarily have the same polarization that the original
electret had.
--scott

So, what is it I'm not seeing when I look at the capsule? Looks pretty
much like a standard brass capsule. Would the electret be the membrane,
or the body?

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Scott Dorsey wrote:
It won't sound the same, though,
because you won't necessarily have the same polarization that the original
electret had.

That should only affect sensitivity, not frequency response.
It will be slightly higher in harmonic distortion, though
because, like all externally polarized capsules, the charge
will migrate to where the diaphragm and plate happen to be
closest together making the biasing field dependant on
diaphragm position. That it can't do that with an electret
is one of the electret's advantages.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

Patrick Covert wrote:
In article ,
(Scott Dorsey) wrote:


Failing that, would it be possible to
convert them to externally polarized?

Maybe. I tried to do this on some Japanese microphones from the early
seventies and I never had any luck with it, but in theory you might be
able to make something like this work. It won't sound the same, though,
because you won't necessarily have the same polarization that the original
electret had.
--scott


So, what is it I'm not seeing when I look at the capsule? Looks pretty
much like a standard brass capsule. Would the electret be the membrane,
or the body?

It can be applied to either the diaphragm or the back plate.
IIRC, there is some small advantage to the plate
application but I can't remember what that advantage is.
I've no idea how you could tell from inspection which it is
but I don't think it much matters even if you are going to
assist it with external polarization.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

On Tue, 30 Aug 2005 22:43:27 -0700, Bob Cain
wrote:

It will be slightly higher in harmonic distortion, though
because, like all externally polarized capsules, the charge
will migrate to where the diaphragm and plate happen to be
closest together making the biasing field dependant on
diaphragm position. That it can't do that with an electret
is one of the electret's advantages.

Now that's very cool. Had no idea; thanks,

Chris Hornbeck

"Bob Cain" wrote in message
...


Actually, you can make them externally polarized. Just apply a voltage
between diaphragm and back plate. The E field will add to whatever is
there from the electret whether it's on the diaphragm or the plate.

Are you kidding?
(1) Good luck finding 1 gigaohm resistors to bias the capsual.
(2) Good luck finding somewhere to attach a wire to the diaphragm.

NS.

Bob Cain wrote:

Actually, you can make them externally polarized. Just
apply a voltage between diaphragm and back plate.

Easy to say, but difficult to do. But how difficult would depend on the
construction of the individual mic. The only one I've actually looked
at inside are my Sony ECM-21s and the capsule is built pretty much like
a single unit. I can't imagine anyone but a really dedicated machinist
successfully making points to which a polarizing voltage can be
connected.

It would probably be just about as easy to build a whole externally
polarized capsule.

Patrick Covert wrote:
In article ,
(Scott Dorsey) wrote:

Failing that, would it be possible to
convert them to externally polarized?

Maybe. I tried to do this on some Japanese microphones from the early
seventies and I never had any luck with it, but in theory you might be
able to make something like this work. It won't sound the same, though,
because you won't necessarily have the same polarization that the original
electret had.

So, what is it I'm not seeing when I look at the capsule? Looks pretty
much like a standard brass capsule. Would the electret be the membrane,
or the body?

The electret is the backplate, which is why strictly-speaking those
capsules are called "back-electrets."

Some early electret types used a thin ceramic electret as the diaphragm
itself. It was pretty high-mass, which meant the frequency response was
poor. This is much of why electrets got a bad reputation.

Some of these microphones are constructed with the diaphragm prestretched
and cemented onto a brass ring, so you can pop the diaphragm and ring
out (or unscrew it) to get inside the capsule. The SM-81 is a great
example of this sort of thing... the SM-81 capsule is designed to be repaired.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Bob Cain wrote:

Actually, you can make them externally polarized. Just
apply a voltage between diaphragm and back plate. The E
field will add to whatever is there from the electret
whether it's on the diaphragm or the plate.

You'd think so, but I could never make this work well. I suspect
you'd have to replace the electret with a conductive backplate.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article ,
"Neon Sound" wrote:

"Bob Cain" wrote in message
...


Actually, you can make them externally polarized. Just apply a voltage
between diaphragm and back plate. The E field will add to whatever is
there from the electret whether it's on the diaphragm or the plate.

Are you kidding?
(1) Good luck finding 1 gigaohm resistors to bias the capsual.
(2) Good luck finding somewhere to attach a wire to the diaphragm.

Now I'm really confused. Wouldn't there have to be a connection to the
diaphragm, in the first place?

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Neon Sound wrote:
"Bob Cain" wrote in message
...


Actually, you can make them externally polarized. Just apply a voltage
between diaphragm and back plate. The E field will add to whatever is
there from the electret whether it's on the diaphragm or the plate.

Are you kidding?
(1) Good luck finding 1 gigaohm resistors to bias the capsual.

Digi-Key stocks them and has for about seven years now. They are a
couple bucks each.

(2) Good luck finding somewhere to attach a wire to the diaphragm.

There's already one attached on the capsule. We're not talking about
an integral-FET capsule here.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article . com, "Mike Rivers" wrote:

Bob Cain wrote:

Actually, you can make them externally polarized. Just
apply a voltage between diaphragm and back plate.

Easy to say, but difficult to do. But how difficult would depend on the
construction of the individual mic. The only one I've actually looked
at inside are my Sony ECM-21s and the capsule is built pretty much like
a single unit. I can't imagine anyone but a really dedicated machinist
successfully making points to which a polarizing voltage can be
connected.

It would probably be just about as easy to build a whole externally
polarized capsule.


I was looking at this. One capsule I'm looking at, looks like the
diaphram is attached electrically to the case. The case sometimes
has a connection as seen from the rear, usually grounded.

http://www.pitt.edu/~szekeres/mic/wm63.jpg

From...
http://www.pitt.edu/~szekeres/mic/mic.htm

greg

Patrick Covert wrote:
In article ,
"Neon Sound" wrote:


"Bob Cain" wrote in message
...


Actually, you can make them externally polarized. Just apply a voltage
between diaphragm and back plate. The E field will add to whatever is
there from the electret whether it's on the diaphragm or the plate.


Are you kidding?
(1) Good luck finding 1 gigaohm resistors to bias the capsual.
(2) Good luck finding somewhere to attach a wire to the diaphragm.


Now I'm really confused. Wouldn't there have to be a connection to the
diaphragm, in the first place?

Of course, and 1 gig resistors are not at all difficult to
obtain.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

Mike Rivers wrote:
Bob Cain wrote:


Actually, you can make them externally polarized. Just
apply a voltage between diaphragm and back plate.


Easy to say, but difficult to do. But how difficult would depend on the
construction of the individual mic. The only one I've actually looked
at inside are my Sony ECM-21s and the capsule is built pretty much like
a single unit. I can't imagine anyone but a really dedicated machinist
successfully making points to which a polarizing voltage can be
connected.

But said connections must already exist. No capsule mod
should be needed. They would be external, in the FET buffer
amp circuitry.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

Scott Dorsey wrote:
Bob Cain wrote:

Actually, you can make them externally polarized. Just
apply a voltage between diaphragm and back plate. The E
field will add to whatever is there from the electret
whether it's on the diaphragm or the plate.


You'd think so, but I could never make this work well.

Interesting. Sure would be instructive to figure out why.

I suspect
you'd have to replace the electret with a conductive backplate.

There is a conductive backplate behind the electret to which
the connection is made. An interposed electret only adds to
the bias potential you can apply between diaphragm and
backplate. It shouldn't diminish any externally applied field.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

Scott Dorsey wrote:

The electret is the backplate, which is why strictly-speaking those
capsules are called "back-electrets."

A pedant would say that the electret material coats the
conductive backplate. :-)

At first blush one would think that the charge embedded in
the electret would be balanced away by an opposite charge
migating to the backplate but since there is a physical
distance between the electret charge and the one migrated to
the backplane, what is called a dipole exists which causes
the requisite field between plate and diaphragm.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein