Just got two Fixer-upper mics for $11 each on Ebay

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Paul678

Well-known member
Joined
Mar 3, 2013
Messages
97
They were both easy soldering fixes.

One is the Audio Technica AT811.  It's actually
a good, darker microphone, with very little
high end.  So it's not very good for acoustic
guitar, but it might be good for someone's vocals,
because it has very warm, full bodied mids, low-mids,
and bass end.  Or it might be good for a really
shrill instrument, when you want to take the high end
off.  Someone mentioned these are good for horns.

The problem is, someone made the piss-poor decision
to make this battery powered only, so it cannot use
phantom power stock.  I'm sure there's a way to
convert it to phantom power, so if someone
knows how, please let me know, as it would likely
increase the output, which is a bit low.

If I don't end up using it, I'll sell it for
$50 or so.

The other mic is a mystery mic:

      http://farm4.staticflickr.com/3686/9113658956_24a58573ca_b.jpg

      http://farm8.staticflickr.com/7387/9111437551_8012b09898_b.jpg

Anyone know what it is?  There are no markings at all.
It works, but has a very tinny tone.  I'm thinking of
replacing the capsule with my old SP-1 capsule.

 
Ok, so apparently electret mic capsules
go bad after a while, so it's possible
my AT811 is too old, which would explain the
low output and very low high end.

Does anyone know what would be a good, modern
electret replacement capsule for the AT811?
 
Ok, sent AT an email......

At the moment, I'm having a hard time accessing the
capsule, just to take a look at it (possibly replace it later).

  There is the metal collar with the threads on each side,
that you unscrew from the main body to install the battery:

        http://img.auctiva.com/imgdata/9/2/8/3/5/7/webimg/661575044_tp.jpg


  It appears that you can also unscrew it from the head-basket,
to access the capsule, but I was not able to budge it.  I
even tried heating it up a bit with my hot-air gun, and still
no-go....
 
Ok, the mystery mic was some sort of AMR Audio
mic.  It doesn't look like they stayed in the
mic business very long, and judging by the sound of this
mic, I can guess why.

The audio totally lacks the low bass end.  Here
is a very rough schematic I drew up (please correct
any mistakes):

    http://farm8.staticflickr.com/7312/9144472886_4c5e3e6aae_o.jpg

It appears the FET is Darlingtoned to the bipolar, and
I would assume the transformer is part of the reason
the bass frequencies are so attenuated.

If someone could enlighten me about the purpose of the
second J231 FET, it would be appreciated.

Here is the electret capsule:

    http://farm8.staticflickr.com/7366/9142246653_0a056e2025_o.jpg
    http://farm6.staticflickr.com/5323/9144473184_a6c6962cdb_o.jpg

It quite likely this old capsule has lost its charge, so
I'm thinking of looking for a replacement.  Any suggestions
appreciated.  Also, I was thinking of making one large
opening, instead of 5 holes, so that there is less
refractions, with perhaps a smoother response, but this
is a sealed aluminum casing, and from what I saw
online, I would likely ruin the capsule if I tried to
to open it up.

Any advice is GREATLY appreciated. Thank you.
 
I have several electret microphones that are more than 40 years old, but they all sound the same as at the moment I bought them.
So I think something else must be wrong with them, IMHO it is not the aging alone.
No idea about the influence of humidity. It might be that this is a factor in the aging of electrets. ( ?? )
 
RuudNL said:
I have several electret microphones that are more than 40 years old, but they all sound the same as at the moment I bought them.
So I think something else must be wrong with them, IMHO it is not the aging alone.
No idea about the influence of humidity. It might be that this is a factor in the aging of electrets. ( ?? )

Well, from what people have been telling me, is that it depends on the manufacturer, but
some have told me their older electret mics are useless now.

It's possible this old capsule has lost its charge, so
I'm thinking of ordering one of these:

      http://www.transsound.net/electrets/tsb-160a.htm
 
Here is an updated schematic:

   
9160363846_1d7ec94150_b.jpg


I believe the unusual 3 band resistor, RED BLACK YELLOW, is a 200k resistor.  It was
completely open when I measured it, so it got damaged somehow, and will need to
be replaced.  Can someone explain it's purpose, in parallel with the 9.1V zener?

And I am aware the second JFET is supposed to be a constant current source,
but where is the current regulation as I have drawn the schematic (which matches the mic)?

If the gate leakage current is in the picoamps, then the gate should pretty much follow the
"source", which we will call the top junction (which is actually labeled the drain, but we assume
the JFET is symmetrical), and the channel should always be "ON" or conductive, because of the
22k resistor.  This is because the resistor is in the gate leg, not the "source" leg. If the resistor was in the "source" leg,
then with large current draw, there will be a large voltage drop across the resistor, and
Vgs will become negative, and pinch-off of the channel will occur.

But this is not how the mic is wired.

Also, isn't there automatically some current sourcing due to the
6.8K resistors in the +48V phantom power?

Remember this mic was really tinny sounding: very little bass.  If anyone can suggest
ways to improve the sound, please do advise.......
 
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