repairing an electret microphone

[Peter]

To replace a wire which broke in the swivel
of the boom, I reconstructed the boom of an
Altec Lansing headset. This included
resoldering the wires to the back of the
electret capsule. The speakers still work
but the mic does not after this repair.

Found a similar failure after soldering a
new electret onto the wires of an inexpensive
Creative/Telex desktop mic.

Is the electret mic particularly heat sensitive?
If so, what technique is recommended? Already
I was careful to apply minimal heating.

Thanks, ... Peter Easthope

 

[Arfa Daily]

To replace a wire which broke in the swivel
of the boom, I reconstructed the boom of an
Altec Lansing headset. This included
resoldering the wires to the back of the
electret capsule. The speakers still work
but the mic does not after this repair.

Found a similar failure after soldering a
new electret onto the wires of an inexpensive
Creative/Telex desktop mic.

Is the electret mic particularly heat sensitive?
If so, what technique is recommended? Already
I was careful to apply minimal heating.

Thanks, ... Peter Easthope

I've never found them to be particularly heat sensitive within common sense
limits, but note that they are polarity sensitive, as they contain a FET
preamp which is phantom powered via the output terminal ...

Arfa

 

[Arfa Daily]

A few electret mic are phantom powered but the majority use a form of AB
power for the capsule.

--
*Gun Control: Use both hands.

Dave Plowman [email protected] London SW
To e-mail, change noise into sound.

I'm using the term "phantom powered" loosely Dave, in that the audio out is
floating on the DC in, because there are only two connections, one of which
is the FET drain terminal, the series DC 'feed' resistor therefore actually
being the drain load resistor. The other terminal (also the capsule case) is
of course ground or FET source. Where there is this type of power / signal
setup, it is often considered generically, to be 'phantom powered' ...
Excuse my ignorance, but what's "AB power" ?

Arfa

 

[Arfa Daily]

Not a good idea to use it where mics are concerned, though. Can cause
confusion.

Other thing is not all electrets have the same ground polarity. But
hopefully are marked.

AB or T power is where the DC is fed up the audio pair on a balanced mic.
Dates from before true phantom and has the disadvantage it puts DC across
a moving coil mic - although when it was common many used a PS for each
mic, rather than getting it from the mixer.

OK, but seems that all of this is tending to look at 'whole' mics as in
something that a newsreader would clip to himself, whereas the original
poster was talking about just the capsule inside, which was also what I was
referring to. As far as the polarity of electret mics varying, I can't
remember ever seeing a capsule where the case wasn't a negative-side ground,
and over the years, I have dealt with and replaced many in cordless phones
and similar.

I appreciate that to a sound engineer, the term "phantom power" has a
slightly special meaning in terms of voltage level etc, but that still
doesn't change the fact that generically, any system where DC power is
supplied to an active signal source, using only the wires that are carrying
the signal rather than any additional power carrying wire, are considered to
be 'phantom' powered, irrespective of the voltage involved. TV antenna
amplifiers for instance, are often described as being phantom powered, as
also are satellite LNBs.

However, all that said, I do take your point that it could give rise to
confusion between a sound engineer reading it, and an electronics engineer,
who might better understand the overall concept. Perhaps it would be better
to call the electret capsule 'line powered' ...

Arfa

 

[Arfa Daily]

Indeed - that's always how aerial pre-amps etc which use this technique
are described. At least in my experience.

No Dave, they aren't. The term "phantom power" is used for many situations
where an active device needs powering and only the signal cable is
available. Antenna amplifiers, satellite LNBs, cable operators' distribution
amplifiers and so on, are all routinely described as being "phantom
powered". There are plenty of web references to the technique of phantom
powering in these applications. Long ago, when I worked in the early days of
cable TV, all of our line amplifiers were powered in this way, and it was
always referred to as phantom powering, both by all of the high-end network
engineering bods, and also our in-house lecturers, responsible for training
of all of the company's engineers.

In fact I would go as far as to say that the technique has been around and
called that for a very long time, and the 'hi-jacking' of the term by sound
engineering to try to mean something very specific, is actually the
questionable use of the phrase.

Phantom power is so called because it is invisible to the actual required
signal. Line power doesn't qualify in this way.

How so ? What do you perceive as being the difference ? If DC is travelling
one way, and signal the other on a single cable, they must be mutually
invisible (or made so by appropriate circuitry techniques at the active
device, and what it's feeding at the far end of its cable). I have never
seen any distinction made before. As far as I am aware, "Line powered" is
just a slightly more technically descriptive way of expressing "phantom
power".

Arfa

 

[Peter]

Arfa wrote, "... they are polarity sensitive, as they
contain a FET preamp ..."

On the Altec, I had the polarity wrong. Should have
trusted the mark I applied before removing the wires.

The headset works as well as ever now.

In this poor picture, the original capsule from the
Creative/Telex desktop mic is on the left.

http://carnot.yi.org/Mics.jpg

The replacement, on the right, has no silver
trace from a solder pad to the capsule. Half of
the head of the capsule is a brown Bakelite color
typical of a PCB. The other half is black. Is that a
conductive layer to connect one pad to the capsule?

In any case, I can buy another mic and try again.

Thanks everyone for the help, ... Peter E.

 

[Adrian Tuddenham]

A phantom circuit is a different thing. Usually only used in desperation
as the quality isn't as good as the discrete pairs.

If the lines were designed to be phantomed, as many long-distance
telephone circuits were, the phantom should meet the same spec as the
main pairs.

 

[Adrian Tuddenham]

They did. The quality didn't suffer, until there were multiple
layers of phantom circuits. Some places used so many phantom circuits
in downtown areas that a problem on a single pair would affect dozens of
circuits. They were quite common in the days when each phone line was a
pair of single wires run between poles. They quickly ran out of room
for more pairs, and had to be creative until multiple pair lead sheathed
cable was developed.

As a slightly quirky aside: starquad cable uses the 'phantom' for the
main circuit with the side pairs short-circuited. The 'phantom' in this
particular case gives better immunity to localised inteference than the
side pairs would on their own.

 

[Tim Phipps]

OK, but seems that all of this is tending to look at 'whole' mics as in
something that a newsreader would clip to himself, whereas the original
poster was talking about just the capsule inside, which was also what I was
referring to. As far as the polarity of electret mics varying, I can't
remember ever seeing a capsule where the case wasn't a negative-side ground,
and over the years, I have dealt with and replaced many in cordless phones
and similar.

IIRC the capsule in the Realistic (Tandy/RadioShack own brand) PZM
microphone had the +ve side connected to the case.

 

[Arfa Daily]

Wasn't the one for the Challenger O rings, was it?

--

Now *that's* below the belt even for you ...

Arfa

 

[Arfa Daily]

IIRC the capsule in the Realistic (Tandy/RadioShack own brand) PZM
microphone had the +ve side connected to the case.

Fair enuff. Still, Tandy / Radio Hack says it all ...

Arfa

 

[Adrian Tuddenham]

... No new phantom circuits have been
installed in decades.

Sorry to spoil your rant, but I used a phantom system to save on copper
when recording stereo from a remote position; I installed it last year.

It runs two high grade line-level audio circuits over dirt cheap 4-core
burglar alarm cable and the phantom can either be used for a third mic
or for talkback.


There's always one awkward so-and-so, isn't there....

 

[Tim Phipps]

Not only RS. Some pro mics which use an electret have the capsule body
reversed from the norm.

BTW, that original RadioShack PZM was a very good bit of kit.

Agreed, it was one of the few decent things you could buy in Tandy.
Although it came with an unbalanced jack fitted, its output was actually
balanced so all one had to do was cut the plug off and fit an XLR instead

 

[Adrian Tuddenham]

My recollection of when they were common in, say, outside broadcasts, was
that they were reserved for talkback use, due to the quality not being as
good as finite pairs. Of course over a relatively short run this might not
be noticeable.

Over a 100 metre run, the phantom was quite good enough for a
full-bandwidth (20c/s - 20Kc/s) mic circuit. As it is usually the
terminating equipment that determines the quality on 'short' lines, I
would not expect much worse over a kilometre or two.

The crosstalk of the phantom to the outer pairs was around 60dB at
10Kc/s whereas the pair-to-pair crosstalk was better than 80dB (both
worsened at 6dB/octave). That might vary with line balance and is also
affected by the physical arrangement of the transformers, which all had
to be mounted in the same box and did not have any screening pots.

It would even have been possible to run a 100v P.A. line as talkback on
the phantom, because the breakthrough would only occur when the side
pairs weren't carrying live programme.

Can I ask why you used burglar alarm cable rather than similar sized
telephone? It's just that telephone cable is twisted pair whereas alarm
usually not.

It was the cheapest four-core flexible cable in the Radiospares
catalogue and it was twisted with a constant physical relationship
between the cores. Telephone twised pair would have been fine for a
fixed installation, but this was for occasional 'outside broadcast' type
of use where a a lot of bending could occur, so a flexible type was
preferred.

Normally cost of the terminating equipment would make such an
arrangement uneconomical for 'short' lines of a few kilometres (or even
less in this particular case), but the mics already had built-in preamps
which delivered 0dBm level and the transformers were part of a surplus
job lot which I obtained cheaply many years ago.

Given the choice between buying expensive mic cables or buying burglar
alarm cable and sticking some spare transformers in a box, I decided to
give phantoming a try - and was pleasantly surprised by the results.

 

[Adrian Tuddenham]

How many miles? The circuits I saw were more than 20.

The test run was only 100 metres, but runs up to a couple of kilometeres
may be used in future. (See my other reply for the reason this was
economical in this particular case)

That wasn't a telephone phantom circuit installed by a phone company,
was it?

No, although there is no engineering reason why a telephone circuit
could not be used.

I ran the same kind of four conductor wire for a microphone, speaker,
transmit switch and a defeat switch for the tone squelch, but only
because there was no way to run new cable between a home brewed remote
head, and the outdoor mounted 330 W low band commercial FM base station.

I moved a school intercom console from one office, to another at the
other end of the building. There was no way to run more cable between
the separate foundations, and there weren't enough shielded lines run
when the addition was built, so I found an abandoned 16 pair 1A1
telephone cable and used it for the additional 12 classrooms. There are
thousands, or even millions of homebrew hacks that have been used from
time to time.

This is where someone with a good wide working knowledge of electronics
can achieve the 'impossible' while the graduate engineer is still saying
it can't be done.

 

[Adrian Tuddenham]

Right. I must admit to being surprised that it's strong enough for that
sort of use. But if it works, that's all that matters.

For the difference in price, it is worth the extra effort of being
gentle with it. I don't let anyone else do the rigging.

Yes - I've played with it here. Acquired a load of rep coils from a
redundant facility.

My comments about a phantom circuit not performing as well as a copper
pair date back to using them from a remote OB location back to studio
centre. And it's quite possible the two pairs that hosted it went by
different routes. So had rather different characteristics.

That sort of routing might work at DC, but I wouldn't trust it above
50c/s. If the phantom is 'official' the telephone people ought to
ensure that it is always run on two adjacent pairs; I have even heard of
a long route where the phantom signal was on a separate pair for part of
the route and only phantomed on the congested bits.

My phantom is always in the same cable, so I don't need to worry about
the pairs getting separated.