Re: repairing an electret microphone

Dave Plowman (News) <dave@xxxxxxxxxxxxxxx> wrote:

In article <1iy4gt1.1q1ewmn1v4u08uN%adrian@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Adrian Tuddenham <adrian@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
... 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.

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 ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
.

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