repairing an electret microphone

So that's still the case? Or did they find ways round this 'impossible' situation? Like running multicores? And hopefully underground?

I can quite see a phantom circuit being cleaner than a faulty copper pair. However, try running that phantom circuit over faulty copper pairs...

The equipment could well have been the subject of a patent. But did they invent the principle? I did do a quick Google but got nowhere.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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IIRC the capsule in the Realistic (Tandy/RadioShack own brand) PZM microphone had the +ve side connected to the case.

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Tim Phipps

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Are you really that stupid? No new phantom circuits have been installed in decades. As I said in other posts, the inability to run more open pair circuits led to the development of lead jacketed multi pair cable, which has mostly been replaced with Fiber optics. Phantom telephone circuits were needed early last century, but technology has passed them by. In areas where there is still copper to the CO, a newer form of phantom is used, by multiplexing multiple lines to a single pair. Typical audio grade is about 16 per pair. The technology wasn't available in the early days of telephone.

Which faults? How far away from the phantomed portion? Give me some real numbers.

WE listed their patent numbers on their products. I never saw anything they built under a license to another company. WE was the manufacturing arm of Bell Labs. I know you like to think that the US stole every idea we ever had, but it just isn't true.

To confuse stupid bastards. It works really well, doesn't it?

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Crikey. How many other versions of 'phantom' are you going to use? Have you ever written a technical spec?

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    Dave Plowman        dave@davenoise.co.uk           London SW
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Yes, but you won't be allowed to read them, without the proper security clearance.

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

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    Dave Plowman        dave@davenoise.co.uk           London SW
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Now *that's* below the belt even for you ...

Arfa

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

Arfa

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.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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No, it was for the communications equipment in use when they announced, "We have loss of Telemetry". Do you know what that means?

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I'm beginning to believe he has EEyore disease.

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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.... :-)

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~ Adrian Tuddenham ~
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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.

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.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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How many miles? The circuits I saw were more than 20.

That wasn't a telephone phantom circuit installed by a phone company, was it? All references to them were removed for the phone company manuals decades ago, and doesn't use WE or other phone company phantom networks.

is a description of the circuit from some old Army manuals.

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.

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or a fast hand ! ).

Not given the size of these babies. They're minute.

Graham

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 :)

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Tim Phipps

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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.

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.

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~ Adrian Tuddenham ~
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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)

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

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.

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~ Adrian Tuddenham ~
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www.poppyrecords.co.uk

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.

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.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.