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Joined: Dec 2001
Posts: 444
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Sandro Offline OP
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A GC asked if I could install some ceiling speakers for the sound system in a cosmetics store we are working on.

The speakers have a 70V xfmr and 6 taps with wattage ratings and I couldn't locate a common.

Anybody have a website, explaining this system and how to wire?

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Joined: Jul 2002
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Sandro,
These systems are rather basic in thier design.
All they really are is a bunch of Mono-phonic speakers all hooked up in Parallel.
The reason for the 70V (or the 110V, we use here) is because of the long runs of speaker wire involved.
All the transformer on the speaker does is isolate the speaker from the 70V line voltage and also match the speaker to the line, hence the tappings. [Linked Image]
I ran my system in Cat-5e. [Linked Image]

[This message has been edited by Trumpy (edited 05-15-2005).]

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Quote
(or the 110V, we use here)

110, or did you mean 100V? The latter is the British standard.

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The reason for the 70V (or the 110V, we use here) is because of the long runs of speaker wire involved.
All the transformer on the speaker does is isolate the speaker from the 70V line voltage and also match the speaker to the line, hence the tappings.


Well, not exactly. The main reason for a constant voltage system is (in order of importance):

1)To allow a number of speakers to be easily connected to the output of an amplifier without regard for impedance matching.

2)To allow the levels (wattage) of each speaker to be easily adjusted independently of the others. This is the reason for the taps, not impedance matching.

3)Allow longer wire runs and/or a smaller size wire as compared to a low impedance amplifier output/voice coil connection. Ohm's law still applies here and you still need to do voltage drop calculations especially with large wattages or long runs.

I ran my system in Cat-5e.

I really didn't need to hear that! [Linked Image]

-Hal

Joined: Dec 2001
Posts: 444
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Sandro Offline OP
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Hal...

Since we are still on the subject...the wattage taps on the speakers, are these Output watts? If not, what Watts are they? The Amp provided to me was output of 35Watts. Using the 2.5W tap that I did, multiplied by 5 speakers total, gives me 12.5 watts. If the 2.5W is actual sound watts, isn't this kind of low? (I never did get to test the system after I hooked it up.)

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Well, let me see if I can explain it this way using actual impedances which work "behind the scenes" with a constant voltage system.

Lets say we have a 10 watt amplifier with the 8 ohm output directly connected to an 8 ohm speaker. One would expect all of the 10 watts to be delivered to the speaker.

Now say we have that same 10 watt amplifier with a 70.7 volt output. In many cases the 70.7 volt output is created using a transformer similar to the ones on the speakers but in reverse- the 8 ohm winding is connected to the 8 ohm output of the amplifier and the high impedance winding is the 70.7 volt output.

If you do the math you will see that the 70.7 volt winding (output) impedance is 500 ohms at 10 watts. So, to deliver that 10 watts to a single speaker we would need a transformer on the speaker with a 500 ohm primary and an 8 ohm secondary.

Now suppose we had two speakers and we wanted to supply them with 5 watts each. The taps on the speaker transformers would need to be 1000 ohms.

To take this a step further now suppose we have a 100 watt amp. The 70.7 volt output will be 50 ohms. Hang that same speaker with the 1000 ohm tap on it and you will still get 5 watts from the speaker.

So the "secret" behind the constant voltage system is that as long as we are talking about the same voltage (hence constant voltage) you can simply mark or identify the taps on the speaker transformer primaries according to the wattage that will be provided to the speaker.

You don't have to worry about the impedances of the amp output or speaker transformer taps, that's all been done for you. Rule of thumb is you simply add up all your speaker wattages and provide an amplifer at least 10% larger to account for transformer losses and head room.

Is 2.5 watts kind of low? Depends. In Yankee stadium yes, in an office probably too loud especially with 5 speakers. Keep in mind that the amp level is usually turned down below maximum output so you won't even get 2.5 watts per speaker except perhaps at low (bass) frequencies if the transformers you used have a frequency response that low. But that's another story.

-Hal

Joined: Jul 2002
Posts: 8,407
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Yeah Paul,
That should actually read 100V.
Hal,
I was merely joking about using Cat-5e for speaker systems.
There's no way that that would fly here. [Linked Image]


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