Constant Voltage System

[redback]

Hi all,
I have 2 identical speakers (Redback C0960) they are of the 100volt type
I also have a AudioTelex AT120R amplifier which has 100volt output
I think have done a minimum safe load impedance calculation correctly, which came to around 83 ohms...

Now I want to connect these speakers to this amp but the resistance of the speakers are different, one speaker, common to the 60watt tap is 4ohms. the other speaker common to the 60watt tap is 16 ohms.
This dosen't seem right?
Could the transformers be faulty?

A bit of Background, these speakers were connected to this amp previously, but somebody decided that more speakers were necessary and added 3 normal 8ohm without transformers.... which eventually blew the amp and 2 of the 3 extra speakers up!

I have had the amp repaired, removed the offending speakers and need to connect these redbacks back up to the amp, and really do not want to blow the amp up again.

Any Help muchly appreciated

[Nigel Goodwin]

You appear confused about 100V line speakers?.

There's no need (and indeed no point!) looking at the impedances, all you need to look at is the wattages.

The output of the amplifiers is via a transformer, which gives the '100V' signal level, and each speaker has it's own transformer that accepts a 100V input, and provides a tapped secondary to select how many watts you want to that particular speaker. All you do is add all the selected wattage taps together, and make sure the amplifier is rated high enough to provide that amount of power. If the amplifier isn't rated high enough, you can either get a bigger amplifier, disconnect some of the speakers, or drop the wattage taps on the speakers until it's within the amplifiers specs.

It's really that easy!.

As you mentioned, an idiot connecting a speaker directly to the 100V line will kill both the amp and the speakers!.

[Enzo]

And i would add that constant voltage audio distribution is not very different from the constant voltage distribution of the mains power in your home. You can plug one, two, or three table lamps into the wall outlets without regard to their impedances. Each draws power as it needs from the constant supply voltage. All you need to do is keep the total draw under the power limits of the system.

The whole point of constant voltage systems is so you can make various combinations of speakers without worrying about impedance. Imagine trying to come up with a series/parallel wiring arrangement for 50 speakers in the ceiling of an airline terminal.

[redback]

Thanks guys

So it dosen't matter what the resistance of each speaker box is, as long as you keep the total WATTS drawn to be equal or below that of the amplifier.

And, I can safely connect the two column speakers up to the amp?


Cheers...

[Nigel Goodwin]

Originally posted by redback

And, I can safely connect the two column speakers up to the amp?

Yes, assuming they have 100V line transformers in them?, and the total watts for all the connected speakers is less than the rated power of the amplifier?.

[redback]

thanks for your help....

I'll do it this weekend

cheers

[redback]

Just one more question, If the speaker transformer primary resistance is only 12ohms on one speaker

I=V/R 8.3=100/12

and if i turn another speaker zone on that has a transformer primary resistance of 12ohms, that would then make a 6ohm load
which would then be drawing a current in the vicinity of 16.6 amps

which I think maybe just a bit too high and would surely blow the amp up...

What would a normal 100volt transformers primary resistance be usually, i'm guessing somewhere up in the 1000 ohm + region

thanks

[Nigel Goodwin]

Originally posted by redback
Just one more question, If the speaker transformer primary resistance is only 12ohms on one speaker

I=V/R 8.3=100/12

and if i turn another speaker zone on that has a transformer primary resistance of 12ohms, that would then make a 6ohm load
which would then be drawing a current in the vicinity of 16.6 amps

which I think maybe just a bit too high and would surely blow the amp up...

What would a normal 100volt transformers primary resistance be usually, i'm guessing somewhere up in the 1000 ohm + region

I've no idea, it's NOT a concern, you just add the watts up written on the transformers - you don't need to know the impedance, that's the whole point!.

[Curmudgeon]

If I understand correctly, you're measuring the transformer DC resistance. However, that is not what the amplifer sees. It sees the speaker impedance multiplied by the transformers winding ratio, squared. For an ideal transformer, the winding resistances would be zero; the DC resistance is just the main factor in the transformer not being 100% efficient.