I went to a company for a walk thru.

Almost 20 speakers in the ceiling.

A toa amp with the 8 ohm output connected. To a radio shack speaker selector. With a 2wire going to each of 4 zones.

Could these be 8 ohm speakers wired in series or are they 70v speakers playing through the 8ohm output.

Can a 70v speaker play thru a 8 ohm output?

yep!

are you sure they were 70V speakers? They could have been tapped as 8ohm as well.

This sounds like I'm scolding you, but anything as dumb as this could be wired in any way whatsoever. Whatever idiot did what you saw did something else idiotic that you haven't seen yet.

Turn it into a 70 volt system. Find out what's there, make a list of whatever specs you can find. Get back to us.

Convert this to a 70 volt system.

brucewayne,

70V is not scary at all. Actually, it's easier to work out amplifier loading than by slogging through the math at 4/8 Ohms. If you have a 50W 70V amplifier and speakers that are set for 5W each, you can have 10 speakers on that string. Need a little more output in one area? Set that speaker for a higher power and add up the total again. That's a whole lot easier than working out a series/parallel arrangement with 4/8 Ohm speakers, possibly with a balancing resistor or two. If you look closely at the transformers used for impedance matching volume controls, they look similar to the transformers incorporated in speakers designed for 70V systems. Actually, in some cases they could be the same transformer with different labeling. Before inexpensive stereo impedance matching controls were available, I would make my own using commercial 70V transformers. (no, they didn't fit neatly in a standard wall box)

If you need some formulas, check out Ernie's post. Work through a few example systems at 8 Ohms, then again at 70V. See, it's much easier to design a large system using the 70V math.

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You don't know what kind of "expert" designed and installed this system. Check out EVERYTHING (don't assume that all of the speakers are wired the same way) because, if you take over this system, any future failure will be your fault. One of my biggest concerns in a system such as this is hidden splices. I don't like to see wire nut junctions in stranded speaker wire runs because they are usually poorly done and become intermittent after a while. Much, much worse is the twist and tape splice.

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Yes, you can use a 70V speaker on a consumer amplifier designed for the 4/8 Ohm world, but the speaker might not play very loud -- depending on its tap setting.

one clarification on Buzz' excellent post. with a 50 watt amp, please don't tap above 40 watts. Headroom is desirable.

If you need more information, my favorite supplier is an excellent source.

Sure it can. Once.

What model of TOA amp?

And then again and again.

The transformer that's part of a 70 volt speaker steps down the voltage fed to it. A 70 volt speaker cannot be damaged by an amp intended for an 8 ohm load unless the amp puts out more than 600 watts.

Details:
If we go back to the Ohm's Law wheel and work out the power that a 70 volt amp would put out into an 8 ohm speaker (if it could), we'll find it's 612.5 watts. So what?

Well, a 70 volt speaker is safe to work with a 70 volt amp -- an amp that would have to deliver 612.5 watts if its load were 8 ohms. If we have an amp of less than 612.5 watts output into 8 ohms, then the power delivered to a 70 volt speaker will be less than you'd get from the power tap chosen when connecting the speaker. That is, a 5 watt tap will give you less than 5 watts output from the speaker. Therefore there's no chance of damaging the speaker.

For instance, a 300 watt (into 8 ohms) speaker outputs 48.98 volts at full power. If we connect a 70 volt speaker to that, we'll get less output than we would with an amp that outputs 70 volts at full power, so there would not be any damage to the speaker.

One point that might be confusing some people is that the 70V amplifier is not always putting out 70V any more than a consumer power amplifier is always delivering its rated power. The "70V" is simply the maximum voltage output. And, of course, if the Volume control is set to zero in either case, the output is zero Volts and any speaker that you connect will be safe and silent.

We go through similar considerations for power wiring, the major difference being that we do supply a constant voltage. In power distribution we usually specify the circuit current because this is easily related to the fire risk. We could note that a particular Romex wire is good for 2400W, but this is true only if we also specify the circuit voltage. Or, we could simply give the minimum resistance that the circuit can support, spec the resistance of the light bulbs, heaters, appliances, computers, etc. connected to the circuit -- then calculate the equivalent resistance of the load. How much fun would that be?


In all cases the goal is to manage current and prevent damage. The specification and math scheme used is mostly traditional, given the context. In home audio four or eight Ohms is easy and traditional. In Pro audio 70V is much easier to work with, and current or watts is easy in home power wiring because of the constant voltage.
 
Edit: As a homework assignment calculate the equivalent resistance for a 50W load at 70V. This is the loading that our 70V amplifier expects to work into. This is why the 70V amplifier might freak out when an 8 Ohm load is connected.

Now consider the converse -- a 5W 70 V speaker connected to a consumer amplifier. Calculate the maximum output voltage of the consumer amp when driving an 8 Ohm load. There's no freak out, but the sound pressure level is a bit low from our 70V speaker. However, this is not necessarily the end of the world because a reasonably efficient speaker driven at a few hundred milliwatts, is "loud" for many livingroom listeners. (But, far from "thumping" for the disco crowd.)

This is why I boringly repeated "at full power" when I was talking about amplifier output voltage.


No, it's the rated output for some particular level of distortion. You can often overdrive an amp and get more power out of it, but it will sound worse than crap.


Right.


I don't see it quite like that. In 70V wiring, you can have whatever quantity of speakers you want, with the limitation being the total power that's drawn. With home power (I assume you mean power amp) wiring we're feeding 4 or 8 ohm speakers, almost always in pairs. It's a different application.
 

This is VERY worthwhile to understand. If you don't understand it, you might get it backwards and have poor results.

Excess current damages amplifiers faster than it damages wire. Step over the output transistor's limit, even briefly, and it will send out smoke signals. Wire is very forgiving for brief excesses.