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70 volt speakers powered by an 8 ohm amp

post #1 of 7
Thread Starter 
The church I work at hired a consultant to redesign the sound system. Gentleman did a fantastic job and really knows his stuff. However, one item keeps driving me crazy because it doesn't make sense. For the lobby there was an existing 70v system. He had me remove the transformers from the amplifier and connect the speakers directly to the mono bridge out (didn't change anything else, so the speakers are still all 70v wired in parallel). When I questioned it, his response was that the amp was powerful enough. Has anyone heard of doing this?
post #2 of 7
Yes. All you need is an amp that will drive the load and has sufficient voltage swing. There will still be transformers at each speaker.
post #3 of 7
It works out to a 600 Watt 8 Ohm power amplifier. But you'll probably never run it at full volume and big power amplifiers are very robust, so even a 300/400 Watt amp should work just fine.
post #4 of 7
The "70V' speakers have transformers so that the volume level of each speaker can be adjusted at each speaker location by changing which transformer tap is connected to the speaker.

This allows you to avoid having a speaker that is too loud or soft at one location relative to another speaker somewhere else.

It also allows 10 or 12 speakers to be connected in parallel without overloading the amplifier, since the transformer coupling makes each speaker load a higher impedance.

The "70v" designation is the APPROXIMATE RMS voltage the amplifier will put out with the volume control TURNED ALL THE WAY UP!! The actual output voltage in use will typically be much lower.

Since you can buy a very good 70v amplifier with lots of desirable features for a low price, I would go a 70V amplifier for that kind of system. It sounds like someone doesn't know WTF they are doing. The way he set it up is stupid.

For example the Pyle PT610 for $130 at Amazon.
Edited by commsysman - 6/1/13 at 6:52am
post #5 of 7
Thread Starter 
Quote:
Originally Posted by commsysman View Post

It sounds like someone doesn't know WTF they are doing. The way he set it up is stupid.

Why is this setup stupid if the amplifier I have meets the the power output requirements as both DonH50 and Speedskater stated? The math does work out.
post #6 of 7
Whatever. Many PA system amplifiers provide both transformer and direct outputs, and a lot of "70V" PA amps these days do not include an output transformer -- just adds weight, volume, heat, and loss. Chances are good a slimline pro PA amp rated to drive 70 V lines does not have an output transformer (last one I put in did not). In the tube days the transformer was required anyway, and early SS units kept them for various reasons (including isolation since those early power transistors were not terribly reliable). Since then reliability has improved, power output has risen significantly, and protection circuits have gotten better so accidentally shorting the input leads while replacing a speaker does not kill the amp. Usually. smile.gif
post #7 of 7
Quote:
Originally Posted by crackley View Post

The church I work at hired a consultant to redesign the sound system. Gentleman did a fantastic job and really knows his stuff. However, one item keeps driving me crazy because it doesn't make sense. For the lobby there was an existing 70v system. He had me remove the transformers from the amplifier and connect the speakers directly to the mono bridge out (didn't change anything else, so the speakers are still all 70v wired in parallel). When I questioned it, his response was that the amp was powerful enough. Has anyone heard of doing this?
I have done this for many years. As long as the amp has enough voltage swing (as stated 600 watts @8 ohms) it will be just fine.

You also eliminate the issue of transformers on the output(very low impedance at low freq among others).
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