The best option would be a proper 70v amplifier, OR a driver transformer which will allow the regular amplifier to drive the system successfully. I strongly advise against trying to change out the volume controls to an "8 ohm" type...connecting multiple speakers to keep the total connected impedance to a reasonable level (MUST stay between 4-16 ohms total connected or the amp can burn out) is very difficult to do.
QSC makes a 70v driver transformer: 70v up to 300 watts
and 70v up to 600 watts
Peavey also makes one: 70v up to 200 watts.
And here's a bunch of info on 70v distributed audio systems: (I originally posted this on another forum) :
There are several myths and misconceptions of what a "70v Distributed Line" PA system is, how it is wired and set up, and what it is capable of. Well these next posts will hopefully answer most of the common (and some uncommon) questions in regards to those systems.
First up, what it is and why it was invented:
70V distributed line systems were invented to allow for the easier transmission of sound over a large area, with a savings in wiring and complexity over conventional (Low Impedance or "Low Z" systems, the 4 and 8 ohm nominal systems used in hi fidelity and other single-speaker per channel) sound systems.
The system is driven with a transformer at the amp end and each speaker has a transformer with wattage taps. This allows you to do two things:
1: Drive a very large number of speakers by simply wiring all of them in parallel without worrying about impedance matching; and
2: By selecting the desired wattage taps at each speaker, controlling the overall level of each speaker to allow for better distribution of the sound levels. (Thus the common use of the term "70v distributed system".)
In theory, if you have a drive transformer large enough (with the corresponding power rated amplifier driving it) there is no real limit on the number of speakers you can connect to a 70v system, the only limit is that the total of all of the wattage taps MUST NOT exceed the amplifier/driver transformer ratings.
For example, a 400w rated amplifier, with the proper drive transformer, can drive 400 1-watt speakers, 800 1/2-watt speakers, 1600 1/4-watt speakers, etc. And you can connect ANY combination of wattage of speaker to the system, but the TOTAL wattage must not exceed the rated power of the amplifier.
These systems are also done in 25v (very rare) and 100v (and higher) versions. The 100v and higher systems are usually done at extremely large venues like motor speedways, large convention centers, etc. The higher voltage allows for greater distances for a given size of wire, and of course greater power for a given size of wire. (Same as the wattage capacity of a conductor is doubled going from 120 to 240 volts.)
The savings in wire (both by allowing smaller conductors, and not requiring individual speakers to be "home run" to the amplifier [or a distribution splice block] for series/parallel connections to get to a 4 or 8 ohm impedance) is considerable and in many cases offsets the slight increase in costs due to the transformers used at the amp and speakers.
Now to address the first myth: 70v systems are frequently run over telephone lines and are perfectly acceptable.
"Frequently run over telephone lines" is the sign of a hack install. Yes, the 70v system allows for smaller conductors in a given installation, but using telephone lines results in wasted power heating up the wire. Simple electrical laws make it so.
First off, using simple math, the current in a 70v system can still reach several amperes depending the the total wattage of the connected speakers. Easily well over what a 24ga wire can handle, plus you STILL have to factor in for voltage drop...you could easily have the speakers at the amp end driving at ear-bleed level and have barely a whisper from the ones at the far end.
Second myth: 70v systems are intrinsically safe and you can't get shocked by one.
Ask any experienced installer and they will tell you it is not so. Even though the amplifier's drive transformer results in what you could call an isolated or SDS type of system, capacitive coupling to the amplifier can and will allow a very nasty shock to occur. (And see my next post about using an amplifier without a driver transformer.)
Further, a short in a 70v system will allow enough current to flow (up to the rated power of the amplifier) which can result in burned conductors, transformers or amplifier failure which can and has caused serious fires.
Third myth: You can use any speaker on the 70v system without a transformer on the speaker itself.
False (and true..see the last sentence.). You may get away with it for a while on some paging systems (where the announcements are short like "Bill, call on line 2") but a standard speaker without a transformer to limit the voltage and power applied to it WILL eventually fail. In extreme cases the voice coils have caught fire. ALL speakers used in a 70v distributed line system MUST have the transformer and it MUST be connected to a tap not to exceed the speaker's power rating.
You can buy (and cheaply) transformers to allow ANY speaker to be used in a distributed line system, juts make sure it has taps rated up to the speaker's power handling.
Fourth myth: 70v distributed systems cannot handle high-fidelity signals (low bass down to 20hz and high end to 20khz.)
Absolutely false. 70v distributed systems are VERY capable of full-range (20-20khz) audio signals..BUT the transformers get pretty pricey. (And big.) Both the driver transformer at the amp and the ones at the speakers MUST be of high quality and made for full range signals. Since the most common use of such systems is background music, paging and outdoor announcing, the most common (and least expensive) speakers are purposely limited in frequency range to save money.
Next post: Is it possible to drive a 70v (or 100v) distributed system without a driver transformer at the amplifer?