I have spoken to (2) technicians who were heavily involved with building and servicing PA system type amps, and one of them also was an installer in large stadiums, hospitals, churches, etc. for 70 volt systems. What we are talking about here is a transformer coupled output. These PA type amps are not direct coupled, and there isn't 70 volts at the output taps at all times.
The voltage ratings for the various taps are for MAXIMUM VOLUME and output from the amp's output transformer on the secondary side- when it's turned all the way up. If there are 4, 8, 16, 32 ohm taps, the maximum output voltage of the secondary also goes up as well- that's why many of them are labelled both with ohm rating and votlage rating.
What happens is, at the lowest ohm rated taps, the full voltage output of the amp is not available. That's why you will see a "25 volt" or "35 volt" tap on an amp with lower ohm ratings like 4 ohm, 6 ohm, 8 ohm, but the maximum voltage tap will be 70 volts.
all those taps refer to voltage output AT FULL VOLUME.
I recently obtained a "70 volt" PA amp that also had 6 ohm/8 ohm, 25 ohm/32 ohm taps, that were also labelled 35 volt and 70 volt correspondiingly.
I set it up with a stereo tape input, and Sony pre-amp from a mini system, and ran stereo into the pre, pushed the "mono" button on the pre, and ran mono from one channel of the pre (now containing the stereo mix albeit in mono) to the amp.
First I connected an 8 ohm speaker and played it via the 35 volt tap, then connected (2) 8 ohm ohm speakers in parallel for about 4 ohms and played it from the 35 volt taps.. Then I connected two 8 ohm speakers in series for a total of 16 ohms and played it from the 35 volt taps, then finally played the 16 ohm speaker load through the 70 volt taps.
The amp played strong and loud from all taps no matter what speaker load was on it, but the fidelity was best with the 16 ohm speaker load on the 70 volt taps- which only makes sense, being with those connections the amp is harnessing its full output potential.
Many of these amps were designed to be connected to a long line of speakers and turned up to MAXIMUM VOLUME the full 70 volts, then the local speaker volumes controlled remotely.
Nothing will blow up, short out, etc. if it is transformer coupled on the outputs. The old PA amps with the HUGE output transformers like above in the pciture, were designed to keep right on playing and not damage the amp and protect it, even if a cable got frayed, shorted, cut, a bird or squirrel chewed through it at a stadium, etc. That's why they were built that way. They were made to keep playing through a cut up damaged line and not damage the amp, and protect it, hence the large transformer coupling on the speaker output. Much more forgiving, much safer.
A direct coupled amp will have better sonics but one touch of the wrong speaker wire and "POOF" it takes out all the final amp transistors in a row. There's no protection.
so to answer the OP, yes you can run an 8 ohm speaker on the 70 volt line, but it will sound a LOT better if you run the ohm rating needed for the 70 volt line, to attain the output in watts the amp was rated for. The field tech told me that right out, volts x volts/ohms = watts so adjust the ohm load of the speakers to attain the equal the watts output rating of the amp- I was already doing that because it was only common sense- per my previous posts.
The amp will sound better on the 70 volt line with higher ohm speaker loads- and it will play much louder without distortion. I have taken a few videos of the tests on the various taps and may upload them to Youtube if need be, and post the links here. You will be amazed at how good this mismatch sounds, using an old tape as a source.
70 volt line ? That is a misconception- that is merely the maximum rating with full input and full output through the amp. It's just a label- because in reality I measured the LINE VOLTAGE while playing the amp VERY LOUD at over half volume on the amp, and about "4" on the pre-amp- , and there was only 2 to 4 VOLTS measured coming from the output to the speakers using the 70 volt output tap line and 16 ohm speaker load, measured with my Fluke digital voltmeter in AC mode. I was filling a 28 x 14 listening room with about 90 db with only about 1 watt or less.
the guys that said this wouldn't work, just haven't tried it themselves. Yes you can play a PA amp through home audio speakers on the 70 volt taps. But you're going to need at least 16 ohms on each amp, and they are usually mono amps- so you will need 2 amps to get stereo.
It's just a lot easier to use a dedicated consumer stereo amp. But if you get the PA amps for free or dirt cheap, go for it.
FWIW, the Sony pre-amp also has a MIC IN jack and mixer knob to adjust the mic level. I plugged my Fender Strat into it, and the damn thing CRANKS. I plugged it into a tube pre and then into the PA amp and really cranked it, overdrive and sounded really cool with a mellow tube distortion.
just like a guitar amp.
amps are amps, nothing mysterious about them
Edited by wall-of-sound - 5/17/13 at 6:00am