Repercussions of choosing a poor audio amplifier

Question

I have a customer who had an audio system setup in his business a couple months ago, and he's had some issues with it.

The relevant bits are that one of the original amplifiers blew upon initial install almost immediately, he tells me. Upon investigation, I've found that he's using an AudioSource AMP-100 amplifier, which is rated for 60W @ 4Ω or 50W RMS @ 8Ω according to the specs. His speakers appear to be 12Ω speakers (or higher), as best I can tell. (I measured the resistance of the speakers, and from the discussion I had in a chat room with a colleague, it appears that from the measured resistance of 9.6Ω, his speakers are somewhere in the ball-park of 12-16Ω. Obviously this is not a true value, but it's a good estimate. I don't have any actual data on the speakers themselves, he no longer has the boxes and they are ceiling mounted.)

Obviously these speakers have a much higher impedance than the amplifier is rated for, so could this be why the original amp failed immediately? Is there something I'm missing here? (I'm not an audio or A/V engineer, but from my understanding mismatching these values could potentially have significant negative impacts.) He currently has one of these running four other speakers in the building. (They are all wired into the same positive/negative contact set on the back of the unit. As best I can tell, they are wired all in parallel.)

I'd love to use the existing amplifier setup and just wire the additional two speakers into the "B" output if possible, but I'm not sure if I should replace that amplifier with something rated in the 8-16Ω range (and probably a higher wattage).

Answer 1

"Obviously these speakers have a much higher impedance than the amplifier is rated for, so could this be why the original amp failed immediately?"

NO it cannot. If it would be a problem then such an amplifier would also fail when you would operate it with no speakers installed. That would be plain silly. Audio amplifiers have a voltage output (low impedance) so only a very low impedance could cause it to fail. But almost all modern amplifiers can handle loads of 4 ohms or higher. Also, most modern amplifiers have some form of overload protection so it would just shut off.

OK, it appears there are four 16 ohm speakers in parallel. This would give a total impdedance of 4 ohms. That would be OK if the speakers are indeed 16 ohms. But if they are bit lower in impedance then the combined impedance will be below 4 ohms and for some amps this can be a problem.

A solution might be to instead of switching the 4 speakers in parallel, to make 2 pairs of 2 in parallel and put those 2 pairs in series. Then the total impedance will be around 8 ohms, that should not give any problems.

Answer 2

"...audio system setup in his business..." "...and they are ceiling mounted..." This sounds like a good candidate for a 70V distribution system. This is different from a simple amp->speaker arrangement, with somewhat unique pitfalls. I don't know if you actually have that or not from the details you provided, but I would not call it unreasonable.

First-off, the reason for the 70V name is because the peak voltage across the distribution wires is absolutely limited to 100V. A sine wave with 100V peak is pretty close to 70V rms, hence the name.

There are modern amps that can drive these lines directly, but 1) older amps, 2) amps based on older designs, and 3) cobbled systems using parts that were not designed for this often use a transformer to step up from the amp itself to the 70V line(s). The speakers almost invariably have individual step-down transformers to match their own individual impedances.

Just like any other component, transformers have limits, and they behave differently outside of those limits. Where an amp may clip off the peaks leaving flat spots with harsh corners where there used to be nice-sounding curves (along with potentially weird recovery artifacts), and where a speaker may distort badly or even tear itself out of its suspension, a transformer will saturate. What this means is that the magnetic core cannot be magnetized any more and any additional volume is effectively short-circuited. This is made worse by 1) loud signal and 2) light load. #2 is somewhat counter-intuitive until you consider that the load current counteracts the source current to reduce the core's magnetization.

If you have a 70V system with a transformer, then it's possible that that's what happened. But I don't know for sure that you even have one; only suspect with good reason.