Originally Posted by lukeamdman
I just want to remind anyone reading about the 21ipal:
IT'S 1 OHM!
That's going to cause major problems for a lot of amplifiers, such as early clipping, power cycles, thermal protection, low impedance protection, etc.
I didn't know how the SpeakerPower amp would handle the load, so this was more of an experiment, and I had already accepted full well that it could have been an epic fail.
One of the ways manufacturers who pay attention get a lot more from an amp is ignoring any "nominal" ratings and look straight to the DCR and in-box impedance of a design. I've seen "4 Ohm" drivers with DCRs ranging from 2.7-4.3 Ohm. It's all about how it's used. This is also a benefit most grossly underestimate in IB solutions, where the system Fb is so low and often leaves a huge impedance peak that doesn't come back close to DCR until the single digit Hz range.
A horn that provides some loading often yields an impedance that is significantly higher than DCR for most of the operating range. Acoustically small horns often have a dip or 2, but still on average are higher impedance than the DCR. I have heard of a few using the SpeakerPower amps to drive IPAL woofers. If you have the DCR and in-box impedance you can get a good idea of how it will do.
The good thing is the amp won't care what load you connect, nor will it likely ever be damaged by a load. The factory burn-in for the amplifiers basically drives a near shorted load that is mechanically attached to the amplifier plate to basically heat itself driving the load and the internal heat generated.
With a reasonably stiff power line, you won't see any sag all the way to around a 1.5 Ohm minimum impedance. Below there you will see some current limiting and reduced maximum power, but it will still work just fine. When I was testing a dual 2 Ohm HS24 with coils paralleled, I left too little time between test runs with a 12-14Hz sine wave in a smaller sealed box, and the SP4k melted the wire right off the former driving the 1-1.5 Ohm impedance.
About the only caution with low impedance loads and raised in-box impedance is to remember that as BL and Cms changes with excursion/power, your impedance curve changes too. IOW, the big bump from high BL drops with the BL curve. Often this can create an accelerated clipping condition once you get to high excursion points on current limited amps.
The high Voltage 3000/6000/12000 models will actually regulate Voltage a little better up until the line Voltage sags somewhere below 200V AC, so until your line sags below that point, the output power remains constant.