Hi Murphyman,
I have posted on the boards in the past and generally steered questioners away from Onkyo receivers and 4-ohm loads, and I answer these questions daily in private emails (advice@axiomaudio.com) as well as taking phone calls on our toll-free line on Wednesdays and Sundays. I've also commented on the inadequacy of power supplies in multichannel receivers on these boards. For the most part, in the multichannel mode, these receivers are measured at one frequency (1 kHz), with one channel driven. The brochures and advertising literature do NOT specify this (no surprise there). And ignore advertising claims of "high current"; that phrase has been tossed around for years by ad copywriters and distributors who don't understand the basics of Ohm's Law and power supplies.
When some of the Onkyos are lab tested by Sound & Vision magazine, with all five or six channels running simultaneously into 8-ohm loads, the protection circuitry shuts down the outputs or severely limits the power output.
For example, the results of independent tests by Sound & Vision magazine (July/August 2002) of the Integra DTR-7.2 ($1,200) vs. the Denon 5803 ($4,300) and Yamaha RX-Z1($2,799) are very revealing of the Integra's current output limiting circuitry and its power-supply limitations.
The Integra is rated at 100 watts X 6 channels. In the bench tests, it produced 141 watts into 8 ohms with one channel driven. With five channels driven, its output sank to 54 watts per channel, and with 6 channels driven, it produced 38 watts per channel and shut down. And this was into 8-ohm loads! Think what would have happened with 4-ohm loads! Onkyo has current-limiting protection circuitry because its power supply isn't up to these admittedly very demanding bench tests. That's why they can sell it a fraction of the price of the Denon and the Yamaha.
(In the same tests, the Denon produced 118 watts per channel, the Yamaha, 109 watts with 7 and 6 channels driven, respectively. They did not shut down.)
As John Henderson has pointed out, the "nominal" impedance rating is a kind of average, usually erring on the side of caution. And one 4-ohm speaker may have quite a different impedance curve than another, and may touch 4 ohms at different frequencies.
You can look at the M80ti's impedance curve at audiovideoreviews.com. Click on the measurement symbol. The M80ti dips to 4 ohms between 40 and 50 Hz, rises, then remains at 4 or 5 ohms up to about 800 Hz, after which the impedance is much higher out to 20 kHz. It never goes below 4 ohms, but it's definitely a 4-ohm speaker, and it will draw more current at the frequencies where it's impedance hovers around 4 ohms.
All of John_Henderson's comments are very accurate, and I can only suggest that anyone thinking of getting 4-ohm speakers who prefers high sound levels, he should scrutinize independent test reports of multichannel receivers very carefully consider separate power amps over a receiver. So far as I'm aware, Sound&Vision is the only magazine that tests A/V receivers with all channels running simultaneously. And even there, the info is in a technical "sidebar" at the end of the report. So read the fine print.
Regards,
