Originally posted by soundhound:
Any amplifier that has the "4 ohm" switch is not robust enough to drive that impedance without the reduction in rail voltages that the switch provides, whether seperate or receiver. This is done for cost reasons mostly, as it would be impractical and costly to make a multi-channel power amplifier with the transistor complement, heavy heatsinking, and large power supply of the more robust stereo amps of 20 years ago. Many of those could drive 2 ohms without complaint.
Certainly can't argue with that. The "low impedance" switch has been a staple of receiver power section design for quite a while, and you've written a good summary of the drawbacks involved in that approach. I've even seen some receiver designs that omit the low impedance "4 ohm" switch and simply rate the amps for speaker loads of 6 ohms or higher. (This is something that I noticed when looking at Denon's site this morning - the rear panel of every receiver in their line, from the $400 AVR-1705 up through the $6000 AVR-5805, lists the allowable speaker impedances as 6-16 ohms. I don't know how much factor of safety they have for loads under 6 ohms, but it is indicative of the lighter design approach you're talking about.) It also makes sense that some of the compromises in transistors, power supplies, and heat sinks carry over to separate power amps that pack five or more channels into one chassis. There still seem to be some fairly robust multichannel amp designs available on the market, although I don't know if many of them include the degree of engineering "oomph" found in the amps you're thinking of.
It reminds me of when a friend of mine bought a pair of Martin Logan's a couple years ago. He bought a two-channel Classe amp at the same time, but the speakers came first and his Yamaha receiver couldn't handle the Aeons (a 4 ohm load that could dip below 2 ohms at times), so he had to wait several days after getting the Aeon's before he could listen to them.
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