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I've done what I can to research but I'm not finding any answers. Is the SC-57 4ohm stable?
 

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fozz,

I'm thinking pioneer is just covering their butts. I have the sc-05 and I run my mini statements, which are under 5 ohms for good portions of the spectrum with no problems. These things run pretty cool anyway, I would search some of the threads for that avr for other people running hard to drive speakers.
 

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I've done what I can to research but I'm not finding any answers. Is the SC-57 4ohm stable?
The SC-57 officially support no less than 6ohm, but you won't hear any of that from the local high-end AV stores that sample the SC-55/57 with Martin Logans which run at 4ohms. I'll tell you now, that you're gonna have heating if not overheating issues with the SC-57 if you run high at 4ohms. Good luck.
 

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Isn't there a FAQ around here somewhere covering "Rated Impedance vs. Real World Use"?

Impedance ratings for both speakers and amplifiers are only "ball park" figures for UL compliance (fire safety). There is no speaker I am aware of that will present a flat 8 Ohm load throughout the frequency spectrum and spl range. Likewise there is no amplifier I am aware of that can handle only an 8 Ohm load, in the strictest sense. System impedance varies with frequencies, and more or less depending on the actual speakers, wires and amplifiers in use. Simple as that.

What was said earlier about "covering their butt" is true and has been true ever since UL safety ratings started showing up on the back of stereo equipment. It's not just Pioneer being clever... it's ANY manufacturer that has a UL safety rating. Pioneer won't tell you that the SC-57 will happily run 4 Ohm speakers because if they can say 8 Ohms (6 Ohms minimum) then they get out of replacement under warranty when eventually some part of the amplification circuits fail... and you tell them you ran a speaker that exceeded their published specifications. But what is really going on is an amplifier's ability to deal with high current demands, which increase with the lowering of impedance, and how (well) it dissipates the heat that it will generate during use. A relatively high end amplifier like the SC-57 should be much better equipped than a much cheaper AVR/amp in this area IF the price tag is to be justified. But I bolded that line about heat for a reason. Even a pro quality amp (2 Ohm stable or less) will over heat if it's worked very hard, probably fail (heat is the number one factor in how long an amplifier section will last) and maybe even catch something on fire if it doesn't have proper ventilation. They put cooling fans on them for a reason!

Here is an anecdote for you: I once came to troubleshoot a customer's setup that was running dual Adcom GFA-5500s as part of a home theater setup. The 5500 is a two channel amplifier rated for 200wpc @ 8 Ohms and costs somewhere between $1000-$1500 each. Adcom's user manual will tell you that 4 Ohm loads are no problem and even speakers causing dips down to 2 Ohms will be fine *with proper ventilation*. Customer was running 8 Ohm rated two-way floorstanders, won't say brand here, just that they were over $1000 each. Amps kept going into protection mode. The 5500 is MOSFET based which typically means more heat than their bipolar cousins. My personal experience has had 5500's running on an open shelf get hot enough (when working hard) that you wouldn't want to leave your hand on the heat sinks for more than a second. This customer had them closed up in what amounted to a sealed high end wooden cabinet and ventilation was the *entire* problem. Ok, so that's a 4 Ohm "stable" amp running 8 Ohm speakers.

One more from the flip side of the coin: My first experience with "big" speakers was my first pair Cerwin Vega DX-9s many years ago. These were 85lb monsters rated at 4 Ohms that featured 15" woofers and an advertised F3 of 30Hz. I ran them for almost a month on a cheap, plastic, garage sale Panasonic bookshelf stereo, the AM/FM/Cassette type with small, removable speakers. I suspect the Panasonic wasn't rated for much more than maybe 15wpc at 16 Ohms? No powerhouse anyway, and yet that Panasonic could make those CVs rock loud enough, and deep enough, that more than once I thought I heard the cops knocking on my door! :yikes: This was all because the CVs were also extremely efficient, rated at 101db/w/m @ 4 Ohms so the little Panasonic never had to work very hard at all to drive them. So yeah, Ohm ratings aren't the whole story either! An 8 Ohm rated speaker with a very low efficiency will make an amplifier work much harder than a similarly rated higher efficiency speaker. Hard work means more heat, heat without good ventilation means a potentially shorter life span of the amplifier.

Another trick was/is circuitry that helped restrain power (and thus heat) as a lower impedance speaker was selected. Vintage McIntosh amps used to have separate power taps on the back for use with 2,4, or 8 Ohm (etc.) speakers that kept the power the same with each different impedance and the heat manageable. Older Yamaha AVRs used to have a manual switch in the back that let you choose between 4 or 8 Ohms for the fronts that basically worked the same way as the McIntosh. Newer AVRs might have a digital switch that can be flipped in a setup parameter on-screen. All these things do the same thing... help keep the amp section working easily enough to prevent heat and failure.

When I go look at different AVRs myself one test I always do is put my hand on them to see how hot they are getting, especially in the big box store demo rooms where they might be getting ran for many hours a day, every day, at varying sound levels (Hey! Watch that five year old!) on various equipment. I won't go into great detail about what my findings have been in the past, only that my last several AVRs have been Yamaha.

Ok, I've rambled enough. Back to the original question... the "Official Pioneer SC-57" thread in this very forum is started with a very detailed review that includes testing with four different speaker setups. The Martin Logans in that review are 5 Ohm nominal and the Axiom towers are rated at 4 Ohm! On a top shelf AVR that's only rated down to 6 Ohms? :heehee:
 
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