Originally Posted by Dave-T
I used XLR’s on the both the Rotel and now the ATI amps. I always thought it had do with the efficacy of class Ab vs class D. Class D is typically around 90% efficient while ab is close to 50% so it takes more power to get the class ab amp to do what the class d amp can do. does the voltage of the xlr outputs on my Marantz 8805 come into play as well? it was also my understanding that 4ohm speakers are more difficult to push then 8ohm speakers and most AVR's can't handle or handle well 4ohm speakers in a home theater setup. I was always told separates are the way to go with 4ohm speakers. I typically Buy what I can afford and sounds good to my ears , alot of my gear is not purchased based on stats. Definitely sounds like you know more about the technical stuff than me.
According to the published spec's, using XLR's on the Rotel requires a higher voltage. You should see a change in your volume control level if you switched to the RCA input on the Rotel. This is not unusual as XLR is more of an industrial connection were the balanced line is used to control EMI/RFI noise on longer cable runs. A higher voltage is typically used as well for consideration to gain structure for signal to noise ratio.
Again if you look at the specification's, the Rotel is rated at a maximum gain of 26.5dB. The ATI AT527NC is rated at 27.8dB maximum gain for a difference of 1.3dB between the two. The ATI has an input sensitivity of 1.6dB for full power on either RCA or XLR input where the Rotel is 1.9v on RCA and 3.8v on XLR.
The output voltage of the Marantz AV8805 is listed as Balanced XLR pre-output: 2.4v and Unbalanced RCA pre-output: 1.2v. So the Marantz AV8805 does not have the necessary voltage to drive the Rotel to full power on either input, RCA/XLR. With regards to the ATI, the Marantz is a little shy on the RCA and you would have to ability to overdrive on the XLR which could potentially cause some distortion.
I would not worry too much about it. You're typically only using watts in the fractions or single digits at normal listening levels anyway.
Speakers with lower resistance (4Ω) require, place a demand for more current for the amplifier to deliver the same voltage. Ohm's law. Separate power amplifiers generally have room for larger power supplies. Larger power supplies can provide more current. Most any name brand AVR can drive a 4Ω speaker. It's when it's called on to produce much higher volume levels where the amplifiers power supply is unable to supply adequate current and the voltage sags. The amplifier heats up as a result and the thermal protection kicks in and shuts the unit down. That's of course if the AVR has thermal protection. If this is done repeatably, components on the board will begin to fail.
FWIW, I run my backyard theater on an entry level Denon AVR E300. I use some older Polk Audio speakers that are 4Ω. I'm on a one acre lot and I've no problem generating sufficient sound to get the neighbors interested with it and it has never shut down from the speakers impedance load. Inside, I've some ridiculously high power amplifiers that never get used to their full power.
With regards to efficiency, less power is converted to heat and into usable "watts" in a class D vs. AB design. Because of the increased efficiency of class D, class D doesn't usually have the big heavy power supply transformer. With either class, you're only using the amount of power called for by the pre-amps input voltage relative to the speakers sensitivity and desired SPL.
Sorry to have intruded, but I hate to see so called "head room" get confused with a difference in input sensitivity when comparing amplifiers.