Dan,
That’s a pretty good analysis you’ve presented and I applaud you for your research, and I hope other readers appreciate the time you put into your post. I do, however, wish to point out what I consider to be an error in your analysis and has to do specifically with your statement (in part): “To look at a real world example Emotivas Class H 7x200 watt channel amplifier (60%efficient) the MPS-2 ($1700) should then only be able to deliver 167 continuous watts per channel assuming the power supply is up to snuff.”
You cannot apply the efficiency factor of 60% that you mention to the MPS-2’s advertized claim of 200 watts times 7 channels because Emotiva’s use of the term “Watts” has already accounted for the overall derating for this particular amp. That is, when you look at the formula for the determination of “watts” (for AC circuits), two derating factors have already been applied: Power Factor (abbreviated PF, a dimensionless value expressed generally between maybe 0.70 to 0.95, that defines the phase relationship between Voltage (V) and Current (A), and with higher numbers approaching 1.0 (unity) the better) and Efficiency (again a dimensionless value expressed from maybe 0.40 to 1.0, 1.0 meaning it’s 100% efficient).
Here’s the formula: Watts = (V) x (A) x (Power Factor) x (Efficiency)
When I use the term “overall derating” I’m referring to the resulting “(Power Factor) x (Efficiency)” value in the above equation because I’ve seldom seen any OEM provide specific values for each of these variables. For example, let’s say the Power Factor is 0.90 (pretty darned good) and the Efficiency is also 0.90 (extremely good, though unlikely unless were discussing poly-phase motors), then the “overall derating” would be 0.81 when converting Volt-Amps ( “(V) x (A)” ) to Watts.
In reviewing the link you provided for the MPS-2, one spec provided by Emotiva caught my eye and might help me explain in a more understandable manner what I’m trying to say. From Emotiva’s site:
· “200 watt power modules feature independent 350VA low noise toroid power transformers w/ high speed rectifiers and low ESR 105°C power supply capacitors”
Using Emotiva’s spec and solving for my definition of “overall derating” yields: (200 Watts)/(350 VA) = ~ 0.57
If the Emotiva’s Class H amps are operating at around 65% Efficiency (0.65), then the amps are around: (0.57)/(0.65) = 0.88 Power Factor…fairly close to the high-end of the range I mention.
Since the MPS-2 is a power amplifier only (and doesn’t have to worry about furnishing any ancillary power for pre-amp/processing/video features as you’ve mentioned), and since each channel has its own dedicated power supply (which I’m guessing probably aids in its overall efficiency), it’s my belief that the Emotiva is likely to be honestly rated (or at least more honestly rated than the offerings of mainstream mfgs).
As a side note, since we also know the supply voltage for the MPS-2 is 120 VAC, each of the power modules’ transformers will draw a maximum of ~ 2.92 amps primary current. For all seven channels simultaneously driven to full-rated output (an unlikely event for this amp in a home application unless you’re running very-low-sensitivity speaker in a pretty large room), we can expect this amp to pull a maximum of ~ 20.4 amps. For anyone considering this amp, you better have a dedicated 20-amp circuit for this fella’ (as another reader has observed). And at 115-lbs you might need a forklift to move this beast.
However, I do find interesting Emotiva’s use of RMS for some of its amps (XPA-5, RPA-2, XPA-2) and just plain Watts for others. I’m guessing the RMS- guys can deliver their claimed power on a continuous basis while the plain old Watts-guys will only be able to do this on some (undefined) short-term basis. Regardless, since Emotiva provides values of “watts” for each of their possible driven configurations (1/2/3/4/5/6/7-channels) I’m inclined to believe them.
To your observation: “What would be the point of a 400 watt x 7 if there is no way in heck your amp can draw enough power from the outlet to support those numbers in the first place?” Emotiva is specifying different levels of power delivered for different values of load impedance (8/4/2 ohms). If you plug these values of impedance into equations for Ohm’s Law, they compute. Given the weight of these brutes (and the values mentioned for power supply components) I suspect they’re pretty accurate.
To conclude: IMO, Emotiva is reaching out to an entirely different (and more discerning) audience than the mainstream mfgs, and I’m more than impressed with them. Like I said…”If I were still addicted…”!!!
Finally, to your statement: “After all, you can only get 1800 watts max continous from you wall (unless you change your circuit breaker)”. Please don’t ever try increasing the breaker rating from 15-amps to 20-amps unless you also confirm that the existing conductors in your wall are capable of handling the additional current. Remember this…fire is a four-letter word!
WTH does any of this mean?
As I review what’s written I wonder how much of this might possibly affect your decision regarding your next upgrade? After all, how many of us actually care about (or even pretend to understand) the differences between Ohm’s Law and L.A. Law? In expressing my opinions on this forum I’ve been challenged by a few who dispute what I say, have chastised/admonished me, and have reminded me that they have never examined the numbers but have, instead, relied on the advice of the “experts” regarding their selections. While I’ve never claimed to be an “expert” I have admitted to being an “avid” audio guy! So for those of you who might rely upon the advice of the “experts” concerning your next purchase, I leave you with the following from Emotiva’s website:
“Here's a typical power spec from a popular name-brand receiver that sells for around $2,000: 120W x 7 (8 Ohm 20-20kHz 0.09%THD) - Sounds great, doesn't it? But look a little deeper into the specifications, and you'll see it again, but listed as a 'Channel Power Rating'. What does that mean? It's called 'fine print...a disclaimer.
It means that one (maybe two) channels can make that rated output, at that specification for a finite amount of time. It doesn't mean that all seven channels, working at the same time, and at the same power level, can come anywhere close to the cumulative power rating of 840 watts that the specs indicate at first glance (7 x 120 = 840). You get the idea. Something had to give. That something was power.”
Wow…Emotiva calls that “Something”: “fine print…a disclaimer.”??? Hells-bells…that’s pretty damned conciliatory! I’m reminded of the adage “numbers don’t lie, but….”. Well, OK Emotiva…I finally get the message! So let me be a little less candid and blunt…and I’ll just continue to call it TRUTH in advertizing.
Case closed.
Fred