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In Reply to: RE: Why More Power? posted by Norman M on July 09, 2015 at 10:58:01
The generally conceded measure for amplifier power is the 'watt'. 1 watt is 1 volt @1 amp.
Certain adjustments need to be made for reactive loads where the VA is more useful and results in a measure called power factor.
You can have different measures, however, based on time limits. 'D' amps, for example, are time limited due to heat sink issues. Some A/B amps will not pass the Stereophile preconitioning period without overheating. Heat Sinks are conceded to be expensive and heavy to ship, and take up more space.
For purposes of this discussion, I'll go with the manufacturers listed 'rms power'.
And since Maggies are GENERALLY a benign, low-reactance load of low sensitivity, I feel fairly safe in using the amplifier manufacturers power rating.
Some interesting things DO pop up when checking further, however. The Pass XA30.5 is rated at 30 watts. However, at redline, fully in A/B mode, it's a LOT more power. Enough more power that I would NOT hesitate for an instant to Biamp my MG1.6s with a PAIR of 'em. Add another (aprox) 3db when going to an active OR passive line level crossover.
I doubt I'd ever make the bias meter even flicker!
Does this help?
Too much is never enough
Follow Ups:
Thanks for your reply but unfortunately it's really no help whatsoever. It fails to consider the potential *current* output of an amp, in amps (peak as well as otherwise).
"For purposes of this discussion, I'll go with the manufacturers listed 'rms power'."
Those numbers, and in particular seem useless. I have 200 watt rms rated/ channel stereo amps, some of which cannot succeed in playing Maggies of 4 ohm impedance to minimal sound level without severe distortion (but succeed in easily playing Maggies with 8 ohm impedance to ear shattering levels, without any problem.) Also an Onkyo TX 8511 receiver, rated "100 watts X 2 channels of clean high current power (Crutchfield.com) which can drive Maggies of either 4 or 8 impedance. (Not implying that I ever use that receiver in playing my Tympani IV-As, but OTOH it works in being able to listen to my smaller MMGs. Much of which got this discussion started, e.g. "power" required to play Maggies as one goes up their line in size, e.g., MMGs vs. the 3.7/i.
Power is volts times amps. period. Given impedance, and watts, it is a simple ohms law calculation to determine voltage and current.
Now, amplifiers DO have peak current capability. My Parasounds have some factory rating of amps output. Since my hearing is till pretty good, I'm not concerning with that measure. In fact, I doubt there is any agreement among manufacturers about HOW to specify that value. It turns into more mis-information than I'd care to deal with.
Those high currents are at a very low voltage, anyway. That doesn't do me much good though it may indicate a robust power supply.
It is ALSO true that music is NOT rms in nature. By this I mean it is very dynamic and the demand upon the amp is always changing. To this end, I look at rise time and slew rate. It is also a good idea to have some idea of 10khz square wave response.
Taken as a whole, a more complete characterization of an amp gives you a better chance of picking one simply by going with the 'on paper' results.
What you are 'up against' is an amp MAY have good output into 8 ohms but simply FAIL to get it done with lower impedance loads.
8 ohm power may be less than the ideal way to buy an amp for maggies. I'd like to know 4 ohm power as well.
I'm not very concerned with the amps ability to charge a battery or work into highly capacitive or inductive (reactive) loads since panels are a generally reasonable load in those regards.
I had a Rotel RB1070 of 125 or 135 x2 @8 which was utterly incapable of making my Panels work right. NO factory 4 ohm rating, but rather a 'bridged' rating of 330 or so at 8 ohms, which IMPLIES a 4 ohm rating of 165x2. Maybe I just didn't like the sound? I sold it within MONTHS. My bad.
The rule of thumb most Maggie owners go by is the 'double up' rule where a true voltage source amp should (or could?) double up power as impedance 'halves'
I'm not fully on board with THAT either, since you could have an amp with some huge power output which Doesn't double down as impedance halves yet still provide enough juice for your panels.
Here is a link to a 'power cube' measuring scheme. Each amp is tested for voltage output at various impedances and at various PHASE ANGLES. I suspect ANY amp which passes this test with good results should power your panels. The quality of sound, is still up to YOU, however.
Too much is never enough
"Power is volts times amps. period. Given impedance, and watts, it is a simple ohms law calculation to determine voltage and current."Uh, no. Power is volts times amps ONLY if the phase angle is zero......as in DC circuits.
It gets a little more complicated in AC circuits.
You're on the right track though. :)The AP testing scenario is a good one because it measures power at various phase angles and dynamic conditions.
Dave.
Edits: 07/20/15
Did I fail to mention Power Factor?
Beyond the scope of necessary answer trying to explain THAT, though when I linked the Power Cube measureing system, you'll get some of that information.
Too much is never enough
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