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Old 16th January 2007, 03:05 AM   #11
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According to my friend who worked at Rockford Acoustics (and is now buying out their equipment from the GR, MI facility), there are set standards when it comes to measuring output power. However, there are no regulations requiring companies to label per specifications. Essentially, an audio company can put whatever number they want on the box, though I'm sure they wouldn't since they'd be sued in a heartbeat for false advertising if anyone with a signal generator and multimeter took a look at it.
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Old 16th January 2007, 04:17 AM   #12
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Videtich:

Well, it might be possible to sue them if they say it puts out 150 watts RMS and it doesn't come anywhere near that.

If they say an amp that actually is rated at 30 watts RMS puts out 150 watts "music power", "max power" or that oldie but goodie, "PMPO", they probably would not be sued because the amp can put out out that power for a fraction of a second.

Fact is, in the seventies the real cheapie amps with unrecognizable names did list wattages way, way over the RMS rating.

I'm just amazed that decades after the power ratings were standardized in the hifi world, the related PA amp world is still messing around with these silly non RMS ratings.
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Old 16th January 2007, 04:34 AM   #13
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Originally posted by AndrewT
Hi,
I think Grollins is alluding to the fact that virtually no amplifier can double it's maximum power output into half load impedance.

This applies when both impedances are within specified range.
Well, I have not designed any large amps like Andrew T, but my understanding is that total wattage is the max available voltage times max available current.

So if the max voltage is eight times the max current, you have an amp that delivers most of it's power into an eight ohm load.

If the max voltage is four times the max current, your amp delivers most of it's power into a four ohm load. So if you hook an 8 ohm speaker up to an amp whose max voltage is four times it's max current, I would imagine you would deliver half the power that you would get if you used a 4 ohm speaker. I think most amps seem to be somewhere in between though, the max voltage is less than eight times but more than four times the max current. So yes, you don't get twice the power if you halve the load.

Incidentally, while we are on the subject, back in the seventies amp manufacturers spoke of "headroom", which meant the amp had extra voltage beyond that required for an 8 ohm load. A friend's big Phase Linear, for example, was really more of a 14 ohm amp than an 8 ohm amp.

I notice that headroom number is not listed these days, so I guess they don't have that extra voltage. Am I correct on this?
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Old 17th January 2007, 06:54 AM   #14
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actually "dynamic headroom" is a real number expressed in db above continuous rated RMS power, and is measured with a special test signal (which was also standardized by the IHF) where a 1khz signal is applied to the amplifier, sufficient to run the amplifier at 50% power. the input signal is amplitude modulated to produce an input envelope that is constant for 80ms, and increased a variable amount for 20ms. the signal "spikes" are then increased in amplitude until the amplifier clips during the spikes. the RMS power at clipping of this test is compared to the continuous rated power of the amp, and the result is expressed in db.

for instance the dynamic headroom rating of the Apt 1 amp was 3db at 8 ohms and 4 ohms. it was 2db at 2 ohms, and 1db at 16 ohms. dynamic headroom has a lot to do with power supply design, and not so much with the output stage.



the unscupulous take the same test signal, and instead of using the rms value of the clipping point, probably use the peak value (or even worse the peak to peak wattage) and use it as the basis for their "wattage" rating. they may even modify the standard test signal by shortening the spikes to 10 or even 5 ms to get even higher numbers
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Old 17th January 2007, 09:11 AM   #15
AndrewT is online now AndrewT  Scotland
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Hi,
I believe that dynamic headroom was invented by designers at the behest of unscrupulous salespersons to make their cheap/underspecified amplifiers seem more powerful.

I further believe dynamic headroom is a measure of the "badness" of an amplifier. i.e. more headroom = more sag in the supply rails when asked to work hard.

It is misused by advertising departments to seem an asset. If you believe in my opinion then low dynamic headroom is the better selection criteria.

Uncle,
I appreciate your contribution on how DH was measured.
Can you reference a way that us DIYers could mimic such a test?
Could a computer generate the envelope for instance?
Is there a software package available as freeware that has such a facility in it?

BTW,
your diagram show a psuedo scope view.
Scaling vertically from that posting, I see a voltage output at the low level of about 25% of maximum, that implies a low power signal of about 10 to 15% of maximum. Similarly the horizontal scale is nowhere near the 20:80 split, it looks more like 6:94. Can you confirm if other ratios are actually specified in the IHF test procedure?
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Old 17th January 2007, 09:16 PM   #16
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Hi all

Here's how I think the old 70's amp power ratings went:

unregulated supplies with small caps = higher voltage than sustained on load

say Vcc (quiescent)=20% more than Vcc (average)

music power = clipping level for short burst (peak)

power rating compared with RMS=1.2 *1.2 * 2 = at least 3x higher.

I think sometimes the clipping level (10% distortion) figure for output power was also used as this allowed the peak output to increase even more.

Peak music power out = peak misleading power out.

cheers
John
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Old 18th January 2007, 01:45 AM   #17
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Looks familiar to power output claims vs. actual RMS ratings in the late 90's to me.
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