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Old 4th April 2005, 06:11 PM   #1
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Default Commercial Product

Hey guys and gals am I unreasonable in my expectations with documentation on commercial product?

Over the weekend I had a pair of Stereophile recommended power amplifiers on the bench. This particular amp is rated at 300 W output. (monoblock)

Sitting in my garage on a wire shelf (58 degrees F ambient) I could get over 300 watts short term, with no problem. Anything much over 60 watts long term (1/2 hour) would cause overheating and self protection.


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Old 4th April 2005, 06:26 PM   #2
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Kind of a grey area...

Back in the mid-'60s you could find all-in-one record players that were advertised as "100 watts peak-instantaneous power". This rating was achieved by connecting the amplifier to a 1 ohm load, raising the line voltage to 140 VAC, and sending a single-cycle tone burst. They would increase the level of the test signal until the amp destroyed itself. In this way, they could take a 7 watt amp and rate it at 100 watts.

It was so out of control that the government (Federal Trade Commission) stepped in. The new rating system was very conservative. All channels had to be driven, and the load, the bandwidth, and the distortion levels all had to be specified. This makes sense, but they also added a "pre-heat" requirement of 33% of rated power for 1 hour, which caused all kinds of problems as this happens to be pretty much the point of maximum dissipation for a class AB output stage.

It sounds like the unit in question has somewhat undersized heatsinks as far as the FTC rating goes, so in that regard it is probably somewhat "under engineered". But I would be highly surprised if you had any problems in the real world when playing music.

So the question becomes rather fuzzy. Does it make sense for the federal government to demand an over-engineered heatsink that will significantly raise the price of a product without any direct benefit to the consumer? If you answer "yes" to this question, then should the federal government also mandate that no capacitor should ever be run at more than 75% of its rated voltage? Where do you draw the line?

I don't know....
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Old 4th April 2005, 06:44 PM   #3
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Me either?
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Old 4th April 2005, 07:52 PM   #4
Netlist is offline Netlist  Belgium
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Default Re: Commercial Product

Quote:
Originally posted by cyclotronguy
Hey guys and gals am I unreasonable in my expectations with documentation on commercial product?
Not, we found same behaviour countless times with countless PA amps years ago.

/Hugo
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Old 5th April 2005, 12:03 AM   #5
Stocker is offline Stocker  United States
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Default There is now an official way to rate Watts...

... that does the same thing as the 100W record player.

It's called PMPO

and largely seems to be a fantastic number slapped on to sell cheap stuff.
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Old 5th April 2005, 02:37 AM   #6
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Amplifiers need to be tested appropriately for their intended application, otherwise the world will be building VASTLY OVERBUILT products consuming excessive raw materials and wasting resources - which should a crime in this polluted/climate change world!

Also it pushes the cost out of the reach of many just for some inappropriate testing regime.

A power amplifier intended for general audio use would be appropriately tested with the already defined 20:1 toneburst signal.

A recent commercial design of mine used one 300W toroid for 150W/channel with 2dB dynamic headroom - so it clips like a 240W/ch amplifier with a stiff supply. It had 5inch square heatsinks with 1.25 inch fins and ran quite warm with high power operation but cool inside with open chassis ventilation and 105C caps.
It was small and efficient and reviewed as a giant killer. Also a fraction of the cost of it's competition.
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Old 5th April 2005, 03:01 AM   #7
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I agree with both your reasoning and conclusions.

Now you also have me curious -- what is the amp you are talking about?
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Old 5th April 2005, 03:11 AM   #8
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sadly Charles the Eidetic amp/preamp never made it to the US. I produced it in Australia in 1989-94 and exported some to Singapore while some were privately taken to the UK and Canada.

It was an OPA627/637 driven nested feedback MOSFET design. It had an impedance plug for 4/8 or 1/2 ohms by series or paralleling the 4 secondary windings for 68V or 34V.
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Old 5th April 2005, 03:24 AM   #9
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OK, thanks for the details. Obviously the output stage had gain somewhere. How was this done?
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Old 5th April 2005, 03:53 AM   #10
sam9 is offline sam9  United States
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I agree that it can be considered wastefull to build and amp that can run at the maximum (either power or thermal) indefinately, but 60W is only 20% of the rated power and 58 deg F (14 deg C) is quite chilly. I don't think you are unreasonable to raise an eyebrow.

I too am curious regarding the make and model. Sounds like an example of Self's Law of High-end Audio Pricing : "MSRP and performance are inversely related". (Myo wmn paraphrasing.)
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