In Reply to: Re: Has to do with UL testing procedures... posted by Bruce from DC on May 28, 2003 at 10:42:25:
I'm not talking about FTC power rating rules - I'm talking about UL product testing. The receiver manufacturers consider the 4 ohm test procedure unfair as it does not reflect real-world operating conditions. They feel it overly stresses the amplifier sections of their receivers and that they are therefore likely to flunk the UL test. Since flunking the test flunks the whole receiver, manufacturers choose to undergo only UL tests that they are sure they can pass. Therefore, 6/8 ohm ratings.
Talk to some folks in the industry - they'll sort things out for you.
What folks would that be?
Unfortunately, accessing the UL standard would cost me $400. But, I am curious, since UL is concerned with safety, not performance issues, as to why the UL test is "unfair" to receivers driving 4 ohm speakers.
Is it because the receivers burst into flames when delivering 1/3 rated power into 4 ohms (1/3 power usually places maximum thermal stress on a transistor amp)?
Or is it that parts of the receiver get dangerously hot under those conditions?
I'll try to dig up some more info for you. As to your safety/performance question - the manufacturers desire UL listings for any number of competitive and liability reasons. There are a number of test suites for which a manufacturer can undergo certification testing.
Let's say (just as an example) that the test procedure for a 4 ohm listing consists of 110% rated power all channels into a purely resistive 4 ohm load for 45 minutes. If the receiver's overtemp protection circuit engages at any time, you flunk. If the temperature of the receiver increases by more than 25 degrees C at any time, you flunk. And IF you flunk the 4 ohm test, your entire test submission fails even though you passed the 6 and 8 ohm procedures with flying colors. See how it works?
Now you're sounding like the FTC test. Lots of inadequately heat-sinked amps will flunk the 1/3 power "preconditioning" aspect of the FTC test because the amps' thermal protection will kick in. (I still think its flaky that UL will certify an amp that does not work to their specification on the great bulk of the speakers out there -- i.e. 8 and 4 ohm nominal speakers. That's almost as good as the sun visor stickers that every SUV manufacturer now includes that say, in essence: This vehicle handles funny; watch out!)
I really would like to know the details about the UL test; it must be fairly recent. I'm just too cheap to pay for the whole thing off their website. Thanks.
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