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In Reply to: 4 ohm speakers for use with HT AV Receiver - A No No? posted by Uncle Pete on May 27, 2003 at 22:52:17:
At least that's what I've heard from some manufacturers. Apparently, the UL testing criteria for driving 4 ohm loads are pretty tough for a receiver to pass. Rather than jeopardizing their UL rating (no prob with 6/8 ohm cert), manufacturers choose to drop the 4 ohm testing even though real-world operation at 4 ohm is not a problem.
Follow Ups:
Well, the test procedures are mandated by the Federal Trade Commission (not UL) which has regulations regarding audio amplifier power output claims in advertising.Power bandwidth must be specified and distortion levels must be specificed. You will notice that many inexpensive recievers specify a power bandwidth of 40-20,000 Hz, not 20-20,000 Hz. That's because the receiver will not deliver rated power at 20 or 30 Hz. Similarly, since the same number of watts at 4 ohms as at 8 ohms requires more amps (current), many receivers do not specify a 4 ohm output because their wimpy power supplies will not deliver enough current to generate a respectable rated power over a decent bandwidth and with respectable distortion figures.
I would think that, for all but the most expensive receivers, the use of 4 ohm speakers would not be a good idea if you're interested in the best sound quality.
Wrongo, wrongo.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?
It seems odd that UL would permit equipment to be certified safe for use with less than all of the speakers that are out there -- and, let's face it, 4 ohm speakers are hardly unusual.
Bruce,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.
The FTC test originated in the 1970s because the IHF (Institute of High Fidelity) test was such meaningless b.s.
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