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The All Channels Driven Amplifier Test Controversy

by September 20, 2004

There has been a lot of controversy on many of the Internet forums regarding power capability of receivers when driving multi-channel speaker systems. Some argue that when a receiver is rated at 100wpc x 7, it should be able to deliver this power to all channels simultaneously. But is this a realistic test condition and requirement to place on such a product whose aim is to provide the highest price/performance in a compact box for the typical consumer?

Given the limited internal space constraints within receivers and the fact they must power a multitude of speakers (7 channels or more) while satisfying UL/CSA requirements, it is often necessary to employ limiting circuits to avoid overheating the amplifiers.

Many audio engineers that design receivers care more about the practical condition in designing the amp section of a receiver and believe customers can utilize more dynamic (sometimes thought of as "peak musical") power than the average power specified or measured. A receiver compliant with THX Select spec for example, is required to output 40V peak to peak (50 watts @ 8ohms / 100 watts @ 4 ohms) when driving multi-channel speakers using 'Tone Burst Signal' method. This suggests that these amplifiers should have no problem with driving a moderately sensitive multi-channel speaker system for most consumer audio applications.

When a reviewer measures amplifier power, it must be noted if the reviewer uses a regulated AC power supply properly, which should be rated to more than 2000W when testing. Since most robust 7.1 channel receivers power consumption is over 1200W when outputting at full power, if no regulation is used, the measurement would simply result in showing the rating of primary AC power. In addition, most reviewers don't specify line voltage or frequency conditions when measuring the power. The power measurement is sensitive to tiny differences of these conditions which will affect the resultant power measurements.

On the flip side, most consumers do not regulate the AC line voltage of the power outlet where they connect their amplifiers, so is it realistic to measure the power with a regulated supply? The answer is yes and no . Since it would require too much space and heat dissipation for a receiver or multi-channel power amp to internally regulate power, the manufacturer must rate their amps using repeatable test conditions. This is a valid argument for using a regulated power supply when specifying power capability of the amplifier.

A Note About Heat Dissipation of Amplifiers
From amplifier testing we have observed in the past, we found that the common mistake some reviewers make in checking multiple channel output power is that they use a mono test tone signal. Since many of the limiter circuits found in receivers look for three channels or more to reach maximum output at the same time for over a half-second, a test tone guarantees that the limiter will be activated. Due to UL/CSA temperature requirements, the limiting would need to be increased with more amplifier channels to remain within the UL/CSA limits.

In "normal" operation with an analog or digital audio source (that is not mono or correlated), it is highly unlikely that the limiter circuit would ever be activated, thereby preserving almost all of the 100W output power per channel. (Actually the limiter circuit does not care whether the source is analog or digital.) That is why the only reliable and realistic measurement of multi-channel output power is by using a "Tone Burst Signal" method such as the one employed by THX.

The controversy behind lower power measurements of receivers implementing limiter circuits reviewed in many magazines is often too familiar but never truly explained. The best way to understand this is to realize the amplifier sections in receivers are usually designed for optimum performance when used with an audio source that a consumer would be expected to listen to, which is NOT a continuous test tone!

If the manufacturer were to remove the limiter circuits, then units would be too expensive to manufacture and still meet UL/CSA temperature standards. This is why many of the lower cost receivers have this limiter circuit but the company flagships do not.

Patrick Hart - Former Product Manager of Yamaha
In the eighties when I was Yamaha's Product Manager the EIA was grappling with the problem of how to measure dynamic power. Dynamic power was the new term for how much power an amp could deliver and sustain for 10 milliseconds with a "down time" in between of 500 milliseconds (at full RMS power). Back then the EIA was trying to banish the old IHF "Peak Power" term which came out of the fifties and sixties and at the same time coin a new one, dynamic power, to show what a real amplifier driving a real load in the real world could really do on musical peaks. Pretty honest, huh?

Now we come back to the "true RMS" reading and it's interpretation, if you want to advertise your $400 receiver as having 100 watts RMS. To be able to say this, many companies will look to that 20 year old mandate for dynamic power and dissect it so that producing 100 watts RMS for 500 milliseconds is okay. And with multi-channel amps and the totally random nature of music or soundtrack power demands spread among so many channels, who is to say they're wrong in rating their receivers in such a way?? It does keep the amplifier's cost down to hit a consumer friendly price point - and "real world" it's still pretty honest.

Remember, these receivers are hardly ever driven so close to their limits for any extended period. Unfortunately, for the few hearing-insensitive consumers that insist on their own continued self-destruction, many manufacturers have to add a limiter.

In a future article we will measure power of a typical home theater system integrated into a consumer's living room to show just how much, (or how little? ), power is being delivered to a loudspeaker system to reach high SPL levels at the listening position. In the meantime, we leave you with a brief perspective of how flawed the "all channels driven test" with a mono test tone is. We also discussed the compromises of how many mass market receivers are rated for power and the obstacles they must overcome to achieve UL/CSA certification in a cost effective package.

To get the scoop on how power in receivers is rated. Check out our article on Product Managing Receiver Platforms

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About the author:

Gene manages this organization, establishes relations with manufacturers and keeps Audioholics a well oiled machine. His goal is to educate about home theater and develop more standards in the industry to eliminate consumer confusion clouded by industry snake oil.

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