The All Channels Driven (ACD) Amplifier Test - page 5

Common Questions about Amplifier Power

Now that we've digested that enlightening Q & A informational with APC AV about power ratings, lets get back to answering some common questions that have plagued the minds of many Audioholics visiting our forums and reading magazine review data and manufacturer literature.

How can my amp claim 200wpc / 400wpc x 7 into 8/4 ohms, respectively, ACD?

Any amp can claim anything… but as for actual delivery - it can't. Unless, that is, it has two individual and independent power cords feeding multiple power supplies. In order to achieve 200wpc x 7 continuously with all channels driven, it would require a whopping 3111 watts, or two 120V dedicated lines consuming almost 13 amps each!

An amplifier claiming 400wpc x 7 into 4 ohms, all channels driven, would require over 6200 watts! That's 3 ½ dedicated 15A lines running at full power - or, perhaps, a mini self contained nuclear reactor in your home! Remember Three Mile Island ? Those guys liked their music LOUD!

Is the ACD Test Representative of Real Music and Movie Program Material?

To answer this question, we have called upon our friends at THX, (namely Senior Fellow John Dahl), who have, to this date, done the most extensive and practical multi-channel real world testing of which we know.

THX: We have a 5.1 logging computer that we have used to play and profile a vast variety of movie, music and game material, identifying how much power is needed in each channel, at what frequencies, and for how long. This is the only way we know that makes it possible to develop a spec and test that guarantee that the product will play any commercial program material at reference level.

The test itself uses programmed bursts of energy that mimic the maximum duration peaks that a product must play. We assume a minimum impedance of 3.2 ohms with low reactance. The test is automated and tests channels singly and in groups with high level burst signals at various frequencies, groups of frequencies, for varying lengths of time.

A THX amp will play any commercial program material to reference level when hooked to an 89dB sensitivity speaker with a minimum impedance of 3.2 ohms. Here are a couple of charts summarizing the numbers for Ultra2 and Select2:

THX employs a proprietary Burst testing scheme - NOT a continuous single tone ACD test at full rated power. They do, however, check to see if the product will swing the required voltage to meet THX guidelines with a given load on 1, 2 or 5 channels driven only long enough to look at the waveform on a scope. Usually that's a few seconds, which is plenty long enough to ensure it will perform similarly on program material. If they see problems on the 1kHz waveform, they check further with higher and lower frequencies.

This is a comprehensive list of other parameters and metrics THX uses to certify amplifiers.

For more input on the ACD Test relevancy, we quote Michael Schenck - Power Electronics Design Engineer of APC AV.

APC AV: I don't agree with testing All Channels Driven - it is not a realistic measure of anything useful. There is no standard means of testing with no valid data to support its veracity. I've never seen any data that says an ACD test is similar to the power consumed by a kick drum or T-Rex. I feel it is important that an amp be capable of driving all the channels at 1/3 or 1/X power concurrently and continuously without overheating or blowing up.

Common sense requires that an amplifier be electrically capable of supplying enough power to satisfy musical and theatrical dynamics without clipping. Therefore, a peak rating with a foundation in reality is also important, I want to know the amp is capable of supplying continuous and peak power to the speakers when needed.

So the ACD is entirely invalid?

The answer to this question is a qualified "It depends." ACD can be thought of as a conditionally valid test to determine if an amplifier is capable of instantaneously consuming all of the power from the wall outlet. In the best case scenario, it can reveal the absolute capability of the amplifier's power supply. If the amp has a robust power supply then you will simply be testing for line voltage sag on an unregulated line up to the fuse limit of the amplifier.

The problem however (as documented in our previous article: The All Channels Driven Test Controversy) is that many budget products are designed for real world performance and must make trade offs for safety and heat dissipation reasons. As a result, they design their amps to be dynamic, but limit the total output capability of the product with a limiter that activates if more than three channels are driven at full power. The result of driving more than three channels at full power is reduced power delivery to all channels to satisfy the heat dissipation requirements of UL, as well as the manufacturer's requirements for dependability and reliability. Thus, when a publication does the classic ACD test into 5 or 7 channels, the reader can get the wrong impression that the amplifier isn't very capable at delivering power despite the fact it exceeds manufacturers specs with flying colors with only one or two channels driven continuously, and also satisfies the old FTC mandate for rating power into two channels.

Let's take the following Scenario when comparing two similarly priced receivers.

• Receiver #1: rated at 110wpc (# of channels driven not specified, but FTC mandates it must apply to at least two channels)
• Receiver #2: rated at 70wpc ACD

If we bench test both receivers with just two channels driven, we more often than not find that Receiver #1 was able to comfortably exceed its rating into 8 ohms and deliver respectable power delivery into 4 ohms (usually higher than the 8 ohm rating). Receiver #2 will usually exceed its 8 ohm rating but typically at a smaller margin than receiver #1 and, more often than not, maintains a similar power rating for 8 and 4 ohm loads. Of course an ideal amplifier should act like a constant voltage source and double power delivery when speaker impedance is halved. Very few receivers, and only the better separates amps, have a robust enough amp topologies and correspondingly robust power supplies to achieve such a feat.

So, in reality, Receiver #1 has the potential to deliver more power than Receiver #2, especially into 4 ohm loads, when driving one or up to three channels simultaneously with a correlated audio source. But because of the thermal limitations of the budget sized receiver, which aims at designing a dynamic amplifier over one that can provide sustained power into all channels simultaneously, it has to limit the power delivery when the amplifier is taxed with a correlated audio source to more than three channels. During loud transients Receiver #1 will likely be able to deliver cleaner output because it has more available headroom than Receiver #2.

On the other hand, the ACD test can very easily boast over inflated power figures for amplifiers that don't employ this form of current limiting. This is true in particular when the audio publications test in the 3 conditions we previously mentioned - instantaneous 1kHz power vs THD test, VARIAC regulated line, and bypassing the internal amplifier fuses. Thus, the consumer is misled into believing the inflated power measurements are continuous and achievable in typical household installations.

Since this test isn't representative of music or movie program material, and it is usually conducted in conditions not representative of normal product usage, its validity is questionable at best. It is certainly not a clear delineation of true amplifier performance in a real world environment.

So Why Do We Need Big Amplifiers?

Despite the fact that few boat-sized multi-channel amplifiers can deliver their inflated 200-400wpc into all channels simultaneously, there is still validity for these amps. An amp with a huge power supply and multiple output devices per channel can typically drive a wider range of speaker loads and usually has vanishingly low output impedance, making it less susceptible to frequency response variations when driving a reactive load such as a loudspeaker. It also has the ability to act more like an ideal voltage source by doubling its power with halving load impedance. This allows the amplifier to deliver more steady state power to low impedance loudspeakers.

Editorial Note about Amplifier Output Impedance
Output impedance is determined by output stage topology and feedback ratio. The internal wiring often (especially if there is internal speaker switching) has more impedance than the amp as does the output choke which is connected after the feedback stage. The ability for an amp to double power with halving impedance is almost all power supply influenced assuming the amplifier output stages can sink the required current to sustain the power levels.

According to Bruno Putzeys of Hypex, it turns out that the output choke is usually the dominant factor in determining amplifier output impedance, because it's outside the loop. Before the output choke, any reasonably designed amp has negligible output impedance. In addition, boat-sized amplifiers typically have such powerful (and hence slow) output stages that loop gain needs to be scaled back a bit, increasing HF distortion. Funnily enough, it is precisely HF distortion that fools the human ear into thinking the bass is tightly controlled.

So What Have We Learned?

• Most of the ACD power figured in magazines and/or in product literature are highly conditional, vary from publication to publication, and are usually not product representative.
• The ACD test is NOT representative of normal program material.
• Compromises in budget gear must be incorporated to produce dynamic amps while meeting stringent UL/CSA heat dissipation requirements under continuous loading conditions.
• Despite many manufacturers' boasts to the contrary, most multi-channel amplifiers' ACD power ratings are, at best, a conditional rating.
• The ACD test isn't a clear indication of the power capabilities of an amplifier designed for dynamic power delivery as a primary metric.

If ACD Doesn't Matter, Then What Does?

The following metrics can provide a far more insightful view of real world amplifier performance:

• Amplifier Bandwidth Linearity
• Output Impedance
• Amplifier Distortion + Noise at Various Power Levels
• Signal to Noise Ratio
• Channel to Channel Isolation
• Amplifier Stability under reactive load testing
• True continuous undistorted power delivery into one or two channels under various loading conditions
• Amplifier power stability with multiple channels driven at reduced power while the primary one or two channels are driven at full rated power.

These are just some of the primary metrics of concern when dealing with true amplifier performance.

A lot can be determined about an amplifier's performance by testing the above criteria with just 1 or 2 channels driven, while a comparison is made with the other channels at idle vs. being driven at a continuous reduced power level. Since the typical program materials we deal with are dynamic in nature, it's important for an amplifier to be able to reproduce large transients. Doing so requires careful design in the amplifier's power supply and output stages.

To test the amplifier's ability of handling multi-channel program material, a tone burst method such as the test THX implements is more practical and accurate in gauging amplifier performance. A slightly modified approach to the new IEC ruling for multi-channel amps is another viable approach (article coming soon).

So, Who is to Blame For Inflated Power Figures?

• First, the consuming public for demanding unqualified numbers over real performance.
• Second, the review publications for pandering to an uneducated demand for numbers without attempting to frame these "test results" in "real world" performance terms.
• Finally, the manufacturers themselves for playing the numbers game despite the fact that they should know better.

Closing Thoughts

Understanding amplifier power ratings can be a difficult task in an industry almost devoid of standards and rife with loose interpretations of existing standards that have exceeded their usefulness and practicality in today's multi-channel environment. Deceptive marketing and inadequately stated test conditions certainly don't help this situation.

So What are We Doing About This?

Audioholics is working closely with industry experts and leaders to formulate a new amplifier measurement standard that is both practical and executable. We are hopeful that our pending amplifier measurement standard will help to level the playing field and achieve universal acceptance among the standards bodies, manufacturers, and other A/V publications looking to adopt more realistic test conditions for multi-channel amplifiers which are more representative of real world product usage. Stay tuned for the new Audioholics Amplifier Measurement Standard coming to a home theater publication near you.

Acknowledgements

I would like to thank the following individuals for their peer review and/or technical contributions to this article:

Michael Schenck of APC AV

Tom Cumberland of Axiom Audio

Dan Banquer of RE Designs Audio

John Dahl of THX

Rod Elliot of Elliot Sound Productions

Joe Cornwall of Impact Acoustics

Bruno Putzeys of Hypex