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The All Channels Driven (ACD) Amplifier Test

by July 17, 2006

More often than not we see folks on our forums and other forum websites debating how powerful a particular amplifier or receiver is based upon a single test popularized by many print magazines and some online publications called the All Channels Driven (ACD) test. Debates rage on from folks that claim their amp or receiver is better because it delivers more power into ACD or that the manufacturers that produce products that don't deliver their rated power in this test condition are either deceptive or inferior. What consumers fail to realize is almost no multi channel amplifier that is rated beyond 150wpc can meet the ACD specification for continuous power delivery on real world AC power lines. Add to this the fact that program material, be it Movie or Music, never taxes all channels continuously at full power, regardless.

Before continuing on, let's define the conditions of this test which most of the magazines and manufacturers use when making this measurement.

  • Power Sweep Test: A single 1kHz sweep of power vs distortion is often used.
  • Test at Clipping: This test is usually conducted at amplifier clipping
  • Regulated Line: This test is almost always conducted on a regulated line held to 120Vrms via a VARIAC device.

The Power Sweep Test

 

 

Let's discuss the implications of the power sweep test for a moment. An instantaneous sweep test will NOT yield a true continuous power rating in an amplifier despite the fact that some manufacturers will claim their power ratings are continuous with ACD and some print publications would have you believe it's a continuous measurement. Instead, the sweep will usually run for a couple of hundred milliseconds in a step progression of increased power until hard clipping is evident.

Closer examination of this graph reveals the amp has engaged a limiter or soft clip circuit above 150 watts yet review publications will usually still publish power figures well into the clipping range of the amplifier and make no notation of what is actually happening when the amp is driven beyond its linear range. In addition, most publications will limit the test to 1kHz only, NOT testing for amplifier bandwidth linearity or distortion problems over the full audible bandwidth 20Hz to 20kHz. More on this later.

Test Conducted at Clipping

Most review publications will publish amplifier power measurements into 1% or even 10% distortion. But, is this an accurate test to conduct? We think not. In our extensive testing of amplifiers, we have found most amps will misbehave when pushed beyond 0.1% THD + N. This is easily shown using FFT distortion analysis as shown below.

FFT @ .1% THD +N (tested at 1kHz) Power Rating

 

In actuality I had to conduct this test at 0.07% THD + N power level on this particular amplifier because the harmonics at 0.1% THD were significantly higher in amplitude. (34.146+54.492dBv)dBv = 88.638dBV or 100*log^-1(-88.638/20) = .004% This is a very clean power level as the harmonic distortion profile is well below audible levels.

 

FFT @ 1% THD + N (tested at 1kHz) Power Rating

At the 1% THD power level, the amp enters hard clipping causing nasty harmonic signatures much closer to the fundamental signal. (34.508+10.754dBv)dBv = 45.262dBV or 100*log^-1(-45.262/20) = .550% which is over 100 times the harmonic distortion of our previous distortion measurement.

As you can see in the plots above, the FFT analysis at 0.1% THD power rating is not full of the harmonic nasties seen in the plot tested at 1% THD rated power which is a result of clipping. The resultant power level is lower at 0.1%, but this is a much cleaner and more representative test of the capability of the amplifier.

Distortion Over Frequency

To understand an amplifiers true power output vs distortion, it should be checked across the entire audible bandwidth and not just one frequency (usually 1kHz). In many cases, an amp will perform ok at 1 kHz but become more stressed at the frequency extremes at full power for a variety of reasons. It is a good measure to run a simple test at 0.1% THD + N @ 1kHz for the amplifier and then apply the same signal amplitude at the frequency extremes (20Hz and 20kHz). You should see similar voltage delivery into our test load and corresponding distortion readings. If the amp is instead driven to 1% THD + N or beyond, at 1kHz, you will likely see the results shown below at the frequency extremes:

Test Limit

20Hz

1KHz

20kHz

 


0.1% Test

0.09%

0.1%

0.08%

THD + N

1% Test

1.2%

1%

1.8%

THD + N

The 1% test clearly shows non linear behavior at 20kHz and at 20Hz which would have likely been missed had we only tested at a single frequency. We have definitely exceeded the Linear Operating Region (LOA) of our amplifier in this test condition. But from a numbers stand point, the 1% 1kHz test looks more impressive on paper since we are lead to believe the amp is more powerful. It's also much easier to test an amp into clipping at one frequency which allows reviewers to pump out more reviews in less time. Don't be satisfied with a quick instant gratification power number. You should demand more information about the amplifier's performance.

Regulated Line

The astute audio enthusiast may wonder how it is possible that magazines and manufacturers generate the power measurements from the Device Under Test (DUT) on a standard household 120V, 15A line. Let's do some elementary math to further elaborate.

References:

Household Line: 120V, 15A can deliver max continuous power of (120 x 15) = 1800 watts (assuming no derating as per UL)

Amplifier Efficiency:

Typical Linear (A/B) Amplifier is between 40-50% nominal and up to 70% under full load.

Rail switching amps such as Class G/H can be as high as 70%

Switching amps (Class D) can see up to 90% real world efficiency assuming a properly designed power supply is utilized.

Note: These estimates assume the amp is under full load and that the power supply transformer doesn't overload, operates in the linear VA curve, and maintains regulation.

Let's focus on linear amps for the moment, since these are currently the most popular type tested and purchased by home theater consumer folks.

Now take our 1800 watt max power from the wall and multiply it by our amp efficiency (let's choose 68%) and we get: 1800 x .69= 1242 watts . This is the max power a typical linear A/B amp can deliver on a continuous basis from a 120V, 15A household line - assuming, of course, the amplifier's power supply can consume the entire 1800 watts of power from the line without causing the power transformer to overheat or go into thermal meltdown and likely trip the breaker.

By now folks from the ACD camp are saying, - well let's use a 20A line instead! That only gets the designer 4 more amps - or 2.6A with a normal power amp! What they fail to realize is most UL compliant consumer electrical devices utilizing the IEC320 receptacle, limit the max current consumption to less than 15A to prevent arcing which can cause a fire, death or serious injury (not to mention voiding your liability insurance). Unless the amp has a specialized connector on the back, or two independent power cords, it is likely it will be limited to 15A continuous consumption for safety's sake. We shall consider those type of devices as an outlier (not typical) and continue on with the more typical consumer devices reviewed and purchased for our home theater systems.

As a side note, other limitations include the AC wall outlet and breaker in the fuse box as well as the potential for 14GA wire run to the AC socket - 12GA wire has to be used for a legitimate 20A breaker.

As we already established, our linear amp best case continuous power delivery is limited to around 1242 watts from a 15A line.

What does this give us under the ACD test?

5 Channels Driven: 810 / 5 = 248 watts per channel

7 Channels Driven: 810 / 7 = 177 watts per channel

Note: This doesn't factor in any additional losses due to processing, and other active devices in a receiver.

So how is it possible that the manufacturers and/or publication reviewers can generate higher #'s than this? The answer is three fold.

  1. They regulate the line voltage using a VARIAC to ensure it maintains 120V under full load.
  2. They negate the fuse rating of the amplifier. Some publications will actually remove the rail fuses and replace them with a short for this test.
  3. Their tests are instantaneous (NOT CONTINUOUS) power sweep measurements into hard clipping at a single frequency, usually 1kHz.

This doesn't sound like a typical scenario (or a safe one at that) in a consumer environment. It also doesn't sound very realistic, does it?

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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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