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Real World Testing: Matching Amplifier Data And Specifications

Converting utility power into audio power

By Pat Brown September 15, 2011

Click here to view/download a printable pdf of this article.

I’ve done enough theorizing and musing about power and power ratings in part 1 and part 2 of this series.

Let’s convert some utility power into audio power. First, some details about the testing.

The Source, The Load
“Source” and “load” are usually associated with amplifier and loudspeaker. I had to zoom out another level and consider the AC source to the amplifier (these amplifiers need a lot of juice). The amplifier has to get its power from somewhere, whether it be the utility power company, generator or a large array of hamsters on treadmills.

I added a new power circuit to the lab for the testing. The line voltage can be 120 volts RMS or 240 volts RMS. The circuit breaker is 30 amps. The electrical panel for the lab is near the amplifier rack, so line losses are small and tripped breakers can be easily reset.

The reason for all of the fuss is that when you are attempting to draw power that is at the limits of what can be safely sourced from the utility company, details that one might normally take for granted suddenly matter. The amplifiers need a hardy supply. Remember that when you design your power distro.

So now all I have to do is plug-in the amplifier, right? At these higher current ratings, there are several choices of power cord plug for both the electrical outlet and the amplifier inlet.

For the outlet, I chose a versatile, 4-conductor connector that can be easily wired for any voltage/grounding needed. The amplifier inlet connector was different for all three amplifiers tested. After a trip to Home Depot, and some rummaging through the parts bin, I fabricated a power cord for each.

Figure 1: Inside the testing – four 1,800-watt water heater elements (purely resistive) mounted into a 25-gallon steel tank.

The audio power must flow to something. The loudspeaker is the obvious candidate, but they get really loud at these drive levels and are expensive to replace. I mentioned a somewhat crude dummy load in part 1, but it had some shortcomings.

As before, when you’re pushing the envelope, the details matter. I decided to build a new load. Figure 1 shows “The Mother-Load,” a 4-channel by 8-ohm dummy load. There are four 1,800-watt water heater elements (purely resistive) that are mounted into a 25-gallon steel tank filled with environmentally-friendly mineral oil.

All of the tests in the article were performed driving the Mother-Load. Yes, I know the name is sexist, but “Father-Load” just doesn’t feel right.

A feature of the Mother-Load is switchable reactive circuits that can be added to each 8-ohm element. I lifted the schematic from the Rane Professional Audio Reference. It is shown in Figure 2.

Figure 2: The “Mother-Load” includes switchable reactive circuits that can be added to each 8-ohm element. (Credit: Rane)

Figure 3 shows the overlaid impedances of the Mother-Load configured at 4 ohms with and without the reactive circuit. I have also included the impedance curve of a real-world 4-ohm load to demonstrate that the average impedance of a typical loudspeaker is higher than its rated value.

Figure 3: “4 ohm” loads compared.

Why do I (perhaps) bore you with these details about power circuits and loads? Because they matter. If someone lays out a bunch of cash for an amplifier that they expect to provide thousands of watts of power, they had better think about where the power is coming from and where it is going to.

If you can’t source or dissipate the power, save some money and buy a smaller amplifier. The voltage and current from these amplifiers will challenge the Mother-Load. What about a “mere mortal” loudspeaker voice coil?


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About Patrick

Pat Brown
Pat Brown

Principals, Synergetic Audio Concepts
   
Pat & Brenda Brown lead SynAudCon, conducting audio seminars and workshops online and around the world. For more information go to www.prosoundtraining.com.
http://www.prosoundtraining.com

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Dan Bavholm says

Besides the clip-detection problem and level determination problem, pink noise isn’t “real world” due to:
-      Pink noise isn’t percussive like music. This will typically increase the frequency at which the clip limiter circuits are triggered and potentially cause the limiters to be in almost constant gain reduction. The results will be misleading.
-      Pink noise doesn’t reflect the spectral content of today’s music. There is an IEC 268 weighting curve, which was an attempt to shape the spectrum to more music like. This is however a bit dated, as it reflects yesterday’s music (e.g. jazz!).

Continuous sine test:
-      Continuous sine tests say more about the protection circuits and efficiency of the amplifier, than how loud it can play and how it sounds.
-      Continuous sine tests shall be used to explore the thermal protection features. Many modern amplifiers may reduce the power after a while, to lower the thermal stress and avoid thermal protection shut down. If this is done right it will never cause any limits to real world program material.
-      In most cases such as live-music, the safety standards (UL 6500 and IEC 60065) level of 1/8 of full power (-9dB) is enough. In some cases such as Sub basses and Disco subs requires 1/3 of full power (-5dB) continuous level.

A repetitive burst test is the best way to judge power amplifiers, as it’s more “real world” than noise and sine tests. Real world music is percussive and this must be reflected with the same beat in the test signal.
-      The problem is that the burst tests according to standards are dated. A new burst test standard is needed for pro audio amplifiers.
-      Bursts standards like IHF/CEA, has too high crest factor (16 dB). EIAJ has to high repetition rate and too short bursts (8 ms) so the lowest frequency component it reflects is 125 Hz. So, they don’t reflect today’s music spectral content or tempo.

With burst tests, it’s easy to see when the amplifier clips on an oscilloscope and determine the peak-voltage. The number of periods in the burst determines the lowest frequency content. To be real world this content shall be at the usual cut off frequency of subs, in the range 20 – 35Hz. At the same time the burst must be tailored with a carrier below 100Hz, as the stress at the power supply is higher below the twice frequency of the mains power.
I have noticed in some class D amps that the second period of a low frequency collapses, due to current protection or self-oscillation. Bad design of bootstrap circuit can also be a cause of this problem. It might also be that the stored energy reserve is consumed and the PSU doesn’t ramp up its mains current draw fast enough.

I recommend judging amplifiers with both burst and sine waves; bursts for real world test and sine test for the thermal protection behaviour. A new burst test standard is needed for pro audio amplifiers.

Dan Bävholm
WaveCapture
(WaveCapture designs audio measurement software. Dan has been product manager and owner of Lab.gruppen, a power amplifier manufacturer in Sweden.)

Chris B says

Where can I get schematics for making a dummy load such as yours for testing amps.  The music store I work for has to routinely test overheating amps & a dummy load is more preferable to speakers obviously.  Thank you

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