None of them had a problem producing their maximum voltage for a few tens of milliseconds, even into 2 ohms with both channels driven. I did trip the circuit breaker a few times at 2 ohms, so this test was really more about the utility power source than it was the amplifier. It’s a shame that this number wields so much influence in the marketplace.
The Truth About 2-Ohm Loads
If you want the maximum output voltage and most linear performance from your amplifier, don’t load it to 2 ohms. I believed this before doing these tests, and I believe it even more now.
You can see how each amplifier’s voltage sags under different loads in the matrix. Voltage sag is bad. I would bet money that anyone who is conducting amplifier shoot-outs for driving their 2-ohm monster sub is mainly listening to each amplifier’s protection circuitry. If you want fidelity, design your subs to 4 ohms or higher (that’s RATED impedance – actual will be higher yet) and buy more amplifiers.
An Important Test That I Omitted
While the data matrix is useful for assessing the performance of each amplifier, it is not the whole story. If you change the program material to music, and drive both channels into 4 ohms or less, there will likely be significant differences in the sound, due to the philosophical underpinnings of the operation of the protection algorithms.
The best way to assess the amplifier’s performance under these conditions may be the FTLC6P test. It requires some Favorite Tunes, a Lawn Chair, and a 6-Pack of your favorite beverage. While the results are subjective, they may the be most revealing test of all for comparing overloaded amplifiers.
The Inevitable Comparisons
While I have stated that this was not an amplifier shoot-out, people will inevitably make comparisons. That is why I suggested the dBW scale in part 1, as opposed to “watts” and have included it in the matrix for the sine wave ratings.
Differences of less than 1 dB are negligible. A 3 dB difference may be audible under controlled conditions. The largest dB differences were for pink noise, which I showed above to be the most difficult signal to characterize with a single number. When we are looking at amplifiers rated at over 1 kW, the relative differences will be small.
Conclusion
The testing validated some important points from parts 1 and 2. Here’s a quick summary:
1) A 1 kHz sine wave rating into 8 ohms is the best measure of an amplifier’s performance for a sound system designer. The amplifier acts as a constant voltage source into 8 ohms. The voltage can be scaled to any crest factor by calculation. This makes accurate sound pressure level calculations at the drawing board possible.
2) Don’t load your amplifiers to 2 ohms to “get more watts.” When the voltage sags, you are losing output level and engaging protection algorithms.
3) Amplifiers don’t like low crest factor signals. If you excessively compress or limit the program material, and drive the amplifier to clipping, you are likely engaging the amplifier’s protection algorithms, with significantly audible ramifications.
4) Don’t compare amplifiers using power ratings derived from burst testing. These are vanity specs that look impressive but reveal very little about the amplifier’s performance.
5) Summing the instantaneous power ratings of all channels and thinking that you can get that much “power” is fantasy. Doing so is misleading at best and it promotes erroneous thinking and bad behavior in the marketplace. It’s putting a modern face on the peak power rating wars of the 1970s.
I was truly impressed in different ways by each amplifier that I tested. The output capability and efficiency given their size and weight is a true miracle of modern technology. It’s my hope that this series of articles sheds some light on how modern amplifiers operate and how their specifications are determined. This should lead to better correlation between the predicted and actual performance of each in real-world sound systems.
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