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RE: Recent and future amplifier measurements

The method of testing audio power amplifiers is invalid because it does not simulate the way amplifiers are used in the real world Connecting the output to a 8, 4, or 2 ohm resistor is a walk in the park compared to what amplifiers really face. For example, to get speaker FR close to flat, most loudspeakers use very complex crossover networks that are highly reactive, not purely resistive. Look at the impedance measurements of YG Acoustics Sonja 1.1 and 1.3. The 1.1 has a reactive load that varies from 90 degrees capacitive in the bass to 33 degrees inductive in the treble, a very difficult load for any amplifier. To get that capacitive, the woofer must have had one humongous capacitor across its voice coil to filter out high frequencies. The 1.3 is better but even there you reported that of the three very expensive amplifiers you had, the best of them costing $44,000 had problems with it. The other factor is reverse EMF especially from woofers. Energy stored in the mechanical suspension as potential energy at its greatest point of excursion on each half cycle is converted back to electrical energy as it travels back through its zero crossing point. This generates a voltage that bucks the amplifier. Depending on the amplifier's power supply this can alter the bias voltage applied to the active devices.

What do these factors alter? Potentially everything including FR and non linear distortion, even short term stability. Exotic wires with highly reactive characteristics only make matters worse. This can easily be seen on an oscilloscope by looking at the output with a square wave input and observing the change in the waveform shape when a speaker is connected. This is why amplifiers that seem to measure about the same on a test bench sound different from each other.

Why are these measurements used? They are a legacy handed down from early times like the 1930s and 1940s when differences between amplifiers were so huge and the loads so simple that the tests showed meaningful differences. But those tests lost their value with time and tell us very little about real worth anymore. Unfortunately no one has bothered to invent better tests that are sensitive to meaningful differences in actual use. Therefore the measurements we do get are of little value.

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  McShane Design  

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