Tuesday, 6 April 2010

Passive Crossovers and the Impact of Amplifier Output Impedance

In previous blogs I've referred to the enormous amount of distortion introduced by passive crossovers and in this one I show how a power amplifier can react with a loudspeaker to change the way it performs. A good quality, competently engineered loudspeaker can have its performance sabotaged by a power amplifier.

If you model second order passive crossover as there may be in a typical two way loudspeaker assuming a typical amplifier output impedance of 0.01 Ohms and then use an amplifier with a higher output impedance, say 0.3 Ohms, you'll find that the bass driver crossover frequency moves up and the tweeter crossover frequency moves down, producing an overlap in the crossover region. The higher the amplifier output impedance the more pronounced the effect.

As an example, using speaker with a nominal impedance of 4 Ohms and a classic 2nd order crossover at 2.3 kHz, increasing the amplifier's output impedance to 0.3 Ohms gave a bass crossover point of 2.4 kHz and a tweeter one of 2.2 kHz (being defined as the frequency at which 90 degrees phase shift occurs). Therefore it is likely that higher output impedance amplifiers will sound noticeably harsher, because of the extra energy produced in this region compared to a conventional amplifier.

In a recent review of amplifier with an output impedance of 0.27 Ohms, the reviewer also noted significant frequency response variations due to the interaction of the amplifier output impedance complex impedance of the speaker load.

We've measured a good quality 15 Watt valve power amplifier with slightly better than 0.1% distortion and it exhibits an output impedance, via the 4 Ohm tap, at 100 Hz  of 0.28 Ohms, at 3 kHz 0.4 ohms and at 10 kHz 0.9 Ohms. Not only will there be even worse overlap at  the crossover frequency, but also a loss of HF output (nearly 2 dB down at 10 kHz).

Single Ended Triodes would be much worse.

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