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Variable Amplifier Impedance

Variable Amplifier Impedance

Is this a project or an article? Since you may have arrived here from either index, you might be a little confused. This is fine, as it has elements of both, but lacks the in-depth analysis of other articles, and also lacks the application of a final design that is common to my projects.
The idea of being able to vary the output impedance of a power amplifier has been around for a long time. I have used these techniques for over 30 years in various designs, and much as I would like to be able to claim otherwise, I was by no means the first.

The technique has been used to drive spring reverb units, and various other transducers where current drive is preferable to voltage drive. The latter is what we all strive for with power amplifiers – a perfect (ideal) voltage amplifier has zero ohms output impedance, and the amplitude does not change as the load varies. Loudspeakers are very non-linear loads, and the impedance will change at different frequencies for all sorts of reasons.

So voltage drive maintains a constant voltage across the load, while a current drive circuit will produce exactly the same current into the load – in both cases regardless of impedance. In reality neither approach is ideal …

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