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In the Forum: Playback Listening
In the Thread: “Harmonic Speed”- friend of foe?
Post Subject: Variable output impedance amplifiersPosted by Duke on: 2/27/2005

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I've played around with a couple of amplifiers that have variable output impedance, namely the Berning ZH-270 and Wolcott Presence P220M.  The Wolcott Presence in particular is interesting, as the constantly-variable output impedance control can be dialed all the way to zero output impedance, which is supposed to pass a square wave with no overshoot.   Zero output impedance = infiinte damping factor, if I understand correctly.  Don't ask me how the circuit works, I don't know, and can't defend the manufacturer's claims.  (Henry Wolcott's regular job is designing and building custom test instruments for the Department of Defense, so it's possible he knows quite a bit about circuit design.)  I presume the circuitry varies the fedback to vary the output impedance, but this is just a guess. 

Now something interesting happens when the output impedance control is dialed to its minimum setting (maximum damping factor, and theoretically "perfect" performance):  The soundstage collapses; the presentation becomes dry, lifeless and even edgy; and any emotion in the music is completely lost.   When the output impedance control is dialed back up (damping factor down), the soundstage blooms, the music becomes "wet", and the emotion in the music begins to move you again.  However, it seems that there can be too much of a good thing - if the output impedance is set at minimum, the bass does become sloppy and resonant and this colors the presentation.  Now it is very hard to say what the ideal setting is because in my room it varies from one speaker to another, and more than likely varies with the room itself and other factors. 

I'd like to see more amps with selectable or variable output impedance, like the Burning and the Presents, so that we can select the best setting by ear rather than relying on the manufacturer to have done so for us, because I'm not sure there is a one-size-fits-all "best" setting. 

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