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Old 6th Aug 2020, 2:20 pm   #1692
Radio Wrangler
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Default Re: The Audiophoolery Thread.

Philips used a piezo accelerometer on the cone, so it was acceleration feedback not position, so with the second derivative under control there would be uncontrolled constants for initial position and initial velocity. I presume they were 'leaked' to zero with long time-constants.

Interesting and said to work quite well. I once heard a pair of the small ones in Wood's in Huddersfield, but not under good listening conditions and not with suitable material.

Driving speakers is interesting. One view is to voltage drive them, and another is to current drive them.

If driven from a perfect current source, the parameter being controlled is the force on the turns of the voice coil. Not all force is available for Newtonian acceleration, some goes into flexing the spider and surround, these are not necessarily linear and can give hysteresis as well.

If driven from a perfect voltage source, the current is not controlled, but then the speaker is not a passive victim of the urges of the amplifier. It is not a pure motor, it is also a generator. Cone motion produces back EMF in the voice coil, proportional to velocity. An ideal speaker would take whatever current necessary to get itself to the dictated velocity where the back EMF cancelled the EMF from the amplifier. Essentially velocity feedback for free. It's not without its imperfections, because the cirrcuit loop includes the DC resistance of the voice coil, wires, crossover inductor and the outpur Z of the amplifier. If all is well, though and these resistances are linear, the speaker velocity is controlled, but winds up as a linear fraction of what you'd expect given the amplifier output voltage, with the loop resistance effectively compromising the loop gain of the effective feedback process.

Let's shoot a sacred cow. What gets called Damping Factor in an amplifier isn't. If you designed an amplifier with zero output impedance, it would be as perfect a voltage source as you could want. It is impossible to dissipate energy in a zero resistance, so it CANNOT do any damping at all. THe damping of cone motion at resonances come from the resistance of the voice coil, crossover windings, wiring.

On the whole, you want a nice low output impedance from an amplifier so you know where you stand, and it's probably what the crossover designer took into account and equalised the drivers with. Also speaker impedanes can vary widely (and wildly) so quoting damping factor against a pure and notional 8 ohms, is a bit thick.

Going the opposite way to a pure current driver, the amplifier has infinite output impedance, and presumably plenty of compliance. To the rest of the circuit, cables, crossover, driver, it looks like an open circuit with current appearing by telepathy or something. There can be no response of the loop current to the motion of thespeaker, no damping of resonances so the tendency is to leave the speaker with more of its imperfections showing. Resonances will be less damped and take longer to decay.

The same thing could happen in the zero ohm output amplifier case, but only if the resistance of the crossover coils and the voice coil were trivial.

But with normal voice coil wire resistance and the coils in the crossover, the low impedance amplifier trying to act like a perfect voltage source, along with low resistance wiring is closer to ideal than would be a current driver amplifier.

David
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