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Old 6th Aug 2020, 11:11 am   #1684
Craig Sawyers
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Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,942
Default Re: The Audiophoolery Thread.

A loudspeaker drive unit is essentially current driven device - the force on a length of wire in a magnetic field is just BIL where B is in Tesla, I is in amps and L is the length of wire. Which is why the T/S parameters for a drive unit include the force factor BL.

Now of course being a resonant device, where the cone mass resonates with the compliance of the roll surround and spider (and the enclosed air of a sealed box) the current falls as the driver approaches mechanical resonance. Or double resonance in the case of a reflex enclosure. Essentially the driver produces a back emf as it approaches and goes through resonance.

Anyway, there are two choices. Either an amplifier and speaker cables with a significantly lower resistance than the loudspeaker voice coil, and allow the driver mechanics to dictate how much current is needed. Or a transconductance amplifier, where the input voltage is converted to a drive current.

The transconductance approach has been used with bass drivers (at the University of Essex, among others), but you need to apply motional feedback by sensing the cone movement and used that to control the amplifier gain. Various sensing has been used - an auxillary coil, capacitance etc. I can't recall if this has even been commercialised.

So the requirements of an amplifier feeding a practical loudspeaker are:

Enough voltage drive to meet the power demands, with a constraint arising from the Xmax of the driver(s)
Enough current capabilities to deal with short term music transients
A low output impedance so it is a flat frequency response into any loudspeaker

And all the design practicalities to make that happen.

High power valve amps with <0.1 ohm output R are likely to work just fine. But low power single ended amps are likely to produce frequency response anomalies into practical loudspeakers just as David has said.

Craig
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