D amps also have much smaller power supplies, so some may lack in the ability to deliver a lot of current. Hence, despite high "power ratings," some may struggle with difficult to drive loads.
Afc - Ref1000 can drive 2ohm delivering 40A. Size of power supply has nothing to do with it since 2" toroidal transformer at 100kHz can pass as much power as 10" toroidal transformer at 60Hz. In addition voltage doesn't drop since it is line and load regulated. Traditional "linear supply" needs a lot of caps because of voltage drop and necessity to filter out 120Hz (difficult to filter out and audible) vs. SMPS high switching frequency (non-audible and easy to filter out). A lot of caps in parallel have a lot of inductance - not desirable since it is in series with the speaker. SMPS is in reality class D amp (origin of class D) and got bad rap only because of cheap computer power supply implementations.
"A Class D amplifier has one output stage fully "off" for half the cycle"
High efficiency of class D is not caused by switches being OFF. Speaker is always connected by Mosfet switches (there is no OFF) but direction of current changes. Icepower uses single supply and full Mosfet bridge reversing output while Hypex uses half bridge connecting output to positive and negative supply. Efficiency comes from the fact that Mosfets in ON state have very low resistance (in order of 20mohm).
Replacing in class D analog voltage with analog time (duty cycle) allows to get rid of nasty problem of output transistor nonlinearity that in class AB is corrected by deep negative feedback that causes transient intermodulation (TIM) and exaggeration of odd harmonics (unpleasant bright sound).
I don't understand mentioned output voltage offset of class D amps. It has as much offset voltage as any other class amp without output capacitor (it uses feedback/servo to zero output). Self zeroing is probably responsible in my Rowland for 5Hz (and not DC) bottom frequency range.
According to R. Vandersteen, DC offset is only a problem in their speakers with a powered woofer (the 5,7, and quattro). Using the class d amps in their other speakers does not cause any concerns. I just spoke with him this afternoon about this.
Pugstub - It might be related to fact that Icepower's full bridge places half of supply voltage (30-40V) on both speaker terminals. Non-powered speaker sees only voltage between terminals (difference) but ground referenced powered speaker might not tolerate such common mode voltage. They also mentioned in Stereophile that some electrostats don't work with Icepower for the same reason. Half bridge schemes like Hypex don't have this problem and can be even bridged while Icepower cannot be (already is).
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