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Don't be too hash to him.

Hi.

No doubt Dan is not prepared in his posts re class D amps without loop feedbacks sounded lousy to him. Apparently he knows very very little how switching amps work. Of coure, he is not alone.

Class D (pulse width modulation) is a 50-year-young concept. Not new at all. It's being not popular mainly due to no active devices to perform the required high speed switching at high frequencies (100KHz & up up) with mininum loss untill the birth of TMOS power MOSFETs in recent years.

Vaccuum tubes can switch pretty fast, but cause too much voltage drop due to their inherent high internal resistance. Bipolar power transistors can handle low frequencies properly but fail to deliver
at high frequencies due to excessive losses & large bias current complexity.

Nothing is perfect. Class D PWM power amp gets its downsides, e.g. its switch mode power supply tends to emit RFI; & pretty sensitive to various reactive speaker load due to its low pass filters.

The low pass filter, a must to remove all higher frequency square wave contamination out the audio signals at the exit of the switching amp to drive the loudspeakers.

It is built of capacitors, inductors & resistors, designed to work on a nominal fixed resistive speaker load, say 8R. But the reactive impedance of any loudspeaker swings up & down like a roller coaster.

An inadequately designed switching amp & its low pass fitler can get lousy sound, possibly due to resonances of the RCL filter circuits on the reactive speaker loads not catered for.

Back to the loop feedback issues. Unlike analogue amps where loop feedback is a gravy to the steak. All class D switch mode amps NEED
feedback loops to maintain the various proper functions of the amp, e.g. switch controller for the switching amp.

Through loop feedbacks, it controls & keeps the output duty cycle
from 5% to max 95% per the error voltage input, & safeguard the switching amp from blowing by monitoring the current sense I/P.

I have just seen class T switching audio amp in the marketplace which claims to beat class D & deliver much better sound.

You said Sharp sounds good, IMO, via digital input saving the A-D convertion error. Considering its USD4,500 vs its lowly 2x50W, it better sounds good. Or who would buy it.

c-J


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