The Class T amps are basically third-order delta-sigma modulators followed by a switching power stage.
In this respect they are similar to the Sharp 1-bit amplifier.
The main differences are the following: Sharp uses a 7th order d-s modulator with the same sample rate as DSD uses (twice that much for the newer generation amplifiers) which is not very easy to design in a stable fashion but gives a high S/N ratio for a given maximum switching frequency (0.5 times the sampling frequency).
Tripath on the other hand uses a very high sampling rate combined with a third order only noise-shaping loop and holds the maximum switching frequency down by a pulse qualification circuit that follows the modulator. In this respect it is quite clever.
Although I could imagine better working principles, the Tripath modules have some advantages. For their price they include quite a lot of functionality and you can get complete evaluation boards and suggestions for board layout etc.
Just remember that any switching amplifier is an RF circuit used to process audio. If you don't have sufficient experience in RF and EMI design you wouldn't get something like that running properly.
Enclosed is the result of the simulation of a third order s-d amplifier model. It's maximum output voltage is just 1 Volt but that doesn't matter for seeing how the priciple works. Shown are the switching waveform at the output of the switching stage and the voltage acros the load. The input frequency is 10 kHz and the amplitude 10% below clipping. I chose such a high input frequency for better visibility.
You can see how the switching frequency decreases with increasing amplitude, that's why there is some ripple on the sinusoid's maximum amplitude (simply because the output filter's efficiency is worse at lower switching frequencies). You can also see how the signal across the load is lagging behind the switching pattern simply because of the output filter's phaseshift.
There is also an FFT of the aforementioned signals enclosed, showing the working principle of the noise shaping. K3 of this circuit is 0.3% approx but would be better for lower input frequencies. One property of noise-shaping delta-sigma modulators is the increased signal quality for lower input frequencies. I think the constant THD figures statted for the class-T amplifiers over the entire audio range is mainly due to the additional filter that is used during THD measurements.
I was able to audition one of the Sharp amplifiers and it was indeed sounding good, but at the same price conventional amplifiers sounds as good IMHO. There is an amp manufactured by Belcanto (Evo) using Tripath's technology, that received good reviews but I wasn't able to hear it so far. But for an almost empty box it is quite expensive at approx 4000 Euros. Also an amplifier manufactured by british ESLAB (also using Tripath technology) got very good reviews apart from the S/N ratio which was 17 dB worse than the manufacturer's statement. It's price is in the same range as the Belcanto
Regards
Charles
