In article , Mike
wrote:


Tripath class 'T' is quite a bit different from a PWM switchmode amp :
http://www.tripath.com/downloads/an1.pdf

"Instead of using PWM, Tripath Class-T amplifier processors use
proprietary algorithms and techniques to create the modulation that
drives the switching transistors. A Class-T amplifier's processors learn
the characteristics of the output transistors. Then, based on the analog
input signal, they switch the output transistors with exactly the right
timing to eliminate Class-D PWM problems: transistors not being perfect
switches, ground bounce, output transistor mismatches, dead-time
distortion and residual energy from the oscillator in the audio band.

The last phrase is an interesing claim. (See below) I wonder what frequency
range they regard as the "audio band"? With people pushing SACD and DVD-A
I'd assume they'd have to assign a higher frequency for this nowdays than
20kHz...

[snip]

The waveform for a Tripath Class-T amplifier would be a complex digital
waveform of varying frequency. A Class-T amplifier switches the output
transistors in a fashion similar to spread spectrum technology, at a
varying rate up to 1.5 MHz and averages 600kHz to 700kHz.

This is the potential problem that concerns me. The method means that power
is nominally present in a 'wash' of anharmonic components - mostly I
suspect in the few 100 kHz to few MHz region. This must be blocked from
reaching the speaker. Hence the method requires an output filter that will
pass the "audio band" with no significant loss, and no significant effect
upon the amps output impedance, yet strongly reject the ultrasonic wash.

The few measurements I have seen make me suspect that the existing amps
have an output impedance in the 5 - 20 kHz range high enough to alter the
resulting frequency response in way that depends upon the speaker impedance
properties. Thus the result may be like a conventional amp with an
excessively large output inductor. The standard network I've seen quoted
for use with these amps (a inductor-cap-inductor 'tee') may give a resonant
peak at HF into some loads as well.

The measurements I have seen, and the data I have seen from the makers do
not clarify the actual total ultrasonic wash power level that reaches the
speakers under normal conditions of use. I'd love to know this as there are
concerns if it is too high. This is complex as it is signal and loudspeaker
dependent. Hence being OK with test sinewaves into an 8 Ohm load doesn't
really cover it adequately in my view.

With sources like SACD and DVD-A the source may also be supplying
significant amounts of signal 20kHz. I wonder if this may find it way into
the power amp, and if so, how it might then interact with the switching
patterns the power amp employs. Again, I have seen no measurements or
analysis of this, but I wonder if it might lead to effects in the 'audio
band'...

Hence my personal view at present is that 'digital' power amps are a neat
idea, but I am not clear that - as yet - I'd prefer one to a conventional
design. There are too many questions which I've not yet seen answered.

Slainte,

Jim

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