[sdiy] ot: class d audio amp

[Oakley Sound]

> Class D audio amps are pure switchers, in that the "amplifier" is
either on or off, resulting in higher efficiency (no crossover) compared
to Class A, B, AB amps...

Whilst they do display potential to be more efficient, care needs to be
taken to ensure that not too much power is wasted in driving the LC
filter at the output stage. Higher switch frequencies also require
quality ferrites for the L parts of the filter.

Things have changed since I worked on a 2KW class-D back in 1987. It was
mainly intended to produce a 'pure' (1% THD) 31.25KHz sine wave
oscillator at 2KW. FETs, high voltage caps and Ferrites have all
improved these days. There's at least one company offering evaluation
boards for class-D amps. But then, my colleague and I at Marconi
Instruments were breaking new ground at that time, we even had visits to
several UK universities talking about our work. Unfortunately, the
parent project was shelved due to commercial pressures. [This pissed me
off so much that I left, only to loose my job two months later when my
new company's directors embezzled the profits!]

I remember reading that the chap that designed the first class-D amp in
the 1930s had to work with relays! We had MOSFETs, six of them I think
in H formation running off rectifed mains. The compound Cgs at nearly
10nF meant that the gate drive current was peaking at around 5A. I think
we used four DS0026 gate drivers chips. But our biggest problem, along
with the new fangled EMC tests, was with high values of d(Vds)/dt; it
would break through into the gate and the FET would stay on too long.
BANG!! Cartridge feed FET systems were imagined.

Regards,

Tony Allgood                         www.oakleysound.co.uk

Oakley Modular Synthesisers      Penrith, Cumbria, England