Hi Bill, Rich, Tom,
First, I suppose I should state that I am not a proponent of the
G2DAF amplifier geometry.
My personal preferences in the amplifier field are in the grid
driven, regulated screen, AB1 club with as much idling current as
necessary to maintain linearity, and heat the shack, or way off in the
other direction: class D, or 3rd harmonic enhanced class C, with envelope
restoration .
The cogent question regarding the DAF concept is "Why does it work as
well as it does? or even at all!"
According the original article, which Ian has posted on his website,
the design goals were for a simplified circuit eliminating the
conventional screen supply coupled with a reduction in standby
dissipation.
Mr. Thornly also seem to be concerned with the linearity of his
creation and supports his findings with a number of charts. He shows
that the output envelope is a fairly faithful reproduction of the input.
The question remains -- how faithful?
Back in the late '50's / early '60's anything with -20dB distortion
products would pass as state of the art.
Today we should ask for a bit better fidelity. -40dB is a good
benchmark.
I would offer the follow "improvements", without warranty!, to the DAF
concept to those interested in improving the design.
1. A tuned input structure to maintain the input waveform fidelity when
being driven from a transistorized exciter.
2. A simple envelope correction circuit appended to the screen supply
to enhance the overall linearity. A couple of simple (diode) envelope
detectors feeding a single op-amp driving a power FET shunt should
suffice.
3. Eschew the use of ceramic tetrodes/pentodes designed specifically
for class AB1 applications -- the grids are generally too lightly
constructed to be driven into AB2. Of course, this comment does not
apply to those parts which are specified for class C operation.
Confirm the results with a standard two-tone measurement and let us
know when you get to -40dB distortion numbers... if you happen to have an
exciter clean enough to make those measurements!
73 & happy DAF'ing!
Marv WC6W
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