>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
> The cogent question regarding the DAF concept is "Why does it work as
>well as it does? or even at all!"
minus 22db distortion is hardly "well".
> 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
> 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?
With a two-tone driving signal, screen potential was constant and
linearity was improved.
> 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
Typical solid state radios are not this good.
> 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
> 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!
A two tone test will not reveal the flaw in the DAF design because it
applies constant screen potential.
cheers, Marv WC6W
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