[AMRadio] 4-1000A Grounded Grid Class-C


Donald Chester k4kyv at charter.net
Thu Oct 3 13:24:03 EDT 2013


> From: Mike Sprenger <mikesprenger at gmail.com>

> Interesting you brought up Asymmetry and how it impacts AM.
>
> I've wondered for some time, is anyone using a "Symmetra Peak" (Khan I
> believe) which was a 4th order "All Pass" filter intended to deal with 
> the
> Asymmetry in speech.
>
> Any "Modern Day" equivalents (perhaps built with Op-Amps, or, emulated 
> in
> software / DSP) out there in use ?


I presently adjust my transmitter to have as much positive peak headroom 
as possible, preferably at least up to about 135%, although not every 
one of  my transmitters has that capability. I modulate as close to 100% 
as possible in the negative without actually breaking the base line, and 
let the positive peaks go where the may, or else if they reach the 
positive headroom limit, cut the audio back to where they don't quite 
flat-top on positive peaks. Positive peak flat-topping causes exactly 
the same kind of splatter and distortion as overmodulation in the 
negative direction. I use an AM broadcast compressor/peak limiter to 
maintain the desired audio level.

I acquired an old Kahn Symmetra Peak from a broadcast station and tried 
it, but didn't like the way the waveform looked on the scope. I also 
have a modern solid state analogue one that uses op-amps, that someone 
sent me, but haven't got  round to hooking it up to try it. The Kahn one 
does just exactly what it is supposed to do with the rotating phase 
shift as I run the signal generator across the audio spectrum, but I 
wasn't happy with the result with my voice as monitored on the 
modulation scope.

I wanted to use it with my 8005 modulated by 838s rig, which modulates 
only up to about 115% positive, hoping I could fully modulate in the 
negative direction without positive peak distortion.

I'm not in the shack right now, but I can look up the information on the 
op-amp one at a later time. It is sold already assembled for only a few 
bucks from a company in Hong Kong. Expect to wait several weeks for it 
to arrive. It requires a separate + and a -DC power supply, something I 
need to throw together before I can use it.


Don k4kyv


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