[AMRadio] AM Amps


W7QHO at aol.com W7QHO at aol.com
Tue Jan 11 02:11:12 EST 2005


In a message dated 1/10/05 9:12:45 PM, w5omr at w5omr.shacknet.nu writes:


> Kinda Sorta.  Depends on the quality of the transmitter.  If he
> reduces the carrier output power of the Ranger, then the audio
> output power will rise, because there won't be as much final
> current flowing through the secondary of the modulation
> transformer.  Depending on the impedance match of the final to
> the modulator through the mod transformer, determins how much
> more audio will be gained (or lost) when power is reduced from
> the exciter.
> 
Hmmm..   Goes without saying that however the carrier level was reduced the 
audio level should be concomitantly lowered to prevent over modulation.   Easy 
to do, just reduce the mic gain control.

>   That, and the natural asymetricalness of the
> operators voice.  Lowering the output power, more audio, a 3:1 SR
> (Symmetry Ratio), the 25w of carrier could contain as much as
> 200w PEP.  In order for the linear to 'cleanly' reproduce the
> audio, it's going to need more than what the plates of a pair of
> 3-500Z's can deliver (*note: I said "Cleanly")
> 
> 
Two hundred watts PEP on a 25 watt carrier comes out to 400% modulation.   
Would take some extreme speech processing to achieve this in the positive 
direction while limiting modulation to 100% negative at the same time.   Far beyond 
the asymmetry of "normal" human   speech.   So, a pair of 
3-500Zs running GG and giving, say, 10db gain, with 35 watts of drive would 
give 350 watts of carrier out and modulating at 100% with a reasonably 
asymmetric audio signal would give 1400 W. PEP.   I have been using a 3-1000Z linear 
in this way for years with excellent results.

Speaking of the human voice, there undoubtedly exist some levels of asymmetry 
depending on the speaker.   I suspect, however that much of the asymmetry 
observed and reported is actually the product of nonlinearities in speech 
amplifier circuitry, particularly in the early, low-level stages.


Dennis D. W7QHO
Glendale, CA



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