[AMRadio] AM Transmitter "quality"


D. Chester k4kyv at charter.net
Sun Sep 20 01:42:48 EDT 2009


> I don't need to send or hear anything above and below about 100 Hz to 4000
> Hz, but extending the response well beyond those points cleans up the 
> audio
> to those points.
> If you build a rig that only goes 300 to 2800, the distortion at 300 and
> 2800 is usually high.

> Brett  N2DTS

The old UTC linear standard series catalogues recommend transformers to have 
a flat response at least one octave above and one octave below the target 
frequency response, to bring phase shift distortion to an acceptable level. 
To achieve reasonably good audio quality at the legendary 300-3000~ 
frequency range, your audio chain needs to be capable of flat response, at 
the very minimum, from 150 to 6000 Hz.  This is necessary to preserve the 
original audio waveform, and substantially reduces listener fatigue.

Using broadcast quality audio transformers  (UTC LS- series or better) and 
"hi-fi" circuit design without negative feedback, my homebrew transmitters 
test flat and relatively distortion-free  from about 40~ to 11,000~.  That 
means they are really optimum only for 80-5500 Hz.  That explains why so 
many vintage AM broadcast transmitters were designed with such a 
tremendously wide frequency response, 30-15K or better, even though everyone 
knew there were few, if any, AM receivers in existence with anywhere near 
that good a frequency response.  The designers were actually shooting for an 
optimum range of about 60-7500 Hz, pretty much typical for a good quality AM 
broadcast signal.  I recall  when I was working in broadcast engineering in 
the mid-60's, the bare-bones minimum acceptable standard for an FCC proof of 
performance was 100Hz  to somewhere between 5000 and 7500 Hz. I forget now 
if the exact high frequency number was 5, 6 or 7.5K.

Don k4kyv





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