[AMRadio] AM Transmitter "quality"

Larry Szendrei ne1s at securespeed.us
Tue Sep 15 15:18:45 EDT 2009

Joe wrote:

> I really don't care whether people chose to modify their old AM
> transmitters to "improve" sound quality, as they can do whatever they
> want with their radio. What does bother me a little is when people use
> words like "nasty" and "awful" to describe the audio quality of some
> classic transmitters, as if thedesigners of the day were not smart
> enough to design a decent transmitter.

Guilty; I used the word "nasty." But in using this word I never meant to
imply the engineers involved in the design were less than competent - I
accept that this was the effect they were trying to achieve, perhaps in an
attempt to make AM competitive with SSB to "cut through the mustard." I'll
still call a spade a spade though - some of them DO sound downright nasty
to my ears ;-)

> At the time these transmitters were built, the goal of amateur pone
> operation was effective radiotelephone service. Human speech falls into
> the range between 300 and 3000 Hz. There are sounds outside this range,
> but they do not contribute to intelligibility. So, in order to make the
> best use of the power available,  the bandwidth didn't need to be more
> than 4Khz.  The next logical step in the quest for efficient use of
> available power was SSB, which has an equally "nasty" sound but does the
> intended job quite well.

A lot of the transmitters we're talking about not only have restricted
bandwidth, but are very distorted in their stock form as well, in part
because of the crude hard clipping methods used to improve "talk power"
(rather than a combination of fast AGC and hard diode clipping only on the
highest audio peaks, as is employed, for example, in the CBS Volumax 410).
Granted, they had a price target for the transmitter they were trying to
meet, so they made the compromises they felt were necessary. I've heard it
proposed that part of this may have been a marketing ploy to help the AM
rigs compete in the marketplace with SSB.

> By modifying a transmitter to increase the audio bandwidth, all that is
> being done is to get it closer to the commercial broadcast transmitters
> of the day, which needed much greater bandwidth to handle music.

Sure, but there is more to audio mods than just opening up the bandpass.
There's plenty of improvements that can be made to reduce the distortion
in the audio chain of the old rigs, for example, adding inverse feedback.
The fact that the stock Ranger has an inverse feedback loop no doubt
contributes to why it is one of the better-sounding rigs of the day.

> Joe


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