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Old 17th Sep 2018, 7:16 am   #63
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Historic AM audio quality

I'm following this with interest. To have designed and put into manufacture an AM transmitter in the 21st century puts me into a bit of a minority. It's readily apparent that you're going against the flow when you try to find suitably rated devices.

The curved characteristic of a valve used as an AGC-controlled amplifier is immediate and applies to all components of its grid voltage. Distortion of an AM signal is inevitable, especially at times when the modulation depth is great and the AGC moves the operating point into the curvier regions. The same goes for transistor stages with AGC controlled bias.

We do, nowadays, have a number of low distortion variable gain techniques. PIN diodes and Gilbert Cells. Back in the day CdS LDRs and neon bulbs would have been great, but the complexity compared to just shoving a variable-Mu pentode in, would have been too expensive.

Diode detectors constitute a non-linear load on whatever supplies signal to them. It's their nature. People would have baulked at the cost of adding adequate buffering.

Delayed AGC tends to make fewer stages run in curvier regions at certain levels and exacerbates the non-linearity of the stages.

Low distortion AM reception, back with wide transmissions, did sound rather good.

AR88s and HROs set wide sounded wonderful on the BBC. The general coverage receivers coming out of Japan for the amateur radio market in the late sixties onwards were grim on AM. They had lousy distortion on SSB as well. Evidently the designers, once they'g got through a crystal filter thought anything goes...

David
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