[Amps] 12V IM3

[Manfred Mornhinweg]

Hi guys,

it seems rather funny to me how you worry about rigs having only a -28dB 
specification for third IMD. Just come here to Chile, and listen in to the kind 
of signals that populate the 40 and 80 meter bands here - the only HF bands used 
by most Chilean hams. -28dB IMD would be super deluxe.

The first thing almost every ham does here, as soon as he gets a new radio, is 
taking off the cover and turning down the ALC trimpot, fully disabling the ALC. 
They call it "freeing up" a radio, in the sense of removing those evil bounds 
that keep the poor radio choked to the 100 watt level.

Second thing they do, is hooking up the radio to one of those super-cheap, 
brick-shape switching power supplies, rated at 12.0V. They make sure there is a 
fuse in the line. And since those power supplies make noise with their tiny, 
fast-running fans, they put it far away and make the 12V cable long. The result 
is that the radio gets maybe 10V supply voltage during modulation peaks. Forget 
the IMD - there are bigger problems, like huge frequency modulation and other 
severe distortions. As long as the other hams can somehow manage to interpret 
what's being transmitted, the signal quality is considered to be fine.

And the third thing they do, is deciding that they aren't getting out loud 
enough, and adding a preamplified microphone plus an external compressor, "to 
help the radio do the job". And now the rage is adding an entire studio-type 
audio console, with multiband equalizers, echo and chorus effects, and the like. 
Of course they still use the radio's internal voice processor too, even while 
adding the external stuff. And how to set it all up? Simple: All knobs to 
maximum, that's sure to produce the loudest signal, right?

The IMD spec of the bare-bones radio, fed by 13.8V, and used with the factory 
power/ALC setting, becomes rather irrelevant...

Fortunately few Chilean hams use amplifiers. But when they do, they hook them 
right to those super improved radios - without an ALC connection, of course. So 
far I have never found any Chilean ham using an ALC connection between rig and 
amp. Not even when using a 100W radio modified to 160W saturated output, to 
drive a tetrode amp requiring 30W of drive.

All my complaining, explaining, educating efforts so far have been fruitless. 
The ham scene here considers that the loudest signal is the best signal, no 
matter how much noise, distortion, splatter, etc, is associated with it. You can 
judge the level of technical knowledge of these hams by this: A typical report 
for a SSB signal would be: "Your carrier is coming in at seven-nine, but the 
audio is weak!"  Go figure.

I find myself acting as a crazy clown, preaching the advantages of clean signals 
in this environment. So I couldn't let this opportunity pass, and blow some 
steam off, at least on this forum, where I can hope that most will understand my 
points!

Manfred
XQ6FOD



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