Keep in mind the fact that one could have the cleanest amp and even with
nfb, alc etc. applied, it still only amplifies whatever is on the input.
What goes into also goes out of but now amplified. So if your exciter is
dirty, you will have a dirty output! We now know that the current group
of overpriced rice boxes are dirty as heck, and they even put out 100+w
spikes that can damage any amp -tube or ss even though the drive level
has been set low, it's the power level dac that must update every time
it keys. They all do it...especially the 10,000+ dollar rigs. All this
talk about imd etc., is usually caused by the exciter not the amp! On
any amp design with any gain factor, if one keeps the exciter sig clean,
at a conservative level it will automatically be clean. No alc is needed
today because the radios do all the alc-ing, the amp is only a
goes inta, goes outa. Alc and ndb only degrades the performance of
the amp. This is why the older hybrid radios which are analog in drive
control, never have a problem! It's when they hook up that 13K$ rice box
do they have any imd and/or drive problems. Perhaps try brown rice, it
may have some other effect. Tom W2CO
On Tue, 2017-04-25 at 13:24 +0000, Catherine James wrote:
> Thanks, Jim. I am learning something new here every day.
>
> I've been clear for some time on the fundamental conflict between good
> linearity (indirectly creating good IMD) and efficiency. With the last couple
> of days of discussion added, I now see that there's also a fundamental
> challenge in getting good efficiency and good thermal handling, and that the
> negative feedback solution isn't necessarily practical for some designs.
>
> The ideal amp would be highly efficient for lower power draw and minimal need
> for heat-sinking, have excellent linearity and IMD, use cheap devices, and
> support ALC, voice compression, and 100% duty cycle modes. Looks like a
> square circle. :-)
>
> 73,
> Cathy
> N5WVR
>
> Jim Thomson <jim.thom@telus.net> wrote:
>
> ### NFB = Negative feedback. 3 db of NFB means you also reduce the gain
> of the amp by 3 db. On paper, to achieve say a 10 db improvement in IMD,
> you require 11 db of NFB.
>
> ## 6 db of NFB on a SB-220 means you now require quadruple the drive
> power. NFB will work on a SS amp, since the gain is sky high to begin
> with.
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--
Sent from W2CO
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