Not necessarily.
When an exciter with IMD is connected to an amp that produces IMD there can
be a sum or difference of the two levels of up to 6 dB of the IM levels.
So it is possible that the overall IMD level can be less.
This summation is for equal levels of IMD produced by the amp and exciter.
Worst case is up to 6dB greater overall IMD and best case is 6dB better
overall IMD.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Thomas
> Walsh
> Sent: Tuesday, April 25, 2017 9:20 AM
> To: amps@contesting.com
> Subject: Re: [Amps] Price per Watt Conversation
>
> 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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