Hi Jim,
According to Collins the IMD reduction is not quite equal to the amount of
feedback in an amp. In other words for a 10 db reduction of IMD you will
need a little more than 10 db of feedback.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim Thomson
> Sent: Wednesday, August 07, 2013 8:46 AM
> To: amps@contesting.com
> Subject: [Amps] NFB vs IMD question
>
> A buddy is building the 250w SS HF amp you see in the last few arrl
> books. Its almost finished. It only requires aprx 7-10 watts of drive
> to the pair of VRF-151G finals.
> He also ran a series of tests on his K3....and found that min IMD
> occurs with 30 w pep out. It increases above or below 30 w pep.
> Its very clean
> at the 30 watt level. The original plan was to use a 5-6db pad between
> the K3 and the hb 250w SS amp. Upon close examination of the 250w ss
> amp,
> it occurred to me that if the NFB was increased by 5-6 db, the 5-6db pad
> between k3 and SS amp could be eliminated.
> I think the base resistor values on the SS amp can be increased to do
> this. Resulting IMD from the 250w SS amp...on paper, should drop a
> bunch.
>
> The 250w SS amp has pretty good IMD as is, at 250w pep out...and a lot
> better at 225w..and better still at 200w. The bias on the SS amp could
> also be
> tweaked a bit to improve it some more.
>
> OK, for every 1 db increase in NFB on the SS amp, how many db will the
> IMD3-5-7-9-11-13 drop ?? I have no info on this anywhere.
> Is it going to be a 1 to 1 deal, or does the IMD drop several db for
> each 1 db increase in NFB ? What about a tube amp, does it follow
> the same pattern as a SS amp re: NFB vs IMD. ?
>
> On paper it appears this scheme should work. What we are after is a
> squeaky clean 180-250 watts pep out. The K3 only puts out
> 100w pep.....and the IMD at 100w pep out is nothing to brag about....its
> lousy.
>
> The thought here was we could kill about 3 birds at once. Increasing
> idle current on the K3 final stage plus its mating driver is not
> appealing
> due to heat issues, plus minimal IMD improvement. Ditto with the 250
> W SS amp. Now whether the 250w SS amp will handle the 30w of drive,
> and no 5-6 db pad....and just relying on a 5-6 db increase in NFB is
> another issue. Any thoughts would be much appreciated.
>
> later........ Jim VE7RF
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