Hi Jim,
Yes, fully agree with your point ## 1 below. But, I'll happily trade a bit
of efficiency in exchange for clean IMD anytime.
Your point ## 2 was implied in my original comment; so we're again in
agreement.
The SPE Expert 2K amplifier with its 6 x MRF151G MOSFETS has decent enough
IMD specs to not cause a nuisance to other adjacent users of the band in
terms of IM crud and buckshot; and the SSB signal is not too wide in
frequency occupancy; if any amp SS or tube meets that key criteria, then I
contend that it is good enough for ham use.
Peter G3RZP and Joe K2XX had recently posted the compliance test data:
http://g4ujs.shacknet.nu/spe_2k-fa.pdf
This is fairly decent performance for a compact SS amp of that 2 kW PEP
power capability and is not so easily disparaged.
Cheers,
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim Thomson
Sent: Sunday, 10 March 2013 11:49 PM
To: amps@contesting.com
Subject: [Amps] Expert 2K / lack of specs / pragmatism prevails
Date: Wed, 6 Mar 2013 11:18:18 +1030
From: "Leigh Turner" <invertech@frontierisp.net.au>
To: <amps@contesting.com>
Subject: Re: [Amps] Expert 2K / lack of specs / pragmatism prevails
When homebrewing big SS amps I've found designing 'em big and de-rating them
a couple of dB below sat Po yields plenty adequate IMD performance for clean
ham band SSB use. Certainly commensurate with the IMD of modern transceiver
PA stages and their associated ALC Tx power control loops.
Inefficiency is the inevitable trade-off one makes when wanting to run the
SS amp clean.
Leigh
VK5KLT
## Leigh, there would be no loss in eff, IF you used bigger devices, or
more of them. The inefficiency starts up when you design a 1 kw amp, and
then operate it at 500w. Its cleaner at 500w, but its inefficient. Reduce
the voltage a bit, like 41%, and eff is back on track, but imd suffers due
to lower voltage used.
## Instead, use devices rated for 2 kw output, design the amp for 1 kw
output, run the highest voltage possible and eff will be as high as it gets.
IMD will be a lot lower, and devices will run cooler. Trade off is you
require more devices or bigger ones. heck, the cooling can be based on 1 kw
output, and also the pwr supply.
## enough of these freescale devices may be the ticket. loads of power
AND low imd for a change. Just cuz you don't hear imd when listening to
an S-9 signal on a noisy band doesn't mean there is no imd. listen to the
same station, but at 40 over S-9, on a quiet band, and presto, imd is easy
to spot. If IMD is only 30 db below pep (arrl method), IMD will be 10 db
over S-9. IMD really needs to be down 60 db below pep, when listening
off freq.
## Its' the IMD 7-9-11-13-15th that causes all the problems. Class A and
some type of sliding bias scheme would work or the eq of EBS. Or just
operate the devices at well below their max ratings, like 1/3 their rating.
## You don't see this lousy imd in the commercial /marine/aviation world.
Jim VE7RF
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