[Elecraft] KPA500 - linear power supply

[Alan Bloom]

One big issue with switching power supplies is their dynamic
performance.  An SSB transmission requires the power supply to be happy
with a load that randomly varies between maximum current and the resting
current of the PA.  If the power supply does not have good dynamic
regulation, that can cause distortion.

With a linear supply, the output filter capacitors have to be big enough
to filter the 100 Hz or 120-Hz ripple anyway, so they automatically do a
good job of smoothing out the variations due to the 300-3000Hz audio of
the SSB signal.

However, the filter capacitors in a switching supply can be much smaller
since they only have to filter out the high-frequency ripple from the
switcher.  So the dynamic regulation depends on the feedback circuitry
in the switching regulator.  If it isn't designed in such a way as to
provide low output impedance in the audio range, considerable distortion
can result.

Al N1AL


On Thu, 2011-05-12 at 19:06 +1000, Gary Gregory wrote:
> I would not be surprised to hear Eric and Wayne state that price was not
> that big a consideration with regard to the power supply even though it
> would have been one of the considerations. My bet is that IMD performance,
> reliability, durability and maybe stable performance played a significant
> role in their final decision.
> 
> I have to say that given the testing I have done here the amplifier works
> better than I expected. The quality I expected, but I still got a surprise
> with the performance.
> 
> I don't find the amplifier heavy but of course a switcher would have reduced
> some weight but I doubt it would have provided any further benefit...ymmv
> 
> 73's
> Gary
> 
> 
> 
> 
> 
> On 12 May 2011 18:45, David Cutter <d.cutter at ntlworld.com> wrote:
> 
> > That's not to say that they didn't try.  Unlike manufacturers who only make
> > linears, Elecraft has a vested interest in making their product line
> > friendly to all their products and a switcher would have to pass muster
> > against their own sensitive receiver.  So, Elecraft internal standard for
> > noise is probably much higher than in the general market place.
> >
> > David
> > G3UNA
> >
> >
> > > Hello George,
> > >
> > > From my reading of all the replies both on and off-the-list, the main
> > > reasons are:
> > >
> > > 1. it is cheaper to use linear PSU so that the price of KPA500 should be
> > > competitive;
> > > 2. maintenance and repair will be easier for a linear PSU;
> > > 3. RFI is so often found in the switching PSU
> > >
> > > I put this question up in the reflector because I find ICPW-1, VL1000 and
> > > TL933 are all using switching PSU. I think items 1 & 2 above have
> > > explained the reason of using linear PSU in KPA500.
> > >
> > > Voltage fluctuation in the AC mains will affect the linearity of the PSU
> > > in KPA500. However, this will not border me because the reliability of AC
> > > power supply in Hong Kong is over 99.9%. We have never suffered from any
> > > power interruption in the past years.
> > >
> > > See you all next week in Dayton.
> > >
> > > TNX & 73,
> > >
> > >
> > > Johnny VR2XMC
> > > >
> > > I can not believe hams who do not want switching mode supplies in their
> > > shack. This precludes the use of a computer in the shack. It also means
> > no
> > > TVs, printers, and a host of other products. There is also a false notion
> > > that so called linear supplies do not cause RFI. Study Dirac's delta
> > > function and you will understand why they can.
> > > Love my two K3s
> > > George, W6GF
> > >
> > >
> > > >
> > > One of the inviolable rules in the W6LX shack is: NO SWITCHING POWER
> > > SUPPLIES.
> > >
> > > Not to make an incendiary statement, but *all* of them generate RFI. Many
> > > people
> > > will tell you that they've never heard RFI from their switching power
> > > supplies,
> > > but often it's just because they don't know what it sounds like or they
> > > haven't
> > > tuned across the right frequency.
> > >
> > >
> > > Why raise the noise floor by even half a dB if you can help it?
> > >
> > > Nothing beats a old-fashioned, big, beefy, and quiet linear power supply.
> > >
> > > Al W6LX
> > >
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