**The frequency response into a dummy load resistor will likely be
pretty reasonable. Where almost all valve amps fall down, is when
driving real-world loudspeakers that present a varying impedance to the
amplifier (which is most of them). Some exceptions include Magnepans and
those speakers which have been designed with Zobel networks to flatten
the impedance curve. Consider the two frequency response graphs. Focus
on the frequency response of two amplifiers (one a valve model and one
an SS model), when driving a simulated loudspeaker:
https://www.stereophile.com/content/quad-ii-classic-monoblock-power-amplifier-measurements
https://www.stereophile.com/content/boulder-810-line-preamplifier-860-power-amplifier-860-measurements
Depending on the two amplifiers, those frequency response errors may be
lesser or greater in different amplifiers. They do, however, illustrate
the fundamental differences between most valve amplifiers and most SS
amplifiers. And, make no mistake: It is the errors in the frequency
response that makes the most audible differences between two different
amplifiers.
The guy who rebuilt my Krell said (before he's experienced the KSA50
-- so that may have changed his perceptions) "The Radford STA25 is
probably the best power amp I've ever heard at any price.. in the top
few anyway.
**Perhaps he hasn't heard many power amps. Perhaps he has a preference
for that particular amplifier. I don't know and, I suspect, neither do
you. Thing is, your Krell KSA100 is, essentially, blameless (ie:
Perfect) from the standpoint of audible flaws. I should add that I have
listened and measured both the KSA50 and KSA100 amplifiers. I felt that
the KSA100 was a truly excellent product. The KSA50 was lacking in my
system, but it was reasonably respectable through other speakers.
Consider an 'ideal' amplifier:
https://www.stereophile.com/content/boulder-amplifiers-2150-monoblock-power-amplifier-measurements
Such an amplifier is (or should be) the amplifier to judge others by.
Coincidentally (ha!) it happens to be the best in electronic
engineering terms, design, and measured performance, with lower
distortion, wider frequency response and higher damping factor than any
other valve power amp I've measured."
>
**Can you supply those measurements? The output impedance is the figure
I am most interested in. I would doubt that the figure would be much
below 1 ~ 1.5 Ohms. I very much doubt his conclusions. I hasten to add
that I have never examined the STA25 on my own bench. I have, however,
had literally hundreds of valve amps cross my bench. ALL display the
kinds of audible flaws I've been discussing to a greater or lesser
degree. And, depending on the load impedance of the speaker, those flaws
may or may not be audibly significant. I should also add that I have had
a large number of SS amplifiers cross my bench that also exhibit audible
flaws,
--
Trevor Wilson
www.rageaudio.com.au