[Trevor Wilson[_3_]]

On 20/09/2019 7:45 pm, Howard Stone wrote:
On Thursday, 19 September 2019 11:00:48 UTC+1, Trevor Wilson wrote:
On 18/09/2019 7:53 pm, Howard Stone wrote:
Well the Radford is back and it has been working well for the past week, with my little Rogers JR149s. The bass is MUCH tighter.

**Than what? The Krell or the unrepaired Radford? The Krell possesses an
almost perfect bass response and low output impedance, thus bass will be
as good as it gets. The Radford, although a decent enough amplifier
cannot hope to match the Krell numbers. That said, the high(ish) output
impedance, common to most valve amplifiers, can lead to a pronounced
bass 'hump' at the resonance peak/s of the speaker system. The bass peak
may be preferred by some listeners.


My plan is to hold on to it for a couple of weeks or so, just to enjoy it and get to know it better. Then I'll send it for a thorough service.

I think the big thing I've learned, thanks to my adventures with the Krell and the Radford, is that the amp really does matter a huge amount. I'd underestimated that before.


**Of course it matters. On one hand you have a 'blameless'
amplifier (Krell) and, on the other hand, you have an amplifier that
insinuates it's own sonic character. The Krell exhibits a ruler flat
frequency response, excellent phase response, inaudible levels of THD
and is load invariant, down to below 2 Ohms. The Radford is the
opposite. It's frequency response varies with the applied load
impedance, it's phase response is not flat at frequency extremes and
distortion is high(ish).


--
Trevor Wilson
www.rageaudio.com.au

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Re the Radford, I can't see the frequency response curve online.

**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/...r-measurements

https://www.stereophile.com/content/...0-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/...r-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