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Trevor Wilson[_3_] Trevor Wilson[_3_] is offline
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Default Introducing a New Horse to the Stable

On 12/09/2019 12:17 am, ~misfit~ wrote:
On 10/09/2019 11:54 PM, Peter Wieck wrote:
OK, OK, I will bite! Minor rant to follow:

Tube vs. Solid State on reliability:

There are not so very many 60-year old components in operation these
days unmodified since-new. My oldest tube item turned 100 this year
and likely works better than when it was new based on a better
understanding of antenna systems, optimum tube voltages and so forth.
Other than moving parts (CD player), the newest component in my office
system was made in 1963. The system runs 9 hours per day, 5 days per
week. Oh, and the tubes are original as well.

On the other hand, and given my hobby, I see a large number of SS
components that have blown transistors, exploded capacitors and much
worse, irrespective of age and source. The well made, well designed
stuff is serviceable, distinguishing it from the rest of the garbage
out there.

I would make a fairly apt comparison: A tube amplifier is much like a
mid-last-century Mercedes or VW - few things were self-adjusting, and
they required regular and attentive care-and-feeding. With such, they
were good for several hundred thousand miles of reliable service. A
contemporary Ford, Cadillac, Plymouth would be considered remarkable
were it to survive 100,000 miles without heroic measures. Might run
very nicely when running, but that would be your basic solid-state
device in comparison.

Put simply, they are different beasts designed with different things
in mind, but for the same basic purpose. That one is or is not
"BETTER" than the other is not relevant to the purpose in either case.

Now, when I here things like "Zero global NFB" and "Critically matched
components", I can smell the snake-oil from a great distance, even the
10,000 miles from here to Australia. I am sure that process also
contains descriptives of "interconnects" rolled on the thighs of
virgins on Walpurgis Night...

Note that even "critically matched" solid-state components drift after
a very short period of time in-service. All of them, such that that
"less than 1%" is meaningful for perhaps 12 hours or so.

Being as this is a hobby for me, I get to try things that are
otherwise unproductive, unprofitable or impractical. Such as
shotgunning a device with single-value capacitors and then comparing
it to the same device with carefully screened and matched caps. Or
matching driver and output transistors and comparing to a similar
device with disparate values. Guys and gals - you would be seriously
shocked to discover how little difference some things make that the
ALL-SEEING, ALL-KNOWING gurus will tell you are critical. Often no
difference at all.


Thanks for your input Peter. If I may ask, do you have an opinion on
'storage capacitors' on an amplifier power supply? What in your opinion
is 'better', a single (or few) very large caps or multiple smaller caps
to the same / similar capacitance?

I have a long term project building my own amp based on PCBs taken from
100w MOSFET (two pairs of J50 / K135 devices per amp) PA amps made by a
New Zealand company in the 1980s. (Craft, Gary Morrison's company before
he went on to become head designer at Plinius until 2005 when he left to
set up Pure Audio). I got my hands on a rack of four of these mono amps
and preliminary testing using a clean source and good speakers suggest
they will make a great stereo amp.

I need to put together a power supply to feed two of these and have some
new 10,000uF caps but was wondering if multiple smaller caps would be
better. (In the PA amps they only had 2,200uF but obviously weren't
called on to reproduce much bass.)

As it is I'll be using fly leads from the rectifier PCB to the caps,
then to the amps and I'm building my own case. I was thinking of maybe
using my 10,000uF caps as well as maybe some smaller ones, perhaps 1,000
in a bank, the best of both worlds. (There are also 100uF electros
across the rails on the amp PCBs that I'll be replacing.) That said I
could also just go to multiple

Cheers,


**Those old MOSFETs were pretty ordinary devices (not very linear).
Evidenced by the fact that Plinius amps have always used BJTs. As Peter
has stated, multiple small value caps will usually provide a superior,
higher speed power supply. However, I would posit that those old MOSFETs
are so horrible (modern MOSFETs are far superior), that it may not be
worth the effort. Craft amps used huge amounts of global NFB, required
due to very low bias currents and the necessity to reduce the huge
levels of distortion caused by the 'knee' at low currents (A Class A, or
high bias MOSFET amp would have been much better). Anyway, the huge
levels of global NFB means that PSRR (Power Supply Rejection Ratio) will
be quite high, thus the influence of power supply changes will be
relatively small.

One more thing: Decent amounts of capacitance placed close to the output
devices is far more influential than caps placed some distance away. In
fact, long(ish) cables AFTER the main filter caps can be a serious
limiting factor on the effectiveness of a power supply in a Class A/B
amplifier. This is because the inductance of the wires can be a factor.



--
Trevor Wilson
www.rageaudio.com.au

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