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~misfit~[_3_] ~misfit~[_3_] is offline
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Default Introducing a New Horse to the Stable

On 12/09/2019 10:18 PM, Trevor Wilson wrote:
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.


I hooked a pair of them up to a preamp while still using their original power supplies and was very
pleased with the sound so decided to go ahead with the build.

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.


Unfortunately I don't own a 'scope so am unable to check a lot of stuff. When I listened to them
with the original power supplies (designed for PA use) they sounded sweet and clean at low and
moderate volume levels but seemed to run out of power at higher volumes, especially when there was
a lot of bass.

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.


Thanks. The fly-leads will only be 6" tops and I'll be using at least 1.5 square mm multistrand
copper conductors. If space allows I'll put a ~1,000uF cap right at the amplifier PCB as well (or
as large as I can get away with). I may end up building a wooden case as I don't have a suitable
metal one and wood's something I have experience and the tools for.

I still haven't finalised my design yet. I might end up feeding them a few more volts than they
were getting from their original power supplies (my only suitable toroidial transformer is 10v AC
higher than original) so may parallel up a third pair of output devices onto the heatsinks using
one of the other amps as a donor. I haven't decided yet, as I said it's a long-term project and I'm
learning as I go.
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy little classification
in the DSM"
David Melville

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