View Single Post
  #16   Report Post  
Posted to rec.audio.high-end
Trevor Wilson[_3_] Trevor Wilson[_3_] is offline
external usenet poster
 
Posts: 139
Default Introducing a New Horse to the Stable

On 13/09/2019 8:02 pm, ~misfit~ wrote:
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.


**I haven't listened to Craft (hi fi) amps in many years. What I heard
back then was pleasing. Very wide bandwidth (ca. 1MHz), as I recall.


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.


**That could be due to a number of factors. Including:

* Insufficient Voltage output.
* Insufficient current output.
* Insufficient power supply.
* An unreasonable speaker impedance.

Don't forget: Those meaty looking 2SJ50/2SK135 output devices are only
rated for a meagre 7 Amps each and 100 Watts PDiss. By comparison, a
typical output BJT of the same time period was rated at a far more
respectable 20 Amps and 200 Watts PDiss (MJ15003/MJ15004). Present
production variants are rated at 25 Amps and 250 Watts.

So, a little Ohm's Law should tell you if you are demanding more current
than the output devices are capable of delivering. 14 Amps is, by high
end audio standards, a relatively modest current ability for a (say) 100
Watt @ 8 Ohms amplifier. Provided the driver impedance is relatively
benign, you should be OK. Fortunately, it is real hard to damage
MOSFETs, by 'asking' them to deliver more current than they are rated for.



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.


**Wiring sounds good. And yeah, caps placed close to output devices is a
very good thing. A wooden case, not so much. Wood is an excellent
thermal insulator, which means heat may not escape too easily.


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.


**Well, the MOSFETs are rated for a decent 160 Volts, so a few more rail
Volts should be OK. And yes, more output devices won't hurt (refer to
Ohm's Law as before). Pay attention to the drive capabilities of the
preceding stages though.


--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus