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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 16/09/2019 11:43 am, ~misfit~ wrote:
On 15/09/2019 1:58 AM, Trevor Wilson wrote:
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 three pairs per side should be fine for a reasonably powerful amp?


**Again: It depends on the maximum Voltage output. 3 pairs allows for a
peak current ability of 21 Amps.

I've studied the PCB and the output devices are paralleled (along with a
resistor for each) so it wouldn't be hard to add a third device to each
(on very short flyleads - or even daughterboards - mounted to the same
heatsink).


**Sure. However, make certain the drive circuitry can cope.


The speakers I'm intending to use with this are Sony SS-K90EDs.
Like these:
https://www.stereo.net.au/forums/topic/260972-fs-sony-ss-k90ed-speakers-rare/


**OK.



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.


That's one of the things I like about MOSFETs.


**Well, a properly designed BJT amp should demonstrate the same
robustness and reliability.


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 have a couple of big heatsinks for the amplifier modules that will sit
either side of the case, fins outwards in free air. They'll easily
handle the power dissipation being 4x bigger than the 'sinks used on the
PA amp. Also I'll ventilate the top and bottom of the 'box' (if I end up
going with wood).


**OK.


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.


Thanks for this Trevor, I have saved it for future reference. My 300 VA
toroid that I'm thinking of using with this outputs 50v AC so +/- 70v DC
when rectified. The original PA transformers were 40v AC.


**+/- 70VDC suggests a maximum power output of around 250 Watts @ 8
Ohms. If you plan on attempting to obtain that much power
(continuously), then you will need two of those toroids.


--
Trevor Wilson
www.rageaudio.com.au

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