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

On 16/09/2019 9:53 PM, Trevor Wilson wrote:
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.


I'm not exactly sure of how to do that?

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.


I intend to use the system in my lounge so won't want crazy SPLs, the speakers likely wouldn't
handle that much power anyway. I actually do have two of the toroids but that would make for a big
amplifier case - and surely then I'd need to consider adding *two* more pairs of output MOSFETs per
amplifier?

I was thinking that, as I don't listen to dubstep or extremely bass-heavy music, using one toroid
and a lot of capacitance (in the region of 20,000 to 50,000 uF per rail) would be enough to handle
transients. If not then I might as well build a pair of monoblocks.

I've got a few coffee-cup sized Mepco/Electra 14,000 uF / 100v caps but they're not new... I also
have 8 new 10,000 uF / 100v Elna caps that are only about 1/4 of the size.

Cheers,
--
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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