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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 11:01 pm, ~misfit~ wrote:
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?


**You need to examine the drive circuitry, the components used and then
calculate if those components can cope with the extra load caused by
extra MOSFETs. It will PROBABLY be OK, but I don't know.


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?


**As Peter has correctly stated, provided you don't need the full
continuous power capacity of the amplifier at all times, then one
transformer will likely be plenty. From my perspective, I am a purist.
If I am presented with an amplifier rated at (say) 200 Watts/channel,
then that amplifier needs to be able to deliver 200 Watts/channel
INDEFINITELY and, possibly more importantly, it needs to be able to
deliver roughly 40% of it's maximum power without thermal distress. With
one transformer in your amplifier chassis, it would fail such a test.
But, your amplifier is not a commercial item. You can make it anything
you want.


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.


**A worthy consideration.


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.


**The amplifier I presently use has a 5.5kVA (yes, 5,500VA), split wound
(one winding for each channel), double C core power transformer,
followed by 92 X 3,300uF filter capacitors. The result is to ensure
that, under full power operation (at any impedance higher than 2 Ohms)
ripple is kept below 100mV. So, discussions of 10,000uF per rail doesn't
excite me. It's what I expect to see in a mass market product from
Yamaha or NAD. However, as Peter and I have both suggested, in a high
global NFB amp, such as yours, huge lumps of filter capacitance will not
be pivotal to performance. Placing a decent amount near the output
devices will be beneficial though.


--
Trevor Wilson
www.rageaudio.com.au

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