"Jim Lesurf"

http://jcgl.orpheusweb.co.uk/temp/amp1.png

http://jcgl.orpheusweb.co.uk/temp/amp2.png

Amp1 is from a paper that Hitachi released circa 1980. "100 Watt super
audio amplifier using new MOS devices" by Tohuru Sampei, et al. Work done
at the Hitachi labs. The main complication is using paired devices to get
enough current and higher power dissipation because the early power fets
were quite limited.

** That is simply not true.

Jim simply has no experience with Hitachi mosfets amps and he is blowing it
out his fat arse.

The stage shown as " Amp 1" could easily deliver over 200 W into 4 ohms, or
any load impedance higher than 4 ohms, no matter what the phase angle.

It would also be immune from overheating damage, show no signs of crossover
distortion and tolerate dead shorts if the DC rails were suitably fused.

A further plus is that even if one mosfet does fail, that does result in the
failure of all the others - as is the case with BJT
output stages.


FWIW They suggest using an output bias current of around 100mA as for the
chosen devices that made the bias current almost temperature independent.
But obviously a designer could choose another value for some other reason.


** With a difficulty, since Hitachi mosfets inherently tend to idle at 100mA
per device.


Amp2 is taken from one of the other Hitach sheets, and is much closer to
the Myst 'design' I think. Also from about 1980. This was claimed by
Hitachi to be an '80 Watt' amp, but I think you'd need to view that figure
as being 'generous' in real world audio terms. :-)

** Nope - it is a very genuine rating.


Personally, I didn't think much of the early power fet designs.


** What arrogant nonsense.

Yaawwwwnnnnnn ...


.... Phil

In article , Dave Plowman (News)
wrote:
In article , Jim Lesurf
wrote:
Personally, I didn't think much of the early power fet designs. Too
current limited. But they were nice and simple to make.

I've got the Maplin ones (which claim 100w into 8 ohms from a circuit
very similar to the Myst) here in the workshop and they continue to
satisfy. Driving some home made speakers using Audax units. Obviously
some fluke of nature.

They should be fine if used within their limits. The problem with
commercial designs is that you have no idea what speakers people will use.
The power fets appeared in an era when the USA in particular was into low-
efficiency, low-impedance highly reactive speakers. They tended to have
both a limited current (Idss) and a channel resistance above an Ohm.
(Sometimes well above! I think I recall some that the makers only specced
as having a high-current channel resistance that was less than about 10
Ohms. Presumably because at the time what they made varied a lot from
device to device with the same part number.)

And in my case I wanted 200wpc for 8 Ohms and 400wpc for 4 Ohms along with
the ability to provide higher currents for peaks into 'nasty' loads. I did
experiment with some of the designs and devices suggested/provided by the
makers back then. OK for an 8 Ohm resistor. But many speakers were rather
different to an 8 Ohm resistor. My comments are about 30+ years ago,
though. Newer devices are, I guess, much better.

Slainte,

Jim

--
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In article ,
Phil Allison wrote:
FWIW They suggest using an output bias current of around 100mA as for
the chosen devices that made the bias current almost temperature
independent. But obviously a designer could choose another value for
some other reason.


** With a difficulty, since Hitachi mosfets inherently tend to idle at
100mA per device.

Not on the Myst amp I've been working on.

I've no setting up instructions for it, so checked the current draw of the
+V line on the working amp - which seems to be the common way of doing
things. That was 60 mA - including the draw of the speaker protection
circuitry, which includes a relay.

--
*I never drink anything stronger than gin before breakfast *

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (Nutcase)


** With a difficulty, since Hitachi mosfets inherently tend to idle at
100mA per device.

Not on the Myst amp I've been working on.


** Run the amp into a load a get the heatsink warm before making an ass of
yourself in public yet again.

****wit.



.... Phil

"Jim Lesurf"

And in my case I wanted 200wpc for 8 Ohms and 400wpc for 4 Ohms along
with
the ability to provide higher currents for peaks into 'nasty' loads.

** Easily done with 3 x 2SK135 and 3 x 2SJ50 devices.

Peak currents of over 20 amps available, with simple zener limiting.

No SOA failure issues and no VI limiting circuits needed.

No over temp failure, parallel device current sharing, bias stability issues
or crossover distortion issues.

Dozens of Hitachi mosfet power amps from the 1980s all had these virtues.

Shame Jim could not see the wood for the BJTs.



.... Phil

In article ,
Phil Allison wrote:
** With a difficulty, since Hitachi mosfets inherently tend to idle at
100mA per device.

Not on the Myst amp I've been working on.


** Run the amp into a load a get the heatsink warm before making an ass
of yourself in public yet again.

****wit.

How warm is warm, Phil? Ok after a couple of hours into an 8 ohm speaker
at normal levels? Because of course I checked it again after initial
setting and use.

Perhaps you only check it under load. Being the ****** you are.

--
*Modulation in all things *

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (Nutase ****wit ) "
Phil Allison wrote:

** With a difficulty, since Hitachi mosfets inherently tend to idle at
100mA per device.

Not on the Myst amp I've been working on.


** Run the amp into a load a get the heatsink warm before making an ass
of yourself in public yet again.

****wit.

How warm is warm, Phil?

** A little above body temp is enough.

Whatever you claim you stupid POS does has NO bearing on my comments.

****head.


.... Phil

In article ,
Phil Allison wrote:
Whatever you claim you stupid POS does has NO bearing on my comments.

You should have that as a motto. It applies to everything you write.

--
*Those who live by the sword get shot by those who don't*

Dave Plowman London SW
To e-mail, change noise into sound.