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Old 31st July 2008, 01:26 PM   #1
cs is offline cs  United Kingdom
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Default Hitachi mosfet power amplifier

Some time ago (1980's I think), Hitachi published an App Note for a power amp using their mosfets. It was unusual in that the VAS was configured as a diff pair, thereby improving the PSRR over the conventional arrangement.

I would like to have a go at this, but using more modern devices. Does anyone have a copy of the original App Note (or schematic) they could post please ?

TIA,
Chris.
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Old 31st July 2008, 01:33 PM   #2
fab is offline fab  Canada
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Do you mean like this:
http://www.ampslab.com/lm60_schematics.htm

or on this forum for "symasym" amp?

There are several threads about the symasym
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Old 31st July 2008, 02:16 PM   #3
cs is offline cs  United Kingdom
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Many thanks, yes that's the one !



Cheers,
Chris.
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Old 31st July 2008, 05:50 PM   #4
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If you use the search function you will find that this classic amplifier has been discussed in much detail many times already

The diff pair style voltage amplifier stage is not really a diff pair as we know it, this topology is used to achieve high gain. And actually this amp has worse PSRR than most.
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Old 31st July 2008, 09:31 PM   #5
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cs,
or this:
http://www.geocities.jp/mutsu562000/...m/e/hp179e.htm
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Old 1st August 2008, 01:49 AM   #6
fab is offline fab  Canada
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Quote:
Originally posted by Lumba Ogir
cs,
or this:
http://www.geocities.jp/mutsu562000/...m/e/hp179e.htm

This is a quasi-complementary design using UHC mosfet N channel only in the output stage. I wonder how they cost and if easily available?

The 150 ohm resistors, are they photoresistors or else?

Is this one really in a Denon amp and which one?
I wonder how it sounds...

I have already used the same design topology but bjt in input stage and vertical mosfet in output stage.

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Old 1st August 2008, 02:14 AM   #7
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fab,
I would rather use laterals and (cascoded) J-FET input.
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Old 1st August 2008, 03:04 AM   #8
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Quote:
Originally posted by fab



This is a quasi-complementary design using UHC mosfet N channel only in the output stage. I wonder how they cost and if easily available?

The 150 ohm resistors, are they photoresistors or else?

Is this one really in a Denon amp and which one?
I wonder how it sounds...

I have already used the same design topology but bjt in input stage and vertical mosfet in output stage.

From Denon Literature

The UHC-MOS has a gain that is even higher than that of bipolar transistors; with a single element, the UHC-MOS achieves the current linearity of 35 MOS-FETs and 3 bipolars

http://www.alldatasheet.com/datashee...I/2SK1297.html

To be honest, I do not see what is so special with the UHC MOS fets that they use. Hitachi does not seem to refer to them as UHC MOS only Denon does. They look to have a transconductance of 9 S at 2A @ Tc 75oC. Capacitances are very high also. It's like they have an oversized die for pulse apps without the case to dissipate the power for long periods. Probably better with multiple IRF devices to get the same tranconductance and lower capacitances.

Or am I missing something ?
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Old 1st August 2008, 03:38 AM   #9
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Fanuc,
Gm is just one parameter of importance. 2SK1297 is a common vertical FET. 2SK1529 is significantly better for audio. However, when sound quality has number one priority I`d go for laterals.
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Old 1st August 2008, 03:40 AM   #10
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"The 150 ohm resistors, are they photoresistors or else?"

Looks like temperature compensation to me.
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