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Old 11th June 2004, 08:23 AM   #1
ekaerin is offline ekaerin  Sweden
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Default When the 6922 married the 2sj49/2sk134

Hello everybody,
Please find attached an image of a 2 stage amp i did some time ago.
The image sucks, the wires got misaligned when I did the good old cut and paste.
It is a tube-MOS-FET design, very much like the one Elector presented back in 1997.
My power supplies are different though and I have used parallel FETs
and no decoupling of the bottom cathode resistors.
Also the component values are different, not shown for clarity.

This is a real simple design schematic-wise but it was kind of hard to
make it work with all loads. The FETs became really nasty at 4 Ohms
and about 3Vp amplitude, throwing out dc bursts (~5V, 40ms).
I have probably used too low ohmic gate resistors (SMDs at the gates).
I had to redo the grounding of all stages and now it is stable with all loads.

This is not any high end topology with ppm distortion. But it is nice to have
different types of amps hanging around.

The sound is very open without artifacts.

/ regards / Mattias
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Old 11th June 2004, 09:41 AM   #2
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Default Similar to a Jap design.

I saw a very similar design from the late 80's or early 90's.
It was a Japanese web site. I can't remember the URL. Probably will come up on a search on Google.
Output stage was identical except for a simpler bias setup.
I've built these and they sound quite nice. Needs lots of work to improve the sound.
Haven't lost one due to oscillation........not yet !
They run pretty hot.
Didn't have the bass end or punch of an Audiolab 8000S. HF also sounded a bit rolled off .........less bite. But nice by itself.

I used 1/4 watt carbon film 1.5K gate stoppers. I did have some problems initially but can't remember now. There was oscillation initially of course but it was brought under control. Lots of sparks and blown fuses . But luckily only two pairs of MOSFET's were lost.
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Old 11th June 2004, 10:20 AM   #3
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looks similar to this one...although the tube section is a bit different
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Old 11th June 2004, 10:08 PM   #4
Zen Mod is offline Zen Mod  Serbia
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look for Tube CAD site-there you'll find some useful info about hybrid amps
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Old 11th June 2004, 11:04 PM   #5
ekaerin is offline ekaerin  Sweden
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Choky, thanks for the advice. I'll give it a look.
I have many half way projects needing to get done so there will
be some time before I'm actually build another of thoose guys.

But dreaming and planning I think is a big part of DIY so......

/ Regards / Mattias
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