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Old 23rd January 2002, 04:07 PM   #1
TJ is offline TJ
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Default FET amp need help !

I found this circuit on a book ! Have anybody build this amp or know who design this amp ? How about the sound ? It stated Class A 100w(4ohm)N0-NFB . This amp original using 4 x 2SK405 , 4 x 2SJ115 and 2SC853/2SD261(one channel only) . I like to build it using 1 x IRF240 and 1 x IRF9240(one channel only).Keep saving $ and space ! Working or not working ? Please help me ! thank Q .
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Old 23rd January 2002, 04:18 PM   #2
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here the circuit diagram !
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Old 23rd January 2002, 07:37 PM   #3
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Default Fet amp

This is not an amp, this is only an Output Stage
of an amp.
It looks like the output stage of a hybrid amp,
driver with tubes.
Think it will work with 240/9240. You must have a look at the bias current. Best sound I think you get with something near 500 ma
Reinhard
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Old 23rd January 2002, 10:20 PM   #4
jam is offline jam  United States
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TJ,

This looks like part of a schematic I saw in MJ magazine.
You have the schamatic of the output stage(follower) that is placed next to the speaker. The missing section is the pre-amp. or voltage gain stage which is placed with the rest of your equipment.
Coda I believe used this scheme with some of their equipment, the advantage is the possibility of getting rid of one active stage in the playback chain.

Jam
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Old 24th January 2002, 01:39 PM   #5
TJ is offline TJ
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jam,
can u tell me which year Mj magazine describe about this circuit ....perhaps u got the back issue can u scan it and e-mail it to me please ! thank .
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Old 17th May 2005, 10:52 AM   #6
Koryusz is offline Koryusz  Hong Kong
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The circuit does work. I just built one. If you just feed the CD signal directly to the AMP, the volume is not enough. The sound volume about at 9 o'clock. So, in order to achive enough sound volume, it is recommed to work with a pre-amp.
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Old 17th May 2005, 11:44 PM   #7
anatech is offline anatech  Canada
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That's because there is no Voltage Amplification Stage. A preamp will not output enough voltage either, you need to build the rest of the amp!

-Chris
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Old 18th May 2005, 12:05 AM   #8
ilimzn is offline ilimzn  Croatia
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For 33V rails use IRFP9140 for P channel, IRF240 for N channel (yes I know one is 100V the other 200). You will need only two pairs, and even that should provide a more robust output stage than the 4 pairs of L-MOS, at a fraction of the price.
You may need to adjust values in the bias generator as VMOS have higher threshold voltages and different thermal characteristics - in particular, if the small BJT is mounted on a heatsink with the VMOS outputs, it will end up overcompensating, which can be solved by replacing the diode with a small resistor (10s of ohms, start low and experiment with increasing the value measuring idle current while heating up the heatsink, eg. with a hairdrier, until you get best stability of the idle current with temperature).
DO NOT use VMOS parts with N at the end for this, so no IRFP240N etc, or low threshold voltage VMOS as there are no reasonable complements for these.
A single pair of IRFP9140 and IRFP240, provided a good heatsink and proper mounting techniques, is good enoug for over 100W output (this will require a 4 ohm load on 33V rails). If only one pair is used, it would be beneficial to use a Vgs multiplier for bias generation instead of a Vbe multiplier, and also use no source resistors on the output MOSFETs.
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Old 18th May 2005, 05:00 AM   #9
Koryusz is offline Koryusz  Hong Kong
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I'm using 6 pairs of 2SK405/2SJ115 with 0.22ohm source resistor. The circuit is quite stable after power up for 10 hours. The DC ouput level lower than few mV.
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