FET amp need help !

TJ

Member
2001-09-05 1:54 pm
MALAYSIA
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 .
 

TJ

Member
2001-09-05 1:54 pm
MALAYSIA
here the circuit diagram !
 

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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
 
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.