Unknown mosfet amplifier

vksamael

Member
2006-12-27 10:03 pm
Hello
I have this amplifier ( which given from my Frend ) , this is mosfet 2sj49/2sk134 amplifier , and driver transistor is 3 pair of bf469/bf470. I have serach over net and dont find anything of this amp. This amp build somewhere of 1980-1990 years in Germany. Amp is very good but i wont build another pcb and , put new capacitors and elektroliytes , resistor and put new transistor ( bf469/470 replace with ksc3503 and ksa1381).
Anyone have schematic or something.
Thank You
 

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It looks very likely that it is a DIY variation of the original Hitachi MOSFET amp developed in the late 70's. If You search the web for that , you will find a zillion schematics dealing with that kind of circuit. The one on the attached schematic is one of them, but I am sure it is close to what you have there.
 

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Hi, The amplifier is the ETI Series 5000 using ETI 477 modules. Designed by David Tillbrook and published in ETI Australia in the early 1980s. Quite a popular design here in Australasia.
100 Watts into 8 Ohms. It is based on the Hitachi app note, the major difference being the VAS consists of two cascaded differential pairs (BF469/470) ala Otala. Source resistors should be metal film types to avoid parasitic oscillations.
Regards,
Steve.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Agreed, the PCB is the giveaway and the larger makeshift heatsinks for the VAS transistors were a necessary modification to some budget kit versions, to reduce the high temperature due to the bias currents used.

FWIW, this is not a particularly good sounding arrangement despite a THD of only 0.007% /1kHz . Changing one type of video driver VAS transistor for another won't change anything however. Believe me, many like myself have tried rolling transistors in this amp over 30 years with no success. It would be better to use a simpler, balanced VAS such as posted by dragonweed but certainly use some decent high speed/ low Cob transistors like the BF469/470 or KSC3503/A1381.

The input pair may be BC556 but the VAS is composed of BF469/470 as described above.
 

owdeo

Member
2003-06-19 6:12 am
Sydney
Agreed - the 5000 (or ETI477) when built properly with decent components is an excellent amplifier that sounds clean at high levels and makes music enjoyable to listen to. It was far better than practically everything else available in kit form at the time. Electronics Australia did their own version, or "update" to the design about 10 years later (the "Pro Series One/Three", actually a different and simpler topology) - it sounded far worse (like a bad PA amp) :yuck:.
 

owdeo

Member
2003-06-19 6:12 am
Sydney
FWIW, this is not a particularly good sounding arrangement despite a THD of only 0.007% /1kHz . Changing one type of video driver VAS transistor for another won't change anything however. Believe me, many like myself have tried rolling transistors in this amp over 30 years with no success. It would be better to use a simpler, balanced VAS such as posted by dragonweed but certainly use some decent high speed/ low Cob transistors like the BF469/470 or KSC3503/A1381.
.

So you've changed your views on THD then - you now admit THD figures don't correlate well with SQ :p

Surely this design is an excellent example of a balanced VAS using the BF469/70 pair. You are proposing to improve the amplifier by modifying it to include aspects that it already has? :confused:
 

vksamael

Member
2006-12-27 10:03 pm
I dont think that i make something better , just i wont change old transistor bf469/470 with ksc3503/a1381 , replace resistor with vishay dale rn60d and mox , replace elektrolyte with elna silmic or cerafine and capacitor with silver mica, wima. That all is possible if i find pcb layout of amplifier