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Reactance 14th November 2005 06:57 AM

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HI all ,,

I found this mosfet amp that i wanna use to power my 80 watt tweeters via a 2 - way linkwitz riley x-over with a crossover point of 3.1 Khz,

I simulated the circuit in using circuit maker 2000 and in worked wonderfully!!. The problem is I build the project on Vero board, and it does not work I get a bad dc offset on the outputs, which fries my mosfets I don’t understand why its doing this ??I checked my wiring 10 times ....... :confused:
I used a +/- 15 Supply just to test and I thought maybe the supply is to weak and I applied the full +/- 30Vdc and bang the mosfets fried Instantly :bawling:

Please Help ,
Thanks in advance…

Amplifier diagram attached.

AndrewT 14th November 2005 08:19 AM

540 is a vertical FET and needs a thermal compensation scheme to keep the bias controlled. Your schemematic has no source resistors which makes the problem even worse. Lateral Fets could work successfully with this layout because they become negative temp coef at about 100mA and above.
Some people use VFETs but the circuit must be designed to suit.
Lateral FETs are much more expensive £4 vs £0.60 in UK.

Reactance 14th November 2005 10:06 AM

Its strange the circuit looked very promising when i used circuitmaker 2000,also i thought a mosfet dont need any bias due to the high input resistance?, the minimal drain to source resistance is about 0.1 ohms so why would i need source resistors ?? is this a bad design if true then i can sleep in peace,without having to diagnose an amp that is not worth troubleshooting. i spend all weekend thinking it will work but sunday i gave up !!!

I thought mosfets are tuff transistors and fast too thats why i use the amp for high frequency applications,not to mention its attractive
design cost and power output .Is there an alternative to this design with the same design cost and output it will really help me a lot.


Upupa Epops 14th November 2005 10:30 AM

It is original schematic designed with old Hitachi SK/SJ , which some nitwit " redesign " for vertical IRF... Andrew is correct, without thermal compensation it will goes into " heaven ".... ;)

Reactance 14th November 2005 10:40 AM

What is vertical IRF ?? eem never heard that before.Can you explain what it means.

zinsula 14th November 2005 10:46 AM

There is also another thread about this amp.....well it drifted a little lately....

Ciao, Tino

zinsula 14th November 2005 11:03 AM

Vertical MOSFET's
2 Attachment(s)
Almost all devices today are verticals (IRF, HARRIS, ....) with very few being horizontals (Renesas = formerly the Hitachi, Exicon, and some BUZ..)

The horizontals have a temperature coefficient which leads to lower transconductance when getting hotter even at fairly low currents. This way, you can bias them "just wit a pot", they will not run away.

With verticals suitable for power amp outputs, forget this, as already stated. The current where you get negative temperature coefficient is VERY high. Look for data as attached. >10A for zero tempco of an IRFP240... :hot: :D
You need to adapt the circuit, eg using a VBE (VGS) multiplier.

Or use horizontal devices, such as the Renesas 2SK1056/2SJ160, which were used in the original design.


Upupa Epops 14th November 2005 11:06 AM

Or BUZ 900 - 906... ;)

Reactance 14th November 2005 12:47 PM

The hell with that mosfet amp it going to give me grey hairs if i pay futher attention to it , why does ppl put diagrams like those on the internet in the 1st place,,

I think i found a good amp ,, P3A looks good !!! any comments on this amp ??This will power my tweeters very well i think ,,

edl 14th November 2005 07:08 PM


Originally posted by Upupa Epops
Or BUZ 900 - 906... ;)

Or 2SK1058/2SJ162, I think this is one of the best audio-mosfet pair even manufactured.

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