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Old 14th November 2005, 06:57 AM   #1
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Question 50 watt MOSFET AMPLIFIER

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 .......
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

Please Help ,
Thanks in advance…

Amplifier diagram attached.
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Old 14th November 2005, 08:19 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
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.
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Old 14th November 2005, 10:06 AM   #3
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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.




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Old 14th November 2005, 10:30 AM   #4
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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 "....
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Old 14th November 2005, 10:40 AM   #5
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What is vertical IRF ?? eem never heard that before.Can you explain what it means.
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Old 14th November 2005, 10:46 AM   #6
zinsula is offline zinsula  Switzerland
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There is also another thread about this amp.....well it drifted a little lately....
Power Amp Schematic

Ciao, Tino
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Old 14th November 2005, 11:03 AM   #7
zinsula is offline zinsula  Switzerland
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Default Vertical MOSFET'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...
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.

Tino
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Old 14th November 2005, 11:06 AM   #8
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Or BUZ 900 - 906...
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Old 14th November 2005, 12:47 PM   #9
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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 ,,
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Old 14th November 2005, 07:08 PM   #10
edl is offline edl  Hungary
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Quote:
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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