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Old 10th June 2007, 08:01 AM   #1
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Default ampli mosfet with irf530/9530

Hi, this is my first post sorry for my poor english
so, I just built a new ampli mosfet like in the picture but when turn on the power the mosfet burn in few seconds....
why?!!?!?
any suggestion?!??!
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Old 10th June 2007, 09:19 AM   #2
Giaime is offline Giaime  Italy
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Ciao ciccio!

Where are you supposed to connect the power supply to the amplifier? From the diagram it's not clear.

I think you should connect the positive end of the power supply (maybe 20-25Vdc) to the drain of the upper power transistor, and the negative end to the drain of the lower power transistor.

Also make sure about the pinout of the power mosfets and the operational amplifier. Just for testing, you can also put a 1000uF 50V capacitor in series with the load (the 8ohm power resistor or a loudspeaker), just in case there is serious voltage offset at the output due to a wiring error.

BTW, I don't think it's a great design. You can't drive power mosfets with an output impedance of 10k (between the output of the opamp and the power mosfet's gate).
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Old 10th June 2007, 11:27 AM   #3
jaycee is offline jaycee  United Kingdom
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I agree, the mosfets are too much of a load for the opamp. There is probably some oscillation there.

Maybe you should look at http://www.wnaudio.com/amos100.pdf
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Old 10th June 2007, 12:12 PM   #4
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Default Re: ampli mosfet with irf530/9530

Quote:
Originally posted by Giaime
Ciao ciccio!

Where are you supposed to connect the power supply to the amplifier? From the diagram it's not clear.

I think you should connect the positive end of the power supply (maybe 20-25Vdc) to the drain of the upper power transistor, and the negative end to the drain of the lower power transistor.

Also make sure about the pinout of the power mosfets and the operational amplifier. Just for testing, you can also put a 1000uF 50V capacitor in series with the load (the 8ohm power resistor or a loudspeaker), just in case there is serious voltage offset at the output due to a wiring error.

BTW, I don't think it's a great design. You can't drive power mosfets with an output impedance of 10k (between the output of the opamp and the power mosfet's gate).

ciao, yes, the power supply is 22,5v from a torroidal transformer and 2 caps 4700mF/63vl
I have only a tester...
the pinout of the mosfet are correct.... about the design; I dont know! you think is not good???
thanks
ciccio.
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Old 10th June 2007, 12:35 PM   #5
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Quote:
Originally posted by jaycee
I agree, the mosfets are too much of a load for the opamp. There is probably some oscillation there.

Maybe you should look at http://www.wnaudio.com/amos100.pdf

ok I agree... and for this reason I want change all my project with a new one!!!
I think the "classic" project are always the best!
any suggestion are welcome.
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Old 10th June 2007, 12:37 PM   #6
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Quote:
Originally posted by Giaime


BTW, I don't think it's a great design. You can't drive power mosfets with an output impedance of 10k (between the output of the opamp and the power mosfet's gate).
Ciao Giaime

The Mosfet output stage is drived by the open loop output impedance of the TL 071 Op amp, usually around 100 Ohms, in series with the 100 uF capacitors.

As the Mosfets at DC and low frequencies have a near infinite input impedance , the 10 K Ohms are perfectly adequate , at high frequencies when the input capacitance come into play , the 100 uF capacitor bypass the 10 K resistor.

But certanly, the circuit maybe upgraded with a bipolar driver before the output Mos-Fets.

Arrivederci
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Old 10th June 2007, 12:43 PM   #7
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opsss... I forget the picture...
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Old 10th June 2007, 12:43 PM   #8
Giaime is offline Giaime  Italy
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Quote:
Originally posted by Tube_Dude


Ciao Giaime

The Mosfet output stage is drive by the open loop output impedance of the TL 071 Op amp usually around 100 Ohms in series with the 100 uF capacitors.
Uhm... you're right, I didn't think about capacitors.

Ciccio, you should also check the pinout of the TL431, just in case the drawing was wrong. Did you set the 47k trimmer? How much quiescent current you wanted to pass in the output stage? I hope you're using big heatsinks if you plan deep class A.

Why don't you take a look at Elliott's site, for projects?
http://sound.westhost.com/projects-1.htm
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Old 10th June 2007, 12:48 PM   #9
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Quote:
Originally posted by Giaime


Uhm... you're right, I didn't think about capacitors.

Ciccio, you should also check the pinout of the TL431, just in case the drawing was wrong. Did you set the 47k trimmer? How much quiescent current you wanted to pass in the output stage? I hope you're using big heatsinks if you plan deep class A.

Why don't you take a look at Elliott's site, for projects?
http://sound.westhost.com/projects-1.htm

ciao Giaime, the pinout of TL431 is ok! I have only read the volt...
is about 2.3v and -12v
the current? still not cheked...
the heatsinks... mhhh... I made the test without....
maibe for this reason the mosfet always burn!
anyway maybe is better the new solution!!!
what you think?!?!
ciao.
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Old 10th June 2007, 01:59 PM   #10
djk is offline djk
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The FETs will burn up if the bias is not set.

Without a heatsink, the FETs will burn up even if the bias is set.

The design of this amp looks marginal.
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