irf 740 replacement

bob_v5 · 9th December 2005, 03:43 AM

Hi, ive blown up the amp i was making. it was good for about 150 watts in 4ohm but then burnt out. i had 2 irf740s on each side. (push pull config)Do you think i should rebuild it with 4 on each side or go for bigger mosfets? if so any recomendations?

CBS240 · 9th December 2005, 04:24 AM

Are you sure it was SOA that caused failure? It is possible that some other circuit condition caused the smoke. A basic schematic topology illustration would be helpful for people here to make better suggestions.

bob_v5 · 9th December 2005, 05:42 PM

hmmm, well it was working fine at lower power. Into 8 ohms it was fine. what is SOA? The circuit is a quasi complimentary mosfet design. It is based on a circuit in a magasine, but i doubled the voltage and uprated all the components. +- 75v, total 150v swing. My friend whos being building amps before i was born helped me design it so im sure hes right when he says i need bigger output devices. Always welcome to a second opinion tho. I'l c if i can post the circuit, but will just be a photo of the diagam so i dunno if it will be readable.

bob_v5 · 9th December 2005, 05:42 PM

here it is, cant really see very gud tho

CBS240 · 9th December 2005, 07:47 PM

Looks readable to me... You did change the top MPSA56 as Vce max is only 80V? This is current source for VAS(which 2N5551 should be fine) The current source will be exposed to rail to rail volts as well, Vce 150V, maybe use 2N5401? Also you may consider a 1K resistor in series with the base of the current source for the differential. If the current source for VAS saturates, it could affect differential bias.

SOA is Safe Operating Area. When you look at a transistor datasheet, it will have a SOA graph that plots Vce vs Ic and shows maximum instantaneous power dissipation allowed before failure. (go by the DC curve) Driving devices hard with lots of voltage on them will cause them to melt.

Your friend is probably correct that you need more transistors. Of course more transistors will require more drive, so a driver stage may be necessary. Your almost in the realm of redesign here.

bob_v5 · 9th December 2005, 09:58 PM

yea i changed all of them to either 2n5401 or 2n5551.

bob_v5 · 10th December 2005, 03:16 PM

what about using bigger mosfets then? I dont really want to redesign it.

9th December 2005, 03:43 AM	#1
bob_v5 diyAudio Member Join Date: Sep 2005	irf 740 replacement Hi, ive blown up the amp i was making. it was good for about 150 watts in 4ohm but then burnt out. i had 2 irf740s on each side. (push pull config)Do you think i should rebuild it with 4 on each side or go for bigger mosfets? if so any recomendations?

9th December 2005, 04:24 AM	#2
CBS240 diyAudio Member Join Date: Aug 2005 Location: K-town	Are you sure it was SOA that caused failure? It is possible that some other circuit condition caused the smoke. A basic schematic topology illustration would be helpful for people here to make better suggestions. __________________ All the trouble I've ever been in started out as fun......

9th December 2005, 07:47 PM	#5
CBS240 diyAudio Member Join Date: Aug 2005 Location: K-town	Looks readable to me... You did change the top MPSA56 as Vce max is only 80V? This is current source for VAS(which 2N5551 should be fine) The current source will be exposed to rail to rail volts as well, Vce 150V, maybe use 2N5401? Also you may consider a 1K resistor in series with the base of the current source for the differential. If the current source for VAS saturates, it could affect differential bias. SOA is Safe Operating Area. When you look at a transistor datasheet, it will have a SOA graph that plots Vce vs Ic and shows maximum instantaneous power dissipation allowed before failure. (go by the DC curve) Driving devices hard with lots of voltage on them will cause them to melt. Your friend is probably correct that you need more transistors. Of course more transistors will require more drive, so a driver stage may be necessary. Your almost in the realm of redesign here. __________________ All the trouble I've ever been in started out as fun......

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9th December 2005, 05:42 PM	#3
bob_v5 diyAudio Member Join Date: Sep 2005	hmmm, well it was working fine at lower power. Into 8 ohms it was fine. what is SOA? The circuit is a quasi complimentary mosfet design. It is based on a circuit in a magasine, but i doubled the voltage and uprated all the components. +- 75v, total 150v swing. My friend whos being building amps before i was born helped me design it so im sure hes right when he says i need bigger output devices. Always welcome to a second opinion tho. I'l c if i can post the circuit, but will just be a photo of the diagam so i dunno if it will be readable.

9th December 2005, 09:58 PM	#6
bob_v5 diyAudio Member Join Date: Sep 2005	yea i changed all of them to either 2n5401 or 2n5551.

10th December 2005, 03:16 PM	#7
bob_v5 diyAudio Member Join Date: Sep 2005	what about using bigger mosfets then? I dont really want to redesign it.

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