Mosfet amp - bridged?

kubeek · 14th June 2006, 01:27 PM

Hello, do you think this amp is designed good enough?
I am planning to use it bridged, but I read here some threads where bridging a certain amp was said to be unstable or what.

More, i want to increase the driver voltage by 10V, for the MJE350 and prats to the left.

The power supply is gonna be +-48V, plus the 2x10V for the driver,
I want to get 350W@8oms, into only a woffer, because it will be amp for bass guitar.

edl · 14th June 2006, 02:18 PM

That's a modified Crescendo amp.
I would rather build one of Quasi's (a forum member) design, or an Anthony Holton design.

kubeek · 16th June 2006, 09:21 AM

Are there other opinions?

djk · 16th June 2006, 11:59 AM

A) Those MOSFETs are expensive, and you will need three pair per half of the bridge amp to drive 350W/8R reliably.

B) Those MOSFETs sound a bit 'muddy' to me. I prefer a 'tighter' sound to my bass.

C) For a similar front end with a more robust output stage that sounds 'tighter' and costs less, I would build the Super Leach.

http://users.ece.gatech.edu/~mleach/superamp/

A similar version of this amp was produced commercially as the Heathkit AA1800. It tested at 400W/8R with ±95V rails.

If you already have your suppy bought, just build a pair of the v4.5 Leach and bridge. It will put out the same power, and a pair uses the same number of output devices.

richie00boy · 16th June 2006, 12:21 PM

No amp becomes unstable by bridging. As long as you observe the impedance considerations you will have no problems from an otherwise working amp.

kubeek · 16th June 2006, 08:04 PM

actually in the previous version of this I used IRFP150/9150, and it sounded good to me, but there was a weak transformer.

kubeek · 26th June 2006, 08:32 PM

Could anyone post here some good schema for a preferably mosfet-amp?
I have a pretty good source of mosfets in TO-247 package, so it´s cheaper for me to use them.

BobEllis · 26th June 2006, 08:48 PM

I like the Pass/Thagard A75 - see www.passdiy.com

I modified it slightly for 65V front end and 59 V output rails (42VAC transformer) You could go a little higher rails, but cooling the front end becomes an issue unless you redesign to reduce current in the VAS. I run 6 pairs of Fairchild IRFP240/9240.

I've also bridged Leach 4.5s on 60V rails - performed quite nicely into 8 ohms. You could easily modify it to run on higher rails, just use MJE15032/33 drivers and MJL4281/4302 or MJL3281/1302 for outputs. Follow Leach's directions for the front end changes.

It wouldn't be too difficult to modify the Leach for a MOSFET output stage - replace the Vbe multiplier device with a mosfet like IRF610 mounted on the heat sink, and skip the compensation diodes, and change the base stopper resistors from 3R3 to 221R or so. Power the front end from a separate regulated supply and away you go. I haven't actually done this yet, so beware.

AndrewT · 26th June 2006, 08:49 PM

Hi,
go Quasi,

He gives good support.

kubeek · 26th June 2006, 08:56 PM

I tried to search, but i didn´t find any of Quasi´s amps. Does he have any website?

16th June 2006, 12:21 PM	#5
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	No amp becomes unstable by bridging. As long as you observe the impedance considerations you will have no problems from an otherwise working amp. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

26th June 2006, 08:49 PM	#9
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, go Quasi, He gives good support. __________________ regards Andrew T.

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14th June 2006, 02:18 PM	#2
edl diyAudio Member Join Date: Oct 2004	That's a modified Crescendo amp. I would rather build one of Quasi's (a forum member) design, or an Anthony Holton design.

16th June 2006, 09:21 AM	#3
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	Are there other opinions?

16th June 2006, 11:59 AM	#4
djk diyAudio Member Join Date: Feb 2001 Location: USA	A) Those MOSFETs are expensive, and you will need three pair per half of the bridge amp to drive 350W/8R reliably. B) Those MOSFETs sound a bit 'muddy' to me. I prefer a 'tighter' sound to my bass. C) For a similar front end with a more robust output stage that sounds 'tighter' and costs less, I would build the Super Leach. http://users.ece.gatech.edu/~mleach/superamp/ A similar version of this amp was produced commercially as the Heathkit AA1800. It tested at 400W/8R with ±95V rails. If you already have your suppy bought, just build a pair of the v4.5 Leach and bridge. It will put out the same power, and a pair uses the same number of output devices.

16th June 2006, 08:04 PM	#6
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	actually in the previous version of this I used IRFP150/9150, and it sounded good to me, but there was a weak transformer.

26th June 2006, 08:32 PM	#7
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	Could anyone post here some good schema for a preferably mosfet-amp? I have a pretty good source of mosfets in TO-247 package, so it´s cheaper for me to use them.

26th June 2006, 08:48 PM	#8
BobEllis diyAudio Member Join Date: Sep 2003 Location: Upstate NY	I like the Pass/Thagard A75 - see www.passdiy.com I modified it slightly for 65V front end and 59 V output rails (42VAC transformer) You could go a little higher rails, but cooling the front end becomes an issue unless you redesign to reduce current in the VAS. I run 6 pairs of Fairchild IRFP240/9240. I've also bridged Leach 4.5s on 60V rails - performed quite nicely into 8 ohms. You could easily modify it to run on higher rails, just use MJE15032/33 drivers and MJL4281/4302 or MJL3281/1302 for outputs. Follow Leach's directions for the front end changes. It wouldn't be too difficult to modify the Leach for a MOSFET output stage - replace the Vbe multiplier device with a mosfet like IRF610 mounted on the heat sink, and skip the compensation diodes, and change the base stopper resistors from 3R3 to 221R or so. Power the front end from a separate regulated supply and away you go. I haven't actually done this yet, so beware.

26th June 2006, 08:56 PM	#10
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	I tried to search, but i didn´t find any of Quasi´s amps. Does he have any website?

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