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Old 14th June 2006, 12:27 PM   #1
kubeek is offline kubeek  Czech Republic
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Default Mosfet amp - bridged?

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.
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Old 14th June 2006, 01:18 PM   #2
edl is offline edl  Hungary
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That's a modified Crescendo amp.
I would rather build one of Quasi's (a forum member) design, or an Anthony Holton design.
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Old 16th June 2006, 08:21 AM   #3
kubeek is offline kubeek  Czech Republic
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Are there other opinions?
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Old 16th June 2006, 10:59 AM   #4
djk is online now djk
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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.
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Old 16th June 2006, 11:21 AM   #5
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No amp becomes unstable by bridging. As long as you observe the impedance considerations you will have no problems from an otherwise working amp.
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Old 16th June 2006, 07:04 PM   #6
kubeek is offline kubeek  Czech Republic
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actually in the previous version of this I used IRFP150/9150, and it sounded good to me, but there was a weak transformer.
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Old 26th June 2006, 07:32 PM   #7
kubeek is offline kubeek  Czech Republic
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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.
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Old 26th June 2006, 07:48 PM   #8
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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.
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Old 26th June 2006, 07:49 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
go Quasi,

He gives good support.
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Old 26th June 2006, 07:56 PM   #10
kubeek is offline kubeek  Czech Republic
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I tried to search, but i didn´t find any of Quasi´s amps. Does he have any website?
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