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Old 20th August 2008, 09:02 PM   #1
mickb is offline mickb  England
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Default Mosfet amp

Hi everyone, i could do with some advice i have two p101 amps
that have Lateral Mosfets as outputs two of each n & p channel.

I built Maplin amps that used these Mosfet's quite a few years ago and it stated not to exceed 56volts.

The p101 boards state that around 70volts can be used.

I have a power supply from an old Yamaha amp which i would like to use which has outputs of +84volt & -84volt, would it be possible to use this with these Mosfets?.

Any advice would be greatly welcome thanks.
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Old 20th August 2008, 09:15 PM   #2
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The voltage limit on the Maplin amps is due to them only using a single output pair. With a benign 8 ohm load you might possibly get away with 84 volt supplies but I think you are seriously pushing it. To be honest I would use another transformer.

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Old 20th August 2008, 09:18 PM   #3
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Hello Mick!

It would be much easier to know for sure if you tell us which Mosfets you mean. Some power FETs can take up to 180 volts, so your 84 volt supply would be ok as the voltage across the FET can reach double that amount. (if we don´t take into consideration any spiked induced by inductive loads)

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Old 20th August 2008, 09:29 PM   #4
mickb is offline mickb  England
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Hi thanks for your replies, I have Exicon 10n16 & 10p16 plastic package mosfets, i also have some K135 & J50 TO3 lateral mosfets which i was thinking of using maybe on another amp with same power supply 84/84volts.
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Old 20th August 2008, 09:46 PM   #5
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With those kind of rails you need to be looking at 3 minimum, 4 ideally pairs of output devices. The voltage of the output devices is not so much the issue, it's the current and power that such high rails will bring.

Although those Exicon devices you have are only rated to 160V anyway, so your 84V rails would be too much.
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Old 20th August 2008, 10:06 PM   #6
mickb is offline mickb  England
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Thank you for your reply, i think i will forget the idea of using this transformer with these boards.

I have a power supply which produces +65 & -65volts would this be a better bet?.
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Old 20th August 2008, 10:12 PM   #7
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Mickb, +- 65 volts still sound too much. It would take several pairs just to avoid blowing them, even with a 8 ohm load.
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Old 20th August 2008, 10:26 PM   #8
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Are the DC voltages 65V? Or the windings 65V? If the DC is 65V then that's perfect for the P101 boards you have. Rod even rates it into 4 ohms load with those rails, but I think it's running close to the edge. What VA is the transformer?
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Old 20th August 2008, 10:34 PM   #9
mickb is offline mickb  England
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Hi the DC voltages are 65volt and i think it is 300VA, thanks for your reply.

Also i was thinking of using these boards in bridged mode, would they be okay for this?
Should i use two seperate power supplies or would the one power supply be okay?.
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Old 20th August 2008, 10:40 PM   #10
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300VA would be a bit feeble to run a pair of boards as you'd be pushing over 200W into a 4 ohm load each one. It would be perfect for a single board running 4 ohms or both boards running an 8 ohm load.

Bridging running those rails you would be limited to an 8 ohm load, although you would still run into supply collapse problems as with running 2x 4 ohm loads above. Bridging should use one supply really.
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