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Old 15th May 2010, 05:42 PM   #1
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Default mosfet schematic

here is a schematic i found on the inetrnet ...the author claims 400W with 80+80 rails but there is a couple of things i dont get ....

1) what determine the bias of this amplifier ???

2) does the D4 1N4148 need to be in thermal contact on heatsink?

3) in this amplifier there is no VI limmiter as is in bipolar amps there is a protection circuit that protects mosfets from overdrive ... is this enough to protect also from the output section ??? in example shortcircuit conditions

4) presuming that its done by mistake and the load for the amplifier is 2 ohms will this circuit be able to protect at this condition also ??

finaly in the complete schematic ( available if needed ) there is an op amp that monitors curent flow on R35 220R and then conected to the output relay

what can it be protected if we monitor the curent flow in this resistor ???

for reference outputs are BUZ901-906


regards sakis
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 15th May 2010, 06:07 PM   #2
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Looks similar to an early Hitachi implementation

1. The 1N4148, not sure this would be very precise tbh. lateral mosfets only need a low gate voltage to turn on & flow some current. 0.7V might be enough, this would be 0.35V per gate. I'm not sure tbh. A variable resitor would have been better imo.

2. No, they are lateral mosfets & thus have a negative temperature coefficient, if they warm up then current through them will fall.

3. Maybe. The mosfets are limited to a 4V gate drive into a short circuit when idling, im not sure at all how much current this would allow a mosfet to pass. When a music signal is applied the gates can go to a higher voltage. Well assume you'll only short things out when connecting them up - not in operation

4. The thing would have a hard time driving a 2ohm load with rail voltages of +/- 80V with only 6 lateral mosfets



I should imagine that monitoring R35 (220R) will show when the VAS enters class B operation. This will only happen if the rather odd protection circuit is forced to operate or gate drive is trying to exceed 12.7V. Either condition is indicative of some kind of short on the output or very heavy load.


Not sure i like the idea of even using laterals with no source resistors but i didn't design the thing
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Last edited by event horizon; 15th May 2010 at 06:35 PM. Reason: added value variable resistor
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Old 16th May 2010, 08:36 AM   #3
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"The thing would have a hard time driving a 2ohm load with rail voltages of +/- 80V with only 6 lateral mosfets "

The Hafler DH500 did 400W/4R, 600W/3R, and 500W/2R with 3 pair, but it ran on 93V at idle. The XL600 increased it to 4 pair and 750W/2R.

Original Hafler outputs were about 1A/1V, so clamping the gates to 4V would be about 4A per device.
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Old 16th May 2010, 10:39 AM   #4
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As event horizon mentions, it's "copy" of the old Hitachi application.

Here's the Maplin amp that's similar, you will see that Lateral FET's don't need temperature compensation like bjt's and HEXFET's. Also those 1k gate resistors seem far to high.
That IN4148 to provide bias is useless IMO.
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Old 16th May 2010, 11:40 AM   #5
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thanks people ...i am terrible with mosfets but 400 W @8R from such a simple circuit looks very tempting
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Old 16th May 2010, 12:12 PM   #6
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"i am terrible with mosfets but 400 W @8R from such a simple circuit looks very tempting "

400W/4R is possible, with a very 'stiff' supply, 400W8R is not possible from 80V.
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Old 16th May 2010, 01:09 PM   #7
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Default Mosfet

Hello


They have just added this MOSFET amplifier...

Its Old, but it sounds very nice..

Clean, Deep , Detailed Sound.


diysmps
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Old 16th May 2010, 01:11 PM   #8
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Old 16th May 2010, 01:22 PM   #9
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 16th May 2010, 09:24 PM   #10
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Hi Sakis, if it is 2SK1058 and 2SJ162 used, then you can throw away that crazy protection circuit utilising Q4, Q5, ZD2, ZD6, D3, D7, ZD3, ZD5, R10, R12, R16, C13 and C10.

The 2SK1058 and 2SJ162 has gate protection zeners built in.

Now the amp is even more simple and trust me it works well and sounds pretty great and power is ample. Mosfets will swing about 5 -7V from the rail without clipping.

You can play with the gate resistor values to adjust the speed. I would use 470R.

I think you will enjoy this amp, it is very lively and sounds much more powerful than it is. If your supply is beefy, it will produce astonishingly clean and powerful low bass

Kind regards

Nico
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