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JFET-MOSFET headphone amplifier J-Mo 3
JFET-MOSFET headphone amplifier J-Mo 3
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Old 4th August 2017, 04:52 AM   #21
ammel68 is offline ammel68  United States
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JFET-MOSFET headphone amplifier J-Mo 3
Quote:
Originally Posted by Osvaldo de Banfield View Post
I always ground any heatsink in order to KNOW its potential. Leaving it unconnected may lead to confusion. Also, it (most of the times) cooperate reducing EMI in switching devices.
With the simple circuit that Richard posted, do we REALLY need to KNOW its potential??
How is leaving it unconnected going to lead confusion??

I don't have to worry about insulating the device from the heatsink by leaving the pins' pads floating. One 6-32 screw through the device and sink and you're done.

You and JPS can do what you want, but having designed several PS boards myself for both IC and discrete regulators, I'm not connecting my heatsink pins to any part of the circuit unless doing so is absolutely necessary for the layout.

Don't know about EMI and switching devices since I'm not into SMPSs.

Now...let's bust out the whiskey and soldering iron and build something!
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Old 4th August 2017, 05:03 AM   #22
ammel68 is offline ammel68  United States
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JFET-MOSFET headphone amplifier J-Mo 3
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Originally Posted by rjm View Post
I see the arguments for grounding it, but I also see its unlikely to offer much real world advantage. I suppose it comes down to whether anyone is likely to want to mount the FETs to the heatsink without electrical insulation.
You already have the heatsink's mounting pads floating in your Eagle layout in post 13.
Why use electrical insulation(both for the back of the transistor AND its mounting hole) when there's no benefit?

Think I'll start looking for a 1kuF MKP output cap for this design...LOL.
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Old 4th August 2017, 06:44 AM   #23
rjm is online now rjm  Japan
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JFET-MOSFET headphone amplifier J-Mo 3
@ammel68

If you don't use isolation pads when connecting the MOSFET to the heatsinks, the potential of the heatsink will be the same as the MOSFET substrate. That's a safety risk, though the voltages are in this instance not particularly high. Let's say you slip up with a screwdriver and short the heatsink to the case, such action would be likely to blow the MOSFET.

So that's the argument for grounding the heatsinks: it forces you to use isolating pads and the heatsinks are safely at zero volts.

The argument against says, well, why not just treat them the same as clip-on heatsinks - just with two screws holding them physically in place on the board. Worst case one of them has around 20 V on it, just be reasonably careful to short it to anything.

I'm screwed either way of course: if connected to ground someone will connect the MOSFETs with no isolation and they will immediately blow on power up. If I leave them isolated someone will still connect the MOSFETs with no isolation and manage to short the heatsink to ground and blow the transistors.

My gut feeling is to ground them though, so I can clearly say "USE ISOLATION PADS" and get the pain over with up front.
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Old 4th August 2017, 06:54 AM   #24
JPS64 is offline JPS64  Germany
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JFET-MOSFET headphone amplifier J-Mo 3
thanks rjm.

JP
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Old 4th August 2017, 06:55 AM   #25
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JFET-MOSFET headphone amplifier J-Mo 3
Most recent version of the regulator and JMo3 BOM. Haven't done the BOM for the regulator yet.
Attached Images
File Type: png m-reg-ltspice.png (144.5 KB, 346 views)
File Type: png mreg20d-brd.png (61.7 KB, 337 views)
File Type: png pcb-mreg20d-sch.png (10.1 KB, 355 views)
Attached Files
File Type: asc M-Reg 2.asc (5.1 KB, 16 views)
File Type: zip pcb-jmo-30g2-ss-bom.zip (116.7 KB, 32 views)
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Old 4th August 2017, 07:00 AM   #26
JPS64 is offline JPS64  Germany
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V+ and V++ to electrolytic not optimal, would change routing.

JP
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Old 4th August 2017, 08:55 AM   #27
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JFET-MOSFET headphone amplifier J-Mo 3
You mean how R2 and R3 are connected, or something different?

/R
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Old 4th August 2017, 08:59 AM   #28
JPS64 is offline JPS64  Germany
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JFET-MOSFET headphone amplifier J-Mo 3
V+ or V++ goes first to electrolytic (avoiding T-routing cause etching problems) then to R and Q.

JP
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Old 4th August 2017, 09:00 AM   #29
JPS64 is offline JPS64  Germany
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and why not put BJT´s on the same heatsink as MOSFET´s?

JP
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Old 4th August 2017, 12:39 PM   #30
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Quote:
Originally Posted by rjm View Post
Unless I'm misreading what you are suggesting, I don't see the need as the output of the circuit is not DC coupled.
I refer at the input. To place a cap in the input to prevent from a non DC free input, to move the bias of both transistors to distortion, or unsafe point (outside SOA).
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