Question - Biasing different types of Mosfets - Page 3

mcp · 27th September 2003, 09:15 AM

Hi Sam

You may want to check out a recent post regarding this amp Q13 in Holton's n-channel amp
Apparently, there's some confusion as to which mosfet is the Vbe multiplier. According to JasonL's pictures, there is a MJE340 legend with a preset beside it. Check the tracing underneath to confirm if indeed this is the Vbe transistor. If it is, jump 3 wires and connect it to the main heatsink. That should prevent the amp from thermal runaway. Cheers.

millwood · 27th September 2003, 11:27 AM

My main complaint about the irf devices is that they tend to be unstable in CFP configuration. In spite of the fact that their input capacitance isn't really that much bigger than the Hitachis.

Once you get them in a EF-like set-up, they are fine stability wise.

Samuel Jayaraj · 21st October 2003, 01:14 PM

I had difficulties posting to the forum for about a fortnight. I hope it works this time around. Please see the attachments:

Samuel Jayaraj · 21st October 2003, 01:15 PM

A (rather poor quality) picture of the assembled PCB of the N-channel amp showing thermal tracking etc.,

Kilentra · 22nd October 2003, 01:35 AM

I think the susceptibility to thermal runaway increases significantly with the bias current, especially since one or two Mosfets start hogging current more as you increase it. The IRF Vbe multiplier would be great, if the thermal coupling was perfect. With imperfect coupling I found the bias would just increase all the time.

I would try the BJT at a lower bias and watch the current through the hottest output device very carefully. If you play loud music for awhile then check the current again after stopping it, the current should be the same or slightly less (not much less).

Samuel Jayaraj · 22nd October 2003, 08:42 AM

[QUOTE]Q12 is not changed to an IRF610[QUOTE]

The above post should read, "Q12 is NOW changed to an IRF610, and earlier in Anthony's schematic it was a BD681".

27th September 2003, 09:15 AM	#21
mcp diyAudio Member Join Date: Jul 2002 Location: Vacation Land	Pelle Post Q13???? Hi Sam You may want to check out a recent post regarding this amp Q13 in Holton's n-channel amp Apparently, there's some confusion as to which mosfet is the Vbe multiplier. According to JasonL's pictures, there is a MJE340 legend with a preset beside it. Check the tracing underneath to confirm if indeed this is the Vbe transistor. If it is, jump 3 wires and connect it to the main heatsink. That should prevent the amp from thermal runaway. Cheers. __________________ Michael Chua

22nd October 2003, 08:42 AM	#26
Samuel Jayaraj diyAudio Member Join Date: May 2001 Location: Bangalore, India	[QUOTE]Q12 is not changed to an IRF610[QUOTE] The above post should read, "Q12 is NOW changed to an IRF610, and earlier in Anthony's schematic it was a BD681". __________________ Sam

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27th September 2003, 11:27 AM	#22
millwood Account Disabled Join Date: Apr 2003 Location: US	My main complaint about the irf devices is that they tend to be unstable in CFP configuration. In spite of the fact that their input capacitance isn't really that much bigger than the Hitachis. Once you get them in a EF-like set-up, they are fine stability wise.

22nd October 2003, 01:35 AM	#25
Kilentra diyAudio Member Join Date: May 2002 Location: Cambridge, MA, USA	I think the susceptibility to thermal runaway increases significantly with the bias current, especially since one or two Mosfets start hogging current more as you increase it. The IRF Vbe multiplier would be great, if the thermal coupling was perfect. With imperfect coupling I found the bias would just increase all the time. I would try the BJT at a lower bias and watch the current through the hottest output device very carefully. If you play loud music for awhile then check the current again after stopping it, the current should be the same or slightly less (not much less).

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