Soft Turn On/Off on Hafler 9500

[partialresponse]

My Hafler 9500 makes a slight thud on turn off. This is probably a faulty component in the soft-turn-off circuit. I've examined the schematic but cannot identify which components of the schematic implement this feature. Can anyone make a suggestion? This is a problem with both channels so it's probably part of the power supply.

The schematic is here: http://k-amps.8m.com/cgi-bin/i/Power...afler_915c.jpg

Thanks

Kofi

[mlloyd1]

TS301.

high resistance when cold, which limits inrush currents as power supply caps charge.

as this device heats up (while the power supply caps charge in the meantime), its resistance drops substantially, reducing its current limiting function.

edit:
perhaps you are also referring to the 2n7000 mosfet and the associated parts in the diff amp. it is part of a circuit that monitors the rails and turns on the rest of the amp when the rails are "stabilized"

mlloyd1

[partialresponse]

Actually the problem is the soft turn-off. From the owner's manual (best owner's manual I've ever seen)...

"The Soft Turn On/Off System operates by monitoring the status of the regulated low voltage power supply. At power up, as soon as this power supply has stabilized to normal operating levels, the Soft Turn On System activates the amplifier circuitry through a small MOSFET switch in the input stage. At power down, the system deactivates the amplifier rapidly before the power supply begins to collapse."

So I think you've narrowed it down considerably. I'm inclined to think it's something that affects both channels. But there is one 2n7000 in each channel so if that's it they must have failed simultaneously. At any rate I'll replace both of them and the inrush limitor TS301.

Thanks so much for your comments.

Kofi