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Old 21st February 2007, 09:37 AM   #1
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Default Fail Safe Power Supply

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The image above, is the schematic for of a simple fail-safe supply which may be used as a direct replacement for many of the Pass Zen/Aleph designs. DIY builders may find the over-current protection useful when setting-up. It can also limit the extent of any damage should a fault develop. A dual, balanced version has been successfully used with LTPZen and is used to eliminate the loudspeaker decoupling capacitors. See http://www.pmeweb.co.uk/audio/psu.html for details. Details also available in pdf format: http://www.pmeweb.co.uk/audio/psu.pdf
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Old 21st February 2007, 09:45 AM   #2
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Neat. It is a good idea to have some current limiting on a power supply in particular if you do some experimenting or if you are tinkering with a new design. Especially if, in the case of this design, the extra cost is quite low.

BTW, I am not able to access the pdf link?

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Old 21st February 2007, 10:44 AM   #3
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I managed to access the PDF fine...

Nice design! cant wait to see people start to implement it and experiment a bit
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Old 21st February 2007, 11:00 AM   #4
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Allen,

Have you thought about a reset switch (pushbutton?) in series with the triac to reset the supply after current limiting?

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Old 21st February 2007, 11:32 AM   #5
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A reset switch isn't a bad idea. I just power down and power up again. With the current schematics, you have to wait about 30 seconds for the capacitors to discharge sufficiently. Long enough to ponder why the amp tripped the PSU in the first place?
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Old 21st February 2007, 12:33 PM   #6
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Quote:
Originally posted by Allen Newby
A reset switch isn't a bad idea. I just power down and power up again. With the current schematics, you have to wait about 30 seconds for the capacitors to discharge sufficiently. Long enough to ponder why the amp tripped the PSU in the first place?

Yes, good point!
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Old 21st February 2007, 03:34 PM   #7
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I like it.

I am wondering the easiest way to trip the protection externally. Say from a DC detector.

Thanks for sharing.

Cheers!
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Old 21st February 2007, 08:20 PM   #8
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Interesting idea.

I designed and built a loudspeaker protection circuit/anti thump ciircuit which uses a relay to disconnect the speakers in the event of a fault. One of the fault conditions is excessive dc. For this I used a second order low pass filter driving an op-amp schmitt trigger. The circuit is relatively simple and it is easy to adjust the threshold values to suit the settling characteristic of the amplifier. You need to scope this first, but generally this will be a hump which rapidly settles to a near zero, followed by a much longer thermal drift to what should be dead-on zero if the amp is correctly trimmed. I hope to publish details of the circuit at a later date.

Instead of controlling an output relay, I don't see why you can't trigger the triac from such a circuit. This would also protect your speakers in the event of an excessive dc fault. The triac gate can be triggerd both internally or externally and operates as a NOR gate. That is the gate of the output mosfet and hence the PSU output goes low when either the internal or the extrnal trigger goes high. I know this will work because the triac gates on the dual fail-safe PSU used on LTPZen work this way; they can be triggerd by excessive currents in either rail.
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Old 21st February 2007, 09:39 PM   #9
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Very nice. I have BJT cap multiplyer supplies on my JLH amps and they work great. Would it be practical to remotely trigger the triac to put the amp in standby mode (keep the caps charged)?
-Mal
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Old 22nd February 2007, 07:53 AM   #10
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Default Re: Fail Safe Power Supply

Quote:
Originally posted by Allen Newby


which may be used as a direct replacement for many of the Pass Zen/Aleph designs. DIY builders may find the over-current protection useful when setting-up.

Thanks for the very useful fail safe power supply.
By the way, Aleph power amps already have the protection
with maximum current limit. Do we need double assurance
of the current limit securities?


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