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Audio power amp protection circuit function?

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The thermal switch monitors the heatsink but what is the function of the depletion type FET, monitoring the negative half cycle of the the AC3 ac line, via D3 ? What happens in a fault mode to that line that can be monitored and could be of use for protection.operation? What does D1 do ? The relay contact cuts in serious attenuation of the signal going to the pa in fault mode

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N Cook
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"N Cook"

** Switch on delay - then un-mute the loudspeaker.

AKA an anti * THUMP * circuit.

** Only if FS1 blows.

** Fast discharge of C5 - so a quick off for the relay when AC supply removed.

Another anti-thump function.

** There is no fault protection in that schem.

...... Phil

Reply to
Phil Allison

So obvious when you point it out, I only assumed it was overload protection and never thought of a mute function. The problem with the amp itself, the relay was falsely cutting in and dropping the gain plus distortion of the pa in normal use. Presumably one of the Cs or the FET playing up, will be looking at it later today.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N Cook

For starters it's not a 'depletion type FET'. It's a plain old JFET (junction fet). JFETs are on with zero gate volts so it conducts when AC is not present holding the relay off by diverting base current from TR10. When AC is present, it turns off due to the negative gate voltage - energising the relay.

Graham

Reply to
Eeyore

pa

(junction

present

present,

I should retain this J112 as a collecor's item then. Circuit operates with the relay energised in normal audio operation , after power-on hold-off, relay contacts are o/c in audio use

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N Cook

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