Audio power amp protection circuit function?

[N Cook]

http://home.graffiti.net/diverse:graffiti.net/prot.jpg
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

 

[Phil Allison]

"N Cook"

http://home.graffiti.net/diverse:graffiti.net/prot.jpg

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 ?

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

AKA an anti * THUMP * circuit.

What happens in a fault mode to that line that can be
monitored and could be of use for protection.operation?

** Only if FS1 blows.

What does D1 do ?

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

Another anti-thump function.

The relay contact cuts in serious attenuation of the signal going to the
pa
in fault mode

** There is no fault protection in that schem.



....... Phil

 

[N Cook]

"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

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.

 

[Eeyore]

http://home.graffiti.net/diverse:graffiti.net/prot.jpg
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

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

 

[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

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