interesting thing about overload protection

[Alfonsol]

Hi
i read in this forums about amplifiers making weird sounds and the speaker coming in and aout when turned on.

i also have an amplifier that made that thing.
if you seach silver sa5030 in this forum you will see the actual amplifier.
it's been working reliably since i repaired it, but i was worried about this behaviour.

i simulated the circuit and now i think this is normal, the simulated circuit have exactly the same behaviour as my amplifier.

i attached apicture of my simulation.

the culprit is a overload protection circuit that also acts as a soft start. it turns off the first stage, and if a load less than 1 ohm is connected it never turns on the first stage.

also this amplifier doesn't have a relay.

 

[qsaudio]

The BEST thing for that problem is to engage the speaker switch after it's been on for 5 to 10 minutes.
There is no other choice.

 

[cademan]

You don't have to wait that long. 10-15 seconds should suffice.

 

[Jon_Logan]

Have you confirmed, in fact, that the protection circuit is disabling the front end for a few seconds at turn on? It could be that a time delay cap has lost it's Farads

 

[qsaudio]

You don't have to wait that long. 10-15 seconds should suffice.

Ya that is a bit too long, I was just thinking of warm up time.

 

[BinaryMike]

That's a whopping turn-on transient, and if your analysis is correct, I think the designer should have done better. Post the details if you want more eyes on this issue. There some folks here with engineering backgounds.

 

[Alfonsol]

Have you confirmed, in fact, that the protection circuit is disabling the front end for a few seconds at turn on? It could be that a time delay cap has lost it's Farads

yes, i simulated the circuit with and without the protection. to simulate the circuit without protection i put a 15v supply in the place of D401, that is a 15v zener, and eliminated the overload protector and muting circuit.
the muting circuit slowly puts voltage in D401.

without the protection and muting circuit the amplifier stabilizes very quick around 300mv and and then slowly (several minutes) stabilizes to -30mv in the simulation.

with protection circuit the amplifier behaves like the simulation i posted, and then slowly stabilizes to -30mv. In my amplifier actually one channel stabilizes to 30mv and the other to -30mv

 

[d3imlay]

what happens when their is an overload or fault? Any protection circuit that dumps DC to the output isn't doing it's job

 

[Alfonsol]

It's a protection for the amplifier, not for the speakers.

i made 2 simulation the first with an 8 ohm load and the second with a 1 ohm load.
blue line is voltage in zener diode, red line is voltage in output, and green line is voltage across emmiter resistors.

the protection circuit measures voltage across emmiters resistors, and if it is too high, it shuts off the zener diode, shutting off the first stage.