amp delay for speaker out

[nucleus]

I realy need help with this one!
I have been trying to find a schematic for ampdelay that can operate on a 2x39V, from transformes made for amplifier. The only schematc i have foud is on the picture.
I tried to put the voltage down with resistor which turned out to be very unstable.
My standard is to take voltage from a amplifiers transformer because if something go wrong than fuses on a secundar (always simetrical) blowes, so the relay for speakers instantly shut of the speakers.
Is this ok, or am i just the only one that uses this kind of a "ampdelay"?

[Centauri]

Basic circuit is fine, but to run on a higher voltage, put the resistor in series with the RELAY coil, not entire circuit. I would also decrease the size of capacitor C1 to 47uF or so to ensure relay drops out quick when power is turned off. I would also feed R1 from the amp's +ve DC supply rail so that time delay starts AFTER the DC rails have risen.

BTW a higher voltage transistor may also be in order.

Cheers

[Cortez]

Check this IC: uPC1237 !

[sek]

I would also feed R1 from the amp's +ve DC supply rail so that time delay starts AFTER the DC rails have risen.

For convenience, this may be true, but for safety it is considered neccessary to feed it from an AC supply rail. Wouldn't the DC reservoir capacitors keep the DC rail voltage too high for too long, so that the shut down (relay turn-off) would come too late... ?

The solution here is not to measure startup at a later point in the sequence, but to increase the circuit's time constant for it's turn-on (and care for a quick turn off).

Sebastian.

PS: Nucleus, what if the other rail fuse blows first? Wouldn't your configuration keep the relay turned on until the second fuse gets a reason to blow too (e.g. huge DC current through the speaker, burnt/shorted output tranistor)? Just a thought.

[Centauri]

Originally posted by sek
For convenience, this may be true, but for safety it is considered neccessary to feed it from an AC supply rail. Wouldn't the DC reservoir capacitors keep the DC rail voltage too high for too long, so that the shut down (relay turn-off) would come too late... ?

While the DC rail, and hence the base voltage, would be held up at switch off, the separately fed supply to this circuit will drop fast (provided the smoothing cap is small) dropping out the relay.

Cheers