How to power a led inside an amp

[AndersJ]

I finished a chip amp last summer, and didn't really use it. I had forgotten all about it until today where i pulled it out, i thought what the heck and gave it a spin.
I was actually amazed at how well it sounds, don't really know why i didn't realize that at first.

Well im going to build a case for it, and i have just ordered a better power supply. But i was thinking about adding a led of some sort to the front panel to tell me if it's on or off. How do i do that the easiest way? Would it be stupid to add it after the rectifier? Should i add a small powersupply just for the led or? (i know how to calculate resistors and all, it's just where to take the power from )

Sorry if this question is really simple and obvious but im not good at figuring such things out

[BWRX]

If you have a transformer that will allow it, you can add a winding that will give you a suitable voltage from which you can power the LED. Or put the resistor and LED between one of the rails and ground.

[AndersJ]

Hmm okay, i might try that out.

[Bazukaz]

Hmz...
Simply use a resistor in series with led.feed it from amp's supply. Calculate its value by ohms law , standard leds need approx 20mA of current.Thats all.
R=U/I,
for u = 30V, i =0.02 A we get 1.5 kOhm resistor(1Watt).

[Nordic]

There ya go Baz, teach a man to fish...

[AndrewT]

Hi,
you could use the LED to give an indication of low power rails or one failed rail.

Wire in a series string of LED, Zener and resistor across both PSU rails.
Adjust the resistor to give a suitable brightness.
When it dims or goes off there is something wrong.

[Nordic]

Care to make a picture Andrew?

[AndrewT]

Hi Nordic,
no.
Just add a Zener to the resistor LED string.

Anders,
Subtract the Zener+LED voltages from your supply voltage and use the Bazukas calculation for the resistor.

By the way, 20mA will be mighty bright, you may want to try 5mA to 10mA range.

[AndersJ]

Thx everyone, but as i said i know how to calculate the resistors and everything, i just thought it might be a bad idea to solder it to the same circuit as the amp. But if you say so, i trust it's okay

[Nordic]

I have been told its worse if you only put it between one rail and ground, than when you put it from rail to rail... maybe someone more clued up can give input.