DIY Mic Pre Kits, Lunchbox Mic Preamp Kits, Power Supply Kits

The power LED on my preamp doesn't work.

I completed this preamp in May but could never figure this out.

The LED is wired as suggested to where one of the board LEDs would have been. One wire shows -20 v . The other registers 3K58 ohms to ground,which seems to match what I see on the schematic.

I assume the 3k58 resistor is to bring the voltage down to 2.8v
for the led, right?

Why don't i get a voltage reading across the wires?

Scott Alger

The resistor is a current limiter. Its to drop the current through the LED so that it doesnt get burned out.

Check the polarity of the LED.

By your description, its on a neg rail, so note that the positive terminal of the LED (the longer lead) goes to the least negative rail (meaning ground). The smaller lead goes to neg voltage.

Thanks for your reply.

I have tried the LED both ways.. It doesnt work.

What I dont understand is why is there no voltage across the leads?

Scott Alger

Maybe the LED is in correctly but burned out. There should be no voltage across it (well... ~.6 volts actually) when the polarity is correct. Try taking out the led and measuring the voltage at the LED pads. Should be -20v when there is an open circuit (again, according to your description). You could try temporarily sticking a new LED in there and see if that is the problem.

Ok I might be an idiot but i swear there was no voltage across the
open leads yesterday. Today -20v as you said.

So the voltage rating of this new LED i'm buying should be what?

Thanks again,

Scott Alger

Haha we've all been there! Happens to me all the time.

Shouldn't matter what the specs of the LED are too much... the current will be the main factor.

Via ohm's law: i = V/R = 20/3k58 = ~5.6mA

Typical LEDs run on max forward current of 10-30 ma... so really any standard radio shack LED should work. You can get packs of LEDs for a buck or two... should be fine.

Well thanks again for your help!

So much to learn and I'm getting too old to learn it all but

as they say I know enough to get in trouble but not enough to get out.


Scott Alger

No problemo. Hopefully it works out for you!

Hehe yeah, its all a constant learning process. Theres just too much info out there!

Anyway, good luck.