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LED + Resistor value for M3 headphone amp

post #1 of 5
Thread Starter 
Hi guys, I am a newbie to DIY electronics but love it!
Ive nearly assembled my M3 heaphone board.
But I am unsure of what LED + LED resistor to use on the board.Or can I just bypass these parts with some jumper wire(i dont really need the LED)
I am using a 24v regulated power supply, and have built the M3 with the recommended parts(2 x opa637s and a opa627). Im not sure what other info I need to give you to work it out??
Any help would be greatly appreciated.
Many thanks!
Boyd
post #2 of 5
You do not need to jumper the LED if you do not want to install one.
post #3 of 5
Quote:
Originally Posted by MisterX View Post
You do not need to jumper the LED if you do not want to install one.
Let's be a little more firm, shall we...

If you jumper both the LED and its associated resistor you'll create a dead short from V+ to V-. This will probably fry something in your power supply, hopefully just a fuse. It wouldn't be a good situation at any rate. If you don't want an LED, leave those positions empty.

If you do want to populate the LED, the main factor in your choice will be the rail voltage of the amp. If you have +/-12V on the rails, the LED and resistor will see 24V. A 'standard' (not superbright) red LED has a forward voltage drop of about 2V. That leaves a drop of 22V on the resistor. You'd then need to look at the LED's datasheet and decide how much current it needs (most will tolerate up to about 20mA, but 1mA-5mA is probably a more appropriate value, especially for superbright LEDs).

Finally, use Ohm's law to figure out the correct resistor value:

R = ((V+ - V-) - Vled) / I

For a +/-12V source and 2V LED with 5mA of drive:

R = ((12 - -12) - 2) / 0.005
R = (24 - 2) / 0.005
R = 22 / 0.005
R = 4.4K
post #4 of 5
Thread Starter 
I love you guys!! thanks so much, now back to the board!
post #5 of 5
IMO, a superbright LED is not always a good choice as they tend to have a narrow beam pattern, not diffused at all.

Since M3 is more of a home amp than portable, you may not have so much of a need to limit current in order to preserve battery life. Thus I feel the best LED option is a frosted or at least colored lens/body, non-super-bright, running at about 10mA. Stated mathematically as error401 did that'd be a roughly 2.2K resistor.
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