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Old 14th August 2008, 08:13 PM   #1
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Default Signal Indicator LED Noise?

I want to add a very simple input signal indicator LED to my amp. This is just to show if signal is present or not. I'm sure the attached circuit will work. But I worry that the LED switching on and off will introduce noise into audio portion of the circuit. I'd like to use a dual op amp where one half is the audio input buffer and the other half drives this LED.

Should I worry about the LED generating noise? Is there a better way? Maybe a shunt capacitor across the LED, a buffer transistor or both?

Thanks!
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Old 14th August 2008, 11:43 PM   #2
dmills is offline dmills  United Kingdom
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Assuming this is not a mic or phono preamp (or the like) I probably would not worry too much, but if you are really concerned, one trick is to have a constant current source feeding the LED (returned to -VE, with a transistor bypassing the LED (also returned to -VE). That way the switching current does not get imposed on the audio reference....

This is also a good trick when implementing bargraph metering on sensitive electronics like the aforementioned mic preamps.

Regards, Dan.
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Old 15th August 2008, 07:32 AM   #3
Mooly is online now Mooly  United Kingdom
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Maybe use the second OpAmp as a "sample and hold" and drive the LED across a diode bridge so either polarity is indicated. A high brightness LED will operate at only a couple of miliamps so there should be no problems there.
Loses it's simplicity though
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Old 15th August 2008, 09:25 AM   #4
lineup is offline lineup  Sweden
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one led has 1.5 Volt drop
one transistor has 0.7 Volt drop
is there any special level - to define for you a signal present?

- one differential input ac voltage amplifier (using op-amp)
- with fullwave rectified output and
- some delay function
- to feed one LED indicator

would be the most flexible and versatile solution
not too difficult to design with ordinary standard compoments
but way to complicated & advanced
for to be a pleasure to put into your circuit
----------------------------------


from your examples
I would take:
-> transistor follower buffer -> LED + Cap across


this would give some delay filtering of output from amplifier
at the same time the small base current for transistor
would not effect the amplifier output, hardly anything
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Old 15th August 2008, 11:13 AM   #5
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There is absolutely no reason to worry about any extra noise, unless you do something really wrong, like messing with grounds.
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