Signal presence LED suggestions

Hi,

I'm looking for a simple and effective circuit which lights an led when an input signal is present.

Any link?

Thanks in advance.

how about this simple circuit (top right section of the circuit diagram)
It's off a RANE mic preamp.
PS. Sorry about the over attachment.

-I think you have to be logged-in to see the attachment-

Hey V9977,

Thanks for your quick reply!
That schema looks really cool and pretty simple. Thanks for your help.
The only thing I don't like is the fact that the rectification diode is directly connected to the 1512 output.
Probably I will use a buffer before, I was thinking to an opamp precision half-wave rectifier.

The Rane schem is the simplest. Very good indeed.

I like to use bi-color LEDs that flash green for signal present, and red for clipping or near clipping.

There are very simple transistor based approaches but often because of diode drop thresholds they don't indicate very low for signal present, and/or may only indicate for one polarity.

I would suggest looking for app notes on a comparator IC like LM339, that should be pretty simple to make signal present indicator with good sensitivity. Note: for a peak detector it is often useful to add some hold capability so you can better see brief transients, for the signal present a hold circuit is not necessary.

JR

Note: comparators have very high input impedance and can be driven directly.

http://www.soundskulptor.com/ has some great LED circuits.  I've used them in some of my projects.
Best,
Bruno2000

Hi,
I've had a look at the french site you recommended but haven't found the led circuits you are talking about... I must have overlooked one part of it. Thanks, Piotr.

+1 on the soundskulptor circuit, which works nicely in my SSL9k
http://www.groupdiy.com/index.php?topic=11320.msg433672#msg433672

I can't see the rectifier in the rane circuit, or is this done by the single zener diode D2?

I looked at the Rane circuit and a couple others.
Does anyone know of a signal presence LED circuit that would work with only +12v.
This is for a tube amp project - I don't expect to build a -12v psu unless I really need to.

you should be able to flip the rane circuit around to work from +12v.. good starter project to learn from..

be careful that current from leds does not corrupt audio grounds, or you'll have an audible signal presence indicator.

JR

What about running the op amp off a single ended supply?
Will it even work?

If the opamp doesn't have a ground pin, how could it know if the supply is split or single?

It's all about voltages relative to the different pins, not the absolute voltage. As long as the relative voltage relationships are valid, the opamp is happy.

JR

Thanks for the help.
I guess the next step it to put it together and see how it works.