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mgburch 12th May 2011 09:37 PM

Replace Power LED with Fuse-Bulb to reduce noise?
LEDs may add noise to the power/ground if used as a power indicator. So would replacing the power LED with a fuse-bulb (aka fuse-lamp) be better indicator?

Anyone know if fuse-bulbs induce any noise of their own into circuit? Those Blue/Green/Orange fuse-bulbs are retro-cool as well :D

What about ordinary incandescent bulb? That would be cheaper but how about any noise?

multisync 12th May 2011 11:40 PM

a led draws very little current 10-20 ma, a low voltage light bulb draws more current so you would think that a light bulb would contribute more ground current. A neon bulb draws very little current but it runs of AC and can possibly induce hum never mind the noise produced when the bulb turns on at the line freq rate. You can put the led after the voltage regulator if you are using them the noise produced wouldn't be any more than a resistor that drew the same current. It's DC anyways and doesn't fluctate so you shoulnd't hear it. If you are that worried about the noise contributed by a led, remove it. Your power indicator would be your ears.

wakibaki 13th May 2011 01:11 AM

I think you're barking up the wrong tree here, LED power indicators would not be as common as they are if they were contributing to audible noise.


tomchr 13th May 2011 01:23 AM

If your amp design is that sensitive to supply noise, you're doing something wrong.

mgburch 14th May 2011 12:27 AM

LEDs are diodes, so yes, they do inject some noise.

Both John Curl and Nelson Pass posted to this thread, so its not just me: It shows the experimental results of actual LED noise measurements. Worth a read.

Yes, I realize the noise can and should be filtered out but reducing noise is better than trying to get rid of it, IMHO.

Anyway, its probably just my OCD making me think about swapping out the LEDs for Lamps. Also, I realize the lamp filaments will also make some noise so could actually be worse. That's why I posted here. To see if anyone has thought about this or actually tried it.

I may just buy a fusebulb and swap out the LED to see if I hear any improvement. Plus it would look nice and retro.

wakibaki 14th May 2011 01:47 AM

Yes, but the LEDs and other diodes in that thread are being used as voltage references. An LED used as an indicator is another kettle of fish.


benb 14th May 2011 02:49 AM

With the pains I've seen on DIYaudio to make fancy active shunt regulators to reduce power supply noise (for preamps), I wouldn't be surprised if an indicator LED added a measurable amount of noise to a good power supply when powered from it with a series resistor. But it seems a small electrolytic capacitor right across the LED would be enough to eliminate it.

An incandescent lamp has lower noise (random variation) in its light output, and so presumably would have lower noise in its electrical current pull, though I'm not sure, and the bandwidth is likely different from an LED.

Regardless, if this is the biggest problem in your audio system, it's either designed badly to make it extra-sensitive to such noise or else it's pretty damn good and you've solved a lot of other problems that get discussed around here.

DF96 14th May 2011 09:35 AM

A quick look at the measurements in that other thread suggests that most LEDS (apart from blue) are not very noisy at all. There will be thermal noise from the dynamic impedance, which gets better with higher current, and maybe other noise too (including some shot noise from random recombination?). I would not worry about LED noise in the PSU. Many people include them much nearer the signal path (e.g. for bias) and seem to have no problem with noise. If you are worried about the noise generated by a forward-biased junction then I guess your equipment has no BJTs in it? "Straining at gnats and swallowing camels"?

A hot lamp filament might generate more noise - because it is hot!

SY 14th May 2011 09:48 AM

If you look at the schematic for the His Master's Noise phono preamp, an LED is effectively in series with the input to the first stage, and another is effectively in series with the input to the second stage. If there were noise problems, this would be the absolute worst place to use one, yet the preamp is exceptionally quiet.

With a dynamic impedance of 5R or less, an electrolytic cap across the LED is a thoroughly useless addition. And any sort of incandescent lamp will be orders of magnitude noisier.

davidsrsb 14th May 2011 12:27 PM

LEDs are forward biased, a very quiet process. Zener diodes are run reversed in avalanche, a very noisy mode.

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