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arnoldc 17th March 2006 10:42 AM

Ping Sy: LED for Cathode Bias
Hi Sy,

I'm reposting this-

Sy, I want to try LEDs. I have a citcuit (417A) that is biased via cathode resistor 100 ohms with cathode voltage of 2V. So that's around 20mA. Can you recommend an LED that can be ordered from Farnell or RS Components?

I found this from Farnell- HLMP-6000. The specs said:

Current, If luminous intensity:10mA
Current, If max:50mA
Depth, external:2.2mm
Depth, lead cross-section:0.5mm
LED colour presentation:Tinted diffused
Length / Height, external:2.15mm
Length, lead:11mm
Voltage, PIV max:3V

In addition, my amp drivers are 417A at 10mA and 2V. It seems to my fickle mind that this LED will work too.

Need you thought on this.

Shoog 17th March 2006 11:23 AM

I've been experimenting with LED bias recently myself. The noise differences between different LEDs seem marginal so I wouldn't worry to much about that. Seems better to run them at the top of there current range rather than the bottom, otherwise the result can be grainy (from my experience). Therefore your current demand of 20mA is perfect - just so long as you don't go for a low current LED.
You would be looking at Orange LED's for your two volts drop (I think green may be OK to). My experience with some orange LED's was that there voltage drop was more variable than RED LED's - I would therefore suggest getting a few extra to match up.

Farnell - TLH04400 656-550


pinkmouse 17th March 2006 11:25 AM

Hi mate

SY's off travelling at the moment, but he generally recommends standard cheap LEDs, usually bought surplus for as little money as possible. From my knowledge of the circuit, that one looks fine.

EC8010 17th March 2006 12:41 PM

2 Attachment(s)
What you want is an LED that doesn't produce much light for the current through it, so look for one producing a luminous intensity of 10mCd (milliCandela) or less. A red LED will probably give a slightly lower voltage drop than you need, so orange might do the trick. As previously mentioned, FEC 656-550 might do.

arnoldc 17th March 2006 01:26 PM

Hi guys, thank you very much for the help.

I've been getting mixed reactions from Audio Asylum about LED vis-a-vis cathode resistor (w/ or w/out capacitor bypass) bias so I'd like to hear it for myself on amps I'm familiar with- mine :)

Thanks for looking it up for me Shoog. Al, thanks for pitching in for Sy, and thanks to you EC8010 for the confirmation.


Shoog 17th March 2006 01:53 PM

Have you got any old orange/green LEDs in dead equipment - that would be the easiest way of trying things out without spending a penny. Mind you don't do what I did and use LED's with built in reistors (doo).

Its good to hear that you want to try it by your own ear. Any statement by an audio freak has to be taken with a pinch of salt. The most violent protests are often from people who "go by the gospel" and have never tested an idea themselves.


rdf 17th March 2006 05:14 PM

My recollectrion is SY does dynamic testing and chose a red LED presenting an AC impedance of approximately 5 ohms at the current of interest. I've been able to find LEDs good for about 7 ohms, unfortunately unmarked models purchased by the bag for about $1.

In my testing red LEDs had lower dynamic impedance than any other colour at the cost of lower bias voltage. Even among reds the impedance varied by a factor of three. Regarding noise, biased with a constant current source no LED of any colour that I tested had a noise level obviously higher than the the measurement floor of my 24-bit card - in other words more than 124 dB below 1 Vrms.

In my circuits, bypassed by a small film and the layout kept tight, I think they work extremely well if the bias point falls where you need it. I have yet to measure an unbypassed resistor that didn't display a ton of extra low-level distortion components over an LED, but that's a taste issue. :)

SY 17th March 2006 06:02 PM

Hello, checking in from Linz, en route to London. As rdf indicated, very few companies list dynamic impedance as a spec; it generally needs to be measured. And, as EC8010 says, the older and poorer light, generally the better. I bought several varieties at surplus (unmarked), set up a jig to test them, then went back and bought a pile of the ones that tested best. But really, for this application, I don't think that there will be much of a significant difference between 4 ohms and 7 ohms, so just buy cheap surplus and avoid anything that is marketed as "high intensity" or "high efficiency."

zarniwoop 18th March 2006 12:09 AM

I thought I was somewhat crazy,.. I'm using a blue LED on the cathode of a 6h30 input stage. Bias is right on, but I don't know if this is better than an unbypassed resistor.

arnoldc 18th March 2006 01:08 AM

rdf, thanks for that additional information. I plan to test bypassed and unbypassed LED in my amp. I'm a sucker for measurements, have a scope but don't know how much to make the most of it. Can you recommend a good, available (i.e. Amazon) book on audio measurements?

Sy, glad to hear from you that my application won't be that picky in the selection of tha LED. Have a nice trip.

zarniwoop, you never compared the two? Hmm, I have a headphone amp here that I plan to use the LEDs on and hopefully I can hear something different.

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