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Old 4th July 2004, 02:39 PM   #1
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Default Using LEDs to bias grid voltage?

I seem to remember someone saying that using a cathode bypass cap when using LEDs was not needed. Is this true? Does anyone have any opinions of using LEDs in this way? Thanks.
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Old 4th July 2004, 03:40 PM   #2
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Bypass caps. should not be used when LED bias is employed. That's a significant advantage. Another advantage is the elimination of the cathode resistor's noise.

Paul Joppa used LED bias in the Bottlehead Seduction phono stage for those reasons. Agilent parts of superior noise factor are specified.
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Old 4th July 2004, 06:17 PM   #3
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Quote:
Originally posted by Eli Duttman
Bypass caps. should not be used when LED bias is employed. That's a significant advantage. Another advantage is the elimination of the cathode resistor's noise.

Paul Joppa used LED bias in the Bottlehead Seduction phono stage for those reasons. Agilent parts of superior noise factor are specified.
Where would I get my hands on some Agilent LEDs Eli? Do you have a link?
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Old 4th July 2004, 07:43 PM   #4
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It would be hard to find a cap that would effectively bypass an LED string without being very large indeed. A properly biased LED has an incremental impedance of a few ohms, much lower than a zener diode of comparable voltage drop. They are inherently less noisy too, as they are in forward conduction mode rather than avalanche or field emission. They also take much less current to get them up past their "knee" than a comparable zener diode.

Where did the information come from about Agilent LEDs being less noisy? Is this true independent of color/voltage drop? I use LEDs routinely as voltage references and to provide a constant voltage source for folded cascode circuits and the like, so this is very interesting information.... I find that the old deep red GaAsP diodes are good for about 1.6V, and the GaP yellow-green diodes drop almost precisely 2V when biased at 10ma.
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Old 4th July 2004, 09:43 PM   #5
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Allied Electronics (www.alliedelec.com) carries the Agilent line.

I have to go through my highly disorganized mess and find the Seduction manual I bought for reference. When I find it, I'll post the part number used in the Seduction.

Joppa and Schmalle have done their share of "leg work" with LEDs. The HP parts "Buddha" Camille spec'ed for the C4S are no longer available. They needed replacements for the C4S and they needed a suitable part for Seduction. I believe (won't swear) Bottlehead is using a several Agilent types.
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Old 4th July 2004, 10:32 PM   #6
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Interesting stuff. I'll have to do my homework. I need exactly 6 volts at 15mA so I guess I will have to use about three LEDs on each cathode. Is the difference between a cathode resistor and LEDs readily apparent when listening to a preamp?
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Old 5th July 2004, 02:50 AM   #7
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Hmm. I'm using a NiMH battery under the cathode. I need 4V @ 20mA. I found some calculations for how to pick a battery (in terms of AH rating) so it would be safe to use with the bias current you'll be passing through it, but I don't remember the math any more. I ended up using Radio Shack batteries originally intended for cordless phones. Anyway, I though it sounded quite a bit better than the R||C combination under the cathode that I had before. I haven't tried diodes though. I wonder how diodes would compare to batteries in terms of impedance, both AC and DC, as well as noise. Also longevity.
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Old 5th July 2004, 03:23 AM   #8
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Quote:
Originally posted by Saurav
Hmm. I'm using a NiMH battery under the cathode. I need 4V @ 20mA. I found some calculations for how to pick a battery (in terms of AH rating) so it would be safe to use with the bias current you'll be passing through it, but I don't remember the math any more. I ended up using Radio Shack batteries originally intended for cordless phones. Anyway, I though it sounded quite a bit better than the R||C combination under the cathode that I had before. I haven't tried diodes though. I wonder how diodes would compare to batteries in terms of impedance, both AC and DC, as well as noise. Also longevity.

I believe that you would divide the AH rating of the batteries( the total AH rating for batteries in parallel ) by the desired current through the tube and the dividend is how much time you would get out of a charge. Not certain though. I've never tried battery bias before. It would be easy enough to do but I don't have much room under my 12" x 12" chassis for batteries
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Old 5th July 2004, 07:33 AM   #9
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I believe that you would divide the AH rating of the batteries( the total AH rating for batteries in parallel ) by the desired current through the tube and the dividend is how much time you would get out of a charge.
Except that the batteries aren't actually powering anything, they only provide a voltage reference. You hook them up so that the +ve terminal goes to the cathode, and the -ve terminal goes to ground. This puts the cathode about 4V (in my case) above ground, which is what I need (since the grid is grounded through the grid leak resistor).

The current through the battery goes from +ve to -ve through the battery, which makes it a charging current. If the battery were supplying power, the current would be +ve to -ve outside the battery. So, essentially, your battery thinks it's always on a charger, even though it's always fully charged. The AH rating of the battery determines how much current it can provide, but it also determines how much current can be used to charge it. You want to pick a battery that can safely handle the tube's bias current as its charging current, over long periods of time.

So it's not like a battery powered supply where the battery would run out if you left the component powered up for a long time. If you put the battery under the cathode, the battery is "getting charged" for the duration that the amp is on. That is an important difference. There is no problem with the charge running out after a certain amount of time, but there is an issue with making sure that the battery can handle the constant current that will be going through it when the amp is on.

I hope that makes things clearer. I hope anyone who is thinking about tryingt his spends some time reading up on it. I think the math has been discussed on this forum, so a search should turn something up. Using a battery with the wrong rating could be a fire hazard.
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Old 5th July 2004, 09:32 AM   #10
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I just picked up 100 ROHM green LEDs and expect to get some old HP units soon. I also have a bunch of no-name green LEDs.
I plan to do some noise measurements when I have time - it never ocurred to me that there would be significant differences in noise voltage between LED manufacturers, and I'm curious about what the noise voltage would be for a run-of the mill LED out of my junk stock. It's not someting manufacturers ever specify. It will be challenging to set up a jig to properly measure the noise voltage of the LED itself - I would want a quiet current source to minimize the error. Once I have the current source, it should also be straightforward to measure the incremental impedance at 10 or 20ma bias and see if there are any differences.
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