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Old 23rd December 2008, 05:05 AM   #11
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Quote:
Originally posted by SY
The LEDs, besides being excellent voltage sources (and very quiet) also give a nice visual indication that current is running through the stage.
They also cost just a few pennies each, and they sound better than even the best electrolytic bypass.
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Old 29th December 2008, 08:19 PM   #12
JesseG is offline JesseG  Canada
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So, LED bias is as simple as putting a string of LEDs in series to get the desired voltage drop? No Caps?

Jess
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Old 29th December 2008, 09:19 PM   #13
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JesseG,

I've been quizzing the helpful folk here on exactly the same thing - read this:

LED cathode bias - how to pick LED?
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Old 29th December 2008, 10:14 PM   #14
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Quote:
Originally posted by dsavitsk


They also cost just a few pennies each, and they sound better than even the best electrolytic bypass.
Hi,

Even better than a thousand uF worth of filmcaps too...
Even twenty five years ago you'd replace electrolytic bypass caps on small signal tubes (2 to 6V drop across the cathode R) with solid tantulum caps or did away with them altogether and used alterantive methods for biasing the grid.

As for leds, you'd better stock up on the older ones. The newer brighter ones aren't all that great. (See EC8010's post on the topic of LED cathode bias, I only confirm his findings).

Cheers,
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Old 30th December 2008, 12:17 AM   #15
SY is offline SY  United States
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Frank, as EC said, the new Agilent jobs are quite bright, but have excellent performance. I'm not as convinced as he is that "bright," "new," and "higher impedance" correlate. I've measured about a dozen different kinds of red LEDs and found high and low impedances in new ones, old one, bright ones, and dim ones.
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Old 30th December 2008, 12:55 AM   #16
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How about my wildman approach to creating a completely isolated negative (or positive) bias voltage?

http://i114.photobucket.com/albums/n...rk/IsoBias.png

Cheers.

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