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LED tube biasing, pros and cons
LED tube biasing, pros and cons
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Old 14th August 2018, 03:55 PM   #221
DF96 is offline DF96  England
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Quote:
Originally Posted by tapehead ted
I wonder if LED noise follows the same kind of curve as zener noise, where noise is proportional to the inverse square of current.
It might be unwise to draw conclusions about noise from forward biased diodes (LEDs) by looking at reverse biased diodes (zeners).

I seem to recall that SY (or was it someone else?) found that the major noise contribution from an LED was simply the thermal noise of the slope resistance. This means that it can be ignored in comparison with valve noise.
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Old 14th August 2018, 05:38 PM   #222
tapehead ted is offline tapehead ted  United States
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I did some calculations from Mr. Jones's measurements based on the assumption that the LED noise followed the same curve as zeners. At an input tube the noise at 1 ma (or less!) could be equivalent to that in quite a large resistor, one large enough to dwarf the valve noise.

Sometimes another few mA is nothing, other times it could be quite expensive- I'm sure it varies. Just noting a situation I ran into where the issue of potential capacitor ringing right into the cathode of the input tube seemed like something to be avoided- again, YMMV. Does anyone say YMMV any more?
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Old 14th August 2018, 10:47 PM   #223
PRR is offline PRR  United States
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Originally Posted by tapehead ted View Post
...assumption that the LED noise followed the same curve as zeners....
It's not remotely working as a Zener (in either mode we call (Zener).

Go fishing. Walk the stream. The LED runs like water through a smooth constriction. The Zener "breaks" like a waterfall. Knowing one does not directly tell you much about the other.
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Old 15th August 2018, 09:46 AM   #224
DF96 is offline DF96  England
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What is "capacitor ringing"? How does it affect audio circuits?
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Old 15th August 2018, 05:50 PM   #225
tapehead ted is offline tapehead ted  United States
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Originally Posted by DF96 View Post
What is "capacitor ringing"? How does it affect audio circuits?
Mechanical resonance.
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Old 15th August 2018, 09:38 PM   #226
tapehead ted is offline tapehead ted  United States
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I know I anticipate problems a lot- my mom calls it "borrowing trouble". Still, I'm expecting an unusual amount of vibration, so looking at a design to accommodate some. Doubtless I should ditch those potentially-microphonic tubes, but at that point love overcomes fear for me and I stumble serenely ahead...
Quote:
Originally Posted by PRR View Post
It's not remotely working as a Zener (in either mode we call (Zener).

Go fishing. Walk the stream. The LED runs like water through a smooth constriction. The Zener "breaks" like a waterfall. Knowing one does not directly tell you much about the other.
Thank you. That helps.
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Old Yesterday, 11:12 AM   #227
DF96 is offline DF96  England
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Mechanical resonance of capacitors is unlikely to be a problem, assuming normal good commercial components (not hand-rolled audiophile items) of appropriate technology (not high-k ceramic). Anyway, I don't think you are yet at the stage in design where such fourth-order effects may matter; you need to concentrate on first and second-order effects such as good grounding.
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