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Old 12th November 2012, 06:51 PM   #1
MartinR is offline MartinR  Germany
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Default Which IR LED?

Hello together,

I would like to test IR LED bias instead of the RC combo in the first stage of my phono preamplifier. Current is 40mA. Bias voltage about 1.4V Question is, which one should I take? Which one has the lowest AC resistance?

http://docs-europe.electrocomponents...6b80325c6f.pdf

http://docs-europe.electrocomponents...6b8025c31a.pdf

Thank you very much in advance.

Kind regards,
Martin
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Old 12th November 2012, 07:03 PM   #2
SY is offline SY  United States
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Depends on the current you're planning to run through it. The critical graph is forward current vs forward voltage. The AC resistance at a given current is the slope of the tangent line to the curve at that current. At 10mA, the 5685 looks like about 6 ohms, the 5594 looks like about 3 ohms.
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Old 12th November 2012, 07:13 PM   #3
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If you re planning to run 40ma through one LED, maybe it is too much, better parallel two of them. Notice that the 5685 graph stops at 50mA
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Old 12th November 2012, 07:17 PM   #4
SY is offline SY  United States
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Ouch, didn't notice that- you're right, 40mA is pretty darn high. Most remote control type IR LEDs have a 30mA max. Paralleling will require a small series resistor to prevent current hogging and runaway.

Or run that stage at a more sensible current!
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Old 12th November 2012, 07:21 PM   #5
wfmali is offline wfmali  Europe
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Quote:
Originally Posted by ;
...in the first stage of my phono preamplifier. Current is 40mA...
Martin, just wonder how the first stage in your phono looks like when you run 40mA through it?
Marcus
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Old 13th November 2012, 07:31 AM   #6
disco is offline disco  Netherlands
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Quote:
... 40mA is pretty darn high. Most remote control type IR LEDs have a 30mA max. Paralleling will require a small series resistor to prevent current hogging and runaway. At 10mA, the 5685 looks like about 6 ohms, the 5594 looks like about 3 ohms.
What value would be practical for the two seriesresistors, 1 ohm or less?
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Old 13th November 2012, 08:10 AM   #7
MartinR is offline MartinR  Germany
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Hello,

the first stage is E810F pentode like Thorstens LCR design but with interstage transformer. Worked fine as long as I connected the step up 1:8. But I´m getting trouble if I connect it 1:16 with a higher impedance cartridge like the DL103. Cathode is grounded so I suspected the grid current but it seems that the low imput impedance of the stage seems to be the problem.

So I tried some other operating points. Works good with 22R/1000uF, resulting in 0.8V grid voltage, but it doesn´t sound good. It is much better with a 1N 4148 (same operating point). So I was wondering how a IR LED would perform.

Soundwise, the best solution so far is with grounded cathode, 180K grid leak and a coupling cap between grid and step up.

Isn´t max current through the 5594 100mA?

Kind regards,
Martin
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Old 13th November 2012, 09:01 AM   #8
euro21 is offline euro21  Hungary
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E810F at 35-40mA and interstage without saturation ?????
It seems that I don't understand it.
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Old 13th November 2012, 09:32 AM   #9
SY is offline SY  United States
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A 20mA current would be more sensible and not cost you much transconductance. And for that, a red LED would work fine- the HLMP6000 series are very quiet and low impedance.

It may be worth reading Vogel's book on phono design as well as his recent article in Linear Audio on triode noise. More current is not always better....
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Old 13th November 2012, 11:38 PM   #10
Ciu is offline Ciu  France
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Maybe Vishay Tsal7400, I've tried up to 28mA (Vf ~1.2v) with E810F Amperex.
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