LED Bias Modifications

[dsavitsk]

Hi Doug,
I would hazard a guess that Rich thought you were talking about using a led as a bias setting indicator, not as the actual source of bias.

That would explain why it didn't make any sense.

Anyhow, thanks for all the suggestions -- I am going to try a few of them and see what seems to work. Additionally, a back of the napkin calculation suggests that the pair of 10uF caps might be big enough to use as ultrapath caps (PS is a cLCRCRC followed by a regulator, so ripple should not be problem) though that might be wishful thinking, so I may give that a shot too.

 

[rdf]

Hi Bob. I experimented extensively with a couple forgotten triode-diode-diode tubes and though it works never got them to measure or sound as good as simple LED biasing.

 

[dsavitsk]

measure the voltage across the LED at each of those operating points, then just take the difference in voltage and R just becomes E/I. (More correctly delta of E over delta of I)


You can take several sets of points (Just 3 points is actually enough) to make sure that R is not too dynamic over the current range you intend to operate. (R varying with current is a distortion generating mechanism.)

I thought this was interesting, and somewhat surprising as I have heard people recommend blue LEDs -- maybe some others are better:

Red

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compared to Blue

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[kevinkr]

I thought this was interesting, and somewhat surprising as I have heard people recommend blue LEDs -- maybe some others are better:

Red

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compared to Blue

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Hi Doug,
Looking at those curves it would seem that the blue leds have a much larger dynamic resistance component in their characteristics - is that what you read out of this too? Or is it just the difference in scales??

 

[Sheldon]

Looking at those curves it would seem that the blue leds have a much larger dynamic resistance component in their characteristics - is that what you read out of this too? Or is it just the difference in scales??

Looks pretty much like a wash to me. Red = 0.5V/.035A = 14.2Ohm. Blue = 0.3V/0.02A = 15Ohm.

Sheldon

Compare to these: http://www.avagotech.com/products/product-detail.jsp?navId=H0,C1,C6469,C5550,C6889,P91120

 

[dsavitsk]

Hi Doug,
Looking at those curves it would seem that the blue leds have a much larger dynamic resistance component in their characteristics - is that what you read out of this too? Or is it just the difference in scales??

I took it to show that the red led's impedance was constant while the blue led's was not. That is, dV/dI for red is constant while blue is a function of V.

 

[SY]

That's higher for the red LED's impedance than I've typically seen with surplus LEDs. In the dozen or so different samples I've tested, the AC impedance was something between 4 and 10 ohms. Surplus rules.

 

[EC8010]

I took it to show that the red LED's impedance was constant while the blue LED's was not. That is, dV/dI for red is constant while blue is a function of V.

All diodes change their dynamic resistance with current, and LEDs are no exception. Having said that, once you're up to about 10mA, things tend to flatten out because although the LED's resistance continues to fall, the resistance of those tiny internal wires connecting to it becomes significant.

 

[rdf]

....the resistance of those tiny internal wires connecting to it becomes significant.

Thx, I never considered that because my construction is always groundplane and the LEDs are tightly cut from pin to plane (and bypassed with a small cap for HF.)

It would be interesting to compare notes on the effect of LED impedance. From my experience the non-linearities aren't usually a factor from the perspective of measured performance. Quite often distortion goes down for me with high impedance diodes due to the small local feedback. Bypassing them with a large cap cause distortion to rise. As always, I also find the upper and more noxious harmonics drop to a much greater extent than the 2nd.

I'm currently laying with a Scott LT-110B tube tuner, replacing the common cathode, loop feedback 12AT7 output stage with a no feedback 5678 biased by two series generic green LEDs. At 4 volts p-p out the 2nd harmonic is below -60 dB and the third well below -100 dB with no higher H visible above -120 dB. If 'bad' LEDs cause measurable problems I'm not seeing them.

 

[kevinkr]

Looks pretty much like a wash to me. Red = 0.5V/.035A = 14.2Ohm. Blue = 0.3V/0.02A = 15Ohm.

Sheldon

Compare to these: http://www.avagotech.com/products/product-detail.jsp?navId=H0,C1,C6469,C5550,C6889,P91120

Hi Sheldon,
To make a valid comparison you would need to compare two points at the same (delta in) operating current, so in a sense you have confirmed my suspicions..

 

[Sheldon]

I just picked a delta that was easy to read from the graph. My point was that the slope is comparable. The blue version does require higher current to be in the linear region, but without a specific application, can't know if that's an issue. In any case, there are LED's available with much lower dynamic impedance, and linear at much lower currents, so we don't have to split hairs on these.

Sheldon