Ping Sy: LED for Cathode Bias

[SY]

Drop me an email- I think we ought to exchange some LEDs and see how our measurements compare on the same devices. I love it when we see the unexpected!

[Shoog]

I just tried a ECC88 with a Green LED as bias, for a current of 4mA. I then took that out and replaced it with a resistor.
I have to say that down at these currents the Green LED had audable softening and smudging of the sound, which would be expected if 2nd harmonic was a significant issue. This is only a rough test which doesn't take into account a number of possable other circuit variables (ie slight shift in bias point).
Conclusion - 4mA would seem to be to low a current, which I had suspected before trying. The question is how low would you say was acceptable?

Shoog

[baggystevo82]

You need to look on the datasheet or measure for yourself current vs voltage dropped, and see how much current is required to make the LED's work linearly, for the example here, not a lot, but can vary hugely between different types. Kind of the same idea as not using tubes near cutoff if possible I guess. Maybe there's some configuration where the tubes and LED's nonlinearities could cancel?
Steve

[EC8010]

Originally posted by Shoog
Conclusion - 4mA would seem to be to low a current, which I had suspected before trying. The question is how low would you say was acceptable?

If you look at the graph on the first page of this thread you can see that you really want to be up at 10mA or more, preferably more.

[Shoog]

As I said before I thought "at least" 10mA would be about right. I just tried the Green LED at 4mA to see what it would do - it didn't !

[rdf]

Hi SY, you have mail.

Before anyone reads too much into my hackery, those numbers are all relative with no absolute reference to real circuits. Could be they translate into distortion components below the residual noise level of any current music media, I have no idea. I won't even hazard a guess at this point if the presence of second harmonic on the cathode is a bad thing. With a sufficiently linear tube it could result in 2nd harmonic cancellation and lower the raw stage distortion. I'm just stumbling about playing with parts.

One thing I am pretty sure though is the comparative I/V curves typically shown for different LED colours are highly idealized. No spec sheet I've seen for visible spectrum LEDs display the sharp-knee/straight-line characteristics shown. On the other hand, take a look at this one from the spec sheet for a Vishay TSHG8200 infrared LED, picked at random for being the longest wavelength and a through-hole mount model. From a purely impedance viewpoint (ignoring noise, etc.) there’s your potential 2 ma LED.

[arnoldc]

On the other hand, take a look at this one from the spec sheet for a Vishay TSHG8200 infrared LED, picked at random for being the longest wavelength and a through-hole mount model. From a purely impedance viewpoint (ignoring noise, etc.) there’s your potential 2 ma LED.

Goodie-goodie Maybe just what we need. Time to order from Farnell again and experiment. I'd love to test this on my 407 preamp which is at 3.5mA current.

[SY]

Shoog, one thing to try is to run a 10mA current source (it can be a very simple one) from some positive potential to the cathode-LED junction to see if that clears things up. The voltage divider action will knock down any noise contribution from the CCS.

[Shoog]

"Shoog, one thing to try is to run a 10mA current source (it can be a very simple one) from some positive potential to the cathode-LED junction to see if that clears things up. The voltage divider action will knock down any noise contribution from the CCS."

This is something we discussed before, and it seems a good idea. However I think the reality is that LEDs have there role at higher currents and thats where I think I will use them. In the same circuit I am using a RED LED in the cathode of a simple ECC88 CCS at about 8mA, and it works great.
I just thought everyone would be interested in the report that there seemed to be an audable distortion at lower currents.

Shoog

[Shoog]

I just had the thought that when substituting a LED for an unbypassed resistor, you will get an increase in output (because of the smaller cathode feedback). This is something to take into account when auditioning the change, because an increase in volume is often confused with an increase in quality.

Shoog