His Master's Noise: A Thoroughly Modern Tube Phono Preamp

[TasoTaso]

Hi, a question about the LEDs.
I have a preohono 2 x ecc83 + 6n6p like CF.
I would like to remove the cathode R of the first two stages and put some LEDs on them.
Are there any LEDs with low voltages, ie below 1.7v of the red LEDs?
So 1.2, 1, 0.8 volts?
Eventually they can be put in parallel to decrease the voltage .. calculating and reasoning as if they were resistors?
I see that to put them in series as possible (to add up the voltages).

Thanks

[6L6]

Infrared LED are approximately 1.2V

You cannot parallel LED to lower voltage. Series LED add the voltage.

[TasoTaso]

Thanks for reply 6L6.

Is there anything less than 1.2v? My first stage (ecc83) has 0.85v on the cathode pin. Are there any other options? For example "normal" diodes instead of LEDs?

Thanks

[mrdave45]

I think normal diodes are in the region of 0.7 volts. They don't have to be light emitting. It's just they tend to have larger band gaps.

Leds are also non ohmic.

If you have 2 resistors, a 5k, and a 10k in series and stick 15v across them, you will get 5v, and 10v respectively.

This would also hold if you had 5r and 10r, or 50k and 100k. The voltage drops would still work out the same but the current would vary.

If we changed the voltage accross the pair to say, 30v.the current would double, as would the voltage drop. The 5k resistor would now have a 10v drop. The 10k a 20v drop.

Leds and diodes are different.

Once they turn on, the voltage drop across them stays at the forward voltage drop.

After that the resistance of the device decreases and so you must use a resistor or similar to limit the current.

Let's take the example of a 1.7 volt led. A 30v power source and say the operating current of the led is 10mA.

So, we are over 1.7v, so it's going to light.

Subtract 1.7v from 30v and that will give you the voltage drop across the resistor.

So 28.3v across the resistor.

We need 10mA. In series like this, every device will see 10mA through it.

Using ohms law, R=V/I

28.3v/0.01A (do calc in amps not milli amps)
And that means you need to use a 2830 ohm resistor.

Closest common value is 2700 or 3300 which would give 10.5ma or 8.6ma.
Either would probably be OK.

Then check the resistor isn't going to melt using i2r

0.0105A*0.0105A*2700 ohm = 0.3 watts
0.0086A*0.0086A*3300ohm = 0.24 watts

Either case probably wants a half watt resistor to be safe.

Having said all that. I think you'll lower head room by changing these diodes. Iirc you raise the cathode voltage above the grid instead of putting negative bias on the grid. Unless you know exactly what you are doing I would leave the hmn stock.

Its a fabulous design that sounds amazing and has very low noise. I use mine with a 0.3mV cartridge and I only start to hear hiss out of it once the volume is way past normal listening levels.

[TasoTaso]

Through the Mouser site you can select the search for a diode based on "VF".
What I ask myself, thanking for the explanation just written, is:

Can a "Normal" diode perform the function of a LED diode ... in a CVS?
.... or is the led "special" for some feature?

...actually (on Mouser) there are infinite "VF" values ​​even between 0.5v and 1v (normal diode = no led)

Thanks

[jan.didden]

A LED is special in that it emits light, which a normal diode doesn't.

You can just try something like an 1N400x, it may be a bit less than 0.8V but your circuit will find a new balance, with slightly higher anode current. You may want to measure that before/after to see how much it changes, and whether that's OK.

Another option would be two Schottky diodes, with the low currents you have they may come out pretty close to 0.85.

Jan

[jackinnj]

Originally Posted by mrdave45

Leds are also non ohmic.

LED's have dynamic impedance.

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[mrdave45]

LED's have dynamic impedance.

Hi Jack, was that a correction or clarification?

[jackinnj]

Hi Jack, was that a correction or clarification?

It's a clarification -- was reading an article on LED drivers in some trade journal.

the impedance changes with frequency and (obviously) current -- for the IR one-LED used here it's trivial in the range of 10Hz to 100kHz. At 4mA with a 10mV signal it's just a few Ohms change.

It does seem to make somewhat of a difference in the "Red Light District" amplifier which uses a 7x7 array of red LED's. Might have some "self-heating" issues in that one too.