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In Reply to: RE: LED cathode bias vs diode 1N4148 in series, Are they the same effect posted by Eric Chan on July 27, 2023 at 04:58:11
Measure the current thought the tube plate load to calculate the operating point. The 1N4148 will have a forward voltage drop around 0.7v. If you wanted 1.5 volt bias you would need 2 in series.People say that different LEDs sound different (in addition to having differences in the forward voltage drop) and they also sound different than diode rectifiers.
Edits: 07/27/23Follow Ups:
Hi Chip,
The LED seems to have constant Voltage whilev4148 may not ad I was told, that is why I want to know their differences in yerm of sound wise and tech aspect
The tech aspect usually revolves around the internal resistance of the part. The 20 to 100 ohms resistance in different LEDs might be audible as the lower resistance will create lower local feedback making the gain very slightly higher. The silicon diode has about the same internal resistance as LEDs, some are a little higher in this regard. For the "constant current" aspect, all diodes have a positive slope for current to voltage, none are completely flat. To get a constant current source you need a constant current circuit.
Once you account for tiny differences in gain due to the internal resistance, the sonic difference falls into the "audiophile ear" world.
You can spend the $1 to plug in a couple of different combinations to see what you like. Just don't be "biased" by the pretty colors.
Try led, diodes and whatever else you fancy and see what you like.
Here's some measurements in the link.
The 1N4007 impedance has even lower, so why use LED then?
To get the same voltage as a red LED, you need two 1N4007s, which doubles the dynamic resistance.
The Conrad Johnson ART preamp use 1N4148 2 in series instead of using single RED LED to achieve 1.4V bias
The recovery time for 1N4148 is 4ns which is very fast, is that part of the reason?
That's a graph of voltage drop. Z of red LED a little lower than 1N4007.
For what it's worth, some LEDs are noisier than others, though not nearly as noisy as Zeners. It matters in a phono preamp.
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