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Post by flacman on Feb 23, 2018 9:16:28 GMT
Emitter resistor of output BJT and 120ohm resistor (in series with the headphone).
At the two positions above, what kind of resistors should I use? Wirewound or Carbonfilm? Which is better? Their price on the mouser is around 3usd/10pcs much cheaper than metal film (>10usd/10pcs). (So I do not put the metal film in the comparison list)
Carbonfilm has high Voltage coefficien ~100ppm/V Wirewound has 0ppm/V but it has inductance
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solderdude
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Post by solderdude on Feb 23, 2018 11:40:30 GMT
Most amplifiers are designed with wirewound resistors in its emitter path. Usually consisting of just a few windings. 1meter of cable in series with the amp will have a higher inductance. The whole design (internal feedback etc.) is designed around this. It is thus pointless to change those resistors to non inductive types as the amplifier in question won't get more bandwidth.
Let's use an example of a very large inductance of say 1uH. With a 32 Ohm resitor (in reality a much higher value inductance) the cutoff frequency (-3dB) = 5MHz. Nothing to worry about really. Metal film resistors typically are around 0.06uH. Nothing to worry about either.
Wirewound resistors with a so-called bifilar winding have little to no inductance. 0.03 μH to 56 μH (quite specialised multiple windings There are even wirewound resistors with incductances as low as 5nH. Handy for high frequency switchers ets.
You will get in trouble in the GHz range. Not in the audio range nor above that.
Most amplifier designs have more problems with capacitive loads than inductance loads.
Headphone drivers are all inductive, and much more so than any coil before it. It would be really weird if amps would start to oscillate when you connect a headphone to it.
So you can pop in any metal-film, wirewound, carbon film, carbon composite, plastic, ceramic type of resistor in that part of the amplifier. Noise levels are so low they can never become audible (not even with the most sensitive headphones around) and bandwidth also is not a problem. The active circuitery will be the limiting factor, not an output resistor. Regardless how loud some people yell they hear vast improvements... there aren't any measurable ones in the audible range and way outside of it.
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Post by flacman on Feb 23, 2018 12:49:01 GMT
Thanks you very much! Then I would not hesitate to use the wirewound resistor.
With the film type, do you think the Voltage Coefficent has a real impact on the audio signal? (I have tried different types of Meta-film resistors with the same Voltage Coefficent, in the input position and LTP's emitter, but the results are very different in the High frequency area)
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solderdude
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Post by solderdude on Feb 23, 2018 13:11:02 GMT
With high voltage tube circuits (electrostatic drive circuits etc) and LTP input pairs working on high voltages it could be a small problem. Not at all in the low voltages in headphone amps and certainly not in the output path,
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