Propeller Head Plaza

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RE: Are the 100 Ohm in series between the cartridge and the input devices? ...

Posted by 13th Duke of Wymbourne on August 17, 2021 at 15:58:21:
Ah, gate-stoppers! I have no experience to know how small a resistance you can get away with. The noise voltage of those resistors will add (~ 1nV.rt.Hz) directly to the equivalent input noise of the rest of the amplifier but, if your input referred noise without the resistors is at least 5nV/rt.Hz then it is not a big hit. The effect of the noise from the series 47k on the output side is lessened by the gain. If the first stage gain is 470x then the noise contributed by the 47k at the output is the same as that contributed by the 100 ohms at the input. A SPICE simulation is a great way to know the dominant noise sources in a circuit.
I did a search on the Vishay website for 'noise' and found nothing relating to resistor generated noise. My conclusion is that 'low-noise' resistors are not a thing - it is purely SQRT(4kTR) V/rt.Hz but I also found the article linked below that states some compositions have a 1/f noise region in addition to the thermal noise (1/f noise is common at low frequencies in semiconductors). I suppose if a resistor with a positive temperature coefficient was dissipating power and got hot, its resistance would go up and then so would the noise generated (noise power is proportional to temperature). In that case a higher power rated resistor might not get so hot but you'd have to calculate to see if the effect is significant. Your gate-stopper resistors will dissipate insignificant power so that is not a problem.
Regards,
13DoW