|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
120.156.168.47
In Reply to: RE: Low noise resistors? posted by 13th Duke of Wymbourne on August 16, 2021 at 14:34:29
Thanks, 13DoW ... they are in fact series resistors.
Andy
Follow Ups:
Are the 100 Ohm in series between the cartridge and the input devices? IIRC, about the best input referred noise for an audio transistor amplifier is approx 1nV/rt.Hz (e.g. MAT-02 and, I think, there is a J-FET can reach that too). Adding a series 100 Ohms is about the same as adding another 1nV/rt.Hz so the system noise doubles.Regards,
13doW.
Edits: 08/16/21
Thank you 13th DoW - exackly! :-)) They are JFET Gate resistors (for stability).
So my problem is ... what are the best reses to use?
I had thought Vishay VSH were good (in terms of low noise) but, as I can't get them any more, maybe I should use:
* Mills ww
* or, say, Holco metal film ... which I use in other places in the circuit?
Or - as I understand higher wattage resistors have less noise - use 1w metal films?
The other place in the circuit where a series res is used is between the Drain of the 1st gain stage JFETs and the passive RIAA circuit - this is 47K.
Andy
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
Edits: 08/17/21
That link is a very interesting article, 13DoW. :-))
I cannot afford the Z-foils ... but I will keep away from wirewound.
So I will use 'standard' metal film - but use 1w versions.
Thanks,
Andy
Andy, just use DALE 1% resistors. They will do the job.
And easily available from Mouser, I see. :-))
Andy
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: