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Low noise resistors?

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Posted on August 15, 2021 at 20:43:06
andyr
Manufacturer

Posts: 12279
Location: Melbourne
Joined: September 2, 2000
I had been using Vishay VSH resistors in 2 places in my Muse phono stage, in a series connection, where low noise is ideal.

My normal supplier does not seem to be able to supply them any more - so I'm wondering what other, reasonable-cost, resistor I could use instead?

I'm thinking I could use 100ohm Mills wire-wound in one place - as I understand wire-wound resistors are rated as 'low noise'. But the other place I need a low noise is with a 47K value - and the Mills MRA-5 range doesn't seem to go up that high.

So what other brands do people recommend?

Thanks,
Andy

 

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RE: Low noise resistors?, posted on August 16, 2021 at 14:34:29
Posts: 2014
Location: Orange Co., Ca
Joined: September 19, 2001
'Low noise' resistor is a bit of an oxymoron. The thermal (aka Johnson) noise power of a resistor is proportional to the resistance value. So, for example, a 47k generates 470x the noise power of a 100 Ohm resistor (approx 27dB). A 'low noise' resistor would be one that doesn't add anything to the inherent thermal noise. Perhaps, look for white papers or specs for 'excess' noise from manufacturers to see if a particular resistor brand is really best for noise.
The impact of noise from a resistor depends on where they are in the circuit. I assume these are loading resistors, for MC & MM respectively, in which case they are shunted by the cartridge source resistance. The cartridge source resistance will be much lower than the loading resistor. In the MC case a typical cartridge source resistance is 5 Ohms. When that 5 Ohms is loaded by a 100 Ohm resistor the circuit noise actually goes down by 0.2dB but the signal goes down by 0.4dB so, overall, SNR degrades by 0.2dB - not much - and, I expect, the active devices in your phono stage will add more than that anyway.

Regards,
13DoW

 

RE: Low noise resistors?, posted on August 16, 2021 at 14:44:31
andyr
Manufacturer

Posts: 12279
Location: Melbourne
Joined: September 2, 2000
Thanks, 13DoW ... they are in fact series resistors.

Andy

 

RE: Low noise resistors?, posted on August 16, 2021 at 17:46:31
Posts: 2014
Location: Orange Co., Ca
Joined: September 19, 2001
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.

 

"reasonable-cost", posted on August 17, 2021 at 02:52:34
1973shovel
Audiophile

Posts: 9515
Location: Greenville SC
Joined: February 25, 2007



You've written a rather ambiguous post. First, what constitutes reasonable cost for one person may not qualify for another.

Secondly, you include no mention of what wattage you require. I tried searching your "Vishay VSH" for some indication, but couldn't find anything indicating what they are (or were), including on the Vishay site, except to see that VSH is their stock market listing.

The Mills are good resistors, but their lowest wattage rating, at least that I'm aware of, is 5-watt. Do you require that much heat dissipation, or are you considering them based on their reputation for low noise?

With that said, the Texas Components "Naked Bulk Metal Foil Resistors" are rated for very low noise, and have a reputation for being very sonically neutral. I've used them in some key areas of phono stages, and like them very much. They're not "reasonable cost" by my standards, unless I compare them to something like an Audio Note Silver Tantalum (which I've never used) and then they begin to look more reasonable to me.

Note that they make several types, the original TX2352 which are somewhat lower in cost, or their best offering, the TX2575 Z-Foil, which are the ones I used. (link to both below)

Another great thing about them, in addition to their low noise and neutrality, is that they will make them to a specific ohmic value. I have an RIAA network, for example, in which one of the values requires a 100K to be paralleled with a 330K. When I replace them I'll be ordering a TX2575 made to 76,744 ohms (the value of those two paralleled) at no extra charge, even in quantities of one or two, with a tolerance of 0.1%

Also take note that since you're a manufacturer, the prices go down as the quantity ordered goes up.

I hope this helps.

 

RE: Are the 100 Ohm in series between the cartridge and the input devices? ..., posted on August 17, 2021 at 05:10:05
andyr
Manufacturer

Posts: 12279
Location: Melbourne
Joined: September 2, 2000
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

 

RE: "reasonable-cost", posted on August 17, 2021 at 05:20:30
andyr
Manufacturer

Posts: 12279
Location: Melbourne
Joined: September 2, 2000
Thank you very much, 1973shovel.

Yes, my definition of "reasonable cost" is quite a bit less than the TI reses you linked to. :-((

I have to put my status on AA as "Manufacturer" - but I am a one-man band (a "hobbyist") ... so I cannot afford to buy in bulk.

Yes, I'm aware that the lowest wattage for Mills is 5w; my thought about using them is based on 2 things:
1. my belief that noise in a 5w res will be less than noise in a 1/2w res, and
2. ww reses are intrinsically "low noise".

Andy


 

RE: Are the 100 Ohm in series between the cartridge and the input devices? ..., posted on August 17, 2021 at 15:58:21
Posts: 2014
Location: Orange Co., Ca
Joined: September 19, 2001
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

 

RE: Thank you very much for the link ..., posted on August 18, 2021 at 04:02:19
andyr
Manufacturer

Posts: 12279
Location: Melbourne
Joined: September 2, 2000
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

 

This is correct, posted on August 19, 2021 at 14:09:04
Triode_Kingdom
Audiophile

Posts: 9033
Location: Central Texas
Joined: September 24, 2006
I'm retired now, but my day job for many years consisted of design and measurement of low phase noise circuity in the region of -180 dBC. This doesn't correlate directly with amplitude noise, but the worst-case equivalent is orders of magnitude better than any vacuum tube. Our products approached the theoretical limits of physics in this regard, and all used thin film resistors. Leaded components aren't usually marked thin or thick film, but resistors like those sold by Vishay with tempcos around 250 ppm/K are undoubtedly thin film. When resistors of this type are used, the noise floor of active devices generally proves to be the limiting factor.

Edit: Just to add some relevance to this, most of our products were RF-related. However, we did sell an opamp-based audio amplifier with an input referred noise of <1 nV/√Hz, and our proprietary in-house design using discrete components was very close to 0.5 nV/√Hz. Both products used standard thin film chip resistors.



--------------------------
Buy Chinese. Bury freedom.

 

RE: Thank you very much for the link ..., posted on September 1, 2021 at 10:58:42
john curl
Manufacturer

Posts: 4708
Joined: May 16, 2000
Andy, just use DALE 1% resistors. They will do the job.










 

RE: Thank you, John ..., posted on September 1, 2021 at 14:41:49
andyr
Manufacturer

Posts: 12279
Location: Melbourne
Joined: September 2, 2000
And easily available from Mouser, I see. :-))

Andy

 

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