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Resistor Noise Can Be Deafening, and Hard to Reduce (2007)

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Re: Resistor Noise Can Be Deafening, and Hard to Reduce (2007)

#101

Post by jhallenworld »

pfdietz wrote: post #69
cryptonector wrote: post #28

> And, of course, we can't change Boltzmann's Constant because Professor Boltzmann is dead[3]. Worth the read just for that punch line.

The opening paragraph of Goodstein's "States of Matter": "Ludwig Boltzmann, who spent much of his life studying statistical mechanics, died in 1906, by his own hand. Paul Ehrenfest, carrying on the work, died similarly in 1933. Now it is our turn to study statistical mechanics."

Hah, I have that book, the next line is something like "we shall proceed cautiously".

Re: Resistor Noise Can Be Deafening, and Hard to Reduce (2007)

#102

Post by mastax »

klodolph wrote: post #66
guenthert wrote: post #64

Earlier quoted context omitted.

Uh, for a looong time already. The LT1028 was available in the eighties and afaik, still unsurpassed (in terms of voltage noise, it's unfortunately a sucker in terms of input current and current noise, so it's for low impedance applications only and it's fairly expensive). The cheaper OP-27 is also old and still available. The challenge is to find a low-noise OpAmp with high input impedance where earlier hybrids with…

Yes, I built a guitar pedal that was designed around JFET gain stages and the JFETs had to be sourced from a specialist. I wonder if they're still being made. Most of the discrete JFETs I see available are for switching applications.

Linear Systems still makes JFETs and all sorts of other discrete parts that everyone else stopped making.

Re: Resistor Noise Can Be Deafening, and Hard to Reduce (2007)

#103

Post by exmadscientist »

ldarby wrote: post #84
jbay808 wrote: post #55

Earlier quoted context omitted.

I think you "well actually"'d a bunch of things I specifically didn't say. I use "Johnson limit" synonymously with "thermal noise limit" because they are the same, and it's the limit of how low noise can be after removing all other sources of noise. Most people, if they even learn about resistor noise, will only learn about thermal noise. If they're lucky enough to identify a resistor as the noise troublemaker in a c…

Elsewhere in this thread there's a link to https://dcc.ligo.org/LIGO-T0900200/public , which backs up what you're saying: "Metal film or thin film resistors have little excess noise in these tests and thick film resistors show large excess noise"

Yep. Any time I actually care about the value of a resistor (which is to say, for precision analog work, not power supplies or logic pull-ups or LEDs or ...), I'll pick a cheap thin-film part, even without doing analysis. This does several things for me:

1. I get a "pretty good" (i.e., >80% of best achievable performance for 2. I get a separate BOM line item for that resistor, so it stands alone (or with similarly important parts) and is easy to single out to, say, ensure correct ordering.

3. The part will be physically distinctive, so I can also distinguish it on the circuit board. If I have space, I'll even go up one size, to make it impossible to swap out with the less critical parts. (It would work fine, if expensive, to put thin-films everywhere. The reverse is not true.) It also sends the "pay attention to this" signal to anyone down the line who only has the board to work from.

I favor Susumu thin-film resistors, as they're generally cost-competitive, very high quality, available in very tight tolerances if needed, and visually easily identifiable as Susumu parts (though the series is not identifiable). This means they have to be thin-film, because Susumu doesn't make anything else. RR and RG series are the go-to parts here. If cost is a concern, Yageo RT is also excellent, but not as noticeable on the board. For analog work, I'll go no smaller than 0603 (1608M) to ensure values are printed on the parts. This pays for itself after only one gain resistor mixup on the bench....

Re: Resistor Noise Can Be Deafening, and Hard to Reduce (2007)

#104

Post by zsmi »

analog31 wrote: post #92
guenthert wrote: post #64

Earlier quoted context omitted.

Uh, for a looong time already. The LT1028 was available in the eighties and afaik, still unsurpassed (in terms of voltage noise, it's unfortunately a sucker in terms of input current and current noise, so it's for low impedance applications only and it's fairly expensive). The cheaper OP-27 is also old and still available. The challenge is to find a low-noise OpAmp with high input impedance where earlier hybrids with…

Indeed the LT1028 datasheet shows a BF862 JFET at the front end of a photodiode preamp using the LT1018. The reason why we're at a plateau of op amp noise performance is that the current crop of chips are operating very close to the theoretical limit for both bipolar and JFET input devices. Ironically I use a LT1028 in a homemade circuit for measuring the noise of things that are more noisy.

Also, a lot of the newer low noise op amps want much better frequencies characteristics and more GBW as they're primary application is sensors. Take for example the ADA4817. It has very comparable noise characteristics but is quite a lot faster.

Re: Resistor Noise Can Be Deafening, and Hard to Reduce (2007)

#105

Post by p1necone »

bcaa7f3a8bbc wrote: post #16
klodolph wrote: post #15

My only experience with this is building and designing guitar amps, which often have 80dB of gain or more, a.k.a., a pain in the ass amount of gain to deal with. It's not something on par with, say, radio astronomy, but it's still a lot of gain to deal with. Usually the main source of noise will be a 120Hz or 100Hz buzz, but with humbucking pickups and careful orientation of the guitar you can mostly eliminate that.…

The experienced and mysterious audio engineer "NwAvGuy" [0] praised the virtue of using two gain stages and moving the volume control away from the first input to reduce Johnson noise in audio amplifier designs [1]. It's a good example of how the basic principle applies both to mundane audio and cutting-edge science: the system noise is dominated by the first amplifier stage. Adding some noise before the first stage…

Has anyone worked out what happened to NwAvGuy yet? Afaik he never posted anything about taking a break or going away for a while. Given his pseudonym was totally anonymous I can only speculate - he could be dead or in prison or something...

Re: Resistor Noise Can Be Deafening, and Hard to Reduce (2007)

#106

Post by bcaa7f3a8bbc »

bsmith0 wrote: post #100
failwhaleshark wrote: post #99

On a tangent: acoustic noise. My HP 32SII made terrible resistor noises. Bzzzzzzz like some sort of bad tinnitus. I could also hear it on an HP 48G when I placed my ear up to it. Wouldn't it be possible to use SMT resistors and pot them in silastic to quiet them down?

It's probably coil whine? https://en.wikipedia.org/wiki/Electromagnetically_induced_ac...

Usually it's inductor coils and transformers, occasionally it's ceramic capacitors (all grades other than NP0 are microphonic, SMD or not), both problems are common in switched-mode power supplies, for example, powering the calculator LCD. I've never seen a singing resistor, very unlikely.

Re: Resistor Noise Can Be Deafening, and Hard to Reduce (2007)

#107

Post by fuzzfactor »

lisper wrote: post #63

One of my first jobs, which I got while I was still an undergrad (in the mid-80s), was designing amplifiers for fiber-optic sensors. I pretty much had no clue what I was doing so I just started futzing around with op-amps and realized very quickly that my signal-to-noise-ratio was much higher than was acceptable. I figured there was some hardware design trick that they hadn't taught me in my EE curriculum, but one da…

>a ten gigaohm resistor

Depending on the voltage across a resistor like that, you may calculate less than one electron passing through the resisitor per second.

Without ceramic or teflon standoffs, the circuit board can often conduct better than the resistor, plus dust can also accumulate on the outside of the resistor and conduct better eventually, which is why they are often encased in glass, so they can be effectively cleaned during a maintenance cycle.

Re: Resistor Noise Can Be Deafening, and Hard to Reduce (2007)

#108

Post by fuzzfactor »

klodolph wrote: post #15

My only experience with this is building and designing guitar amps, which often have 80dB of gain or more, a.k.a., a pain in the ass amount of gain to deal with. It's not something on par with, say, radio astronomy, but it's still a lot of gain to deal with. Usually the main source of noise will be a 120Hz or 100Hz buzz, but with humbucking pickups and careful orientation of the guitar you can mostly eliminate that.…

Not only high gain but high power also.

Shot noise was originally attributed to electrons hitting the anodes of vacuum tubes, but transistors turned out to make the same kind of noise so it applies to them too.

And it is current-dependent so sometimes you can be quieter at idle by biasing lower.

For resistor noise, nothing wrong with a megohm referencing input to ground since there's not any significant current flowing there.

But in a tube preamp the typical 100K anode resistor will conduct a bit and can get hot. These should be carefully auditioned. Plus higher wattage rating parts give less noise in this service than they put in most commercial amps.

If you increase gain using something like 150K, 220K, or even 330K there will be less current (through the resistor and the tube) but the increased amplification factor will equally multiply any noise which occurs before that gain stage.

Then with power you've got the ability to broadcast audio, naturally over extremely short distances (compared to radio frequencies which hopefully you are filtering out) but those are the distances inside your chassis where different parts of the wiring layout can interact beyond a cetain point as broadcast and receiving antennae, and either provide negative or positive feedback, stabilizing or destabilizing respectively parts of the circuit whether you intended it to happen or not.

On top of the expected acoustic mechanical feedback from the speaker at high volume, which reverses polarity based on distance, you've also got magnetic feedback. Once a very high-power output transformer goes wild it can reach out a lot further and touch your pickups directly from a few feet away.

At this point the noise at idle is usually as loud as as ten pounds of bacon frying and I hate that.

So get out the soldering iron and fix it so you can only tell it's on if you put your ear close to the speaker, when it's actually set loud enough to play with a heavy drummer.

Hearing protection required beyond this point.

Re: Resistor Noise Can Be Deafening, and Hard to Reduce (2007)

#109

Post by Zenst »

KeytarHero wrote: post #96
Zenst wrote: post #80

Earlier quoted context omitted.

Yip - not looked into any IC's, though kit with that functionality been trickling past couple of years and slowly picking up pace. https://zoomcorp.com/en/us/field-recorders/field-recorders/z... one example.

Not exactly - Zoom stuff does a clever trick of using 2 ADCs at different gain levels (essentially the same way that HDR photography works), but ultimately it's still using fixed-point ADCs. This isn't actually a new idea - I know Line 6 has been using it in guitar pedals since at least 2006. Of course, ADCs have gotten better since then - I doubt they were getting even 24 ENOB back then.

Ah the F6 has dual ADC and it uses that to cover a larger recording range - best explained here https://www.youtube.com/watch?v=NTuJ1fk3PsY

Also nicely explained by Sound Devices here for their offerings: https://www.sounddevices.com/how-is-a-32-bit-float-file-reco...

Still pondering the F2, does it have dual ADC to capture that input range or is this why they sell it with a lavier mic included (hmmm).

Re: Resistor Noise Can Be Deafening, and Hard to Reduce (2007)

#110

Post by lisper »

fuzzfactor wrote: post #107
lisper wrote: post #63

One of my first jobs, which I got while I was still an undergrad (in the mid-80s), was designing amplifiers for fiber-optic sensors. I pretty much had no clue what I was doing so I just started futzing around with op-amps and realized very quickly that my signal-to-noise-ratio was much higher than was acceptable. I figured there was some hardware design trick that they hadn't taught me in my EE curriculum, but one da…

>a ten gigaohm resistor Depending on the voltage across a resistor like that, you may calculate less than one electron passing through the resisitor per second. Without ceramic or teflon standoffs, the circuit board can often conduct better than the resistor, plus dust can also accumulate on the outside of the resistor and conduct better eventually, which is why they are often encased in glass, so they can be effecti…

> Depending on the voltage across a resistor like that, you may calculate less than one electron passing through the resisitor per second.

If memory serves (this was a very long time ago) the output signal was a couple of millivolts, so it wasn't quite that bad. But one other thing that saved us was that we only needed a few hertz of bandwidth.

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