Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

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

analog.com

Post reply on HN
Print view View original

61–70 of 110 posts

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

#61

Post by fsh »

thaumasiotes wrote: post #18
ISL wrote: post #7

Earlier quoted context omitted.

It is now partially incorrect, too. Now we can no longer change Boltzmann's constant not because he's dead (indeed, until recently, it was a measurable quantity), but because k_B is now defined as a part of the redefinition of the SI unit system in 2018. The value is 1.380649×10^{−23} J/K, exactly.

> k_B is now defined as a part of the redefinition of the SI unit system in 2018. > The value is 1.380649×10^{−23} J/K, exactly. Hmm. This is a trendy thing to do with SI units, indeed. But this definition makes me wonder if there are also definitions of the Joule and the Kelvin. If there are, it seems like they could easily conflict with this definition of k_B. And if that happened, we'd have to admit that k_B was a…

The underlying reason is that we don't actually need a unit for temperature. The temperature of a substance is simply the mean kinetic energy of its molecules which can be given in Joule. The problem is that historically thermometers were calibrated using the triple point of water and not by measuring the kinetic energy of the molecules. This is how the Kelvin scale used to be defined. The Boltzmann constant was simply a measure of the triple point of water in energy units (which could be measured). One problem was that the isotopic composition of water influences the triple point and was not well defined in the old SI system. Nowadays, we can actually calibrate thermometers by measuring the kinetic energy of molecules, so we no longer need to use the triple point of water. This is why the Kelvin is now just a rescaling of the Joule with a fixed coefficient (the defined Boltzmann constant). So the Boltzmann constant can no longer be measured. On the other hand, it is now possible to measure the triple point of water in Kelvin (this used to be 273.16 K by definition in the old SI system).

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

#63

Post by lisper »

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 day I decided to do the math on resistor noise and discovered that that was in fact my limiting factor and the only way were were going to get it to work was to either cool the first-stage resistor or to use a ridiculously high value because the gain goes up linearly with the resistor value but the noise only increases with the square root. We ended up with a ten gigaohm resistor, which was just enough to get the S/N ration we needed to make it work.

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

#64

Post by guenthert »

klodolph wrote: post #19
fallingfrog wrote: post #14

I usually put a single 470 ohm resistor in line with the gate of a discrete jfet in common collector mode as the first gain stage in my projects. Once you boost up the signal voltage it’s way easier to maintain a good signal to noise ratio. The resistor is there to prevent the jfet from being burned out by over voltage on the gate, which is very sensitive to static electricity. But, I can easily hear the difference i…

> ...a discrete jfet has a better noise floor than an op amp or a regular transistor. This used to be true, but you can get really good low-noise op amps these days.

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 discrete JFets fronting a low noise OpAmp were often used. These days its rather a challenge to find low-noise discrete JFet pairs and one has to use an integrated OpAmp instead (the causal chain might be reversed there).

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

#66

Post by klodolph »

guenthert wrote: post #64
klodolph wrote: post #19

Earlier quoted context omitted.

> ...a discrete jfet has a better noise floor than an op amp or a regular transistor. This used to be true, but you can get really good low-noise op amps these days.

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.

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

#67

Post by bcaa7f3a8bbc »

kazinator wrote: post #47
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.…

I was surprised to find that after replacing most of the op-amps in a ADA MP-1 pre-amp, most of that orientation-sensitive remaining buzz that you still get with humbucking pickups was seriously reduced. When I'm playing at a low volume, I can just mute the strings and put the guitar on a stand to get it to be quiet. On a high-gain program using the tube board and all. The reason for some of the buzz is that the circ…

I remember seeing an interesting Audio Engineering Society's presentation (2005) [0] on a similar problem in balanced audio interfaces. Interestingly, an old-school audio transformer is more robust, it has higher CMRR in the real world when there's some common-mode impedance imbalance in the system, on the other hand the CMRR of an opamp seriously degrades. Designs which naively rely on the opamp CMRR were responsible for many noise problems in balanced audio.

> Where Did We Go Wrong? TRANSFORMERS were essential elements of EVERY balanced interface 50 years ago ... High noise rejection was taken for granted but very few engineers understood why it worked. Differential amplifiers, cheap and simple, began replacing audio transformers by 1970. Equipment specs promised high CMRR, but noise problems in real-world systems became more widespread than ever before ...Reputation of balanced interfaces began to tarnish and “pin 1” problems also started to appear!

> Why Transformers are Better. Typical “active” input stage common-mode impedances are 5 kΩ to 50 kΩ at 60 Hz. Widely used SSM-2141 IC loses 25 dB of CMRR with a source imbalance of only 1 Ω. Typical transformer input common-mode impedances are about 50 MΩ @ 60 Hz. Makes them 1,000 times more tolerant of source imbalances – full CMRR with any real-world source.

> CMRR and Testing. Noise rejection in a real interface depends on how driver, cable, and receiver interact. Traditional CMRR measurements ignore the effects of driver and cable impedances! Like most such tests, the previous IEC version “tweaked” driver impedances to zero imbalance. IEC recognized in 1999 that the results of this test did not correlate to performance in real systems... My realistic method became “IEC Standard 60268-3, Sound System Equipment - Part 3: Amplifiers” in 2000. The latest generation Audio Precision analyzers, APx520/521/525/526, support this CMRR test!

[0] https://www.aes-media.org/sections/pnw/pnwrecaps/2005/whitlo...

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

#68

Post by michael1999 »

Zenst wrote: post #23
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.…

I was messing about with contact microphones last year and very much akin area to guitar amps as high-impedance, so very much the same issues. If you run on batteries you will find it works best, as with anything mains, you will want a good ground. What I did find was that if you use peizo's back to back you can effect a balanced signal and that in itself helps immensely in eliminating much of the noise. You can also…

Wait. We have ADCs that produce floats now? That’s amazing.

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

#69

Post by pfdietz »

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."

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

#70

Post by analog31 »

jbay808 wrote: post #33

> In fact, a remarkably common response to a diagnosis of resistor noise is to seek a source of "good" resistors, with "good" being defined as without thermal noise. This is impossible. It's impossible to make a totally noiseless resistor, but it's also important to understand that all resistors are not created equal. Most resistors have noise levels that are orders of magnitude above the Johnson limit. Potentiometer…

I recommend measuring the noise. Systems guru Phil Hobbs said that you should know where every dB of noise comes from in your design. Of course a dB could be a little or a lot in your application, but the point is that you should perform a noise budget and then test your assumptions.

It's not necessarily easy, but recommended if possible. I was doing it with DIY equipment, so I don't claim traceable results.

In one case, I literally measured the noise of some resistors, and within the parameters of what I cared about, I found no measurable difference between metal film and carbon film. I was passing no DC current through the resistor. Some sources of "excess noise" are proportionate to DC current and can be corrected by appropriate filtering.

Post reply on HN