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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)

#31

Post by pvitz »

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…

I think I remember seeing this in "The Art of Electronics" for actually amplifying shot noise for a hardware random number generator.

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

#32

Post by DiabloD3 »

wishinghand wrote: post #24
bcaa7f3a8bbc wrote: post #16

Earlier quoted context omitted.

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…

That's really interesting. Is that a common design in headphone and other consumer amps?

It basically is now. He didn't invent the technique, but popularized it in inexpensive devices.

The O2 has been surpassed by many great designs, but the O2 really did start that arms race.

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

#33

Post by jbay808 »

> 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. Potentiometers are especially bad.

If you want "good" resistors for noise-critical applications, I recommend metal thin-film resistors. They hardly cost any extra anyway.

Also, in cases where resistors are used to set DC signals such as offsets and biases, you can add capacitors to filter the heck out of those lines to decrease their noise contribution.

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

#34

Post by DiabloD3 »

peter_d_sherman wrote: post #21

There's a strange relationship between Resistance, Noise (audible and above the auditory spectrum, such as RF), and the concept of Impedance... I'll put a wager that future (or perhaps even current!) scientists are able to engineer complex waveforms such that the complex waveform effectively negates the resistance/noise/impedance -- effectively turning the resistor into a conductor -- but only for that specific compl…

You overcomplicated it, and somehow badly described LVDS at the same time.

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

#35

Post by h2odragon »

peter_d_sherman wrote: post #21

There's a strange relationship between Resistance, Noise (audible and above the auditory spectrum, such as RF), and the concept of Impedance... I'll put a wager that future (or perhaps even current!) scientists are able to engineer complex waveforms such that the complex waveform effectively negates the resistance/noise/impedance -- effectively turning the resistor into a conductor -- but only for that specific compl…

Like the maths of trying to urinate silently on a tin roof.

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

#36

Post by hughrr »

p_j_w wrote: post #30
madengr wrote: post #12

Earlier quoted context omitted.

Since the amplifier has both voltage and current input noise sources, there is an optimum source impedance that provides a minimum noise figure, and it’s not an impedance match. This is called a minimum noise match, along with associated noise contours where noise is traded off with impedance match. Also, the noise from an antenna is dependent on it’s efficiency and what it’s pointing at. Even if it’s input impedance…

>This is called a minimum noise match, along with associated noise contours where noise is traded off with impedance match. You also get to trade off your impedances with gain and stability. RF design is fun!

It’s not which is why I now write software :)

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

#37

Post by mgleason_3 »

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 pinnacle on a wonderful respite from the mire of politics and opinions that often invades hacker news.

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

#38

Post by StavrosK »

JamesBryant wrote: post #17
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.

It was correct when I wrote the RAQ in 2007. In fact I've been making the crack about 'not being able to change it because he's dead' in my lectures for at least forty years.

It's correct now too, since we can no less change the value now than before the redefinition (we can't change it in either case, and not because Boltzmann is dead), so the joke works fine!

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

#40

Post by prpl »

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…

… and overspec them everywhere reasonable
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