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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)

#21

Post by peter_d_sherman »

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 complex waveform -- which very possibly would change over time...

Also, future (and perhaps current!) scientists should be able to use an electrical signal of known characteristics -- to determine exactly what the complex impedance of the resistor/resistance element/impedance element -- in a circuit is, exactly...

In other words, given one of the above things (complex waveform, complex impedance) -- derive what the other one is, from it...

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

#22

Post by minitoar »

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…

Not in SI. Kelvin is defined in terms of k_B. Joule is defined in several ways, eg force & distance.

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

#23

Post by Zenst »

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 use a contact material sandwich in-between the piezo discs and effect how it works tone wise as well as become more zoned in the pick-up area.

But impedance matching is, as with guitars, very much key for pre-amps.

As for input levels and clipping - the rise of 32bit float has made a huge difference and means you can not worry about mic input levels at the ADC stage as much and normalise everything in post, sorting the levels out then without any fear of clipping at all.

Some nice low-noise preamp designs to check out here: http://www.richardmudhar.com/piezo-contact-microphone-hi-z-a...

Though those just unbalanced input designs, alas I'm not aware of any balanced contact mic's on the market - but can easily make them yourself using the above approach.

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

#24

Post by wishinghand »

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…

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

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

#25

Post by OnlyOneCannolo »

JamesBryant wrote: post #20
jeffbee wrote: post #11

Earlier quoted context omitted.

I don't know how this one made it, but if there's an Analog Dialogue article on the front page of HN weekly for the next five years it will still barely scratch the surface of all the treasure that's buried in that archive.

You can find all the RAQ (Rarely Asked Questions) at:- https://www.analog.com/en/analog-dialogue/raqs/raq-issue-xxx... Where xxx (or xx or x - no leading zeros) is the number. Highest is presently 190. I wrote all the first ones, then I shared slots, and after I retired I just wrote the occasional one.

I binge-read the RAQs a few months ago. So much great information. Thank you!

I also appreciated the consistent URL scheme because it let me easily scrape all the PDFs to read them offline. The one bummer is that the links at the end of the PDF versions seem to be broken now.

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

#29

Post by isatty »

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?

I'd say a higher end DAC/amp would consider it. My Benchmark devices (which nwavguy uses as a reference to build his O2 and ODAC) does it the right way.

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

#30

Post by p_j_w »

madengr wrote: post #12
bcaa7f3a8bbc wrote: post #9

Fun fact: for the most demanding RF applications, namely, radio astronomy, the front-end low-noise amplifiers are indeed cooled to cryogenic temperature by liquid nitrogen. Here's how it's done at NASA for the Deep Space Network [0]. It's a long paper, see Chapter 4 Cryogenic Refrigeration Systems , PDF page 179 (text page 159). Also, nice photos in page 183 and 188. [0] https://descanso.jpl.nasa.gov/monograph/series…

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!

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