[HN Gopher] Resistor Noise Can Be Deafening, and Hard to Reduce ...
___________________________________________________________________
Resistor Noise Can Be Deafening, and Hard to Reduce (2007)
Author : bcaa7f3a8bbc
Score : 217 points
Date : 2021-06-09 13:47 UTC (9 hours ago)
(HTM) web link (www.analog.com)
(TXT) w3m dump (www.analog.com)
| buescher wrote:
| And yet, there is such a thing as a low-noise resistor, because
| thermal noise is not the only resistor noise. This application
| note, also from Analog, explains:
| https://www.analog.com/media/en/technical-documentation/appl...
| ng55QPSK wrote:
| At my workplace one R&D engineer had the last lines printed out
| as poster (i guess the AD note is older than 2007)
| cushychicken wrote:
| Chapter 8 of _The Art of Electronics, 3rd Edition_ is a great
| resource on electrical noise. The first section is almost
| exclusively about resistors and noise budgeting. Fun read. I
| should get back to it at some point.
|
| Lots of great tricks on bootstrapped filters, capacitive
| multipliers, precision transistor noise measurements with some
| outboard circuits fed into a spectrum analyzer.
| jbay808 wrote:
| > 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.
| prpl wrote:
| ... and overspec them everywhere reasonable
| wellthereitis wrote:
| Great tips, thank you!
| xenocyon wrote:
| Electrical engineer here. _Thermal_ noise is the same for all
| resistors with a given R, regardless of their method of
| manufacture. You cannot have thermal noise either less or more
| than this, so it 's not a Johnson "limit" but rather a definite
| value. You are correct that there are other sources of noise
| such as 1/f noise. But more importantly, the manner in which
| noise manifests in the end result has to do with the circuit as
| a whole.
|
| For noise critical applications you should do a noise analysis
| of the circuit as a whole rather than make ad hoc selections of
| components.
| jbay808 wrote:
| 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 circuit,
| they might not have any idea that they can potentially cut
| the noise 1000x by changing the resistor from thick film to
| thin film, with no other design changes, at the cost of one
| cent. It's not common knowledge, as illustrated by the fact
| that an article like this, dedicated to the topic of resistor
| noise, doesn't even mention it. And instead laughs at someone
| even considering to look for a "good" resistor as though it
| were superstition.
|
| In fact, thick film resistors are far more common, so if
| you're in the situation where you're reading this article
| because you have a noisy resistor in your circuit and don't
| already know about Johnson noise, you almost _definitely_ don
| 't know about current/flicker noise, and since you got here
| because of a noisy resistor in the first place, a "good" thin
| film resistor is overwhelmingly likely to be the cure.
|
| I'm not advocating ad hoc selection of components to reduce
| noise any more than selecting ad hoc components to reduce
| cost. A noise analysis can help you find the problem but
| won't help you solve it if you think your only option is to
| change the resistor's value, rather than its type.
| ldarby wrote:
| 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"
| robocat wrote:
| Your comment looks like it disagrees, but actually you are
| both agreeing that thermal noise is not the only noise source
| and that other design factors matter.
|
| For other readers: some resistors have much lower flicker
| noise[1] where "wire-wound resistors have the least amount of
| flicker noise. Since flicker noise is related to the level of
| DC, if the current is kept low, thermal noise will be the
| predominant effect in the resistor, and the type of resistor
| used may not affect noise levels, depending on the frequency
| window."
|
| https://en.wikipedia.org/wiki/Flicker_noise
| colin_mccabe wrote:
| Thanks for the comment... good to have some EEs drop in.
| analog31 wrote:
| 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.
| Unklejoe wrote:
| I was watching an interview with Tom Christiansen (he owns
| Neurochrome, a company that makes very high-end DIY amplifier
| designs/kits, with THDs of 0.0001%).
|
| He mentioned something about how resistor noise can actually
| track with the low frequency portions of the audio signal due to
| the resistor literally heating up and cooling down as the current
| through it varies. I thought that was interesting. I knew that
| noise was proportional to heat, but I didn't realize the
| temperature could vary that quickly, but I guess it makes sense
| when you're dealing with tiny parts. There are probably also
| localized hot spots that have less thermal mass than the entire
| resistor as a whole.
|
| The interview is posted in this thread:
| https://www.audiosciencereview.com/forum/index.php?threads/t...
| FriendWithMoon wrote:
| Similar to what having permanent tinnitus sounds like. Take care
| of your ears!
| pontifier wrote:
| Excellent article! I feel like I understand so much more about
| analog signals than I did before.
|
| It seems very obvious now, that if you want to have a high signal
| to noise ratio, you should get as much signal as possible, and
| keep your signal voltage as high as possible as long as possible
| before amplification.
|
| This fundamental resistor noise is something that I'm probably
| going to start seeing everywhere when I look at any analog
| signals, and will have to take into account when designing
| things.
| fallingfrog wrote:
| 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 if I put a 10k
| resistor there instead. It's really important to get that first
| gain stage really, really quiet, a discrete jfet has a better
| noise floor than an op amp or a regular transistor.
| klodolph wrote:
| > ...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.
| guenthert wrote:
| 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).
| klodolph wrote:
| 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.
| squarefoot wrote:
| Low noise JFETS are hard to find today. Devices like the
| Toshiba 2SK369, 2SK117, 2SK170 etc. went out of
| production years ago for being mostly THT and possibly
| non ROHS devices, while purely functionally speaking they
| would still be great parts. They can be seen online on
| Ebay, Aliexpress, etc. but those are almost always
| relabeled fakes; real ones are hard to find and not
| cheap. Before learning about counterfeit components the
| hard way, I bought two bags of 2SK117 from two vendors on
| Ebay: they were identical, but when fit in a circuit, I
| had to recalculate the drain and source resistor with all
| the parts from one vendor because it clearly had
| different specs than a real 2SK117. Ugh! ...lesson
| learned. Somebody suggested the use of the BF256 JFET in
| audio circuits, even in RIAA phono cartridge preamps. It
| is a RF part (the natural successor and direct equivalent
| of the venerable BF245) but it seems it is also a low
| noise part capable of working down to audio with no
| issues. It went out of production more recently than
| other models and some vendors still have stocks
| available. Probably not as quiet as the others though.
| fallingfrog wrote:
| I bought 4000 J201's when I saw they were becoming
| obsolete, I still have most of them. But, I think there
| are better choices out there anyhow, that's just what I'm
| used to using. They're really easy to bias.
| adammunich wrote:
| What is your favorite?
| klodolph wrote:
| I don't really have a favorite op amp. There are so many
| good ones. Some audiophiles like to swap out op amps and
| talk about how good different ones sound, but I think
| that's a bunch of hogwash. If you believe that choosing the
| right op amp for an audio project has an impact on the
| sound, I'm the wrong person to ask about it.
|
| There are some exceptions, like if you need a microphone
| preamp with 60 or 80 dB gain, but I have no experience in
| that area.
| [deleted]
| ChuckMcM wrote:
| I read it more like a whine "don't blame our opamps, they better
| than resistors" :-) But it is something I've become much more
| familiar with building RF frontends for software defined radios.
| You want as much gain as you can get to pull in weak signals but
| keeping the whole thing noise quiet is really really hard.
| peter_d_sherman wrote:
| 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...
| DiabloD3 wrote:
| You overcomplicated it, and somehow badly described LVDS at the
| same time.
| h2odragon wrote:
| Like the maths of trying to urinate silently on a tin roof.
| lisper wrote:
| 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.
| analog31 wrote:
| >>> I figured there was some hardware design trick that they
| hadn't taught me in my EE curriculum
|
| https://patents.google.com/patent/US4744105A
| [deleted]
| zihotki wrote:
| The article, actually a single Q&A, is dated Sep 2007.
| jeffbee wrote:
| 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.
| JamesBryant wrote:
| 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.
| OnlyOneCannolo wrote:
| 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.
| zihotki wrote:
| And that's only the AD, if we add old articles for the last
| century from other sources, the HN will be flooded over. But
| would be they news?
| segfaultbuserr wrote:
| > _if we add old articles for the last century from other
| sources, the HN will be flooded over._
|
| HN has been doing it for... I think a decade already? The
| situation so far looks good to me.
|
| * "Corn-Pone Opinions" by Mark Twain (1901)
|
| https://news.ycombinator.com/item?id=10822133
|
| 127 points, 56 comments
|
| * Five Hundred and Seven Mechanical Movements (1908)
|
| https://news.ycombinator.com/item?id=19968114
|
| 798 points, 147 comments
|
| * G.K. Chesterton: The fallacy of success (1909)
|
| https://news.ycombinator.com/item?id=6512288
|
| 198 points, 97 comments
|
| * The Basic Problem of Democracy (1919)
|
| https://news.ycombinator.com/item?id=21893486
|
| 206 points, 189 comments
|
| * Why I Quit Being So Accommodating (1922)
|
| https://news.ycombinator.com/item?id=14418877
|
| 866 points, 316 comments
|
| * Why I Never Hire Brilliant Men (1924)
|
| https://news.ycombinator.com/item?id=7437643
|
| 387 points, 264 comments
|
| * War Is a Racket (1933)
|
| https://news.ycombinator.com/item?id=22012255
|
| 336 points, 181 comments
|
| * A Mathematical Theory of Communication (1948) [pdf]
|
| https://news.ycombinator.com/item?id=23035107
|
| 197 points, 39 comments
|
| * Claude Shannon Demonstrates Machine Learning (1952)
|
| https://news.ycombinator.com/item?id=25919006
|
| 209 points, 51 comments
|
| * Three-dimensional model of electricity consumption in
| Manchester (1954)
|
| https://news.ycombinator.com/item?id=25959571
|
| 135 points, 31 comments
|
| * Kurt Godel's Letter to John von Neumann (1956) [pdf]
|
| https://news.ycombinator.com/item?id=19281633
|
| 183 points, 85 comments
|
| * An Algebraic Language for the Manipulation of Symbolic
| Expressions (1958) [pdf]
|
| https://news.ycombinator.com/item?id=14885779
|
| 171 points, 12 comments
|
| * America's Cult of Ignorance (1980) [pdf]
|
| https://news.ycombinator.com/item?id=14778856
|
| 169 points, 113 comments
|
| * How does a gas pump know to shut itself off? (1981)
|
| https://news.ycombinator.com/item?id=24741029
|
| 384 points, 390 comments
|
| * Will cable TV be invaded by commercials? (1981)
|
| https://news.ycombinator.com/item?id=22977448
|
| 288 points, 354 comments
|
| * Have You Ever Tried to Sell a Diamond? (1982)
|
| https://news.ycombinator.com/item?id=4535611
|
| 273 points, 229 comments
| Causality1 wrote:
| To the best of my knowledge time-independent articles don't
| need a date stamp. I never see one on Wikipedia articles even
| if the last edit was years ago.
| zihotki wrote:
| My comment was only in regards with adding the date to the
| headline.
| IgorPartola wrote:
| I am old enough to remember when naked Wikipedia articles
| were discouraged (or maybe even disallowed) on HN.
| xbar wrote:
| Thanks. I will be sure to pay my respects at Boltzmann's grave
| the next time I am in Vienan.
| sunjain wrote:
| It is surreal that this article showed up now. I am going back
| and forth currently with our electric utility company. They
| upgraded capacitor pack on the pole by my house. And these new
| ones are generating so much of this kind of low buzz sound, it is
| unbearable. I was researching into what generates this noise, and
| found this article very timely. Only solution is to move these,
| as nothing else can be done about this sound. Which is turning
| into quite a project.
| exmadscientist wrote:
| It is quite rare for capacitor banks to hum. Much more likely
| that you're hearing the magnetics (transformers), which indeed
| have reason to be where they are. Good luck.
| sunjain wrote:
| Thanks. Indeed those are transformers, upon further
| digging(for some reason, I think of transformers as big,
| these are not big but boy they emit this low-buzz sound). And
| indeed it has been very challenging to have them moved,
| because as you said they probably picked this location based
| on a reason.
| mattkrause wrote:
| The last line got a good chuckle out of me!
| ISL wrote:
| 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 1.380649x10^{-23} J/K, exactly.
| JamesBryant wrote:
| 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.
| StavrosK wrote:
| 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!
| thaumasiotes wrote:
| > k_B is now defined as a part of the redefinition of the SI
| unit system in 2018.
|
| > The value is 1.380649x10^{-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
| measurable quantity all along -- the only way to demonstrate
| a conflict in the definitions would be by measuring the
| quantity more accurately.
| minitoar wrote:
| Not in SI. Kelvin is defined in terms of k_B. Joule is
| defined in several ways, eg force & distance.
| fsh wrote:
| 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).
| carapace wrote:
| Not really related (I think) but I was reading about "an
| electrical resistor that has no residual mutual- or self-
| inductance at high frequencies."
|
| The trick is to twist it into a Mobius strip!
|
| http://www.rexresearch.com/davis/davis.htm
| Footkerchief wrote:
| The underlying principle:
| https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise
| klodolph wrote:
| 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 next source of noise will be a
| low-level white noise (sounds like a hiss), which is from the
| amplifier, and consists of a mixture of Johnson noise and shot
| noise.
|
| In older amps you may hear a louder hiss/crackle which is from
| old carbon comp resistors, which is an inferior type of resistor
| that produces additional noise through a different mechanism.
|
| If you're trying to record your guitar directly through a digital
| interface, you may run into clipping issues and have to enable
| the pad (a built-in attenuator). Unfortunately, my experience is
| that the pad often introduces an unacceptable amount of noise,
| and I believe that it's just plain Johnson noise from a resistive
| divider.
| bcaa7f3a8bbc wrote:
| 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
| significantly degrades signal-to-noise ratio, but adding the
| same noise after the first stage is often acceptable since the
| signal is much stronger now. To reduce noise, you move the
| noise-generating resistor away in an audio amp, or
| cryogenically cool the resistor in a radio telescope front-end.
|
| > One of the big claims for many audiophile op amps is lower
| noise. The chip manufactures make a big deal about it and
| audiophiles, not surprisingly, have jumped on the bandwagon.
| But, in reality, it's often the Johnson Noise that limits the
| noise performance of a headphone amp, not the op amps. Johnson
| Noise is, literally, self generated noise that's present in any
| resistor. The larger the resistor value, the more noise you
| get. Many DIY headphone amp designs have the volume control at
| the input to the gain stage. And it's, at the lowest, usually
| 10,000 ohms. By comparison the O2 has 274 ohms in series with
| the input. That's a huge difference in Johnson Noise. The way
| volume controls work, the noise is typically worst at half
| volume where you have 5000 ohms in series with the source and
| 5000 ohms to ground. So, at typical volume settings, you get a
| fair amount of Johnson Noise from the volume control that's
| amplified by whatever gain your amp has. That noise typically
| exceeds the op amp's internal noise. If you put the volume
| control after the gain stage its Johnson Noise is no longer
| amplified. And, as a bonus, the volume control at lower
| settings now attenuates noise from the gain stage. For more,
| see O2 Circuit Description and Circuit Design.
|
| > To put these numbers in perspective, referenced to the old
| 400 mV they're -105.3 dBr and -108.2 dBr. On the exact same
| test, at half volume, the Mini3 had nearly 11 dB more noise and
| measured -94.5 and -97.5 dB. Noise of -113 dB below 1 volt is
| under 3 microvolts.
|
| [0] https://spectrum.ieee.org/tech-history/silicon-
| revolution/nw...
|
| [1] https://nwavguy.blogspot.com/2011/07/o2-headphone-amp.html
| pvitz wrote:
| I think I remember seeing this in "The Art of Electronics"
| for actually amplifying shot noise for a hardware random
| number generator.
| Gibbon1 wrote:
| > the system noise is dominated by the first amplifier stage.
|
| In radio receiver design you have an LNA, low noise
| amplifier, as the first stage. It's designed for low noise
| and to be linear. Idea is take the energy from the antenna
| and amplify it with as little noise and cross modulation as
| possible[1].
|
| [1] If the amp is non linear you end up folding out of band
| signals into your band of interest.
| MegaDeKay wrote:
| The former bit is illustrated by the Friis formula.
|
| https://en.wikipedia.org/wiki/Friis_formulas_for_noise
| sobriquet9 wrote:
| It's relatively easy to make a headphone amp quiet because
| its input is typically already quite strong.
|
| Noise is more of a problem for microphone preamps, guitar
| pickups, etc., where the input signal is weak.
| kazinator wrote:
| Bingo. In any piece of music gear that is after the
| guitar/mic preamp, where it is working with line level
| signal (a higher voltage than consumer audio line level, by
| the way), it hardly matters where you put the volume knob.
| If you design it halfway well, it will be quiet as a mouse.
| wishinghand wrote:
| That's really interesting. Is that a common design in
| headphone and other consumer amps?
| DiabloD3 wrote:
| 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.
| klodolph wrote:
| It was also popular in the 80s, 70s, 60s, and yes, the
| 50s. Everything old is new. The real question is, "Why
| did people switch to single stage amps in the 1990s and
| 2000s?" The answer is that a bunch of chips appeared on
| the market around that time which could do everything.
| isatty wrote:
| 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.
| floxy wrote:
| >I believe that it's just plain Johnson noise from a resistive
| divider.
|
| Any reason for guitar pads couldn't use a capacitive divider in
| place of a resistive divider? (I have no idea what a guitar pad
| is)
| Zenst wrote:
| 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.
| klodolph wrote:
| One common solution for piezo pickups / contact mics is to
| put an amplifier or buffer near the pickups, powered by a 9V
| battery or 48V phantom power.
|
| The piezo pickup is not balanced but it's not necessary, you
| can make the output of the amplifier balanced. This is the
| same way that condenser mics work. The microphone capsule
| itself is not balanced, but it doesn't need to be... the
| output of the amplifier or buffer is balanced and that's all
| you need.
| michael1999 wrote:
| Wait. We have ADCs that produce floats now? That's amazing.
| ansgri wrote:
| It seems we do not, not quite. A quick googling got quite a
| few research papers and prototypes, but nothing mass-
| produced.
| Zenst wrote:
| 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.
| kazinator wrote:
| 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 circuits are
| amplifying common mode. The op-amps are operated in feedback
| meaning that the - and + inputs are at nearly the same voltage.
| However, the incoming common mode noise moves that entire
| voltage; and it's possible for the common movement of +/- to
| itself be amplified.
|
| So that is to say, suppose you have this representative single-
| ended stage: |\ in
| --)|---+------- |+\ | | > ---+--- out
| < ,- |-/ | > | |/| |
| | | GND | GND `----------'
|
| The guitar cable's shield is connected to GND. Now suppose that
| GND is oscillating at 60 Hz due to the cable shield picking up
| EMI. (The cable shield is a big area of copper bathed in noisy
| electric fields, with nothing shielding it, and is galvanically
| connected to your amp!)
|
| This means that the (+) node of the OP amp is seeing this
| fluctutation, and due to the feedback the (-) node is
| following.
|
| An ideal op-amp will not amplify any voltage offset that
| equally affects (+) and (-). But real op-amps do. The degree to
| which they do not is the CMRR (common mode rejection ratio)
| which is a data sheet parameter that is better in some parts
| than others.
| bcaa7f3a8bbc wrote:
| 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 kO to 50 kO at 60 Hz. Widely
| used SSM-2141 IC loses 25 dB of CMRR with a source imbalance
| of only 1 O. Typical transformer input common-mode impedances
| are about 50 MO @ 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...
| dekhn wrote:
| Yet another unexpected sighting of Boltzmann's tomb.
| OnlyOneCannolo wrote:
| They have a cool PDF cheat-sheet related to this which doesn't
| seem to be linked from the page.
|
| https://www.analog.com/media/en/selection-guides/Equation_Pu...
| cryptonector wrote:
| > And, of course, we can't change Boltzmann's Constant because
| Professor Boltzmann is dead[3].
|
| Worth the read just for that punch line.
| pfdietz wrote:
| 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."
| Applejinx wrote:
| "...for as long as we can"? D:
| Workaccount2 wrote:
| Ehrenfest didn't just die similarly, he took his (disabled)
| son with him!
| matthewdgreen wrote:
| https://en.wikipedia.org/wiki/Breeds_There_a_Man...%3F
| mgleason_3 wrote:
| The pinnacle on a wonderful respite from the mire of politics
| and opinions that often invades hacker news.
| jeffrallen wrote:
| If you did not read to the last sentence, go back an do so. It's
| worth it.
| kazinator wrote:
| Should say "Now that he's long dead, we could easily get away
| with changing Professor Boltzmann's constant, but it would be
| disrespectful".
| bcaa7f3a8bbc wrote:
| 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/series10/Reid_DESCAN...
| exmadscientist wrote:
| I haven't crunched the numbers, but I seem to remember that
| LIGO has the RF guys beat in terms of requirements.
|
| Fortunately for the rest of us, that meant they had to make
| some measurements, which they were kind enough to share:
| https://dcc.ligo.org/LIGO-T0900200/public
| tedd4u wrote:
| Hew boy, don't click through to that site unless you have a
| few hours to check out all the custom LIGO part drawings ...
|
| Thanks for the link.
| madengr wrote:
| 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 is 50
| Ohms, it can generate far less noise than the equivalent
| resistor.
| p_j_w wrote:
| >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!
| hughrr wrote:
| It's not which is why I now write software :)
| jhallenworld wrote:
| The reverse problem is interesting. Consider Voyager 1- its
| high gain antenna is pointed essentially directly at the sun, a
| powerful wide-band noise source. How does it detect anything
| from earth? The DSN has to outshine the sun within the tiny
| S-band window that Voyager listens to: 20 kW and 62 dB antenna
| gain.
|
| https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_e...
|
| https://ipnpr.jpl.nasa.gov/progress_report/42-175/175E.pdf
| [deleted]
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