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.
> 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
Noise is more of a problem for microphone preamps, guitar pickups, etc., where the input signal is weak.
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.
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.
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.
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 `----------'
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.
> 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...
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.
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.
For noise critical applications you should do a noise analysis of the circuit as a whole rather than make ad hoc selections of components.
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.
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.”
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.
[0] https://descanso.jpl.nasa.gov/monograph/series10/Reid_DESCAN...
https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_e...
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
Thanks for the link.
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.
You also get to trade off your impedances with gain and stability. RF design is fun!
Worth the read just for that punch line.
"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."
Just a couple hundred bucks a month for a decade, unlimited reading and real life trial and error, and infinite fun.
Give it a consideration
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.
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.
The trick is to twist it into a Möbius strip!
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.
This used to be true, but you can get really good low-noise op amps these days.
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...
Lots of great tricks on bootstrapped filters, capacitive multipliers, precision transistor noise measurements with some outboard circuits fed into a spectrum analyzer.
The value is 1.380649×10^{−23} J/K, exactly.
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.
> 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 measurable quantity all along -- the only way to demonstrate a conflict in the definitions would be by measuring the quantity more accurately.
https://www.analog.com/media/en/selection-guides/Equation_Pu...
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.
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?
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'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...