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

> 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

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.

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.

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.

https://www.youtube.com/watch?v=Z2GUoi63pJs

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)

For noise critical applications you should do a noise analysis of the circuit as a whole rather than make ad hoc selections of components.

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.

https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_e...

https://ipnpr.jpl.nasa.gov/progress_report/42-175/175E.pdf

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

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.

"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."

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.

https://patents.google.com/patent/US4744105A

This used to be true, but you can get really good low-noise op amps these days.

The value is 1.380649×10^{−23} J/K, exactly.