[klodolph]

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]

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

[sobriquet9]

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]

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.

[pvitz]

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

[Gibbon1]

> 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]

The former bit is illustrated by the Friis formula.

https://en.wikipedia.org/wiki/Friis_formulas_for_noise

[p1necone]

Has anyone worked out what happened to NwAvGuy yet? Afaik he never posted anything about taking a break or going away for a while. Given his pseudonym was totally anonymous I can only speculate - he could be dead or in prison or something...

[wishinghand]

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

[DiabloD3]

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]

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]

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.

[Zenst]

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]

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]

Wait. We have ADCs that produce floats now? That’s amazing.

[Zenst]

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.

[KeytarHero]

Not exactly - Zoom stuff does a clever trick of using 2 ADCs at different gain levels (essentially the same way that HDR photography works), but ultimately it's still using fixed-point ADCs.

This isn't actually a new idea - I know Line 6 has been using it in guitar pedals since at least 2006. Of course, ADCs have gotten better since then - I doubt they were getting even 24 ENOB back then.

[Zenst]

Ah the F6 has dual ADC and it uses that to cover a larger recording range - best explained here https://www.youtube.com/watch?v=NTuJ1fk3PsY

Also nicely explained by Sound Devices here for their offerings: https://www.sounddevices.com/how-is-a-32-bit-float-file-reco...

Still pondering the F2, does it have dual ADC to capture that input range or is this why they sell it with a lavier mic included (hmmm).

[ansgri]

It seems we do not, not quite. A quick googling got quite a few research papers and prototypes, but nothing mass-produced.

[kazinator]

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]

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 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...

[YZF]

IIRC that's where you'd use an instrumentation amplifier.

[fuzzfactor]

Not only high gain but high power also.

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.

[Renaud]

While not exactly for guitar, Phil’s Lab on youtube recently posted a low noise headphone amplifier design that's both over-kill in some ways and interesting and cheap to make.

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

[floxy]

>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)

[msds]

Wouldn't the input impedance be annoyingly variable?