11–20 of 110 posts
The article, actually a single Q&A, is dated Sep 2007.
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…
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.
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.
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.
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.…
> 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
The last line got a good chuckle out of me!
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.
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 last line got a good chuckle out of me!
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.
> 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.
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 i…
This used to be true, but you can get really good low-noise op amps these days.
The article, actually a single Q&A, is dated Sep 2007.
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.
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.