How much difference do "low noise" op amps really make?

[ayayay!]

I had a job at one time where I designed analog filtering in the audio frequency range for military infrared detectors mounted on a rotating platform and used to spot aircraft.  Achieving a low NETD (noise equivalent temperature difference) was the main design parameter and we took no shortcuts.  It got to the point where an aircraft could come toward the sensor and shut off its engine and the thermal imaging would still spot it due to the gray-body frictional heating of the fuselage and wings.  Great fun back in the '70's.

And that is why guitars and pedals win!  To have a career like that and still work on little snippets like pedals,... that says something amptramp.  

[R.G.]

You know, it's almost like noise, it's sources and eradication, is a complex subject. Maybe enough complexity there to write a textbook.

Oh, wait! There are textbooks on this!  I like Henry Ott's "Noise Reduction Techniques in Electronic Systems", a copy of which I keep handy... (he says, fondling his hardcover edition). There are copies of this available at Amazon for as little as $2.20 plus shipping if you can stand to read a used copy.

The basics of noise are that anything that is resistive or active and has a temperature above absolute zero generates noise. In addition to their thermal noise, they can generate a few other kinds of noise, depending on their internal structures. Reactances (inductance and capacitance) do not generate noise, being merely storage places for energy. The resistive imperfections in real inductors and caps do generate noise. There is current noise and voltage noise. That implies, correctly, that there may be an optimal impedance for minimizing the total noise power in a setup depending on the nature of the source impedance that generates the signal you're dealing with. And since noise power is related to the noise bandwidth, restricting the bandwidth amplified will restrict the noise amplified, too.

Aside from the various explanations, equations, and delving into that "internal structures" and "few other kinds of noise", that's it. Of course, that "aside from" thing makes up not only the rest of the book, but a whole bookshelf on its own, so there's always more to learn. Which is good. 

...  The question to ask yourself is how much bandwidth (particularly at the upper end) does the pedal truly need?  I find that many of the favourites here can retain more bandwidth than is necessary to do the job.

I hate to keep harping on it, but the Distortion+ and DOD 250 are excellent textbook examples of this.  The 10k series resistor and .001uf cap to ground provide a lowpass filter function (and only a 6db/octave one at that) that starts rolling off high frequencies at just under 16khz.  If it was a home stereo system and I listened to symphonies or other acoustic music only, and I had both my hearing and top-quality tweeters in my speaker system, then I'd be concerned about keeping all that top end, but who the hell is all that HF content for when it comes to distortion boxes going into speakers that roll off starting around 6khz?  Why NOT insert some caps here and there to constrain bandwidth, and in so doing reduce a lot of audible noise?

Oh, sure, Mark. Do the reasonable, thoughtful, correct thing! 

Interesting discussion. What are good bandwidth narrowing values for such a noise reducing mod that still preserves the desired tonal characteristics of the Dist+ and 250 still using a 741 op amp?

In general, leave out every possible bit of input that doesn't contribute to the sound you want. I know that's not component values, but a little tinkering and listening will give up what sounds good to you. The Technology of the Tube Screamer points out some places to tinker with bandwidth on an opamp stage, and there are many other references.

...Fender Yngwie Malmsteen model and it is very quiet 'compared' to my Fender Standard Strat because of the Dimarzio stacked noise reducing single coil pickups in the YJM.

No pickup can be noise reducing, at least in the sense of eliminating noise it generates inside itself. The best it can do is to reject external pickup of unwanted stuff, for which "humbucking" is a more accurate description. You're stuck forever with the noise the resistance part of the pickup generates. All you can do is twiddle with the amplifier which follows it to not make that any worse. Notice that the high impedance forced by the inductance part of the pickup source impedance makes you use a high impedance  amplifier, which is worse for noise than you'd want if you had only the 4K-20K resistance of the pickup wire to deal with. So it's not optimal for noise by its very nature.

I've been curious about why, prevailingly, there's an apparent distaste for "noisy" passives, like ceramic caps, but an apparent loyalty to "noisy" chips like the 741. This thread has shed some light on that; at least regarding potential noise sources in front of high gain amplification. Cool!

The pure capacitance part of a ceramic cap can't generate noise. The issue with ceramics is a certain gritty distortion some people claim to hear. Frankly, it's easy to demonstrate piezoelectric sound pickup on some sounds, and I wonder about that as an origin of the "ceramic = noisy" myth.

The obvious way to fix that would be to use a FET opamp buffer (or just a single FET source follower I suppose) before the 741 distortion stage. Eventually intend to build a discrete fuzz  effect that is similarly buffered, to avoid the annoying "sounds great with this guitar at these settings, but not with that guitar with those settings" effect.

Yep. Both personally and professionally I've been doing that for a few decades. Works if done right.

[familyortiz]

I'm guessing that Hammer's point 3.b) with the feedback cap is going to hammer on the hiss (pun intended), given that it should roll off the high frequency gain.

[petemoore]

  Topic: How much difference do "low noise" op amps really make?  (Read 417 times) ?
  Low Noise or other opamps types make no difference whatsoever until measured.
   Often ears are found to be the preferred final measurement devices for noise and tone, because these devices vary greatly [as do guitar/cable/speaker etc.] comparing notes may lead to communication confusion.
  Fortunalely opamps swap from sockets quickly and treble control is easily made adjustable [adj.-treble knob is valued added D+ feature].
 

[HOTTUBES]

I swapped out all 4 op amps in my Boss GE7 to TLO72's and it made a huge difference in the Overall noise level ....

[Mark Hammer]

I'm guessing that Hammer's point 3.b) with the feedback cap is going to hammer on the hiss (pun intended), given that it should roll off the high frequency gain.

Correct.  You've got prevention, and you've got cure.  "Low noise" components can prevent noise from occurring, when the stars align, but the cap cures the accumulated hiss when it is otherwise unpreventable.

Incidentally, the TL06x series is hissier than the TL07x series.  They also draw less current.  So, you can decide whether you want to use less current, or whether you want less noise.  Some manufacturers will make the choice to opt for less current draw,for whatever reasons.

Note as well that there are many circumstances where whatever inherent noise properties the chip might have, replacing it is not going to improve circuit noise, because the chip is not in the audio path.  So, it is common for modulation effects to use either a TL022, an LM358, a TL062, or other similar chip, for the LFO.  These op-amps are not in the audio path so whatever hiss they produce goes unheard.  On the other hand, replace them with something that draws more current and there is a good chance you will hear them, or more particularly the LFO ticking than can result in.

On a related note, this is why some designs will use dual op-amps when it seems like the circuit could be more compact if a quad was used.  Use of a quad may mean that whatever you use for one part of the circuit may oblige you to use something with similar parameters for another part that actually "wants" a different sort of op-amp.  To connect to the above paragraph, the John Hollis Zombie is a perfect case in point.  It uses 4 op-amps overall.  The audio path would like a low noise chip, but the LFO would like a low current op-amp.  Using a quad for the whole thing may mean you have to forfeit something about performance.  So a pair of duals allows for the tailoring of op-amp characteristics to suit the circuit section, and can provide an improvement over use of a quad.

[WGTP]

Bad solder joints or parts?