What is headroom?

[smoguzbenjamin]

I keep hearing people say 'headroom' whenever talking about powering op-amps and effects... What is this headroom? Should I interpret this as a sortof high-cieling incase something decides to jump about or whatnot? Or am I merely confuzzling myself?  :?

[Mike Burgundy]

Well, actually that's not a bad metaphor at all.
See your "room" for electrical signals (ie music) like this: way on the bottom (lowest volume) is the so-called noise-floor. It is really called that. Anything this low will literally drown in the fuzzy carpet of background noise. Thats the lower threshold of signal intensity, it's "zero", ground level.
The ceiling limits how high you can go. anything that tries to go higher bangs it's head and gets stopped dead, until it's time to go back down again. Same applies for a signal although without the headaches. An opamp tries to churn out signal that's as high or higher than it's powersupply, cannot go further, boom. The signal get's stopped on the max level until it can get down again. This is clipping - distortion!
Headroom is the amount of play you have without banging your head on anything - from no appriciable signal to *just* before the onset of clipping.

Edit:
Zo leer je nog eens wat. Groeten uit Delft

[Mark Hammer]

Of course, you're both right.  I will add, however, that headroom is always defined relative to the input signal.

Case in point.  At the dawn of the vintage amp craze, one writer at the time pointed out that in the 50's playing styles and pickup output levels presented a much more modest signal to amplifiers.  As a result, he wrote, preamps tended to be designed for more gain because there was enough headroom to do so.  Of course, when you fed in the hottest humbucker Larry DiMarzio could offer and hit a Pete Townsend style power chord, that headroom all but disappeared and the amp went into delightful clipping.  That, and the desire to have amps that went up to 12 (or 11 if you're Nigel Tufnel) sent players scurrying around looking for older amps.

In many (though certainly not all) instances, contemporary amps have plenty of headroom and can tolerate much higher input levels than they normally see.  In the case of keyboard amps, of course, they are supposed to be able to cope with not only tinkling two upper keys but also slamming down on the keyboard with both forearms, which is a pretty big dynamic range.  As a result they are generally designed with much more headroom than guitar amps.

The same power supply and gain structure can have gobs of headroom if one is simply amplifying a 10mv voice mic signal, but considerably less headroom if handling a line-level 1v signal.  In the one case, you have the freedom to amplify the input signal many many times before running out of headroom, where in the other case, even a small amount of amplification may use up the available headroom.

The other thing to consider is voltage swing.  Not every semiconductor can provide a swing from one power supply limit to the other (e.g., a 9v battery supply would permit a maximum vooltage swing of 9v and a +/-15v supply would permit a maximum voltage swing of 30v).  In some cases they can, but in many instances they can't.  Some op-amps can only swing to within a volt or two of the supply "rails".  So, if a given op-amp could only swing to within 2v of the supply rails, that would mean that a 9v supply would permit the device to swing from 2v to 7v, or 5v in total.  If the typical input signal is 500mv, then the most you could expect the device to do would be to amplify by a factor of 10, and even then you'd be taking a big risk if you were striving for clean tone.

In some instances, then, one can improve headroom with the same supply by changing semiconductors that permit more voltage swing.