Need Explanations

[Tony Edwards]

I have been playing steel guitar for over 35 years now. But there are some terms that many forum members use that have never been explained to me. Would someone please explain to me the meaning of these two terms pertaining to steel guitar and amplifiers:
1. What is "Cabinet Drop" on a pedal steel guitar?
2. What is "Head Room" on an amplifier?

Thank you for explaining these terms to me. Maybe this will help me to improve myself when it comes to playing steel.

Tony

[Richard Sinkler]

Cabinet drop is when you push a pedal and an unaffected string goes slightly flat.

I know what headroom is, but someone else will explain it better.

[Ken Morgan]

Headroom is how much power is available for more than a split second, before an obviously unpleasant distortion becomes apparent.

Example: a 4 cylinder car and a turbo v8 can both travel at 70 MPH. Jump on the accelerator, and the 4 cylinder probably won’t speed up to 85, whereas the turbo v8 will get to 85 in the blink of an eye.

[Bill Hatcher]

In the amp world a small powered amp will break up and distort at a lower volume…..it has very little headroom. A powerful amp will play much louder before you reach distortion.

[Ian Rae]

Much has been written about cabinet drop. There are many factors that cause some strings to go flat when others are raised and the body of the guitar is the easiest suspect to blame.

The fact is that the total system of components that holds the ends of all the strings apart is sensitive to individual changes in tension. What matters is whether the effect is noticeable in practice.

[Richard Sinkler]

The fact is that the total system of components that holds the ends of all the strings apart is sensitive to individual changes in tension. What matters is whether the effect is noticeable in practice.

Only 1 of the 8 guitars I have owned had audible cabinet drop. It was a new (2018) Mullen Royal Precision SD12. When pressing the A pedal, you could actually hear the 6th string detuning. Others I have owned, you could see a slight drop on a good tuner, but couldn't hear it.

[Jerry Overstreet]

I like to think of headroom as reserve power. Insurance against sudden spikes, transients and big power demands such as big C6 grips and rakes or big 6,8,10 triads on the E9 with lots of volume.

These types of attacks on the amp will let you know quick if you're short of headroom with unwanted distortion and whimpering as stated above.

I don't know if speakers have such a thing as headroom, but they also need to be very efficient and be capable of withstanding those same demands. Generally, you want a speaker with higher power handling rating than your amp's output. FI, NV400 rated at 210 RMS W and the Black Widow capable of handling 350W.

[Dave Stagner]

On the subject of headroom, there's apparent headroom as well as real headroom. This gets into just how an amp distorts under load. Second harmonic distortion is just generating a harmonic an octave above the actual note, and our ears hear that as an increase in apparent loudness and brightness. Third harmonic distortion is an octave + fifth above, so it sounds like musical complexity. Fourth harmonic is two octaves, so more brightness. Fifth harmonic brings a natural third in (which is between minor and major), so it can sound rather dissonant. So an amp that produces mostly low-order, even harmonics just sounds bigger and brighter as it starts to distort. This is particularly true of loud "clean" tube amps like blackface/silverface Fenders. Apparent headroom!

Harmonic distortion isn't the only kind of distortion, though. There's intermodulation distortion, hard clipping, and speaker distortion working its way back into the amp circuit through the global negative feedback loop.

Speaker distortion in particular is nasty stuff. Think about what a speaker is - a coil of wire suspended in a magnetic field. When current is applied to the coil, it becomes an electromagnet, and moves relative to the fixed magnet. This is an electric motor. And it's connected to the cone and suspension (and frame, and cabinet), which is basically a spring. When you put mechanical energy into a spring, it returns to its original position when released - eventually. But it overshoots and oscillates, which also moves the coil. And what is a coil of wire moving in a magnetic field? A generator! So our speaker is responding to signal from the amp, but it's also feeding signal back into the amp - a delayed, distorted funhouse-mirror version of what went into it. And most amps have a "global negative feedback loop" that feeds an inverted copy of the output signal back into the input of the amp. This reduces harmonic distortion. (This is also where the presence control sits, changing the frequency balance of the global feedback.) But that negative feedback loop, connected to the speaker outputs, is picking up this weird motor-distortion from the speakers and feeding it back into the amp. It's not much, but it's there - and it's REALLY distorted, in ugly, nonmusical ways. I think this is why tube power amps sound better than most solid-state! The transformer output of a tube amp handles the back EMF (that's what it's called) from the speaker better than the transistor output of solid-state amp. And of course, this leads to apparent headroom! A solid-state amp might be audibly reflecting back EMF long before it runs out of power, while a tube amp may just go into harmonic distortion as it runs out of power, while the back EMF is buried.

Note that I may be wrong; I am often wrong. I am not an electrical engineer, just some guy who used to build DIY tube amps, with enough knowledge to be dangerous. This is an exotic theory of "amp tone", not at all mainstream. It has some proponents in DIY tube hi-fi circles. (Those same proponents argue for simple speakers with simple crossovers, to reduce distortion.) In the commercial world, it's not good to talk about, because it can't be easily measured. Much easier to talk about THD, which is measured by feeding a sine wave into a resistive load - a signal wildly different from the 10 octaves and million-to-one dynamic range of real musical signals, and a load that completely obliterates the EMF and impedance variability of actual speakers. It's like measuring how well a car can drive exactly 60mph in a straight line and thinking this describes its performance for acceleration, braking, and curves. But I digress...

At any rate, when we talk about "headroom", I think we human musicians are talking about the apparent headroom of an amp, not the "real" headroom of "will this run out of power?" The same thing that makes some amps sound "warm" and "musical" makes them sound "loud", and other amps are "cold" and "harsh" even when they have plenty of on-paper headroom.

[Tony Edwards]

A great big THANK YOU to everyone who posted a reply to my questions. Now I can understand more clearly the meanings of these phrases. Thanks again!

Tony