Speaker Troubleshooting, Impedance, and Reactance
/Well, it was bound to happen eventually...
After loaning a small but oft used bass amp out, the thing finally had enough. Amidst the tragedy, however, is the glimmer of hope that something better will come of what's left behind.
So what is left behind?
Before salvage comes discovering what's still useful on the amp and more importantly finding out what scared off the magic smoke in the first place. Troubleshooting is what separates the engineers form the sound guys, at least partially. In troubleshooting, there's exactly one question to ask:
- Does it work the other way?
Provided you know what "the other way" is, troubleshooting is easy. Speed is another critical element when working in the wild but for now only worry about finding the problem. Since ther are only 3 major components to choo, it's easiest thing first. Eliminate as many possible failure modes in the shortest amount of time. For a stacked instrument amplifier, the list of sections in terms of diagnosis time are
- Cables & external connectors
- Speakers & hardware
- Amplifier electronics
Incidentally, this is also the order of cost to repair. New speaker cable is sold for $1.50/foot even at big name stores. A new transformer can be half the cost of a stack this size... assuming you can still buy them.
On more complex systems, troubleshooting becomes more of a bracketing game. Go roughly halfway up the signal chain and swap a pair of inputs. If the issue follows your switch, the problem is downstream, if it doesn't follow, the problem is upstream. Regroup and repeat until you find the trouble component.
Wires
First step for the Ampeg was swapping all the cables with a set I know works before even giving the amp an initial test. Since this didn't solve the noise problem or reduce it, I knew there was something else worth looking over. Even then I still checked my cable for signs of damage before returning it to service
Speakers
Once I eliminated wires, I moved to tackle the speaker cabinet. Use a 1/8" to 1/4" adapter to plug a phone into the speaker input. Once I confirmed the speaker cab was the culprit, I just used my ears to find the worse sounding speaker. One speaker sounded perfectly healthy, but the other one has clearly seen better days. A quick resistance check shows a petty major short in the coil winding. No need to check the electronics now, the mystery's solved.
A Word on Specs
Sadly, the specs on the speaker aren't readily available to the public. Lucky for me, there was another speaker to benchmark basic electrical properties like DC resistance. Notice this is not impedance. Standard ohmmeters use a DC charge to measure voltage drop and the calculate resistance, this will be lower than rated nominal impedance.
What Went Wrong Though?
Before writing everything off the important question must be answered- what caused the failure? If there's a defect in the amp it'll fry anything we stick on it. Simply replacing the bad speakers is throwing away money. Autopsy results show a pretty clearly charred speaker coil, much more use and a fire would've been the likely outcome.
As I mentioned in the video, key clue in this mystery was the fact that a guitar was hooked up to the speaker and the variable nature of impedance.
Z, X, and R
Most people are familiar with resistance, often abbreviated R. The more observant may even be familiar with impedance, Z. While often used interchangeably with the general public, impedance is actually a vector sum of resistance and a third component, reactance. This X factor is additional resistance to current flow, and the 3 variables are related via the equation
Sound familiar?
So clearly reactance is a big part of impedance, but what is it exactly? Well, reactance is a blocking force that prevents current flow. It's much like resistance, except it only occurs in AC circuits, it can be positive and negative, and it only appears in certain components. One such component is the inductor, which is essentially a coil of wire that stores energy in the form of a magnetic field. Each wrap in an inductor is insulated so current has to travel the full length of the coil to reach the other terminal.
Inductive components resist current changes in the form of reactance, which is calculated like so:
Where 'f' is the frequency of the signal going through the circuit and 'I' is the inductance of the coil (it's just a specification, don't worry too much about inductance.) You'll notice that at low frequencies, there is relatively low reactance, in fact at 0Hz (aka DC) there is no reactance, but as frequency increases so does reactance. When using a guitar with relatively high frequency output you get a significant reactance.
How does reactance manifest itself in an inductor? Voltage. The magnetic field surrounding an inductor (a voice coil) actually produces a counter voltage to resist the changing current. For this particular amp, the voltage was enough to case arcing between the windings of the coil. As soon as a low resistance path was available, current poured into the voice coil like never before. The tiny delicate wires couldn't handle the current flow and began to burn which then further shorted the coil. The result in this case was a blown speaker but if the poor speaker would've continued the cycle it very well could have caught fire. Speaker cones are pretty much paper.