1987 - 2017: Thirtieth Anniversary!
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The Pitfalls of Hot Swapping
(A lot of this essay will seem so woefully obsolete in such a short time - I expect that soon, apart from some remaining AC power cords, it will be possible for an entire home entertainment system to be assembled without any wires. Unless you want high fidelity, of course. That will take a bit longer.)
One of the drawbacks to the ever-advancing world of computers, smartphones, and consumer electronics in general (besides the perception they create that anything that is two years old is obsolete) is the improvement in the user's ability to add and remove devices without turning anything off.
This is called "hot swapping", and it used to be anathema across the board for anything with wires.
USB was the first widespread, popular computer connection that, by definition, was hot-swappable, followed by the myriad forms of memory cards and their readers (there were others, like FireWire, but they never became such ubiquitous standards).
Before that, any hardware changes, from motherboards to printers, had to be done with all power switches off, and, ideally, all power cords unplugged. Errors in this procedure could easily turn an expensive computer into a doorstop.
Likewise, users were strongly instructed to disconnect the power from all the components of a stereo/home entertainment system before making or changing any connections or components. Errors in this procedure could likewise damage or destroy expensive equipment in a fraction of a second.
Fast forward to the modern era, and it seems like every electronic device we own is designed to have things connected or disconnected with no concern for what is turned on or off at all. In fact, it has become an encouraged and accepted "given" that it will happen. If I want to unplug my card reader and plug in my phone on the front of my computer, it takes less time to do than it just took to type.
So what's the big deal, you may ask? The big deal is that some low level and high level audio connections are highly sensitive to "live" connection and disconnection. I will be focusing on the amplifier/speaker relationship here, and more importantly, the influence of upstream activity on it.
The basic issue is that an audio amplifier that is turned on takes very tiny signals from its inputs and increases their voltage a hundredfold (or a hundred thousandfold or more for phono cartridge inputs), then backs this voltage up with enough current to melt twenty gauge wire. And there are places in the amplifier where the volume control ("attenuator") makes no difference to this. For instance, on my bridged Adcom GFA 555 amplifiers, any signal present at the input jack - at any time, even with the power off (!) - can result in a signal that can run a toaster at the speaker terminals (I haven't tried it, but one of these days...).
What this means is that great care must be taken when doing certain obvious, and a few less obvious, procedures.
I think most people realize it is "obvious" that when adding a component to a system, like a new tape deck, that all the power switches should be off. Ideally, speaker switches should also be off, and all level controls turned all the way down. Some learn this the hard or very hard way. Touching a "live" component input wire with everything turned on can result in a very loud noise coming from the speakers. That is hard on the ears, and hard on the speakers. If the gain stages involved happen to be sufficient and attenuation insufficient, it can be very hard on the speakers and/or amplifier, even to the point of starting a fire. Simply connecting the first "RCA" plug is fraught with danger - the center pin, the live signal connection, makes contact before the cup-shaped outer connection does. The cup is the ground connection, so when the first pin connects, it can act as a live antenna and dump unreferenced stray voltage into the amplifier input. (The second one usually isn't such a problem, since the first one has grounded the two pieces of equipment to each other.)
The point of this essay
What isn't as obvious are the perils introduced by the long-awaited merger between home entertainment systems and computers (and phones, tablets, refrigerators, garage door openers, etc.). Nowadays most control centers (receivers, preamplifiers, etc.) come with front panel connectors for our new toys - jacks for MP3 players, USB ports, etc. (they have often had a set of video inputs for a long time, of course). These provide access to low-level signal paths in the chain without any real warnings or hardware precautions (circuits to mute inputs during this process) as to what can happen to the unwary.
First, the common activity of hot swapping electronics has become so mundane we have been lulled into a false sense of safety. Second, some of these hot swappable connections are on our audio devices themselves.
What it comes down to is this - before plugging in or unplugging any "portable" device from a system - and this can include connecting via WiFi - make sure the main volume control is all the way down at the very least. Ideally, also turn off the power if possible, or at least turn off the speakers.
What compelled me to write this warning is an increasing number of customers who are having more frequent "innocent" accidents than in the past. In the bad old days, it took a real mistake to kill a pair of tweeters - like the three-year-old turning the volume to 11 when the stereo is off, or dropping a phonograph needle on a record, or messing with the preamplifier to power amplifier interconnects with the amplifiers turned on. Now all it requires is, for example, unplugging your phone from the front panel input, a procedure that needs no special planning with most of the other electronics you might encounter.
Another case to be careful of is turning on or off a computer that is connected as the source to an audio system - the shutting off and/or activation of the sound card can produce unplanned popping, also.
A related issue, that was part of the reason I wrote this, is the wildly different "recording level" of many audio sources on the internet. While quiet sources are, as I have explained a thousand times, the reason for the "second half" of the volume control, it is very easy to turn the volume up to accomodate, say, a very quiet video clip, and then play something that is at a much more "normal" level (this happens to me a lot when I switch back to the radio). This can result in playing a signal at a level far beyond the ability of the system to handle cleanly, and is a very fast way to damage speakers.
Things like this are not rare or occasional events, like installing a new component used to be - they happen multiple times a day to most of us.
Please be careful out there if you value the survival of your speakers!