For a second or two, anyway?
I’ve noticed this with guitar amplifiers, and stereo amplifiers - when you switch the amp off, the music or whatever continues for a second or two before fading. What causes this? Is it some sort of capacitance in the transistors or similar?
Thanks in advance etc…
There are large capacitors in their power supplies that will hold enough of a charge to run for just a second.
Why do their power supplies contain capacitors? It’s not a phenomenon I’ve ever noticed with say, a microwave, or the CD player in the HiFi separates set. Why do amplifiers need these capacitors?
Maybe a very old amplifier with tubes cooling off? WAG
That’s how they amplify.
The capacitors serve to smooth out ripple on the DC output of the linear power supplies. Amplifiers need large capacitors, because any ripple on the DC power will result in a 60 Hertz hum being heard in the output of the amp. Guitar amps use linear power supplies because they are cheap and reliable, but they are heavy and require large capacitors for filtering. Switching power supplies are used in computers and many VCRs, DVD players and other electronics, and are much smaller and more efficient, but cost more to make and are more difficult to repair. Switching power supplies run at much higher frequencies than linear supplies and so can use much smaller capacitors to filter out ripple.
You’re thinking of transistors, the active elements in an amplifier. Transistors are semiconductor devices which can use a very small current (or voltage for MOS types) to control a much larger current. They can be used as either switches, as in digital electronics or as amplifiers.
Capacitors are used to store energy. AC cycles on and off at 60 cycles per second, but the amplifier needs power constantly. Energy is stored in the capacitors while the AC is in the “on” part of its cycle, then released by the capacitors when the AC is in the “off” part of its cycle. The more capacitance you use, the more it is going to smooth out the off and on (actually a sine wave) of the AC and get it closer and closer to a pure DC supply. The less smooth the DC power is, the more it is said to have “ripple” in it. Any ripple in the power supply becomes audible noise in the amplifier, so you want to have as little ripple as possible.
A cheap way to get rid of ripple is just to use more capacitance. This is why your amp will stay on for several seconds even though it’s trying to filter off changes in the AC power supply that happen at 1/60th of a second. The reason your CD player doesn’t exhibit the same phenomenon is that it has a more complicated power supply and may rely on multiple stages of filtering before it gets to the auidio circuitry. The same could be done for your guitar amp, but it would cost a lot more money since you are dealing with much higher power levels.
The big caps in a power supply also help the amp handle transients that would otherwise be too much for the power supply to feed by providing some reserve power. Another term for this is “headroom”
When you turn off the amp, that stored power in the caps is consumed, enabling the amp to make sound for a short while after the power is switched off.
If you have an amp where the sound cuts out instantly, there’s a relay that disconnects the speaker(s) - when you turn the amp on, the relay is on a time-delay - this protects the speakers from that “WHUMP!!” when the amp first turns on and hasn’t stabilized.
Q.E.D. and ECG are correct.
Here’s a layman’s explanation:
One of the problems with the AC power available at your wall receptacle is that the power goes to exactly 0 watts 120 times a second, regardless of what’s plugged into it. And it’s very close to 0 watts for a significant part of the cycle.
As you might imagine, this sucks if you’re trying to power something that needs power continuously. In other words, just about every appliance needs to use power (or have power available to it) 100% of the time. But the power at the damn outlet is at 0 watts (or around 0 watts) for a significant period of time over each cycle of the 60 Hz sine wave. See the problem?
To get around this problem, just about every appliance needs to take some of the energy that’s coming from the wall receptacle and store it in an onboard “energy storage device.” That way, if the power coming in from the outlet is at (or around) 0 watts, the appliance can grab the extra energy it needs from it’s on-board storage device.
So what are these storage devices?
In an AC motor it’s rotational inertia. In most all other electrical appliances, it’s the capacitor. Batteries are also sometimes used. I think I’ve even heard of designs using flywheels.
Thanks guys, for all your help - that makes a lot of sense. Minor nitpick is that over in the UK, my AC supply’s running at 50Hz so all your explanations are wrong 
Thanks again!
The flip side of that is that when you turn the amp on the big capacitors have to get charged up. Back in my PA running days we had a 3000 Watt amp that would dim the lights in the room when you turned it on. In one club every time we turned it on it drew more current than the main breaker would allow so it threw and cut off the power. In order to get it powered up one of us switched the amp on and the other reset the breaker repeatedly, building charge in the caps.
Since the OP is well and truly answered I thought I’d just chuck in an old war story.