I push the power button on my home stereo (or on its remote), and maybe five seconds later a relay closes, and it finally starts driving the speakers. Contrast this with a clock-radio or some equally cheap/small sound system, which starts putting out sound within a fraction of a second of hitting the power button, with no obvious relay closing to precipitate audio output.
So what’s going on in the big home stereo? Why the delay?
If your home receiver has surround/DSP, it’s waiting for everything to boot/load (firmware, DSP code, etc.) before turning on the audio outputs. avoids pop/click noises.
My Mirantz receiver has a very loud relay closing* before the audio turns on, but there is still that 5 or 6 second delay. It’s probably, like jz78817 said, firmware and everything else booting up, plus any caps inside charging.
*I mute my TV quite a bit and it bugged me that every time I muted and unmuted it I could hear the relay opening and closing. I was always worried that it would wear out prematurely (my Onkyo receivers didn’t do this). Oddly, there’s a setting so you can change the function of “mute” so that instead of the sound going do 0% it goes to something less that that. Now the relay doesn’t open, but OTOH, if whatever’s going on is loud, I can still hear it, it’s a bit unsettling sometimes.
The power supply in the power amp is waiting for the filter capacitors to charge.
These capacitors are so large that if they were placed directly across the output of the rectifier, the amp would blow it’s fuse when the power was turned on. So, they have a slow-charge circuit that is bypassed when the caps are fully charged. That relay also enables the output to the speakers.
The delay is to let the turn-on transients in the amplifier die out before connecting the speakers. Otherwise you get a loud “thump” through the speakers, which in addition to being annoying can also damage the speakers.
This. Good receivers/amps (and even middling ones) have had delays on audio output for years and years. Far more pleasant to wait a moment and then have the sound come on cleanly (even ramping up) than listen to CLACK! POP! RUMBLE! “…just can’t kill the beast…”
No, it’s a stereo amp/receiver only.
Um, who are you?
This is the right answer. Filter cap charging is also a consideration.
My current receiver does have all that - but the behavior I described was true even on older receivers, before the days of surround-sound, so the filter cap/turn-on transients explanations seem more apropos.
Curiously, I have just been working on an amplifier I build for a friend many many years ago, and the fault turned out to be in exactly the turn on mute system. (Looks like a dried out capacitor is causing the circuit to misbehave - given its age I’m not surprised.)
Summarising the above. When a larger amplifier is turned on it takes a while for the power rails to come up to full voltage. This is the power supply capacitors charging. A very powerful amplifier may include inrush limiting as well, which further increases the time the capacitors take to charge. Whilst the power rails are at some intermediate voltage the amplifier can be in a somewhat hard to define state. Some elements of the circuit will come up and become operational earlier than others. In many amplifiers but especially those with negative feedback, this can be a bit of an issue. Before the circuit has established itself at the correct operating levels, the partially energised bits can behave oddly, and swinging the output to one rail or the other is not uncommon. (Careful design can avoid these issues, but it is remarkable how common they remain.) This is not a happy occurrence for your bass driver, as it is not protected from a pure DC signal. Minimally you can get a disturbing thump, and at worst, actual damage can be done. The same can occur at turn off as well. So a common design is to incorporate a relay on a timer on the output. A more modern receiver may run the relay from the internal microcontroller that runs the rest of the amp. In this case the micro will wait for the entire DSP system to initialise and stabilise as well. Many higher end amps monitor the output and will drop the relay in the presence of significant distortion or DC being detected on the output. This is a good thing.*
Corblimey, back in the days of thermionic valves, you expected to have to wait for the wireless (never radio in those days) and the TV to “warm up”…
I still have a mid-range receiver from around 1980 and this is what the crew at the stereo shop told me when I bought it. No digital boot-up going on here.
As already noted, there are at least two different things going on – the capacitor charging/transient stabilization at powerup, and muting during the transition between different signal types, notably digital and analog. It happens on mine every time there is a transition, and is different from the power-on relay. If I’ve been listening to something in stereo and a Dolby Digital movie comes on, there is an audible relay click at the same moment that the Dolby channel indicators light up.
Actually, more like, there used to be. The receiver is getting old and at some point recently the digital-signal relay broke, and seems to be stuck in the closed position. With the result that everything works just as before, but now when there is a switch to Dolby Digital, instead of a click inside the receiver there is a pop from the speakers. One concludes that this is what the relay was there to avoid. It’s been going on for several years and hasn’t done any harm. The great big relay sound when you first turn on it on is still there. In fact, rather surprisingly, the table lamp dims briefly when the receiver powers up, so I assume there is quite the initial power draw, and this is only a modestly powerful amplifier.
They really did need to warm-up … run the tube filament circuit for a minute or two before we turned on the rest of the circuits … kept the tubes happier longer …
Even now many guitar amps have a standby position on the power switch. Whilst it is mostly understood to be a way of getting instant sound, it keeps the heaters going, but with the HT off. The problem with slamming the tubes with full anode voltage and a cold cathode is that the HT is capable of dragging ions out of a cold cathode, and this “cathode stripping” reduced the cathode’s useful life, as it wrecks the coatings on the cathode used to promote electron emission.