I have a new piano. Similar to the old one but the adapter supplied is a massive transformer thingy of 250V 2A capability, the AP6. The previous one was 1.5 A, the AP5D which was big enough, and recommended for the instrument I have now. But when I see these and others advertised, they seem to be for a multiplicity of Yamaha instruments, some big, some small. Indeed, some adapters seem no bigger than mobile phone chargers .. .

How critical is this? Does it matter if the adapter is a bit on the big side?

4 issues: As long as the voltage is within 5% of spec, and the amps is at least high enough as stated on the original adapter, and the plug is the same type, and the polarity is correct, then yes you can use any adapter.

If I had the choice I would prefer a larger power supply with a transformer in it to a smaller 'switched-mode' power supply (ie. the sort of thing you get with a mobile phone) for a piano or any audio device

The reason being that the 'chopping' of the waveform which a switch-mode power supply performs introduces all sorts of high-frequency signals (lookup 'step functions' and 'fourier analysis' if you want an explanation of why) into the power supply which can create issues with both the audio quality and the electronics if they are not properly filtered-out. Good power supplies are very important if you want good amplification

It's nothing to get too upset about and the smaller, switch mode power supplies normally 'work' fine, it's just you might notice your piano sounds a bit better with a larger, transformer-type PSU

Thank you, both. The reason I asked was because, once on an outdoor location, I ran a Yamaha keyboard (PSR630 I think) through what I thought was a suitably connected car battery. Suitably connected through the adaptor input that is.

And it was never right since then . . . maybe "switch-on" was a bit too savage given the capacity of the battery, I don`t really know ..

Hang on, are you saying you connected this keyboard to the car battery through AC adapter? Or are you saying you bypassed the adapter and had the battery connected straight to the keyboard?

Battery straight to keyboard through the socket where the adapter usually goes. Well, it is 12V . . or maybe 13 or so. It worked.

Ok, that sounds fine. A car battery will typically sit around 12.5V on a low current draw, so there's no reason that would cause any trouble. If you ran the battery down to a large degree, you might have problems though. A lot of electronic gear will not like a low voltage. It's equivalent to a "brown-out", which is what happens when your household electricity supply isn't cut off (blackout), but one or two of the phase lines to your local exchange is lost and your voltage drops to between 1/3 and 2/3 of normal. A lot of electronic circuits will fry if operated at low voltage for too long. It's possible something was damaged, but is partly functional in your keyboard and that's why it works but doesn't sound as good anymore.

Thanks Ando. The battery may well have run down a bit during THAT afternoon . . . !

A regular car battery has RAW power! It's meant to start a car and doesn't have any means of regulation, which most electronic devices need and use.
A plug in power supply has at the least a RL or small Pi filter in it after the bridge rectifier [if that is what it uses to change ac to dc] or it may use two diodes with a center tap for 12 volts.
A regulator would be ideal. A zener diode is a regulator of sorts.

A standalone car battery will work fine. Most devices don't need tight regulation at all. And the battery's voltage will remain reasonably constant until it drains significantly.

But what does it matter? The OP is asking about power supply adapters, not car batteries.

"A regular car battery has RAW power! It's meant to start a car and doesn't have any means of regulation"

Tis is what I`m getting at. The initial power surge when the switch is operated. Like a pressurised mains water supply whooshing into an empty pipe system until it pressures up . . . And components which may be compromised somewhat because of it. Like a reduced life . . .The same principle for adapters, too.

Much ado about nothing.

A battery is the smoothest source of power.
It has no need for regulation.
And if there is any power surge when the equipment is turned on, it has NOTHING to do with the battery.

Well, he sort of asked about car batteries.

A few points I would add:

Car batteries are unsuitable for continuous use between charges. When the voltage starts dropping (battery drain), you could experience erratic or unstable behavior. This could damage the instrument if it is not designed to deal with this. Use a deep cycle marine battery instead. They are designed to hold a constant charge longer, and can take more abuse when it comes to being run 'dry'.

A raw battery is not fused. Your keyboard may be - or it may not be. Either way, I would make sure there is a fuse somewhere in the equation. If you instead connect the battery to a power inverter, and plug the piano (with its factory power supply) in to that, you would get your fused circuit. However, some inverters spit out some serious chopped sine waves, which some electronics absolutely hate.

Consult the factory if your instrument is under warranty as any of this could void it.