Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
I have two Yamaha keyboards powered by 12 V supplies. I just noticed they both read 16 V when measured when not under load, and at least one of them reads around 14.5 V when under load (I can't measure the other one under load easily).
Why?
Other supplies measure a nice ~12 V so it's not my meter.
This suggests that these are unregulated power supplies; they probably have a transformer, a bridge or full-wave rectifier, and some filtering (capacitor and maybe an inductor) but no voltage regulation.
Instead, the keyboard unit has the voltage regulator(s) required to make it operate reliably. This is simply a design decision. Some factors in favor of this pattern might be:
This is common; most supplies like this have a higher voltage when not under load. If you want to properly measure them you'll need to connect something like a 1 kΩ or 100 Ω load, or maybe lower resistance.
It is a 12V DC unregulated power supply, with just a transformer, rectifying diodes and a bulk reservoir capacitor.
Such a power supply would read about 16V when measured without load.
12VRMS AC has peak voltage of almost 17V. That is rectified into a capacitor through one or two diodes so it will drop by some amount, which including some tolerances would be exactly the 16V DC you happen to measure.
Why it is an unreglated instead of something else? It likely is cheaper to have a single type of transformer which fits to many kind of devices. Devices have the regulators for the required voltages inside.
Another thing is, if it were a regulated supply, it would likely be a switch mode power supply that might be more difficult in an environment with audio devices being connected together. Having a standard cheap transformer generates less electromagnetic interference than cheap switch mode power supplies.
Most analogue processing circuits are capable of working adequately over a reasonable range of DC supply voltages so, why bother with a regulator inside the power-pack?
There is also quite a substantial case for not using an SMPS (which would of course regulate) because the switching artefacts present on the output will inevitably interfere with the audio circuits and cause audible noise problems. These problems may come and go depending where you are using the equipment and what you are connecting audio outputs to.
Yamaha have been using this type of power supply for at least 20 years (to my knowledge) and the only motivation to change it (with some risk of interference) is to use an SMPS and be a little greener/ecological.
My 12 volt Yamaha power supplies barely get warm so it's not like they are taking much more than 10 watts from the AC. So, an unregulated old-fashioned transformer, bridge and smoothing capacitor does the job just nicely even if its output regulation leaves something to be desired.
Other supplies measure a nice ~12V so it's not my meter.... probably true, but it is faulty logic \$\endgroup\$