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).


Other supplies measure a nice ~12 V so it's not my meter.

  • 3
    \$\begingroup\$ Do they feel heavier than your "exactly 12V" supplies? \$\endgroup\$ Nov 7, 2022 at 14:22
  • \$\begingroup\$ My Yamaha keyboard and drum kit are the same if that's any consolation. \$\endgroup\$
    – Andy aka
    Nov 7, 2022 at 14:25
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    \$\begingroup\$ Other supplies measure a nice ~12V so it's not my meter. ... probably true, but it is faulty logic \$\endgroup\$
    – jsotola
    Nov 7, 2022 at 16:25
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    \$\begingroup\$ I think the close votes about this being about use of electrical devices is not accurate - it simply asks why these behave so, and it is a valid thing for a beginner to understand. If you need a reason to close this, it would likely be due to being a duplicate. \$\endgroup\$
    – Justme
    Nov 7, 2022 at 17:01
  • \$\begingroup\$ I have a Yamaha power supply exactly like this. It's from a 1989-dated FX-500 effects unit. Original owner, bought in 1989. \$\endgroup\$
    – Kaz
    Nov 8, 2022 at 0:27

4 Answers 4


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:

  • Lowers the cost of the power supply, at least a tiny bit
  • Allows the power supply to be smaller, perhaps a tiny bit
  • Protects the main unit somewhat in case someone substitutes a power supply that isn't quite right
  • If the main unit requires several different voltages -- for example, 12V for the power amplifier and a lower voltage for the digital logic -- then generating and regulating multiple voltages in the power supply would mean a multi-conductor power cord and special, more costly power connectors.
  • \$\begingroup\$ That makes perfect sense. I'm gonna make a quick guess that these were more common in the past? I've measured a few more and newer ones seem to generally be more accurate. \$\endgroup\$
    – iHnR
    Nov 9, 2022 at 9:40

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.

  • \$\begingroup\$ Did you mean 'or maybe less'? \$\endgroup\$
    – HandyHowie
    Nov 7, 2022 at 15:18
  • \$\begingroup\$ @HandyHowie A lower resistance means a higher current, a larger load. So "more" and "less" in a context like this is a bit subjective. \$\endgroup\$
    – Hearth
    Nov 7, 2022 at 20:16
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    \$\begingroup\$ Yeah in this case more is less \$\endgroup\$
    – Voltage Spike
    Nov 7, 2022 at 22:07

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.

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    \$\begingroup\$ I wouldn’t call it “linear”, since that implies a regulator. It’s a rather a rectifier supply. \$\endgroup\$ Nov 7, 2022 at 22:22

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.


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