We have laying around one of the Korg SP-300 (which were made in Italy - they weigh a ton!)

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  • Have misplaced the original power supply. That thing was a HUGE black cube, getting on for the size of a small toaster. (We did in fact purchase it in I think France if that matters, maybe they are bigger there!)

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  • Notice it is marked "21" volts. (And a whole amp, too.) Can this really be correct? I may just be completely wrong but device inputs are usually some small amount of volts?

  • Checking sweetwater etc, you really can't buy the original any more it seems

  • You can easily google "replacement for SP-300 power supply". But ............

  • .... example, link they are all weenie little boxes like this -

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I've found in the past if you buy a "weenie" replacement for any device (example, a cheap weenie replacement for a modern laptop that needs a lot of juice), they just hopelessly fail.

(Indeed notice as well for some reason the voltage is not even marked on those.)

My questions are two,

  1. Does anyone know if it's a hard fact that it needs "21V" input; perhaps someone else has one and/or the original (perhaps European) power supply

  2. Does anyone happen to have tried one of the "weenie" replacements, which are only like $20 - will it actually even work?

  • 1
    Have you spoken to Korg yet..?
    – Tim
    Commented Jan 16, 2021 at 18:05
  • Note the polarity on that thing. The label next the plug hole says the outside is positive, while in my experience the inside is quite commonly positive. Check before you try.
    – Peter
    Commented Jan 17, 2021 at 14:09
  • @Tim it's a good point, i will phone them. But I surely checked their product range and they basically don't bother selling power supplies ("ac adaptors") now - they offer like one crappy one! I was really surprised that some company doesn't offer, you know "stage and studio quality" power supplies for stuff - I figured all you pros would say "oh yes, just go to blah" for the best in ac adaptors! But no!
    – Fattie
    Commented Jan 17, 2021 at 16:19

3 Answers 3


Best guess: the old one was entirely old-school coil-wound. New one is a switch-mode supply. So long as it's correctly earthed to reduce hum*, it should be fine. That's actually quite a substantial supply for a switch-mode, so it presumably has sufficient rectifying/power smoothing.

*This may not be an absolute requirement, but checking pictures of similar supplies on other sites (eBay etc), it does look like the unit itself is earthed, not just 2-pin.

BTW, yes, it's a hard fact that it needs 21v and at least 1A. You can use a supply of more than 1A, as amps are 'pulled' by the equipment not 'pushed' by the power supply. Voltage, on the other hand has no such tolerance. You might get away with 20 or 22v, but really you should be looking for the correct nominal voltage.
Additionally - but now we're getting into esoterica best dealt with by competent electronics experts, not me;) - you cannot measure voltage on a power supply by any consumer-available means. Voltage must be measured under load, not just by sticking a multi-meter in the plug. The power supply, therefore, has been calculated by someone smarter than me to deliver 21v whilst under load from that specific piece of equipment (or at least calculated to be within tolerance).

The reason the picture doesn't have a model number or voltage marked is one of convenience for the supplier - they probably make 15 versions in the same casing. One set of info-free pictures is all they need to sell any variant.

  • I'd have thought that a 24VDC psu would do the job. That'e 15% over, which is within tolerance. 1Amp isn't a huge amount of current. How the unit is earthed - do you mean the psu or the Korg? And that polarity is critical.
    – Tim
    Commented Jan 16, 2021 at 17:36
  • 1
    @Tim - Thing is, how are you going to gauge what is 'in tolerance' with no way to measure it? Plug it in & see if smoke comes out? It probably won't but why risk it? PSU is earthed, equipment can't be, there are only two wires going from PSU to keyboard. Polarity is critical, it's DC not AC where you can get away with it. It should be diode-protected in case of inversion, but again, do you want to risk it?
    – Tetsujin
    Commented Jan 16, 2021 at 17:41
  • This is amazing info, blokes! Aside - in fact, are the old-school "WOUND" thingies you talk of actually better ??! Do musicians / engineers prefer them? Can you still buy them "even if more expensive" sort of thing? Cheers !
    – Fattie
    Commented Jan 16, 2021 at 17:49
  • tolerance - indeed notice some of the ones you see on EBay are like "22.5V" which makes me nervous :O
    – Fattie
    Commented Jan 16, 2021 at 17:49
  • 1
    It's unlikely to make any real difference compared to a half-decent modern switch-mode. PSU design has come a long way since we all got mobile phones. The only 'general guide' is that the heft of the unit still has some bearing on its power-smoothing ability, so don't go for some 5 buck weeny wall-wart from eBay. Think of the difference between a laptop supply & the one for your xmas LED lights ;)
    – Tetsujin
    Commented Jan 16, 2021 at 17:51

The interesting thing about 21V DC is that it is 12V + 9V. These are two commonly found voltages for musical equipment, e.g. many power supplies for guitar pedals offer both 9V and 12V.

There are two ways to combine two outputs of a power supply, each with their own type of Y-cable. One that puts the outputs in series will combine the voltage, one that puts them in parallel will combine the current (ampere).

If you find a guitar pedal power supply with both 9V and 12V outputs of at least 1A, and the right voltage-summing Y-cable, this could be an easy solution. These power supplies are also specifically made with audio equipment in mind, so one from a reputable manufacturer should have decent shielding against electro-magnetic interference and other things that may introduce unwanted noise into the signal path.

More info at e.g. https://stinkfoot.se/archives/4242


Result ...

For anyone ever googling here who has this problem.

I did grab one of the "cheap-ass" ones, and, it does seem to be fine. It does not conk out after hours of use; volume seems the same as much as I remember.

So, the cheap ones are amazingly poorly made, the plug barely fits etc, but it gets the job done!

(I have no knowledge about whether a cheap one introduces hum, or such, that might be relevant in a pro setting.)

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