Replacing Power Supplies

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Dr. Phibes
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Replacing Power Supplies

Post by Dr. Phibes » Tue Jan 20, 2015 11:27 pm

The power supply board on the dead Omni 2 I got recently is a mess. Someone in the past did a real hatchet job on it - pulled traces and dribbles of lumpy solder everywhere. Needless to say, it's not doing what its supposed to. I could probably get it going again but I don't think it's worth it.

I'd much rather replace it with a new, modern switched-mode power supply. The Omni requires a 15V/-15V supply (max 1A I believe). I've found two that might be suitable:

This Meanwell PD-2515 is small, cheap and available within the E.U. Unfortunately, it has a maximum power rating of 24W and I think the Omni 2 is rated at 40W which leads me to...

this one from the U.S. More expensive and comes with an additional 5V rail I don't need but is rated at 65W.

Do any of guys have some additional advice I should consider (this is the first time I've done something like this). Obviously I'll need to drill a few holes in the chassis to fit a new PSU but I'm not too worried about that.

P.S. I'm not going to make my own, I don't have the skill.

Thanks

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Re: Replacing Power Supplies

Post by Stab Frenzy » Tue Jan 20, 2015 11:41 pm

If you don't have the skill to build your own power supply you shouldn't be messing with mains voltages. Take it to someone who isn't going kill themselves/burn their house down with it. Power supplies are so incredibly easy to build.

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Re: Replacing Power Supplies

Post by Dr. Phibes » Wed Jan 21, 2015 1:09 am

Safety is precisely what I'm thinking about. I could throw together a simple flyback design or something like that (and have done so previously for purely DIY stuff) but I want something with a reasonable degree of assured efficiency, longevity and safety. If I do fry myself (in the event that the isolation transformer & RCD disappear) I, or my closest kin, will not hold anyone here accountable.

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Re: Replacing Power Supplies

Post by synthroom » Wed Jan 21, 2015 3:17 pm

While I could, if I really had to, make a PSU myself, but why do it!

I replaced the PSU in my Emulator II with a Meanwell T-60. It's been working great!

I bought mine from http://www.Jameco.com . I've not deal with these people, but look around here: http://www.meanwelldirect.co.uk/
or these guys, who I bought a Raspberry Pi from when they were released:
http://uk.rs-online.com/web/
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Re: Replacing Power Supplies

Post by HideawayStudio » Sun Jan 25, 2015 1:15 pm

I would strongly recommend replacing an ancient inefficient SMPSU in a hybrid or digital instrument with known heavy supply loads eg. Emulator II or DK Synergy where the reduced heat, size, noise, option of fan removal and increased safety plus often universal mains supply inputs are all advantages (not least because they pose much less of a threat to cherished circuitry!)

BUT...

I've had some tricky/unfortunate experiences replacing linear PSUs in analogs with SMPSUs.

One thing that is often overlooked is just how much ripple is present on an SMPSU (even modern ones) and this can cause real issues in a voltage controlled synth.

I cannot recommend this retrofit to any vintage monosynth....

However...

There are cases where I feel its worth the effort though in big polysynths. Two examples immediately spring to mind but even these are tricky namely the Polymoog and the Rhodes Chroma. Both are instruments with notorious large linear PSUs supplying heavy loads which run hot and are known for having issues. In the case of the Chroma there is now a superb SMPSU retrofit available. In the case of the Polymoog I've been on my own somewhat but rapidly ran into issues with excessive ripple which I managed to counter in the end by reverse connecting the floating split-rail outputs (ie. using the -ve rail output as +ve by swapping their polarities). To my relief this had a nulling effect which provided just enough reduction in ripple to counter a very noticeable warbling effect in the upper registers which is what I suspect you will encounter in many such retrofits in voltage controlled synths unless the PSUs feature linear post regulation (found in some lower power audio/medical grade SMPSUs).

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Re: Replacing Power Supplies

Post by sneakthief » Sun Jan 25, 2015 3:46 pm

I put this in an Arp Omni and it works great:

http://www.mouser.com/ProductDetail/Int ... G4Jg%3D%3D

Listen to HideawayStudio - he knows the score.
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Re: Replacing Power Supplies

Post by synthroom » Sun Jan 25, 2015 4:08 pm

HIdeaway - how hard would it be to add some filtering to remove the ripple in the monosynth situation?

I have no experience with modern modular rack PSUs, but what about adapting a PSU from one of those?
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Re: Replacing Power Supplies

Post by Dr. Phibes » Mon Jan 26, 2015 10:33 am

HideawayStudio wrote: There are cases where I feel its worth the effort though in big polysynths. Two examples immediately spring to mind but even these are tricky namely the Polymoog and the Rhodes Chroma. Both are instruments with notorious large linear PSUs supplying heavy loads which run hot and are known for having issues. In the case of the Chroma there is now a superb SMPSU retrofit available. In the case of the Polymoog I've been on my own somewhat but rapidly ran into issues with excessive ripple which I managed to counter in the end by reverse connecting the floating split-rail outputs (ie. using the -ve rail output as +ve by swapping their polarities). To my relief this had a nulling effect which provided just enough reduction in ripple to counter a very noticeable warbling effect in the upper registers which is what I suspect you will encounter in many such retrofits in voltage controlled synths unless the PSUs feature linear post regulation (found in some lower power audio/medical grade SMPSUs).
Interesting, I got a new SMPSU installed in polymoog keyboard a few years ago; I didn't hear how it sounded with the original but looking back it would have been interesting to compare. Actually, it's funny that the stuff about power supplies popped up in the other thread when it did. After rummaging around some old junk I found a power supply from a knackered jx-3p and hooked it up to the omni just for the interim. It works fine but the filtering caps are old and there's a bit of ripple getting through. While scoping for dead logic chips (only 1 so far!) I could see the subtle effects this ripple had at various points in the signal path which must inevitably affect the overall tone.
sneakthief wrote:I put this in an Arp Omni and it works great:

http://www.mouser.com/ProductDetail/Int ... G4Jg%3D%3D

Listen to HideawayStudio - he knows the score.
I think I've seen someone selling that on ebay as an Omni PSU replacement for something like £200 :?

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Re: Replacing Power Supplies

Post by HideawayStudio » Mon Jan 26, 2015 1:10 pm

synthroom wrote:HIdeaway - how hard would it be to add some filtering to remove the ripple in the monosynth situation?
I have no experience with modern modular rack PSUs, but what about adapting a PSU from one of those?
If the SMPSU is providing the correct voltage you need then its almost impossible to excessive remove ripple as there is no headroom to work within (eg. for low dropout linear post regulation) and even the biggest of caps won't remove the ripple to levels not perceivable in an analog with VCOs. Unfortunately the situation is worse in monosynths because the PSU loading is usually so small that many SMPSU are bordering on instability under such light loads (they work best in terms of efficiency and stability when nearing their power rating). Even with dump resistors fitted, many SMPSUs just don't like near zero loading which is why their use in big polysynths or complex digital synths/samplers makes much more sense.

In the case of a linear modular PSU then it may well be ok but it must be up to the power needs of the synth and don't forget even monosynths often have 5V logic rails as well as the usual +/-15, 12 or 10 volt split rails.

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Re: Replacing Power Supplies

Post by rschnier » Sun Feb 01, 2015 11:55 pm

A related thought: I've always wondered why ARP didn't just use 7815/7915 +-15 volt, 1A regulators in the Omni-2's PS. I've built lots of +-15 volt supplies to go in various analog gear I've constructed over the years and those parts are very reliable. The only reason I can think of is that those regulators might have been considered "too new" at the time...or perhaps they didn't exist? Those seem unlikely though, as I recall that series of regulators (78xx and 79xx series) being around since at least the mid-1970's. If anyone has any insight, would be interesting to know.
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Re: Replacing Power Supplies

Post by HideawayStudio » Sat Feb 07, 2015 2:45 pm

rschnier wrote:A related thought: I've always wondered why ARP didn't just use 7815/7915 +-15 volt, 1A regulators in the Omni-2's PS. I've built lots of +-15 volt supplies to go in various analog gear I've constructed over the years and those parts are very reliable. The only reason I can think of is that those regulators might have been considered "too new" at the time...or perhaps they didn't exist? Those seem unlikely though, as I recall that series of regulators (78xx and 79xx series) being around since at least the mid-1970's. If anyone has any insight, would be interesting to know.
7815 & 7915 have indeed been around a very long time now. They are convenient, reliable and easy to built/design around but what is often overlooked is that their performance from a stability/regulation/ripple rejection point of view is not as good as several other band gap reference based series regulators such as the LM317/LM337 pair and many of the slightly more discrete solutions (using external series power transistors) found in synths such as the Polymoog which are also often adjustable with sensing to dynamically compensate for voltage drops in the PSU cabling under load changes.

In a bigger design, where there is a fair load on the PSU reservoir caps, the ripple currents can be sufficiently high that a 78xx/79xx series pair is insufficient to reject it.

In audio circles the LM317/LM337 pair is without question considered a better choice of integrated regulator.

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Re: Replacing Power Supplies

Post by Steve Jones » Sat Feb 07, 2015 4:32 pm

I took an old Lambda SMPSU out of a rack mount Prophet 5 recently that had failed. I replaced it with a new SMPSU and ran into the described issue, too much hash on the +/- 15V rails for the VCO's (strangely the old Lambda unit didn't have that issue). I think that in order to use an SMPSU in that scenario it might be necessary to use an SMPSU with a higher rails and then do some post-regulation and filtering using op-amps such as an LM324 and some driver transistors to bring it down to +/- 15V.
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Re: Replacing Power Supplies

Post by Dr. Phibes » Sat Feb 07, 2015 7:33 pm

I went with a pre-assembled linear supply in the end after reading the advice herein. Still need to properly mount the new transformer but otherwise it seems to work well.

I've replaced the decoupling tantalums but I can't be bothered replacing all the individual keying ones. I don't intend on selling this one off for awhile so I'll just deal with it on a step by step basis :)

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