Last year, a customer brought me an ARP 2600 that had just been serviced with the exception of the power supply. The customer said that the synth would sometimes freeze and be unresponsive so I gave him a choice; I could either service the original power supply or design something a little more up-to-date. We both decided to take advantage of the situation and my Sirius replacement power supply for the classic ARP 2600 was the result.
To be honest, I took on way too many design projects in 2024 . So much so, that my repair work seriously slipped and I found myself continually apologising to my good customers and thanking them for their patience. I figured I'd done enough design work for a while and vowed that 2025 would be different. Time to take things easy.
Well, almost...
Sirius is one of two projects from last year that kind of overspilt into the New Year, the other being Nebula-X, my balanced outputs jack-board for the Roland JX-10.
Anyway, life is great so enough moaning...
The original ARP 2600 power supply, including the transformer but excluding the fuse (no idea why they did it like that but , hey), is attached to a metal plate which is screwed to the inside of the wooden 2600 case. This keeps things very tidy, focused and easy to work on. That being said, the power supply is at the rear of the instrument and can be a bit awkward to get to. The construction of the 2600 is such that you have to kind of reach over the synthesiser front-panel, to get to the back of the instrument.
With only +/-15V lines, on the face of it, the original power supply couldn't be more simple. I measured the static current consumption to be about 170mA per rail so that wasn't at all a big deal, especially when I had 25W converters in mind for this project.
In the previous paragraph I used the phrase "on the face of it". That's because things are potentially a little more involved. The original PSU has a pair of potentiometers which allow for adjustment of the supply lines. The electronics will run fine with a reasonable margin of error but annoyingly, many synthesisers of this era, including the 2600, took reference directly from the power lines. The 'reference' voltage is used for things like the VCOs and scaling of the keyboard. Wanting to use a modern, modular switched-mode design like my other power supplies, it wouldn't be easy to emulate this feature.
I was able to include independent voltage references on both my Aurora PSU for the Roland MKS-80 and my Supernova PSU for the Roland Juno-106 for example, as the original power supplies for these instruments didn't take references directly from the supply lines.
It's quite possible to use sufficiently accurate devices so as not to need adjustment and so I looked at how I could do that. The Vigortronix AC/DC converters I intended to use and which I'm very familiar with, have a specified accuracy of 2%. That's pretty good. I measured the voltages of several 15V AC/DC converters that I had in stock. Coupled with my experience of my other power supplies, all of which are sent out with a test certificate, I could see that the tolerance was no worse than 1.6% with the vast majority falling within a tolerance of 0.75%. This was really quite encouraging.
Okay, let me give a bit of perspective...
There's no denying that accuracy and tolerances of electronic components and systems has jumped leaps and bounds since the ARP 2600 appeared on the scene in the early seventies. In fact, we should consider the accuracy of the equipment that was used to calibrate the supplies at the factory in the first place. With this in mind, I decided to continue with my Sirius replacement power supply for the classic ARP 2600 prototype but I also decided to hand-pick the converters, choosing those which had an accuracy of no less than 1.0%. That would give a worst case scenario of 14.85V or 15.15V. It would be interesting to see what would happen.
After a couple of weeks of testing, it was a great relief to note that things seemed to work just fine. My ears, my oscilloscope and my precision tuner, didn't reveal anything strange. With a view to offer Sirius to the wider ARP 2600 community however, I thought I could do better and so the executive decision was made to use converters with an accuracy of 0.7%. Yes, I know... it won't make any difference in the real world but I'll be able to sleep better! 😴
Weighing just 215g my Sirius replacement power supply for the classic ARP 2600 is less than a third of the weight of the original (727g). This meant that Sirius could easily be secured using the original mounting plate. Indeed, although the Sirius PCB is attached to the plate using M3.5 fixing hardware, I didn't see a problem using the original screws to secure the plate to the inside of the 2600 case. Not having to supply a custom solution would help keep the cost down.
"M3.5 fixing hardware?" I hear you ask! Glad you spotted that. Indeed, just like most manufacturers, 99% of my stuff uses M3. Since I wanted to use the original mounting plate which had been drilled to accept countersunk screws however, I had no option other than to choose my fixing hardware to be as close to the original as possible. Adequate to support Sirius, M3 screws and hex spacers would have been too small and I wouldn't have been able to use the original mounting plate.
Removing the original power supply was straight-forward and simple. Securing Sirius to the the original mounting plate and then fixing that into the position of the old power supply was also straight-forward and simple. I was however, concerned about soldering the connections from the mains input, synthesiser and the keyboard port, to Sirius. It would mean leaning over the synthesiser and I wanted to reduce the risk of an accident for others. I therefore chose to use screw terminals for all connections.
With over-voltage and over-current protection built-in, advanced noise filtering on the back-end and nearer 95% efficiency as opposed to perhaps 15% efficiency of the original PSU, Sirius will also allow your ARP 2600 to be powered from virtually any domestic power supply. That means that there's no need to change transformer taps, should you suddenly find yourself on the other side of the planet, alone with your favourite synthesiser! 🙂
In contrast to the image above, the image below makes Sirius look like its always been there! You'll notice that I took the opportunity to change the power input cables to something a little more 'standard'.
You may also notice the addition of an extra earth wire which is strapped to the voltage lines going back in the direction of the synthesiser. This is important. Unlike a linear power supply, 0V on the output of a SMPS, is not always connected to earth through the power supply. For the filters on Sirius to work properly, the return path for the each supply has to be direct. Earthing 0V through the PSU will break this path and the filters will be ineffective. Instead, 0V is earthed through the 'load' (the synthesiser) and the additional earth wire is strapped to the metal chassis of the 2600. Of course, this maintains safety as well as providing the required return path integrity. Earthing in this way also ensures that the effect of any parasitic capacitances on the outputs of the AC/DC converters are removed.
I made sure that after several hours of operation, the ARP 2600 still worked fine. As far as Sirius was concerned, it didn't even get warm.
I'd only just got back from spending New Year's Eve in the Bavarian Alps two days ago and officially, I'm back to work 'till Monday but I simply couldn't wait to tell my customer the good news. In fact, he asked if he could come over the next day. Of course, that wasn't a problem as Ryan had been very patient and appreciated all the precautions I took with his beloved ARP 2600. The first thing he noticed was that there was no hum! Yeah, I kinda forgot to mention that, LOL. 🤣
Trying to remember how it all worked and then after 'painting' some sounds, Ryan commented that he wouldn't have been able to do that before as things would lock up after only a few minutes, requiring a restart to sort it all out.
And finally...
It should be no surprise that in recent years, various clones have been made of the iconic ARP 2600. Please be warned however, that my Sirius replacement power supply for the classic ARP 2600 was designed for the original, now vintage, ARP 2600 from the 1970s. I don't imagine the Korg ARP 2600 FS or the Korg ARP 2600M to bear any resemblance to the original instrument. As for the Behringer 2600 type of thing, forget it. So... DON'T even think about fitting Sirius to any of these machines!
Yes, the original power supply can be fixed and in fact, it's not at all difficult to service but Sirius changes things big time offering reliability, stability, longevity and... NO HUM.
Sirius is available to purchase here:
All I need to do now, is compile the installation manual. 😕
In the meantime, here's some interesting reading regarding the legendary ARP 2600:
https://en.wikipedia.org/wiki/ARP_2600
https://www.vintagesynth.com/arp/2600