Building a replica of the ARP Pro Soloist synthesizer

[Gurao]

So my current project is building a replica of the ARP Pro Soloist synthesizer from the 70’s. The project is mostly going very smoothly, but I am having some issues with the power supply section of the synth: Even after many searches, I don’t know what kind of power transformer to select.



As you can see on the schematic, the original unit used a transformer with a center tapped primary and two center tapped secondaries. This seems unusual to me, because most transformers I can find have either only one center tapped secondary winding, or two secondaries, but not center tapped. Other than that, there are almost no other indicators of what kind of transformer was used. The only other things that caught my attention are that the power supply uses a 250 mA fuse, which means - if I understand it correctly - that the instrument doesn’t use more than 250 mA of total current, because the fuse would blow otherwise. Also, the 1N4448 diodes(http://www.kynix.com/Detail/966618/1N4448.html) used here as a bridge rectifier have an average forward current IF(AV) of only 150 mA.
The power supply outputs 3 individual power rails of + 5 V, + 15 V, and - 15 V, so the transformer must be able to handle that.



Another thing that may help you to help me is something I found in the service manual for the instrument. The section about the power supply mentions what kind of dummy loads should be used to test the power supply. Using Ohm’s law to calculate the current for each rail, I got the following results:

• + 5 V = 1 A
• + 15 V ≈ 0.06 A = 60 mA
• - 15 V ≈ 0.05 A = 50 mA

One final thing to mention is that I would like to use a toroidal transformer, since they are less noisy and better suited for audio applications like this one.
So my questions are:

• What kind of power transformer should I use?
• How can I know the voltage ratings for each of the secondary windings? Aren’t they supposed to be a little bit higher than the needed voltage?
• What current rating is the transformer supposed to have?
• And most importantly: Where can I buy it?

I really appreciate any help!

[cjd]

My vote goes to building a quality power supply that meets the needs of the project, and ignore completely how they got there on the original.

I would build with a single toroidal transformer that will provide adequate current (you can never have too much capacity... you just never need/use it. Though you need a bigger fuse on the line voltage side to survive inrush current).

Use voltage regulators to get it where you need it. Two on the + side, at +15v and then down-regulate further for the +5v. And then -15v.

Parts Express has a handful of nice toroids. They have other transformers as well. There are certainly other sources once you figure out exactly what you want (I love apexjr.com if they have what I need - usually just wire though.) Lots of useful data online about building power supplies, and they're crucial for good results, so take some time researching and understanding.

You can certainly go unregulated, but I no longer remember what exactly you'd need to find; also, it may be a bit tough to source the right parts.

Sounds like a fun project. I've mostly built chip-amps, but also a DAC (wolfson) and a couple pre-amp attempts (neither fully successful). Lots of crossovers, since I actually do the design work on those. Also they're a bit less finicky.