Hi, I have a project that is to build 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!

 

 

 

This is very far afield from unRaid topic, which is the purpose of these forums. Don't think we have people here that will be able to help. This post will be moved to the bilge, but wanted to reply and let you understand why. If anyone has the knowledge and wants to reach out @Tendste, please do so via PM.

Just some notes.

 

It was quite common earlier to use custom-made transformers - you ordered the exact type of windings you wanted.

 

250 mA on the primary side is plenty of power, since the current needs on the primary side is the inverse of the voltage conversion between primary and secondary side.

 

The synth draws more current from the 5V, which is why it uses more powerful diodes to rectify the 5V.

 

Note that when you rectify and filter, the top voltage of AC is sqrt(2) of the rms voltage. So a transformer that gives out 12V will have 1.4*12 = 16.8V peak voltage. The ripple voltage over the capacitor will reach this peak value and the size of the capacitor compared to the current load will decide how much ripple there will be. And the size of the ripple decides how much margin there is to produce the filtered and regulated DC output.