Electronic – Can someone help me to find the right transformer for this power supply

linearpowerpower supplysynthesizertransformer

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.

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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 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.

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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!

Thank you in advance,
Martin

Best Answer

The transformer primary centre-tap is for 115 / 230 V operation. If you don't need it you can just choose the voltage for your region.

The transformer may have been custom wound and you could have difficulty finding a match from any of the large suppliers although +5 V and +/-15 V are common requirements. You can of course use two separate transformers - one for the 5 V supply and one for the +/-15 V supply. Parallel the tranformer primaries to the fused mains supply and connect the common ground points after the rectifier.

If you need to create your own supplies the 7805 (1 A) and 78L15 / 79L15 (0.5 A) voltage regulators would suffice. There are plenty of designs and tutorials on these devices on the web. Pay attention to decoupling capacitors.

Since +5 V and +/-15 V are so common you could also just purchase readymade PSUs. Look out for low noise power supplies so that you don't get audio interference.