A new PSU for your old Synthesizer.

I've been looking for a power supply that can replace the ones found in some vintage audio equipment for some time. But I never found the type of product that was right for me. There are quite a few DIY-made solutions but again, these solutions didn't really suit me.

For example, I used a power supply whose primary part is a switching PSU that converts the main AC to 28V DC which is then transformed into + 5V + 12V and -12V. This system works very well. I used it in a Dynacord ADS sampler :

But the main AC is converted by a switching module. It's certainly an easy to use solution but always ends up causing problems because of the main capacitor directly subjected to a high electric potential. If this capacitor is not of very good quality, the power supply quickly loses its performance.

Others are creating new switching power supplies without an intermediate stage. In the following example, the main AC is directly converted into the different standard low DC voltage. This is manufactured by synthronics and is intended to replace the original power supply of a Prophet-VS :

This power supply is certainly of very good quality, but if something goes wrong, I wouldn't really feel able to fix it.

Other topologies are also used, for example the power supply for the JX10 from supersynthprojects.com, made up of several switching power supply modules:

In this case, each module is directly connected to the main AC, which has the effect of multiplying the potential sources of problems related to the direct connection to the main AC.

In addition, all these power supplies are specific and rather dedicated to a particular device. Not really flexible, in fact.

And by the way, what is the main problem encountered when creating this type of power supply? and why does everyone seem to want to use switching power supplies? 

The reason is in one sentence: heat dissipation. And especially in digital / analog machines where voltages of 12V and 5V are required. The 12V can be generated from a rectified and filtered voltage of 15V for example.

But mostly, the 5V is generated from the same primary 15V. And in a classic linear regulator topology, this therefore gives 10V to 'lose'. Under 1A, 10W of heat is produced. The regulator heats up, it is then necessary to equip it with a heatsink to dissipate this heat flow so that it is not destroyed.

The solution I adopted is a mix between low voltage switching regulation and linear regulation. The lowering of the mains voltage is carried out by a standard transformer. There are therefore no longer any capacitors subject to the main AC and in the event of a problem, the power supply can be easily repaired because the components are standard and easily available. 3D image of the power supply board:

This power supply provides + 5V, + 12V and -12V. Each output is able to provide 2A, possibly 3A but I have not carried out tests at this last intensity. 

In addition, I added the availability of a battery-backed voltage, particularly useful to have a 'place' already planned to insert this kind of backup in older machines that do not have a battery holder. I also added a circuit capable of providing write protection usable for SRAM protection, as well as a reliable RESET signal. Two power connectors are available plus a floppy drive type connector. 

All these characteristics should make this power supply usable in a large number of machines, while providing a very good level of safety since the main AC is no longer directly handled.

This gives a fairly compact circuit board :

This is the prototype of the power supply. For the tests some unavailable capacitors were momentarily replaced by non cms models. The final version will of course include all the right components. 

After checking that everything was working fine, I connected this power supply directly to a Korg T1 motherboard:

Everything works fine and after a few hours the power circuits are barely hot. Finally, the only modification I will have to make for the final version will be to replace the current limiting resistors of the LEDs. They really light up too much ;-)

Depending on the current expected to be supplied, an 18V and 4A transformer per winding should be sufficient for most uses. This PSU will be available in the coming weeks ... depending on the activity of the Covid ...