The new three output PSU is validated.

The initial idea was to replace linear power supplies which sometimes get very hot, as well as the switching power supply directly connected to the mains which can present security risks.

I therefore opted for an intermediate solution which consists in using a standard transformer to lower the mains voltage, and a triple low-voltage switching power supply to take advantage of a better performance than that provided by a purely linear power supply. The two outputs provided for supplying analog parts are fitted with linear regulators following the switching regulators for better noise rejection.

The result is this reduced size power supply (160 x 80mm):

I have already had the opportunity to perform some tests when I had not placed all the defined components, especially the output filter capacitors. I had to temporarily place models with through legs. For these tests, I used all the boards of a Korg T1. 

It works fine!

This time I used the final version of the power supply to power the board set of a Dynacord-ADS sampler, a famous vintage German samper :

www.deepsonic.ch

This sampler consumes more current than the T1. In fact, a little more than 2A for the 5V part for the version I own:

For this test I did not use the floppy disk drive which actually consumes very little on standby. I was however able to test the audio output which does not present any residual noise. The ADS has a switch on the front panel allowing the generation of a nice sinusoidal output.

Final thoughts: This power supply works perfectly and is able to deliver at least 2A on all its outputs without requiring a cooling system. Note that if the + 5V AND the + 12V should each output 2A, one of the diode bridges should be capped with a small cooling system since it should provide 4A, but this situation rarely happens. For ease of implementation, I may change the output capacitors to through-leg models in the production version of this power supply.

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 ...