freestompboxes.org

plush wrote:

marshmellow wrote:There are isolated DC-DC topologies that use transformers which are much smaller compared to the transformer in a standard AC supply. Could be that, or they are just as generous with the term isolated as Joyo.

You can't isolate DC from DC via transformer. Transformers work only with ac. Since this isolator runs on DC, it's based on pure bullshit.

https://en.wikipedia.org/wiki/Flyback_converter

Ok then.

plush wrote:You can't isolate DC from DC via transformer.

Quite correct.

The way (isolated) DC to DC converters work is to first chop the DC (so it has an AC component). You then apply this through a transformer which will block the DC and allow the AC will pass through. The output voltage is determined by the turns ratio of the transformer (and the current drawn) so you can have a different voltage to your original DC voltage. Finally rectify and smooth back to DC again.

To get a stabilised supply you can apply feedback (this also has to be isolated so either a small transformer or an opto isolator) from the output to control the chopper.

You are correct. But if you do isolation using THAT many DC-DC isolated converters, the odds of having switching frequencies close enough to have difference frequencies in the audio range are pretty high, and you had better do it in a steel box, or perfectly-lined with mu metal (seams can make slots that act like antennae and radiate RFI). For a audio DIYer, the headache and frustration level of troubleshooting that kind of high tech stuff is pretty high.

Why not simply buy a surplus 100W toroid transformer, take off the insulation tape, unwind the secondary, and wind 8 or 10 secondaries of smaller wire, add leads, and re-wrap the transformer tape? Then make you a PCB with multiple fullwave rectifiers and smoothing capacitors for each output, Add IC regulators if you want to fancy it up. The 100W toroid core will provide fairly stable AC voltages that won't change much with load current.

Result:
No switching noise, easy to filter the 120 hz. hum, AND you can make special outputs to power those rare birds that require AC voltage inputs, AND you can make output voltages other than 9v to run effects that shine on 12v or 18v. And if you make more secondaries than you need to plan for future acquisitions, you can just tape the end up and leave them until you need them.

Yeah, it will take most of a day to wind, but you can listen to music while you do it, right?

I've rewound many toroids over the years, and calculating turns is 8th grade algebra.

If you want isolated, bite the bullet and wind your own transformer.IMHO.

mr coffee