Parallel split rail supply with only positive regulators

Question

I'm designing a +/-12V split rail supply for a eurorack (modular) sythesizer. I was adviced to put a pre-regulator block at a higher voltage a connecting to it several point-of-load regulators. Because of the transformer I have, I was required to use LDOs, and to save cost I tried to use only positive one's (the negative one's are three times as expensive, at least in through-hole version).

Here is the simplified schematic of what I've come up with:

The pre-regulator provides two 13.5V floating pairs of nets, which are then reduce to 12V at the load, and both GND nets are put together at that point. I think it works fine when only one POL regulator is connected, but it starts to break down when several of them are connected in parallel. The problem I think is that each U4 will not necessarily provide exactly 12V at their output.

Is there a solution that would allow me to keep the same regulators (which I already have)?

Answer 1

This all looks over complicated.

Pro analogue mixing consoles from the 1970's to the present typically use a single high current PSU that provides +/-17V (for powering standard audio opamps like NE5532, TL07x or modern equivalents). They often used the "upside down" regulator trick as you have here to reduce parts count.

The +/-17V is then distributed to the modules and at the input of each module is a 10R resistor mounted standoff - this is basically a cheap fuse (hence the standoff).

The advantage here is that the voltage everywhere is the same. Having local regulators, you can have perhaps 0.25V difference between supplies at different places, if one op amp then clips and feeds another with a lower voltage you can get weird things going on.

It's also just simpler and has worked fine in probably tens of thousands of devices sold for four or five decades.

NOTE : your schema is maybe simplified, but don't neglect decoupling caps and protective diodes around your regulators. Consult the device datasheet and don't skimp on this.

NOTE 2 : If you want to save time and hassle, you could consider building your module into an API module frame - there are compatible ones from many manufacturers and pretty good value. Save you a lot of grunt work, as long as you have enough front panel space for your interconnects.

NOTE 3: the higher the supply voltage the better your maximum output level, and therefore headroom and/or operating level, which in turn helps get the best possible signal to noise. There is nothing magic about 12V unless you have a very good reason to use that voltage.

Answer 2

It would be useful to know the power levels in question, and whether this for a one-off unit, or for commercial production.

If it is for commercial use, you will almost certainly need features such as safety certification, variable voltage input (110-250V), low weight (so no transformers), high efficiency and a good power factor. Unless your volumes are very high, and margins low, the easiest approach is to use off-the-shelf SMPS. These units can either be external (with 12V supply to your kit), and internal mount, with a direct IEC input connection, and internal DC connections.

If it is for your own one-off use, then I would also go the route of an off the shelf unit. You can easily get something that will "just work" and allow you to focus on the rest of the system. You will also avoid all the wasted heat you are going to generate by using linear regulators direct from a transformer output.

If you are concerned about switching noise, there are very good SMPS out there, but you can just use the SMPS as a pre-converter to say +/-18V, then go linear for the last bit. This is the approach used in many commercial audio systems.