I found several Dell laptop chargers on Finn ("Craigslist"), rated at 180W and 240W, 19.5V.
I have no idea if they're any good when it comes to switching noise, but I've bought a 180W one for testing.
The adapters have a 7.4mm/5.0mm barrel connector, with an inner pin of < 1.0mm. I've found a 7.5mm/5.0mm connector from JLCPCB rated at 15A/30V, which should be ok even for 240W:
https://jlcpcb.com/partdetail/XUNPU-DC_505ACL060/C30607513
Also, I've found second hand wire harnesses from Dell laptops, that converts the barrel into 6 x 19.5V, 6 x GND and a single wire for signaling. It is hard to figure out what the ratings of these are. I've tried searching for the ones used in the top-end laptops, the ones most likely to use the 240W PSU. So far I've landed on the "0J60G1 / J60G1 / DC301015A00", which is used in some of the Dell Alienware PCs.
I've ordered a cheap version off Ali Express, along with a 7.4mm/5.0m PCB connector and a 7.4 to 4.5mm converter with 20AWG wires inside - these are too thin for my fully spec'ed synth, but will let me cut the wire to get access to the wires, letting me measure ripple etc.
I may also end up cutting the plug of the 180W charger if I get impatient, it was only about $15 on Finn.
As for the center signaling pin, this is quite interesting - it lets me read the type of charger plugged in - I could for example check if the wattage rating is high enough for the synth. Not a big deal but a nice addition if I want to do proper power management - after all, I think I have full control of powering up the secondary SMPS'es - if the PSU is too weak I can leave the voice card unpowered.
So - Dell adapters, are they usable for what I want to do? Time will tell...
Making the circuit tolerate switching noise
This is a big one. Even the secondary SMPS'es will have some switching noise, perhaps as much as 80mV. 80mV on the VCO pitch CV will be a disaster.
Let's assume for a second that the ground rail will stay somewhat clean and the noise is only present on the 12V/-12V power rails (not sure if this is true though).
As far as I understand, most op amps will not care much about this noise, they go about their business regardless, so I will have to look at other places the supply voltages are used. One place immediately springs to mind - the base voltage for the 1V/oct input on the VCO. This is connected via a resistor to the power rail. I may be able to get around this by using a voltage reference instead.
Most other stuff in the circuit is hopefully more resilient to noise, as the noise will mostly affect the amplitude of the signal. We will probably have the same issue around filter frequency and resonance controls.
In short, I have to go through the voice card and modules to look at where the power rails are used directly.
I suspected this would come back to bite me - I've read several times that one should not use the power rails as reference voltages, but it's so darn convenient, they're already there. Oh well, you live, you learn...
