Yikes. That mixer doesn't look like it would chew up that much current. There are much bigger consoles that don't use 5A on one rail. Those fader motors are hungry.Hmm... I guess my measurings were off....
I managed to measure up to about 5A when using all faders at once, and up to 8.5A on the 5V line.
What's the voltage across it? I would think the transistor would be taking most of it. But between the two it is 12W or so. You should probably stick to the safe side and get a little piece of aluminum and use resistor and transistor packages that you can mount on there. You could probably get away with a ceramic deal and a PCB layout heat sink for a TO-220. But I would over design for a one-off especially if you don't have a lot of experience with power bits.Anyways, I've tried to simulate that shunt circuit. The 10R resistor will get quite toasty, but I can manage that heat.
If 12V is just for the motors, that'll be fine. If the voltage dropped even more it probably still wouldn't be a problem, the faders would just get a little slugish. But at low 11's you won't even notice.The 12V line does go down to the lower 11's during heavy work, but I think it will ok.
I would simulate it with a variable load. Meaning put a current sink in place of R9 and modulate the current up and down between 0A to 5A at 1Hz or so. Then look at the dynamics of everything and verify that the shunt is really doing what it's supposed to. I've never made a shunt regulator like that. This is all theoretical.Here's the circuit simulation:
You could try it. But again, you then cannot have a lot of capacitance on the line because the SMPS might go into hiccup mode. And if you were hearing a whistle, that implies that something is bleeding into the audio. But that could just be a completely separate problem. We probably should have explored that to completion first before designing circuits.Yeah, I'm using some heavy duty wires for the 5V. Luckily there's a large solid aluminium tray running through the gut of the mixer, that will work nicely for dissipating the heat of the transistors - I think I'll just get one of those small but beefy power resistors, that can be mounted on that tray.
I do wonder if I should just forget about the CM on the 12V line - since this is only supplying the motors, would there be any real benefit of adding the CM after the current shunt?