boji said:
Still trying to learn how to implement RC iso. If I understand correctly, the regulator is the iso, and the RC stabilizes? Not much online specific to audio consoles. I'll see if there's something in Self's Small signal design.
The RC isolator isolates the transient currents away from the power supply, and re-routes those transient currents through the shunt capacitor so that they are sourced directly from the local ground. The price of all of this is that the voltage at the top of the shunt capacitor can bounce along with the transient current drawn by the amplifier from the isolator cap - that cap is now driven by 100Ω and not a low impedance supply, so if you draw current from the shunt cap, it cannot re-charge instantly, since it is powered through the series isolator resistor. However, that's what you want - that's how the transient current is kept away from the power supply.
To counteract this load current induced 'voltage bounce', you use series regulators between the isolators and the amplifier circuits you're powering. This provides a nice low impedance supply to the amplifier circuits, isolated from the actual power supply by the isolators.
One thing to keep in mind is that the regulators driving the circuits need a voltage margin (dropout voltage) to work properly, so the desired output voltage plus the regulator's dropout voltage should be at least a volt or two less than the main supply voltage minus 2x the idle current drop through the isolator resistor. As an example: say you have ±24V and you want ±17V at your circuitry: you have a combined 24-17=7V of drop that you can lose across your series resistor and in your post isolator regulator. Modern regulators require little voltage drop, maybe only a half volt, but they can work better with several volts. The isolators also should not drop too much voltage when the circuit is idling, since the circuit will probably draw more supply current when it supplies signal to a load. So, you should do some thumbnail calculations and then once you get things running, verify that your assumptions made sense with actual operating conditions using actual signals.
So the ADP7142 and ADP7182 are good up to 200ma so that would mean a 2 IC's per channel (A ton of them).
Yes, the ADP7142 and 7182 are really tiny. In my world, they work well for one amplifier circuit plus a servo and maybe a buffer, or maybe a couple of these if you're lucky. But, they're small, and their output capacitor requirement is very modest, making them work with my world well. In my quest to make current loops among multi-module circuits exceedingly small, using one regulator per "tiny circuit chunk" makes sense.
However, LM317/337 are a lot cheaper and produce much more output, so those can be preferable if you use the regulator to power more than just a tiny circuit. TI makes some nice modern regulators as well - the TPS7A33x and TPS7A47x are similarly modern and high performance as the AD regulators, but can source 1A and have larger packages to dissipate the heat. There are relatively few + and - regulator pairs, but these should suffice for most uses.
What I don't get is if the PSU is rated 800 watts and 24 channels are drawing roughly 200w, does the crosstalk come from the supply (slew?) or the fact that the bars have a small amount of inductance?
The crosstalk isn't gonna be huge, but it will come from the fact that the power supply will have a finite output impedance, the power rail leads will have some inductance and resistance, and the local circuit ground is so far away from the regulator's ground that the two won't be tightly coupled. None of this is huge, but with many lower end, virtual earth mix bus consoles from the 70s and early 80s, a lot of 'gunk' seemed to build up using 16+ channels at a time, making a multitrack mix get pretty murky. It seems clear to me that if you could make the current path of each amplifier in a console much smaller and more local, this 'gunk buildup' would be reduced. The isolators do this by removing transient current from the power supplies and sending it to the local ground. The regulators are there just to make up for the voltage bounce that the isolators cause, and to provide quality, low impedance rails to each chip, which can be only a few millimeters away if you lay it out like that.
Edit: Ok so the LM317 and 337's can supply roughly six times as much current as the AD's, but they would be working fairly hard per 8 channels. Also I'd need to replace the two PSU's to get the higher voltages required. If I go with what I got, (Wheatstone TV800, Amek Big SMPS) would large lytics near each point of distribution be at all helpful?
No, large post-regulator caps aren't needed in this system since the RC isolators will eliminate the transient load to the supply anyway. Use them if the supply requires them for stability or for low noise, but again, that's the point of the isolators - you don't need the supply to do much more than source the bias current of your amplifiers, along with a heavily lowpassed "extra draw" when power is sourced to a load. The shunt C moves all of the transient current to ground, so it does not have to be sourced from the supply.
Many supplies can be re-worked to provide a higher regulated output voltage by adjusting a pot or a couple of gain resistors, but the drop across the series R might be too much to 'squeeze out' of an existing design. On the other hand, 24V is a somewhat standard supply voltage, so you might be able to ditch the huge old supply and get something smaller, maybe even a switcher, since its output will be heavily filtered, and it's not doing much more than providing 2x the idling current.