Using pulsed power source and capacitors for driving solenoids
The principle of PWM drive is basically sound, but not as you've drawn it. 

That shunt capacitor you've shown is either not necessary, or is not sufficient. In either case, you don't want it connected to the FET drain.







simulate this circuit &ndash; Schematic created using CircuitLab

If those solenoids are laminated, ie designed for DC or AC excitation, then they can work with rapidly changing DC, and you don't need the capacitor at all, as shown in the lefthand diagram. The PWM frequency will be fairly tightly constrained. It must be a) fast enough to keep the solenoids pulled in without chattering/buzzing and b) slow enough for the solenoid laminations to not overheat. This may limit you to a 100Hz or few 100Hz PWM, which may cause an audible buzzing problem.

Your capacitor will store the rail voltage when the FET is on, and drive the solenoid at more or less full power regardless of the PWM duty cycle, so should be removed. The solenoid current will be continuous due to its own inductance. When the FET is off, inductor current will continue to flow through the shunt diode.

If the solenoids are not laminated, or buzzing as mentioned above turns out to be a problem, then they must be fed clean(ish) DC, as in the righthand diagram. The inductor and capacitor will act as a filter. The diode D2 is still required to supply the inductor current when the FET is off. This is not a SMPS, so the DC to the solenoid does not need to be clean to mV.

This allows a wider choice of PWM frequency, as the solenoid is not seeing the switching frequency directly. Something much higher, like 25kHz, would be appropriate. Being above 20kHz, you won't have to worry about audibility, and it would use much smaller filter components than 100s Hz.

As you have total control over the PWM, you could arrange for the switching across the mulitple channels to be out of phase, so reduce the peak current drawn when there are multiple keys driven.