phono preamp hum - ground loop?
Lets suppose the power supply need to find a return path for the fast-edged diode currents. Out through the power transformer of the turntable is a fine path. Assume 100pF primary-secondary, with 20 volts from the WallWart, spiking with 10 microsecond edges. [1uH don't do much to stop 50KHz spikes.] What is the current, seeking a return path?

What is the impedance of 100pF at 50KHz? Z = 1/2*piFC = 10^+10/50,000 = 200Kohms[error: 10^10/(6.28*50,000) = 30K ohms]

Assume 50mA total average current for the RIAA; the raw-DC droop is 
dV/dT = I/C, dV = I*T/C
$$dV = 0.05 * 1/50 / 0.005Farad = 200 * 0.05amp/50Hz = 4 *0.05 = 0.2 volts$$
This droop is substantially larger than 0.026 of the diodes; we'll assume a 1amp surge to refill the capacitors (1amp is easy math).

We need some voltage somewhere, to drive current to the Turntable transformer.
Use the inductance of WallWart power cable: assume 1uH.
V = L * dI/dT = 1uH * 1amp/10uS = 0.1 volt. Aaahhhh now we can predict how much current needs to leave the region of the VDD filtering, flow through the RIAA opamp "grounds", and continue on over to the Turntable.

0.1volts/ 30Kohm = 3uA................we just predicted 3uA through the "GND" foil. How many squares? at 0.0005 ohms/square?

The only GND path I see is at the top, approximately 20 squares, or 0.01 ohms.
3uA * 0.01 ohms, is 30 nanoVolts. [we ignore inductance of that thin PCB region]

For a 3 millivolt MovingMagnet, that is -100dBc. 

For 100uV Moving Coil, that is -70dBc. With all the high-frequency diode surges.



The Turntable Transformer path explains why some turntables "cause more hum": their power transformers are wound differently.

Note the WallWart voltage plays into this. With 20 volts peak, the slewrate is 20*377 ~~ 8,000 volts/second. The diodes need 0.026 volts for 2.718... factor of current increase. Thus 8,000/0.026 = 320,000 or 3uS turnon. We assumed 10uS. Of course, up near the peak of the 50Hz power, slewrate is much slower; 0.2volts droop puts our diode-turnon right near the peak. So maybe 100uS Trise.



Go to diyaudio.com, to the forum/thread "Simplistic NJFET RIAA" and check out their bulk_DC_rectification methods. They achieve, by my estimate, 1 nanoVolt floors.