Hum help for newbie

[meche]

For my first DIY audio project, I assembled a somewhat modified version of the Elliott Sound Products phono preamp and power supply.
(Much appreciation to ESP for the free schematics)
I hooked it up and turned it on and got nothing but 60Hz hum.
The attached PDF has diagrams & photos of the setup.
By trial and error, I played around with the grounding and settled on leaving the signal and preamp grounds connected, but separating them from the power supply/center-tap ground, and not connecting anything to the mains ground. My preamp box is plastic, so the incoming RCAs are insulated from the chassis. This got rid of the hum, but I still don't get any of the phono audio. If the ground wire from the turntable isn't connected to the signal/preamp ground, I pick up radio interference (audible voices), so I know my amp is amplifying *something*.
I've checked the turntable & poweramp; everything else is functioning properly.
I have a feeling that the issue is somewhere in the grounding or power supply design, so that's why I'm posting here.
BTW, my power amp has a 2-prong power cord, so isn't connected to the mains ground.

Many thanks for any help.

 

[ph_christensen]

if you seperated the grounds from the signal and the preamp and are using a plastic box, you created an artifical ground. The ground from your signal, preamp, and PS have to be common. I would suggest reconecting all of the grounds and using a 60hz filter on your PS.

 

[gmphadte]

There are other issues with the assembly...

The box should be of some metal
Edit: U can paste some metal foil inside the box
The transformer should be shielded
The input and output wires should be shielded.
The ac wiring should be away from the IC and its connections.

Gajanan Phadte

 

[jan.didden]

You need to run the gnd wires from the input and output RCA's to the points labeled 'gnd' in your diagram, preferably as near as possible as the actual signal pin on the board. That should do it.

Did you measure the supply voltages at the chips?

BTW The value of C4 seems excessive. You sure that's OK?

Jan Didden

 

[AndrewT]

BTW The value of C4 seems excessive. You sure that's OK?

why?
820r & 82nF gives 67uS.
A bit more impedance in parallel to 820r will raise the filter even higher.
It might require a slightly larger cap to give the 75uS for RIAA.