3
\$\begingroup\$

I'm designing an op amp based phono preamp. It has very high gain (60 dB for 20Hz, 40 dB for 1kHz) and the signal from phono cartridge is very low (like 5mV amplitude). So it's a very sensitive circuit.

As you may know signal from the turntable goes with coaxial cable + separate grounding wire to minimise hum and noise - current returning to cartridge has different path than noise going through the separate gnd wire.

I wanted to apply this philosophy to the input stage of my preamp - my goal is to make the cartridge return current path separate from the ground plane with all the contaminated currents that it picks due to EMI, 50Hz hum etc.

Is making a separate ground for the input stage and then linking it to main ground going to help me to lower the noise floor? I want to make the grounds separate and join them with low value resistor (like 10 Ohms to make sure that none of the "contaminated" current will flow through the phono cart return path.

I also include the schematic and PCB design.

The schematic:

schematic

PCB:

PCB

\$\endgroup\$
3
  • \$\begingroup\$ Gnd2 is the "bad ground"? \$\endgroup\$
    – Sim Son
    Oct 17, 2020 at 2:32
  • \$\begingroup\$ I think I would try it with a solid ground plane first. But if you want you can use a diff amp in the front end of your system. Connect clean ground from the stylus to the negative input and signal to the positive. Don't tie the grounds together at all on your board. I am not in a position to guarantee that this will work well but it seems like a solid idea. After the diff amp you can put your gain stage. The diff amp output impedance will be much lower so it will be less sensitive to noise pickup. You can have some gain in the diff amp if you like, too. That may also help with noise. \$\endgroup\$
    – user57037
    Oct 17, 2020 at 3:12
  • \$\begingroup\$ GND is contaminated ground. GND2 is a cartridge return path. \$\endgroup\$
    – cubix
    Oct 17, 2020 at 8:30

1 Answer 1

-1
\$\begingroup\$

Attach that fifth WIRE, from frame of the tone_arm, to the chassis of the PreAmp.

This greatly lowers the electric fields attempting to couple from the tone_arm metal, onto the twisted pairs (Left and Right pairs) inside the tonearm.

The source of these fields? In one case, the Efield trash was from primary_secondary coupling (thru fish_paper) of the tiny transformer powering the turntable motor.

\$\endgroup\$
1
  • \$\begingroup\$ I will do that anyway, i want to use some extra techniques lowering the noise floor more. \$\endgroup\$
    – cubix
    Oct 17, 2020 at 8:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.