Hi there! i already have all the parts needed for building this pre: Op-amp Based RIAA Phono Preamp for MM and MC Phono Cartridges
This will be my first project and as you can imagine i have lots of doubts... Many of them are probably silly but the one i think is important is where should i connect the turntable ground (the turntable has a separate ground wire alongside rca outs).
The creator of the pre says "A ground terminal is affixed to the case for connection to the turntable ground" and two wires i cant tell where they come from are connected to that terminal. Where does this two cables go?
It troubles me that in other place he says "The 150 ohm and 0.1 uF type X2 capacitor are the only things that should be connected to the case" since the ground terminal isnt isolated from chassis.
Any help would be welcomed since im already late for the building (the pre was intended to be a birthday gift
)
Regards!
This will be my first project and as you can imagine i have lots of doubts... Many of them are probably silly but the one i think is important is where should i connect the turntable ground (the turntable has a separate ground wire alongside rca outs).
The creator of the pre says "A ground terminal is affixed to the case for connection to the turntable ground" and two wires i cant tell where they come from are connected to that terminal. Where does this two cables go?
It troubles me that in other place he says "The 150 ohm and 0.1 uF type X2 capacitor are the only things that should be connected to the case" since the ground terminal isnt isolated from chassis.
Any help would be welcomed since im already late for the building (the pre was intended to be a birthday gift
)Regards!
Well apparently i needed to write the thread to realise maybe the to wires come one from the resistor and one from the cap... Still not sure though. Probably for some strange reason its better to have the node at the chassis terminal rather than at the board.
Need to stop overthinking stuff and start frying stuff i guess (this may be a good advice to many firstimers, as long as u dont fry yourself)
Need to stop overthinking stuff and start frying stuff i guess (this may be a good advice to many firstimers, as long as u dont fry yourself)
The circuit signal ground is not directly connected to the case to prevent ground loop currents (magnetic interference) which would impose hum directly on the signal. The 150 ohm resistor breaks the loop and reduces any currents by a large factor. (The loop involves the external wiring, without series resistance fairly large currents can flow in the signal wire shields, causing IR voltages that directly add to the signal as perceived by the preamp input stage).
The capacitor is needed to short out high frequency noise which could otherwise capacitive couple from chassis to preamp circuit.
The chassis ground post should be connected to the turntable via a wire running closely alongside the signal cables (there's special twin-coax plus ground wire cable available for this purpose). The provides shielding to reduce capacitive interference on the cartridge and its wiring. No other ground connection to the turntable should be made - the mains side of a turntable for the motor is commonly double-insulated so the turntable chassis floats until the preamp ground wire is connected - failing to connect the ground wire will lead to obvious levels of hum.
Note that the photo shields are very specifically not connected to the chassis in this design.
The capacitor is needed to short out high frequency noise which could otherwise capacitive couple from chassis to preamp circuit.
The chassis ground post should be connected to the turntable via a wire running closely alongside the signal cables (there's special twin-coax plus ground wire cable available for this purpose). The provides shielding to reduce capacitive interference on the cartridge and its wiring. No other ground connection to the turntable should be made - the mains side of a turntable for the motor is commonly double-insulated so the turntable chassis floats until the preamp ground wire is connected - failing to connect the ground wire will lead to obvious levels of hum.
Note that the photo shields are very specifically not connected to the chassis in this design.
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.