Where to connect the chassis lead from a record turntable?

[JohnJohn20]

I have a record turntable and an oldish amplifier which has an earth connection for the turntable (as well as two RCA leads for the signal) which works well except that the amplifier is getting noisy (old capacitors maybe?).

My new NAD amplifier had no phono connection so i dismantled another amplifier, took the phono preamp out of it and connected it internally in the NAD amp inline with one of the inputs.

This works good except that I now have this annoying 50Hz hum when I use the turntable connected to that input.

I tried connecting the turntable earth/chassis lead to the amp input ground (which made no difference), the amp 0V (which increased hum) and the amp chassis (which increased hum).

And if the turntable RCA leads are disconnected the hum stops.

I would like to know:
1. Where should the earth lead connect to inside the turntable? It looks like all the earth terminal (where the earth lead connects) has wires going to two places on the stylus arm only.
2. I would have thought 0V in the amplifier (which is also the common connection for the phone preamp input and output) was the obvious place to connect the earth lead from the turntable. What other place could possibly be better?

Thanks for reading.

 

[kam1787]

could be cable grounding problem, is the cable shielding grounded? how? both ends? one end - which end?

or power supply/ground induced - how did you connect the pre-amp to the amp's power supply - any filtering?

 

[JohnJohn20]

Hi kam1787, thanks for your response.

To connect the preamp inputs and outputs I disconnected the L and R tabs of the RCA sockets where they connect on to the amp mainboard (leaving the common/earth connected) and used unshielded wires (50mm long) to connect the preamp inputs to the RCA socket tabs and the preamp outputs to the mainboard.

The preamp power in V+, V- and 0V connects directly to the amp power supply through wires about 150mm long. No filters, just 1k 1W resistors in the V+ and V- lines and 15V zeners to drop the voltage down from the 50V. The preamp board does have 10uF capacitors between 0V and the V+ and V- connections.

So the common/ground signal connection from the turntable connects to the amp 0V which connects to the common/ground of the preamp via the 0V power connection to the amp power supply.

So. I had a poke and I noticed that if I touch or hold my finger near to the unshielded preamp input and output connecting wires increases the hum so I think the first step is to replace these with shielded wires.

I intend to connect the shielding to the preamp 0V and leave the other end of the shielding unconnected (so to avoid earth loops).

Is that good practice?

Thanks.

 

[kam1787]

sounds about right

is the grounding all at a common point?

 

[chuckey]

From memory, the wires inside the pick up arm are normally unshielded, they will then terminate on a tagstrip under he turn table where the "live conductors got to the centre conductors of coaxs and the earhty wire And the braids are connected to an earthed tag (screwed to chassis).
Somewhere around there should also be a safety mains earth to protect against mains faults. A proper NAB input preamplifier will have a big bass boost of 20-30 DB to get the sound right.
It could be helpful if the coax braids were not connected to the chassis, a similar effect can be got by disconnecting the mains earth but it is unsafe to do so.
Been there done that :evil: what a pain!
Frank

 

[JohnJohn20]

Thank you both.

Yes kam1787, all the ground wires goto a single point on the preamp board.

My turn table has two of the signal wires from the cartridge going to the coax center conductors and two to the outer ones.

I will try disconnecting the coax outers at the turntable end and then have the earth side signal wires from the cartridge connecting to the tone arm casing which will then connect via the earth/chassis cable to 0V on the amplifier.

Changing the signal wires inside the amp to coax ones has made a big difference.

Thanks.