Ground Wire - Signal or Chassis?

[TriodeLuvr]

Do most turntables connect their ground wire to the internal turntable chassis, or is it connected to the signal (coax) ground? Or both? I'm building a phono preamp that will have its ground post connected only to the chassis and earth ground. Signal grounds, including the input/output RCAs, are isolated from this. It's not clear to me whether this is what turntable manufacturers expect, or if the ground wire is intended to connect to signal ground at both ends.

Jack

 

[jusmee]

I don't think they adhere to any standard. It seems it is different each time you combine new equipment. Some don't even have a ground wire, the signal ground doing both jobs (or looked at another way, only signal is grounded and the equipment is double insulated and doesn't require a ground). Other factors, such as the cartridge being grounded to metal at the headshell or not, also change the situation. It's always a case of trying multiple ways of connecting the ground wire (or not) and using whatever results in the lowest noise.

 

[muovimies]

It varies, most turntables have a separate ground wire and keep them separate. Notable exceptions would be Rega and at least some (vintage) Thorens, which have the ground wired to one of the RCA shields. Actually I think a lot of European vintage turntables from the era of DIN-connectors used to do it this way at least when wired for RCA.

I think keeping them seperated in the preamp you might run into scenarios where it becomes a problem especially with turntables which ground the signal negative? I might be mistaken but I think the common practice is to have all the grounds together in the preamp.

 

[gadget73]

The Thorens shares one signal ground with the chassis ground, but they can be modified to split the two circuits. There is a service bulletin that explains this as a hum fix. I'm not sure the specific conditions that require this though. The only time I've ever had hum issues with mine is when using a cartridge with metal mounting ears that also had a ground strap between cartridge body and one of the signal grounds. That creates a ground loop but insulating the cartridge or removing the strap shuts it up.

For most setups it probably wouldn't matter though since the outer ring on the RCA and the chassis of the phono pre are low ohms to one another. I don't honestly know what a table with split connections would do if the chassis went to the phono pre chassis and had no connection to the signal ground.

 

[TriodeLuvr]

The Thorens shares one signal ground with the chassis ground, but they can be modified to split the two circuits. There is a service bulletin that explains this as a hum fix.

Yes, I read that in the Thorens service manual just before posting the question. It says separate grounds might be necessary with some combinations of components, and that a separate turntable chassis ground wire might or might not even be needed. Not very helpful for designing a preamp.

My current all-in-one preamp with phono ties the RCAs and signal grounds to the chassis. This initially created a hum problem with my Citation V in all modes, but isolating the internal power supply/signal ground of the Citation from the chassis resolved it. Since then, I've been building gear with separate grounds to eliminate the possibility of 3-wire ground loops.

Interesting, I just measured the ground wire from my mid-'80s Denon turntable. It's infinite ohms to the RCA grounds. That apparently means it's only connected to the turntable chassis. I don't see how that could cause an issue regardless of how the phono pre is configured. Guess I'll continue forward and mount the ground post to the chassis.

Jack

 

[gadget73]

Turntables typically don't show any connection from signal ground to chassis ground if they have the separate lead, but phono preamps often do. If there was a connection to ground at both ends you get the loop problems.

 

[ripblade]

Turntables with interchangeable arms usually have their own dedicated ground wires. The arms themselves will likewise have their own dedicated ground leads which may or may not be bonded to the signal grounds. Record decks that are sold with integral arms often have the table and signal grounds bonded together, whether or not they use a dedicated ground lead.

I prefer to keep chassis and signal grounds separate, if possible, but so long as there's no hum either arrangement will work.

 

[cannext]

and to make things even more complicated ,some cartridges connect one signal ground to their outer metal case (shield ) this can potentially get electrically connected to the headshell (metal screws ) which connects to the tonearm which in most cases is connects to the turntable chassis which if this is connected to the amp with a separate ground wire makes a nice ground loop .
Welcome to the wonderfull world of turntables .
I would build it as you are stating in the first post .
Regards F.