Hi All,
I have a turntable (TT) that has its metal chassis grounded to protective earth (PE), as would be expected. Also grounded to this chassis (via a tiny ~30AWG wire) is the tone arm, internal cartridge wiring shield and the “grounding wire” – a small diameter wire (~20AWG) that is intended to be connected to the chassis of the pre-amp. The pre-amp consists of two metal enclosures – one is a PSU and is connected to PE, the other is the amp itself and is galvanically isolated from the mains via the trx and it’s appropriately rated insulation (the design and build has been checked by competent and qualified persons). If I connect the TT to this pre-amp without the “grounding wire” I get a 50Hz hum (as I expect). If I connect the “grounding wire” to the chassis of the pre-amp the hum goes away (again, as expected). The problem is that by connecting the “grounding wire” (referenced to PE in the TT) to the pre-amp the galvanic isolation of the pre-amp is lost, as the pre-amp chassis is connected to the 0v rail of the secondary of the trx (see attached sketch). I am trying to figure out the best way to solve this issue. The options that I see as possibilities are:
I have attached a sketch of the wiring.
Many thanks in advance.
I have a turntable (TT) that has its metal chassis grounded to protective earth (PE), as would be expected. Also grounded to this chassis (via a tiny ~30AWG wire) is the tone arm, internal cartridge wiring shield and the “grounding wire” – a small diameter wire (~20AWG) that is intended to be connected to the chassis of the pre-amp. The pre-amp consists of two metal enclosures – one is a PSU and is connected to PE, the other is the amp itself and is galvanically isolated from the mains via the trx and it’s appropriately rated insulation (the design and build has been checked by competent and qualified persons). If I connect the TT to this pre-amp without the “grounding wire” I get a 50Hz hum (as I expect). If I connect the “grounding wire” to the chassis of the pre-amp the hum goes away (again, as expected). The problem is that by connecting the “grounding wire” (referenced to PE in the TT) to the pre-amp the galvanic isolation of the pre-amp is lost, as the pre-amp chassis is connected to the 0v rail of the secondary of the trx (see attached sketch). I am trying to figure out the best way to solve this issue. The options that I see as possibilities are:
- Possibly the ~30AWG wire connecting the tone arm et al to the TT PE is unlikely there for safety as such a small wire couldn’t carry any significant fault current so perhaps this wire could be removed and the tone arm floated (it has no other connection to the chassis apart from this tiny wire);
- Possibly an isolation trx could be used between the Pre Amp and the TT’s grounding wire;
- Possibly the TT could itself be isolated from the mains with an isolation trx (this seems expensive);
- This I’m not entirely sure of, but could a “ground lift” device be used? As previously explained on this forum and others C and R in parallel along with a diode bridge is used to float a signal GND from PE. If this could be used, would the diode bridge be needed as the wires involved are effectively signal GND’s , although one is referenced to PE in the TT?
I have attached a sketch of the wiring.
Many thanks in advance.
Attachments
I don't see that you have a problem if you have removed the hum.
The grounding wire puts the turntable and the zero volt rail of the pre-amp at the same ground potential (the zero reference level used to apply and measure voltages).
This is what I have done with my Thorens turntable:
It had a two-wire fixed mains cable, and all metal parts were connected together, and there was also a "grounding wire" of the tonearm. Incidentally the tonearm base is mounted on an isolation block, and the whole tonearm is electrically isolated from the rest, but the grounding wire connected it to the subchassis. So everything was floating until the grounding wire was connected to somewhere.
I installed an IEC socket with protective earth instead. I connected all metal parts (motor chassis, subchassis) to the PE. The grounding wire (and only the grounding wire) coming from the arm is separated from the PE, and goes along with the L/R phono cable to the chassis of my MC SUT. It could go equally well to the chassis of the phono preamplifier. There is zero hum.
I see you have done something similar.
It had a two-wire fixed mains cable, and all metal parts were connected together, and there was also a "grounding wire" of the tonearm. Incidentally the tonearm base is mounted on an isolation block, and the whole tonearm is electrically isolated from the rest, but the grounding wire connected it to the subchassis. So everything was floating until the grounding wire was connected to somewhere.
I installed an IEC socket with protective earth instead. I connected all metal parts (motor chassis, subchassis) to the PE. The grounding wire (and only the grounding wire) coming from the arm is separated from the PE, and goes along with the L/R phono cable to the chassis of my MC SUT. It could go equally well to the chassis of the phono preamplifier. There is zero hum.
I see you have done something similar.
Whilst the hum has been removed, the connection of the PE referenced "Grounding Wire" from the tone arm to the pre-amp defeats the galvanic isolation of the pre-amp as the pre-amp 0V rail would now also be referenced to PE. So the problem is that I want to maintain the design philosophy of the floating and galvanically isolated pre-amp but wish to connect a TT that, from the manufacturer, has its tone arm ground wire referenced to PE.
I tried this, but it made little difference (perhaps slightly worse) to the hum. This seems what I would expect as the tone arm is already tied to PE in the TT - connecting the ground to the PSU box, also bonded to PE, only creates the potential for a ground loop. As I understand it, the tone arm and internal shielding grounding wire needs to be connected to the reference of the signal it is protecting.
It sounds like you have done something similar to the first option I thought might be a possibility - floating the tonearm and associated shielding.
Most modern T/T's use low voltage DC motors and wall warts of some description. In older T/T's, a mains cable ran to to a PSU in the plinth that then powered a DC motor (Technics, Pioneer etc). The power earth (PE) is usually separated from the tone arm ground and connected to the mains plug. The tone arm ground attaches to the back of the amplifier at the 'T/T Ground' and should be completely isolated from the T/T PE. By grounding the arm like this, you neutralize any capacitance between the pick-up wiring inside the arm and any external electric fields - they impinge on the arm, and are essentially conducted to the amp housing and ultimately the amp PE (assuming the amp PE is connected to the internal 0V). If you don't ground the arm in this way, noise is coupled common mode into the pickup wiring. Most unbalanced phono amps do not have good common mode rejection - easily proved by plugging an RCA terminated cable into the phono amp, shorting the other end out and then just holding it. Invariably, a lot of hum can be heard from the speakers.
Magnetically coupled noise can also be a problem - this is best resolved by correctly dressing the cabling - eg keeping mains cables away from the pickup and interconnects from T/T to amplifier. Most T/T's address this aspect quite well from what I've seen.
Note, if you connect the tone arm ground to the PE on the T/T, you are likely to get an earth loop and severe hum.
Magnetically coupled noise can also be a problem - this is best resolved by correctly dressing the cabling - eg keeping mains cables away from the pickup and interconnects from T/T to amplifier. Most T/T's address this aspect quite well from what I've seen.
Note, if you connect the tone arm ground to the PE on the T/T, you are likely to get an earth loop and severe hum.
Hi Bonsai, thanks for the detailed reply. It sounds like I should be able to remove the ~30AWG wire that connects the tone arm to the TT PE, floating the tonearm with respect to the rest of the TT, but grounding it to the pre-amp chassis and thus the signal reference via the "grounding wire".
The TT in question is a late 1970's Denon DP1200. According to the schematic I found for it, the tonearm was connected by Denon to PE via the ~30AWG wire at time of manufacture.
Yes, you should be able to float the arm and ground it to the T/T.
If the main metalwork of your T/T is connected solidly to PE, I would not worry that the tone arm grounding wire is thin. I cannot imagine a scenario where you would get a safety problem the arm. The thin wire is there purely to ground the arm for noise purposes - not safety.
Seems strange Denon did it that way, but if it works, you should be good to go.
If the main metalwork of your T/T is connected solidly to PE, I would not worry that the tone arm grounding wire is thin. I cannot imagine a scenario where you would get a safety problem the arm. The thin wire is there purely to ground the arm for noise purposes - not safety.
Seems strange Denon did it that way, but if it works, you should be good to go.
