New Posts New Posts RSS Feed - Phono stage ground and electrical safety
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

Welcome to the Graham Slee Audio Products Owners Club

 

Open to all owners plus those contemplating the purchase of a Graham Slee audio product wishing to use our loaner program: join here
Subscribe to our newsletter here (Rules on posting can be found here)


Phono stage ground and electrical safety

 Post Reply Post Reply
Author
mw_lake View Drop Down
New Member
New Member


Joined: 18 Feb 2020
Status: Offline
Points: 3
Post Options Post Options   Thanks (0) Thanks(0)   Quote mw_lake Quote  Post ReplyReply Direct Link To This Post Topic: Phono stage ground and electrical safety
    Posted: 24 Mar 2020 at 11:33pm
Hello,

My new Gram Amp 2 SE has been such a revelation that I recently acquired a Systemdek IIX 900 to provide a better source.  On opening up the turntable I discovered that the motor casing, aluminium sub-chassis and tonearm are all connected (by daisy-chaining) to the audio ground post and there is no earth through the mains plug (the mains cable is two-core).  I can of course connect the turntable audio ground to the ground on the Gram Amp, but given that the latter appears to be double insulated with no route to earth of its own, am I correct in thinking this means that when used with the Gram Amp (or any other double insulated phono stage) the turntable is unearthed from a safety point of view?  I'm guessing that the design assumed grounding through a (pre)amplifier with a route to earth via a three core mains cable, but my use of a double insulated phono stage falls outside the usage scenarios envisaged back in the 1980s.

My Systemdek is one of the later models with a rectangular aluminium armboard.  Judging from various photos of the innards that I've seen online it appears  that at least some earlier Systemdeks which had a metal cover on the bottom of the plinth were in fact earthed via a connection from the motor to a three core mains cable.  Curiously, those photos also show daisy-chaining of the metal cover, motor and sub-chassis, meaning that the audio ground and safety earth are connected, which if I understand correctly is inviting a ground loop (although it is not clear to me how the tonearm was grounded given that the earlier armboard was wooden).  Indeed, I wonder if that is why the designers switched to a single ground?  If so, then I guess they assumed that electrical safety would be provided by a route to earth through a (pre)amplifier with a three core mains cable, along with replacing the metal cover on the bottom of the plinth with a sheet of hardboard.  If I've got this right then my usage scenario defeats the former and I doubt the latter would pass contemporary regulations for double insulation (not least because the motor adjustment screws on the top of the plinth could become live if a fault caused the motor casing to become live).  I imagine a better arrangement would be to connect the motor and sub-chassis to earth via a three core mains cable and then connect the tonearm alone to the audio ground.  Is that the way to go?

Thanks, Mark
Back to Top
John C View Drop Down
Admin Group
Admin Group

gspaudio moderator

Joined: 13 Jan 2008
Location: Sheffield
Status: Offline
Points: 135
Post Options Post Options   Thanks (0) Thanks(0)   Quote John C Quote  Post ReplyReply Direct Link To This Post Posted: 25 Mar 2020 at 6:14pm
Hi Mark,

A manufacturer should not reply on another piece of equipment to provide a safety connection elsewhere in the system and if they have supplied a product with a 2-core cable it should be 'Class 2 - double insulated'. This type of equipment does not require a safety earth connection, just like all our phono and headphone amps. It is not always a good idea to make an earth connection to a piece of equipment where it should not be connected.
An audio grounding connection is not the same as a safety earth and these are not always connected together. In some cases where hum may be apparrent it would be OK to try a connection from your turntable ground to mains earth but this should really not be necessary. 
The turntable ground connection on our amplifiers is connected internally to the power supply ground (0 volts) which should do the job.
You may also see on some amplifiers a 'ground lift' switch which will disconnect safety earth from audio ground circuits internally where hum problems may be apparent.
I spent many years building and installing pro audio recording and broadcast studios where we would generally keep all audio grounds separate and connect to their own grounding rod/spike. Also 7 years in electrical inspection and testing means I am very familiar with these kind of issues.
I would be interested to know what motor is in there so if you would like more information on what to do with your specific turntable please give me a call on either of the first two numbers listed at the bottom of each web page or drop me an email to john{at}gspaudio.co.uk and I'll be happy to help.

Best wishes
John
Back to Top
Graham Slee View Drop Down
Admin Group
Admin Group
Avatar
Telling it as it is

Joined: 11 Jan 2008
Location: South Yorkshire
Status: Online
Points: 9274
Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 25 Mar 2020 at 9:35pm
Class II double insulation is a guarantee of electrical safety and provided it has been done to the letter, no safety earth is required.

Approval of some description will help settle the mind. Still, it can be self-declared by the manufacturer (at "pain of death" if discovered to be false), and either way, the box in box double insulation mark must appear somewhere on the product, or at least on its packaging or instructions.

Any touchable metalwork on class II double-insulated product must have been flash tested at high voltage to ensure no connection between the mains side and user side can exist. Creepage and clearance distances must be complied with.

If the product complies, then it should be safe to collect all possible sources of ground potentials together. Motor frame; arm; and any other exposed metalwork, to the phono stage (or say an integrated amplifier input) metalwork, such that no noise potential can arise due to it being shorted together as such.

As this is not electrical safety earth, the receiving equipment (the phono stage, etc.) is not required to provide that function. And, indeed, many integrated amplifiers might also be class II double insulated, without safety earth.

You might ask, then where does the noise go? It cannot exist where all grounds are connected such that there is no potential difference between them. Should the ground be left off the phono stage, then the noise potential will be different between the turntable and phono stage, allowing a noise voltage to develop in the screen or shield of the turntable interconnect.

Sometimes a turntable can be quiet enough for all the grounds to be connected through the screen or shield, as with some plastic and wood designs.

The grounding post connects to signal ground within the phono stage or other receiving device, and the input ground should be a direct connection with any metal chassis.

Admittedly all this is a bit technical, but I trust I have explained it understandably.

Not simple enough for Google-Bot to understand...
Back to Top
mw_lake View Drop Down
New Member
New Member


Joined: 18 Feb 2020
Status: Offline
Points: 3
Post Options Post Options   Thanks (0) Thanks(0)   Quote mw_lake Quote  Post ReplyReply Direct Link To This Post Posted: 27 Mar 2020 at 6:04pm
Thank you both for your comprehensive and helpful responses.  

Closer scrutiny of various photographs online suggests to me that Systemdeks have been earthed in three different ways: i) cover plate, motor, sub-chassis and arm connected to both safety earth and audio ground; ii) cover plate alone connected to safety earth, everything else to audio ground; iii) no safety earth, wood (hardboard) cover plate, everything else connected to audio ground.  The third arrangement clearly treats the turntable as if it were double insulated, but on mine there is no class II symbol and I'm not convinced it would past the tests (for instance, exposed screw heads provide an electrical  connection to the motor casing).  I can only speculate whether all three arrangements are ex-factory, or whether one or more are the result of subsequent user modification.

Following discussion with John I am inclined to disconnect the motor and control board from the audio ground and instead connect them to safety earth.  It will be interesting to see what effect, if any, this has on hum.

Again, many thanks, Mark

Back to Top
John C View Drop Down
Admin Group
Admin Group

gspaudio moderator

Joined: 13 Jan 2008
Location: Sheffield
Status: Offline
Points: 135
Post Options Post Options   Thanks (1) Thanks(1)   Quote John C Quote  Post ReplyReply Direct Link To This Post Posted: 27 Mar 2020 at 6:58pm
Always happy to help Mark.
Please share your findings on here as you carry out the modifications.

Cheers
John
Back to Top
 Post Reply Post Reply
  Share Topic   

Forum Jump Forum Permissions View Drop Down

Forum Software by Web Wiz Forums® version 12.01
Copyright ©2001-2018 Web Wiz Ltd.

This page was generated in 0.078 seconds.