|
|
Post by Priapulus on Dec 30, 2013 18:04:26 GMT -5
I've been rebuilding my turntable. Turntables often have a ground wire to connects to the preamp. After some Googleing , I've come to the conclusion that this wire has nothing to do with the signal ground of the cartridge connections, and nothing to do with the motor power safety grounding. It seems the purpose if the ground wire is to connect the preamp chassis to the metal hardware of the turntable: tonearm, ,motor, frame, platter. The idea is that metal parts can act as antennas and collect microvolts of ac powerline radiation, etc, which can cause hum if coupled into the cartridge circuits. (i.e. it isn't for the usual ground faults).
Am I correct? |
|
|
|
Post by novisnick on Dec 30, 2013 18:27:20 GMT -5
sounds plausible.I'm not sure.
What kind? year, model, etc. are you working on?
I listened to WYWH last night on my Technics SL-23 and swear it sounded better then a CD.Ive been pondering a newer TT but am having a hard time believing it could get better. No refference to listen to.
Nick
|
|
|
|
Post by mgbpuff on Dec 30, 2013 18:35:50 GMT -5
No. It has everything to do with the signal. If there is a ground potential difference between the TT and the pre and there is no ground wire between the two devices, ground currents will flow via the RCA interconnect shields causing noise in the very low level signal. A heavier ground wire between TT and pre will reduce and maybe even completely eliminate the ground current in the RCAs.
|
|
|
|
Post by Priapulus on Dec 30, 2013 18:56:14 GMT -5
> What kind? year, model, etc. are you working on?
Acoustic Research "The Turntable" (1985) , SME-3 arm, Shure V15-III cartridge
I love the solid wood base. This is the classic '80s audiophile turntable, highly rebuildable and upgradable. I'm refinishing the woodwork, rewiring it, new motor, pulley and belt. I've already installed a rebuilt SME-3 tonearm, which is the tonearm of choice for the classic Shure V15-III cartridge. |
|
|
|
Post by Priapulus on Dec 30, 2013 19:15:03 GMT -5
> No. It has everything to do with the signal. If there is a ground potential difference between the TT and the pre and there is no ground wire between the two devices, ground currents will flow via the RCA interconnect shields causing noise in the very low level signal. A heavier ground wire between TT and pre will reduce and maybe even completely eliminate the ground current in the RCAs.
What you say is true. But when I connect a grounding wire between the metal platter and the signal ground, where is the second ground that is required for a loop? There is only one wire (my grounding wire) to the platter. Aren't I simply draining any noise the platter attracts (as an antenna) to the signal ground? I can make it hum by simply touching my finger to the "hot" pin on the cartridge, am I a ground loop?
|
|
|
|
Post by mgbpuff on Dec 30, 2013 19:31:04 GMT -5
The tt ground wire does not attach to the platter, it connects to the tt platten (the base). The RCA connector shields provide a ground path to the preamp also. Usually the tt is grounded via the power connector as is the preamp. The ground wire will not be needed if these power grounds are adequate, but distance and perhaps different receptacles may cause a slight ground potential difference to exist between the two components. If that is so and the noise/hum is noticeable, then the separate ground wire between components may solve the problem
|
|
|
|
Post by Priapulus on Dec 30, 2013 20:59:20 GMT -5
> The tt ground wire does not attach to the platter, it connects to the tt platten (the base). The RCA connector shields provide a ground path to the preamp also. Usually the tt is grounded via the power connector as is the preamp. The ground wire will not be needed if these power grounds are adequate, but distance and perhaps different receptacles may cause a slight ground potential difference to exist between the two components. If that is so and the noise/hum is noticeable, then the separate ground wire between components may solve the problem
Everything you say is true, except that I don't know of any turntables with a three pin (grounded) power plug. I don't mean to be difficult, but it affects the way I decide to ground my TT. I'm also replacing the power cord; changing to a three pin may be a bad thing; I notice EMO uses 2 pin cords on their XSP-1, probably to avoid ground loops.
|
|
|
|
Post by mgbpuff on Dec 30, 2013 22:24:28 GMT -5
"Everything you say is true, except that I don't know of any turntables with a three pin (grounded) power plug"
True, even my tt is dc driven - no 3 prong ac power cord going to the tt. Well,then, the tt platform and tonearm are isolated from the electrical system ground except through the RCA shields to the preamp which may or may not be tied to ground through a 3 prong ac plug but which will be tied to ground via the RCA output interconnects going to the power amp which IS grounded via 3 wire plug. Therefore the ground makes it way through all components and may or may not cause hum and we are back to my first post. If hum occurs, add the wire connection tt to pre and perhaps pre to power amp. Ground problems are not always easy to solve.
Don't add a 3 prong power cord if the original was two prong and don't add the tt to pre or pre to amp ground wire unless hum is present.
|
|
|
|
Post by lionear on Jan 2, 2014 3:56:18 GMT -5
I think you're correct in most areas, Priapulus. But I disagree about connecting the motor. There is no "signal ground" that comes out of the cartridge. The four wires that are connected to the cartridge all carry signal: white = Left + blue = Left - red = Right + green = Right - You'll want to connect the tone arm's Ground connection to the Ground point on the preamp's phono input stage. This allows the arm tube and arm bearing assembly to act as a shield to the four signal wires. (You can experiment with having the Ground connection and leaving it unconnected.) You should NOT connect the drive motor's Earth connection to the Ground connection on the tonearm. - If the drive motor is an AC synchronous motor, the drive motor's Earth connection could theoretically be as high as 120VAC, 16A in a short circuit scenario. Even in normal operation, the drive motor's Earth connection may be carrying an AC leakage current. You certainly don't want this to go to the preamp's phono stage (or the tonearm). The drive motor's Earth connection has to go through the motor's power cord to the power receptacle in the wall.
- If it is a drive motor is a DC motor, then the Earth connection of the motor's power supply has to go through the motor's power cord to the power receptacle in the wall.
- Even if the drive motor's cable is a two-pin plug, the body of the motor may carry a high AC leakage current. This CANNOT be allowed to find its way to the tonearm and the Ground point on the preamp's phono input stage.
You should NOT connect the drive motor's Earth connection to the turntable frame and platter. The motor should be pretty much electrically decoupled from the platter and turntable frame. If there's an electrical connection, then I would NOT connect the turntable frame and platter to the tonearm's Ground. If they're electrically decoupled, you may want to experiment with connecting the turntable frame and platter to the tonearm's Ground connection. I don't think you'll need it. You might want to run all this past a Linn tech. |
|
|
|
Post by Priapulus on Jan 2, 2014 14:33:55 GMT -5
> lionear
> I think you're correct in most areas, Priapulus. But I disagree about connecting the motor.
Yes, I agree. A turntable ground has nothing to do with the AC power safety ground, and should not be connected. I will connect my turntable ground only to the metal parts of the tonearm, turntable frame and platter. Thank you.
|
|
KeithL
Administrator 
Posts: 10,273
|
Post by KeithL on Jan 2, 2014 15:19:55 GMT -5
I would usually agree with what you said.... but not always. The safety ground is generally intended to provide a safe path to ground for AC powered devices (if the internal insulation shorts, the power is shorted to the safety ground rather than going through the user when they touch it). This is a different purpose than the signal ground, which is indeed intended to ground all the shields and carry any noise intercepted by any shield or other metal component to ground (the metal tonearm counts as a shield). Most motors these days are well insulated - or isolated - and so don't need or "want" to be grounded. Often, it works best to keep these two (signal and power) grounds separate, but not always. The signal coming from a cartridge is very tiny, and a huge amount of gain is applied to it. This makes turntables very sensitive to hum and noise. Sometimes the theory falls a little short in terms of the best way to connect the grounds with a particular turntable and cartridge (and preamp). So, sometimes it does work best to connect signal ground to power ground; other times it doesn't. Likewise, one of the virtues of using a two-pronged plug on a turntable is that you can reverse it and see if doing so gets you less hum. (At the signal levels we're talking about, the relationship between the ground of the turntable and the electromagnetic noise fields in the air of your room itself is significant, and things like which prong on the power cord is connected to which wire in the wall do affect this relationship - which is why sometimes theory falls short and some experimentation is required. If I were designing (or re-wiring) a turntable, I would connect the signal grounds together - in a line (try to avoid loops and stick with a "star" configuration). Some turntables actually cut the ground on ONE of the RCA interconnects to avoid creating a ground loop there. I would not start with the signal ground connected to power ground, but would be prepared to try connecting it if I heard any hum. I would also try connecting one or both to a "real earth ground" (a metal water pipe or a ground stake), to see if that gave me lower hum. The ground connection or stake provided for a cable or satellite box can also be a good ground - but, because of the current it carries, it can also be a poor ground in terms of noise.... so, again, be prepared to experiment a little. > lionear > I think you're correct in most areas, Priapulus. But I disagree about connecting the motor. Yes, I agree. A turntable ground has nothing to do with the AC power safety ground, and should not be connected. I will connect my turntable ground only to the metal parts of the tonearm, turntable frame and platter. Thank you. |
|
|
|
Post by tchaik on Jan 2, 2014 21:48:36 GMT -5
turntable grounding? is that what you do when your turntable stayed out past curfew one too many times?  tchaik............ |
|
