In the process of wiring the new tonearm on my restored SL1200. I'm using balanced wiring from the headshell forward with two mini XLRs out from the arm base. The audio grounding scheme will be centrally tied from the preamp back to the arm, floating at the TT / headshell. For the time being, I'm using SE (floating ground) MC preamp inputs until switching over to fully balanced inputs with a center tapped ground per channel.
My question is, with floating SE cartridge connections, what is the best way to internally ground the entire system? The headshell won't be tied into ground, as is typically the case with SE outputs from the TT.
My question is, with floating SE cartridge connections, what is the best way to internally ground the entire system? The headshell won't be tied into ground, as is typically the case with SE outputs from the TT.
That sounds right to me. Thanks! I usually run just a single 5 pin mini XLR from the arm and then to both sides.
Ideally, you want each channel's signal wires run together, maybe with a twist, inside of their own shield, which is itself connected on only one end, normally at the electronics end. A separate wire connects turntable and arm metal parts to electronics chassis. You can even separate this out into separate wires from various pieces of turntable and arm (all independent of signal wiring and sheilds), but usually nobody bothers - but can't hurt.
An "instrumentation" differential input (equal impedance to ground from each terminal) has a nominally 3dB thermal noise penalty, so isn't always automatically "better". The floating nature of a MC or MM/MI cartridge makes all inputs, including single-ended, truly differential, although circuit impedances aren't equal on both terminals of a conventional single-ended input, slightly spoiling its ability to reject external noise impressed across the signal wiring feeding it. The choice is a balancing act.
All good fortune,
Chris
An "instrumentation" differential input (equal impedance to ground from each terminal) has a nominally 3dB thermal noise penalty, so isn't always automatically "better". The floating nature of a MC or MM/MI cartridge makes all inputs, including single-ended, truly differential, although circuit impedances aren't equal on both terminals of a conventional single-ended input, slightly spoiling its ability to reject external noise impressed across the signal wiring feeding it. The choice is a balancing act.
All good fortune,
Chris
ps: The only limitation I can think of to 5 connector pins (for a single-ended electronics sytem) is the secondary issue of where the electronics signal ground meets the chassis (and therefore the connection from the turntable and arm metalwork). If the connection point is right at the input jack, all shields, bleeds and electronics "ground" connections meet in a useful version of a "star ground".
The modern implementation, circa 1995 or so, of a "star ground" ignores all issues occurring outside of the chassis, so running signal around inside the electronics box becomes fraught with peril. But I digress.
All good fortune,
Chris
The modern implementation, circa 1995 or so, of a "star ground" ignores all issues occurring outside of the chassis, so running signal around inside the electronics box becomes fraught with peril. But I digress.
All good fortune,
Chris
Twisted pair for each channel, inside its own screen. Use the screen as the earth bond (it has a nice high cross-section). You don't need to worry about earth loops in the screens because they don't carry signal, so it's best to consider RF issues, and that means tying everything down to a low impedance earth, screens to earth at each end. Where I would split the earths is to provide a mains safety earth to the (mains-driven) turntable, no link to the arm at turntable, but earth bond the arm to the pre-amplifier's chassis. An MC step-up transformer can allow true balanced operation, but too many connect one side of the input to earth and are not wound balanced. For MM, the 3dB noise penalty and higher signal level makes a balanced input unnecessary, but balanced wiring still helps because induced currents produce equal interference voltages that cancel at the pre-amplifier input even if it's not balanced.
First, check if the arm is connected to turntable chases ground. Arm should not be connected to the same chasses ground. Because the turntable body is diffeferent system without elecrical connection with tone arm. Ysusally contains a electronic for motor control...
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Arm ground is audio chases ground that should be connected to GND at phono input.
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Coil is single ended, one coil without C.T. BUT it can be treated as balanced.
It will be the best to use sheilded twisted pair cable. Shield to connect from guard shield electronic circuit, and left open at the side of arm.
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twisted pairs as ussual fo the phono coil...
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Arm ground is audio chases ground that should be connected to GND at phono input.
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Coil is single ended, one coil without C.T. BUT it can be treated as balanced.
It will be the best to use sheilded twisted pair cable. Shield to connect from guard shield electronic circuit, and left open at the side of arm.
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twisted pairs as ussual fo the phono coil...
The problem is that inside the arm, from cartridge to the xlr, the cables are not twistead.
So the use of cable with screen is not useful.
Then is possible to connect central pin of the 5 poles connector with the screen of the signal cables that goes to chassis phono, one for each channel
Or use the central pin of the 5 poles to connect the chassis and toneram to phono chassis ground with a single wire; then use a cable with screen (2 wires + screen) , one for channel, where the screen is connected on pin 1 of XLR only on phono side .
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I'm not a fan of common 5 pin XLR outputs being used for balanced input stages, where there are transformers isolating each channel separately. This causes inter-channel capacitive coupling and a host of other symptoms that come with it.
If one was running a low output MC cart, it would be common place to use step up transformers at the preamp inputs. This would provide galvanic isolation and therefore truly balanced, fully floating inputs.
In the above scheme, the transformer shields would be connected together, running to a star ground point on the chassis and PS earth. Separate from this, SU transformer output center taps would go to the local input ground, then to the chassis star ground.
Separate input shields using two XLR connectors from the TT avoid ground loops into the preamp if the first stages are coupled to ground through large capacitors. Sometimes the headshell is also connected to ground through the L channel negative lead, which causes a lot of noise issues with MC carts. I think its unnecessary to have the headshell grounded to the L channel if there's a dedicated TT chassis ground.
My tonearm is connected to chassis earth, but I have the option to lift it from the PS earth if needed. This was due to the motor PS having a grounded mains plug being in a metal chassis, so I installed a ground lift switch.
The tonearm is also grounded through its gimbal bearings, but thats a rather noisy connection, so I bypassed it with a large capacitor.
We have alot of RF noise in our mains supply so I installed a snubber in the mains breaker juncton box in addition to a whole house surge suppressor. The only way to run a low output MC cart without alot of noise is.t use SU transformers, so I have to adapt my setup.
If one was running a low output MC cart, it would be common place to use step up transformers at the preamp inputs. This would provide galvanic isolation and therefore truly balanced, fully floating inputs.
In the above scheme, the transformer shields would be connected together, running to a star ground point on the chassis and PS earth. Separate from this, SU transformer output center taps would go to the local input ground, then to the chassis star ground.
Separate input shields using two XLR connectors from the TT avoid ground loops into the preamp if the first stages are coupled to ground through large capacitors. Sometimes the headshell is also connected to ground through the L channel negative lead, which causes a lot of noise issues with MC carts. I think its unnecessary to have the headshell grounded to the L channel if there's a dedicated TT chassis ground.
My tonearm is connected to chassis earth, but I have the option to lift it from the PS earth if needed. This was due to the motor PS having a grounded mains plug being in a metal chassis, so I installed a ground lift switch.
The tonearm is also grounded through its gimbal bearings, but thats a rather noisy connection, so I bypassed it with a large capacitor.
We have alot of RF noise in our mains supply so I installed a snubber in the mains breaker juncton box in addition to a whole house surge suppressor. The only way to run a low output MC cart without alot of noise is.t use SU transformers, so I have to adapt my setup.
In general, the wires within the arm are lightly twisted. Ideally, each channel would be individually tightly twisted (that's what I do). Centre tapped generator is not relevant; what is important for balance is impedances, not voltages. Ideal balanced generator is floating. A surprising amount of hum is picked up by the cable from arm base to RIAA stage, and a good screen is very useful. You don't get crosstalk between channels with a balanced set-up - that's how ancient telecomms was able to squirt many signals down miles of cable without screens between pairs. With step-up transformers and the wiring I've described, you can touch one pin of a moving coil cartridge at full volume and not hear a sound.
I have an SMEIV arm. The arm is terminated in a 5 pin DIN. That is 2 cartridge wires L and R, and arm ground. From a Cardas DIN plug I go via
two lengths of Cardas 33x4 Shielded Tonearm Wire. I wire each as a star quad, with the two braids going to arm ground. Each of those lengths of coax goes to an XLR, and into the phono stage.
Zero hum of course.
two lengths of Cardas 33x4 Shielded Tonearm Wire. I wire each as a star quad, with the two braids going to arm ground. Each of those lengths of coax goes to an XLR, and into the phono stage.
Zero hum of course.
If the arm ground can't be separated from the TT chassis, there's not much else I can do than continue and extend the preamp ground to the interconnect shield, ending it at the TT. If the TT chassis isn't earthed to anything else than the preamp, it shouldn't cause any noise.
Any flaws in this logic?
I could isolate the arm base from the TT chassis ground with insulated washers, but I'm not sure if this is worth the hassle.
Any flaws in this logic?
I could isolate the arm base from the TT chassis ground with insulated washers, but I'm not sure if this is worth the hassle.
Ah - now I see the problem. If the safety ground on the turntable from the mains power connection is the same as the arm ground, there is indeed a bit of a problem. I'm fortunate that my plinth is quartz composite, as is the arm board. The the Garrard 401 safety ground is completely isolated from arm ground.
Is there any way in which the arm can be insulated from the turntable? That would eliminate any ground potential conflict.
The original SME3009 had little rubber washers that isolated the arm ground from the turntable. These were often reportedly for vibration isolation, for which they had zero effect. But their electrical isolation was real.
Is there any way in which the arm can be insulated from the turntable? That would eliminate any ground potential conflict.
The original SME3009 had little rubber washers that isolated the arm ground from the turntable. These were often reportedly for vibration isolation, for which they had zero effect. But their electrical isolation was real.
So the potential issue is the PE ground loop? Another way to attack that is to run the TT's mains power cord parallel to the (shielded, twisted) signal wiring, then plug it into a mains outlet on the preamp. Maybe add a special one, just for the purpose.
All good fortune,
Chris
All good fortune,
Chris
Star quad? Someone worked in broadcast television, methinks. I've never gone that far, but a twisted pair in screen from arm base is way better than standard coaxial cable.
Isolating the arm base from the turntable chassis should be possible. The hard clear plastic that was often used for the front cover of document folders that had flat metal bits you bent to retain the pages would be suitable for making a large washer above and below the chassis. You also have to prevent the arm touching the turntable chassis as it goes through the hole. The problem is easily solved if you have a lathe, but not all homes have one. You don't need a stunning electrical insulator, but you don't want a squidgy material that creeps. Curved nail scissors enable surprisingly neat round holes to be cut.
Isolating the arm base from the turntable chassis should be possible. The hard clear plastic that was often used for the front cover of document folders that had flat metal bits you bent to retain the pages would be suitable for making a large washer above and below the chassis. You also have to prevent the arm touching the turntable chassis as it goes through the hole. The problem is easily solved if you have a lathe, but not all homes have one. You don't need a stunning electrical insulator, but you don't want a squidgy material that creeps. Curved nail scissors enable surprisingly neat round holes to be cut.
The idea of keeping the mains PE in close proximity to signal PE was first visible to me from comments by member jneutron, a very interesting guy from CERN, in the old Blowtorch threads.
Another approach, possibly only applicable in America, depends on local codes, is to make a mains outlet with no PE Earth connection back to panel, and the safety aspect left to a GFI device in the outlet. This is within code in America, and useful in older housing lacking the third wire - better than nothing and nominally safe enough for kitchens and bathrooms. Possibly not code elsewhere.
All good fortune,
Chris
Another approach, possibly only applicable in America, depends on local codes, is to make a mains outlet with no PE Earth connection back to panel, and the safety aspect left to a GFI device in the outlet. This is within code in America, and useful in older housing lacking the third wire - better than nothing and nominally safe enough for kitchens and bathrooms. Possibly not code elsewhere.
All good fortune,
Chris
Yes, I'm able to isolate the mains earth from the chassis and arm. I'm running the motor PS separately from the chassis, then have a switchable AC coupled ground just in case peripheral connected preamps need this. The safety earth is permanently connected to the PS primary with galvanic isolated secondary.
The arm setup could be isolated with some light machining. The arm is essentially a SL1200 Mg arm with ceramic bearings, so the arm tube is isolated. The ceramic bearing material does however possess a small amount of conductivity. It would need to be isolated to be on the safe side. The bearing lubricant is also slightly conductive.
I have all sorts of nylon washers and sleeves. Also have access to shrink tubing and Teflon stock.
I'll post pictures when I get around to it.
The arm setup could be isolated with some light machining. The arm is essentially a SL1200 Mg arm with ceramic bearings, so the arm tube is isolated. The ceramic bearing material does however possess a small amount of conductivity. It would need to be isolated to be on the safe side. The bearing lubricant is also slightly conductive.
I have all sorts of nylon washers and sleeves. Also have access to shrink tubing and Teflon stock.
I'll post pictures when I get around to it.
So you're in clover! Except that "ceramic bearings, so the arm tube is isolated", so the arm tube would ideally like some nice fine wire to it to earth it. Beware that PTFE creeps. You don't need an especially good insulator between motor and arm, but you probably want to avoid changing the compliance between the two.
PTFE creeps under pressure even at 4.2K liquid helium temperature. How this was found out is too long a story.
If you have an issue with noise in your AC lines.... how about just running a home run to your stereo and then use an AC regenerator off that line? Use the regenerator's ground.
Seems like a lot of what your are doing is fixing the symptoms, not the root problem.