Hi all, my first post here, after years lurking.
I have a plan to improve my Technics SL-7. I've already upgraded it by reducing the audio signal path and with the addition of a couple of good RCA connectors and a connector for the ground cable coming from the cartridge. Now I'll try an additional improvement, changing the audio signal cables coming from the tonearm to the RCA connectors. My idea is to change them from a typical unbalanced scheme (with cable shields used as the - conductor for each channel) to a semi-balanced one (using shielded two-conductor cables).
Where shall I connect the signal cables' shields for best audio performances? I think three connections would be feasible:
I have a plan to improve my Technics SL-7. I've already upgraded it by reducing the audio signal path and with the addition of a couple of good RCA connectors and a connector for the ground cable coming from the cartridge. Now I'll try an additional improvement, changing the audio signal cables coming from the tonearm to the RCA connectors. My idea is to change them from a typical unbalanced scheme (with cable shields used as the - conductor for each channel) to a semi-balanced one (using shielded two-conductor cables).
Where shall I connect the signal cables' shields for best audio performances? I think three connections would be feasible:
- to the turntable chassis;
- to the ground connector coming from the cartridge (which is connected to the amp ground, as expected);
- to the - conductor of the audio signal cable, then would both be connected to the RCA connector ring.
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Not exactly sure what you mean. I'm afraid the details didn't quite make it across the language barrier.
Anyway, there are only about 2 grounding schemes in a turntable that would be considered correct. The SL-7 probably came wired for one of them from the factory, roughly like this, and I'd advise replicating it:
Cartridge L+ --> conductor RCA L
Cartridge L- --> shield RCA L
- separate from -
Cartridge R+ --> conductor RCA R
Cartridge R- --> shield RCA R
- separate from -
Tonearm tube --> separate ground wire
There are no connections between L, R and extra ground. They are only merged at the phono pre. Usually nothing much should happen if you connect L and R ground at the turntable, as the loop area formed by an RCA cable tends to be very low, but still, it might increase hum levels under adverse circumstances.
If there is chassis ground separate from tonearm ground, I'd try hooking that onto the ground wire as well - not sure whether it'll do much but is worth a shot. There isn't an awful lot you can do about static charge buildup when using a rubber mat (and I like rubber mats for their platter damping abilities), besides using one of these Clearaudio brush things I guess.
Oh, now I get what you meant. In my opinion there is zero benefit from using shielded twisted pair here, quite the contrary - it tends to exhibit markedly more capacitance than some decent coax of same length (with some bad luck you looking at some ~150 pF/m conductor to conductor plus another 100 pF/m conductor to shield though only part of that would be effective, vs. as low as 60 pF/m - plain thin shielded audio cable seems to be around 200 pF/m). If you've got touchy MM cartridges (like higher-end ATs or their older models), cable capacitance is your enemy. Adding some more parallel capacitance is dead easy if needed (either in the preamp or externally) - removing some, however, is anything but.
If you were to used STP wiring, I'd say the shields should connect to the ground wire alongside tonearm ground - definitely not anywhere else.
I would upgrade to some lower-capacitance but still nice and flexible coax with some decent-quality connectors and pretty much leave it at that - maybe RG-174 (2.8 mm OD, PE dielectric, ~100 pF/m, 50 ohms), and some Reans perhaps (nothing too fat in diameter anyway to avoid nasty surprises). RG-179A/B as 75 ohm coax would have substantially less capacitance and be even thinner, but it uses teflon dielectric, which is a bit notorious for its triboelectricity, so that might not be the lowest-noise cable - and the last thing you need in a high-impedance, low-level application like this is cables that generate their own signals when moved or vibrating. If you want PE dielectric and lower capacitance, you'd have to go with the much thicker RG-59 at 6 mm OD. Not particularly flexible in my experience but among the lowest-capacitance commonly available coax (60 pF/m) as it is 75 ohm video stuff. I'm very much assuming that you'd have to look a lot harder for some RG-62, RG-71 or RG-210 (93 ohm stuff at 14 pF/ft or 48 pF/m, foamed PE, ~6 mm OD).
These Technics 'tables usually weigh in at 130-150 pF of total cable capacitance stock (tonearm wiring + RCA), that's just about low enough for a fussy cartridge insisting on 100-200 pF of load when used on an input with 47 pF (reasonably common around 1980 but unusually low these days - typical standard phonopre inputs are 220 pF || 47 kOhm). Not bad for stock with some decent-length cable anyway. You'll probably be able to knock a few dozen pF off, even more so when deciding that the best place for a phonopre is right next to the turntable and choosing cable length accordingly.
Going twisted pair would only make sense if phonopre inputs were actually balanced, but a balanced input comes with a 3 dB noise penalty right from the get-go, plus there's the generally higher-capacitance cabling.
Anyway, there are only about 2 grounding schemes in a turntable that would be considered correct. The SL-7 probably came wired for one of them from the factory, roughly like this, and I'd advise replicating it:
Cartridge L+ --> conductor RCA L
Cartridge L- --> shield RCA L
- separate from -
Cartridge R+ --> conductor RCA R
Cartridge R- --> shield RCA R
- separate from -
Tonearm tube --> separate ground wire
There are no connections between L, R and extra ground. They are only merged at the phono pre. Usually nothing much should happen if you connect L and R ground at the turntable, as the loop area formed by an RCA cable tends to be very low, but still, it might increase hum levels under adverse circumstances.
If there is chassis ground separate from tonearm ground, I'd try hooking that onto the ground wire as well - not sure whether it'll do much but is worth a shot. There isn't an awful lot you can do about static charge buildup when using a rubber mat (and I like rubber mats for their platter damping abilities), besides using one of these Clearaudio brush things I guess.
Oh, now I get what you meant. In my opinion there is zero benefit from using shielded twisted pair here, quite the contrary - it tends to exhibit markedly more capacitance than some decent coax of same length (with some bad luck you looking at some ~150 pF/m conductor to conductor plus another 100 pF/m conductor to shield though only part of that would be effective, vs. as low as 60 pF/m - plain thin shielded audio cable seems to be around 200 pF/m). If you've got touchy MM cartridges (like higher-end ATs or their older models), cable capacitance is your enemy. Adding some more parallel capacitance is dead easy if needed (either in the preamp or externally) - removing some, however, is anything but.
If you were to used STP wiring, I'd say the shields should connect to the ground wire alongside tonearm ground - definitely not anywhere else.
I would upgrade to some lower-capacitance but still nice and flexible coax with some decent-quality connectors and pretty much leave it at that - maybe RG-174 (2.8 mm OD, PE dielectric, ~100 pF/m, 50 ohms), and some Reans perhaps (nothing too fat in diameter anyway to avoid nasty surprises). RG-179A/B as 75 ohm coax would have substantially less capacitance and be even thinner, but it uses teflon dielectric, which is a bit notorious for its triboelectricity, so that might not be the lowest-noise cable - and the last thing you need in a high-impedance, low-level application like this is cables that generate their own signals when moved or vibrating. If you want PE dielectric and lower capacitance, you'd have to go with the much thicker RG-59 at 6 mm OD. Not particularly flexible in my experience but among the lowest-capacitance commonly available coax (60 pF/m) as it is 75 ohm video stuff. I'm very much assuming that you'd have to look a lot harder for some RG-62, RG-71 or RG-210 (93 ohm stuff at 14 pF/ft or 48 pF/m, foamed PE, ~6 mm OD).
These Technics 'tables usually weigh in at 130-150 pF of total cable capacitance stock (tonearm wiring + RCA), that's just about low enough for a fussy cartridge insisting on 100-200 pF of load when used on an input with 47 pF (reasonably common around 1980 but unusually low these days - typical standard phonopre inputs are 220 pF || 47 kOhm). Not bad for stock with some decent-length cable anyway. You'll probably be able to knock a few dozen pF off, even more so when deciding that the best place for a phonopre is right next to the turntable and choosing cable length accordingly.
Going twisted pair would only make sense if phonopre inputs were actually balanced, but a balanced input comes with a 3 dB noise penalty right from the get-go, plus there's the generally higher-capacitance cabling.
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Thanks sgrossklass for your kind reply. I'm sorry for my poor technical english, I'll try to edit the original message to make it easier to understand.
My idea started with a twisted pair cable because, in my experience, two separate conductors work always better than the usual RCA scheme: conductor for +/ shield for -
After all, the internal wiring takes only 20/30cm: is capacitance an issue for this lenght?
Your answer is useful also because I didn't know anything about triboelectricity, and I was planning to use a PTFE-insulated Habia cable. Probably I have to find another product. Is PE insulation the best solution for moving cables like these?
What about using 4x RG-174 to replace the original cables? After all they are slimmer, I think I could manage to let them pass from the cover to the main chassis (this point is the narrower passage).
Both with twisted pairs and coaxial cables, if I understand correctly, the better way to connect shields is to the ground connector?
My idea started with a twisted pair cable because, in my experience, two separate conductors work always better than the usual RCA scheme: conductor for +/ shield for -
After all, the internal wiring takes only 20/30cm: is capacitance an issue for this lenght?
Your answer is useful also because I didn't know anything about triboelectricity, and I was planning to use a PTFE-insulated Habia cable. Probably I have to find another product. Is PE insulation the best solution for moving cables like these?
What about using 4x RG-174 to replace the original cables? After all they are slimmer, I think I could manage to let them pass from the cover to the main chassis (this point is the narrower passage).
Both with twisted pairs and coaxial cables, if I understand correctly, the better way to connect shields is to the ground connector?
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