High End Phono Cable: The Millivolt Link and Its Load
Quick answer / TL;DR: A high end phono cable carries a few millivolts from a moving-magnet cartridge, or under one millivolt from a moving coil, into a phono stage whose input sets the load. For moving magnet the load is typically 47 kilo-ohms with 100 to 300 picofarads, and the cable capacitance is part of that total alongside the arm wiring and the stage input. The grounding wire is a separate path, and the length adds capacitance. For moving coil the load resistance matters more than the capacitance, and the cable is judged on shielding and on the low signal it must carry.
A high end phono cable carries the smallest signal in the system – a few millivolts from a moving-magnet cartridge, or a fraction of one from a moving coil – through a circuit whose behaviour is set by the load the cartridge sees. The cable is part of that load, so its capacitance and its grounding are not refinements: they are the figures that decide whether the cartridge performs to its specification.
This is the phono link's view: the cartridge's output and its load, the capacitance the cable adds to the total, the grounding path, and the length beyond which the loading drifts outside the manufacturer's range.
The Signal, and the Load It Sees
The phono signal is the smallest in the chain, and the load it sees is specified by the cartridge maker. A moving-magnet cartridge puts out a few millivolts and expects a load around 47 kilo-ohms with a capacitance inside a stated range – commonly 100 to 300 picofarads – because its response above the midrange depends on that capacitance. A moving-coil cartridge puts out a fraction of a millivolt into a much lower impedance, and its load is a resistance rather than a capacitance, set by a transformer or a head amplifier. The two cartridge types therefore judge the same cable on different figures, and the first purchase question is which type the deck carries.
| The cartridge | The output | The load that matters | The cable figure |
|---|---|---|---|
| Moving magnet | A few millivolts | 47 kilo-ohms with 100 to 300 pF | Capacitance, and the shield |
| Moving coil | Under a millivolt | A low resistance, set by the stage | Shielding, and the low signal's integrity |
| The arm wiring | Included in the total | 20 to 40 pF in the common arm | Part of the capacitance budget |
| The ground wire | A separate conductor | The chassis and the stage ground | The hum path, and how it is routed |
The table is the phono link in four rows, and the reading is that the capacitance budget is shared. The arm wiring, the cable and the stage input all sit in parallel, so the cable's figure is not judged alone – it is judged against what the arm and the stage already contribute. A medium-capacitance cable on a long arm can still land inside a 300 pF range if the stage input is low; the same cable on a low-capacitance stage and a short arm leaves room. The budget is the figure, and the cable is one line of it.
The Capacitance Budget
The capacitance budget is the moving-magnet case's central figure, and it is arithmetic rather than taste. The cartridge states a range; the arm wiring contributes its own figure, often 20 to 40 picofarads; the cable adds its capacitance per metre times the length; and the phono stage's input adds a figure of its own, sometimes switchable. The four are added, and the total is checked against the range. A cable that pushes the total past the top of the range tilts the top octave upward and can make the cartridge sound bright; a total below the range softens it. Both are audible, and both are correctable by choosing the cable or the stage setting.
Where the cable figure sits in the total
The cable's share of the budget is the part the purchase controls, and it is usually the largest single line after the stage. A one metre cable at 100 pF per metre contributes 100 pF; a two metre cable of the same stock contributes 200 pF, which can consume the whole range on its own. That is why phono cables are sold short and why the low-capacitance stock matters here more than anywhere else in the chain: the figure is not a refinement but a term in the load the cartridge was designed against.
The Ground, and the Hum
The ground is the phono link's second figure, and the cable carries it as a separate conductor. The turntable's chassis, the arm's earth and the phono stage's ground are tied together by that wire, and the connection is the path the hum current takes when the potentials differ. A missing or high-resistance ground wire produces the classic hum that changes when the arm is touched; a ground loop between the deck and the stage produces a hum that persists when the arm is lifted. The cable's figure is the ground conductor's quality and its routing – kept away from the mains leads and connected at both ends – and the check is the hum's behaviour rather than the meter.
Moving coil, and the different figure
The moving-coil cartridge changes which cable figure matters. Its output is a fraction of a millivolt, so the cable's shielding is the first requirement and its capacitance is largely irrelevant because the source impedance is low. The load that matters is the resistance presented by the transformer or the head amplifier, and the cable's contribution is a small series resistance rather than a capacitance. The purchase is therefore a shielded, well-constructed cable with a reliable ground, and the exotic dielectric arguments from the moving-magnet case do not carry over.
The Length, and the Practical Build
The length is the figure that ties the phono link together, and short is the rule for a reason that is arithmetic rather than stylistic. Every extra metre adds capacitance to the moving-magnet budget and picks up more of the room's noise on a signal that is already tiny. The practical build keeps the cable as short as the deck's position allows, routes the ground wire alongside it rather than through a mains bundle, and terminates with plugs that grip the stage's sockets firmly. A short, low-capacitance, well-grounded cable is the whole purchase, and the exotic conductor is the last figure rather than the first.
High end phono cables are stocked in short lengths with low capacitance per metre, a separate ground conductor, and RCA or DIN terminations as the deck requires; the catalog lists the capacitance per metre, the shield, the ground conductor and the length per part number.
Real-World Applications
The moving-magnet deck on a low-mass arm: the capacitance budget added up from the arm, the cable and the stage, and a short low-capacitance cable chosen to keep the total inside the cartridge range. The moving-coil setup through a step-up transformer: a shielded short cable with a solid ground, and the capacitance question set aside because the source impedance is low. The vintage deck with a hum that changes when the arm is touched: the ground wire checked end to end before any cable is replaced. In each the phono cable is one term in a load the cartridge specifies, and the capacitance, the shield and the ground are the figures that decide whether that load is met.
Related reading: The line-level link one stage later: high end interconnects — high-end-interconnects
Related reading: The chain the phono stage feeds: high end cables audio — high-end-cables-audio
Frequently Asked Questions
Does the phono cable really affect the sound?
For a moving-magnet cartridge, yes, and measurably: the cable capacitance is part of the load the cartridge sees, and the total against the manufacturer's range changes the response above the midrange. For a moving coil the capacitance matters far less because the source impedance is low; the shielding and the ground are the figures that matter there.
What capacitance should a phono cable have?
Low enough that the total stays inside the cartridge range, which is commonly 100 to 300 picofarads including the arm wiring, the cable and the phono stage input. A one metre cable at 100 pF per metre contributes 100 pF, so the cable can consume the whole budget on its own at two metres. The figure to work with is the total rather than the cable alone.
Why do phono cables have a separate ground wire?
Because the phono signal is tiny and the ground path is part of the noise floor. The separate conductor ties the turntable chassis, the arm earth and the phono stage ground together, and it carries the hum current when the potentials differ. A missing or high-resistance ground produces the hum that changes when the arm is touched, which is the classic symptom.
How long can a phono cable be?
As short as the deck's position allows. Every metre adds capacitance to the moving-magnet budget and picks up more noise on a signal of a few millivolts. A metre is typical, two is the practical limit for most moving-magnet setups, and longer runs push the total past the cartridge range before the shielding question even arises.
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