Cable & SUT Capacitance Calculator: Line, Phono and MC Step-Up Loading

Will a long interconnect audibly dull the treble? That is the question this tool answers, with a mode for each kind of cable. At line level (a DAC, CD player, or preamp feeding an amp) the cable's capacitance works against the source's output impedance, the small internal resistance the source drives the cable through. Together they form a low-pass filter, which is a gentle treble cut. That mode finds the −3 dB corner, the frequency where treble is down 3 dB, and plots the curve. On a moving-magnet turntable (the common cartridge type) capacitance is a loading budget instead. Phono mode adds your cable and phono stage together, then checks the total against the figure your cartridge's maker publishes. Running a moving-coil cartridge into a step-up transformer (SUT)? Switch that on in phono mode: the transformer multiplies the phono stage's capacitance by the square of its ratio before the cartridge sees it, and the tool shows what that costs in treble.

My setups

Save a whole configuration: mode, source or cartridge, cable, and receiving input. Useful when you're picking between cable choices, checking a long run, or matching a cartridge to a phono stage.

−3 dB corner - The frequency where treble is down 3 dB. Only a problem near or below 20 kHz.
Total loading - Cable capacitance plus the receiving input's.
Audibility - 5 tiers from "Serious loss" to "Inaudible".

A roll-off like this is not only a level change: the treble also shifts in phase, starting well below the corner, and as the reflected capacitance grows the leading edge of high-frequency transients (cymbal strikes, plucked strings) softens with it.

A simplified single-pole model: the cartridge's source impedance against the reflected load and the tonearm cable. It ignores the transformer's own leakage inductance and interwinding capacitance, which shape its real top octave and can partly offset or deepen this roll-off, and it does not model the SUT's recommended primary loading.

Source

Ω

The series output impedance from the source's spec sheet. DAC or solid-state line out: 50-600 Ω. Tube stage without a cathode follower: 1-5 kΩ. A volume pot peaks at a quarter of its track resistance, so a 10 kΩ passive attenuator is 2.5 kΩ.

Quick source presets

Cartridge

Picking a cartridge fills in the total loading capacitance its maker recommends. Not listed? Leave Custom and type the figure from its manual.

pF

The total the cartridge should see: cable plus phono-stage input, added together. It is on the cartridge's spec sheet. Low-output moving-coil cartridges publish no figure, and the presets record that as 0.

Each preset carries its maker's published turns ratio. Picking one fills the ratio in; a ratio you type yourself switches this back to Custom.

The N in 1:N. Type 20, or paste 1:20 as the maker prints it. A 1:10 step-up adds 20.0 dB of gain.

Ω

From the cartridge's spec sheet, listed as internal impedance or coil resistance.

Reflected capacitance -

Cable

Sets typical pF/m for the family. Override below for your spec sheet's exact value.

pF / m

Budget RCA: 120-250. Quality: 50-100. Specialist low-capacitance: 20-60.

Typical interconnect: 0.5-3 m. Long studio run: up to 30 m / 100 ft.

Receiving input Phono stage

pF

The capacitance the receiving component puts across its own input, from its manual. Set 0 if it is not specified. On a phono stage this is the input loading figure: typically 100-300 pF. Through a step-up it sits on the transformer's secondary, and the cartridge sees it multiplied by the square of the ratio.

Typical input capacitance

High-frequency response Why phono mode shows no curve High-frequency response through the step-up

Log axis 1 kHz - 100 kHz. The −3 dB point is where treble is half-power; below it the response is flat, above it rolls off at 6 dB/octave. The audio band (≤ 20 kHz) is lightly shaded. Aim to keep the corner above 20 kHz - past the top of the audible band.

Log axis 1 kHz - 100 kHz. The cartridge's source impedance driving the tonearm cable and the phono stage's capacitance as the transformer reflects it: a single-pole roll-off, flat below the −3 dB point and falling at 6 dB/octave above it. The audio band (≤ 20 kHz) is lightly shaded.

There is no curve here on purpose. A moving-magnet cartridge is an inductive source, so its top octave is set by a resonance between the cartridge's own coil inductance and the capacitance you load it with. That is a second-order L/C response, not the first-order RC roll-off drawn in line-level mode, and the two do not give the same answer. This tool holds no cartridge inductance figures, so any curve it drew for a cartridge would be a guess. What it can do honestly is add your capacitance up and compare the total with the figure the cartridge's maker publishes, which is exactly how that maker expects you to set the load.

Show the math

Three loads that look like one

Line level: an RC low-pass filter

A source drives the cable through its own output impedance. That series resistance and the shunt capacitance of the cable plus the receiving input form a low-pass filter with corner frequency f = 1 ÷ (2π x R x C), where R is the source's output impedance in ohms and C the total capacitance in farads.

Total capacitance = cable pF/m x length + the receiving input's pF. Shorter, lower-capacitance cable moves the corner up; so does a source with a lower output impedance. Most line outputs sit between 50 and 600 Ω, which puts the corner somewhere between a few hundred kilohertz and several megahertz on a normal interconnect: nowhere near the audio band. What bites is a high source impedance into a long, high-capacitance run - a passive attenuator, a tube stage without a cathode follower, or a 20 m studio cable.

Phono: a loading budget, not a corner

A moving-magnet cartridge is an inductive generator, so the capacitance hanging off it does not set an RC corner at all. It tunes a resonance with the cartridge's own coil inductance. Add capacitance and that resonance moves down: past a point it drops into the top octave, first as a lift and then as a steep fall above it.

That is why cartridge makers publish a total loading capacitance instead of a frequency. Meeting it is arithmetic: cable pF/m x length, plus the phono stage's input capacitance, compared with the maker's figure. Phono mode does exactly that sum and nothing more. The 47 kΩ a phono stage presents is a load resistance, not a series source resistance, so it never belongs in the RC formula above. Low-output moving-coil cartridges publish no capacitance figure and do not need one; their generator inductance is far lower, so this resonance sits well clear of the music.

MC through a step-up: the load is multiplied

A step-up transformer with a turns ratio of 1:N raises a moving-coil cartridge's voltage N times, and it transforms impedance by N² between its windings. The phono stage's input capacitance sits on the secondary, so the cartridge, on the primary, sees it multiplied by N²: its reflected capacitance. That load, beside the tonearm cable, meets the cartridge's own source impedance as a single-pole RC filter, f = 1 ÷ (2π x R x C).

With a 1:20 step-up transformer, your phono stage's 100 pF input capacitance becomes a 40.0 nF load on the cartridge. Against a 100 Ω cartridge that rolls off at 39.8 kHz, roughly a 1.0 dB loss at 20 kHz. The multiplier is the ratio squared, 20 x 20 = 400, so a phono input that would be harmless on its own becomes the load that sets the corner. A low-impedance cartridge of a few ohms pushes the same corner far above the audio band, which is why most step-up pairings never hear it; a higher-impedance moving coil into a high ratio is where it starts to show. The model leaves out the transformer's leakage inductance and interwinding capacitance, which shape its real top octave.

Typical capacitance, by cable type

pF/m is the number to look for on the datasheet. Multiply by length and add the receiving input's pF: at line level that total is what the source has to drive, and on a turntable it is what you compare with your cartridge's recommended loading.

Cable typeCapacitanceWhere it shows up
Generic shielded RCA (PVC dielectric)120-250 pF/mBargain-bin AV cable. Worst case on long runs.
Standard hi-fi interconnect80-130 pF/mMid-tier RCA. Fine at line level and normal lengths.
Quality interconnect (PE / PTFE)50-90 pF/mReference audio cable. Headroom on longer runs.
Tonearm cable (specialist)20-60 pF/mThe lowest-capacitance family sold. Built to leave cartridge-loading headroom.
Studio mic cable (Star-Quad XLR)35-60 pF/mPro audio long runs. Low pF/m is part of the spec.
Coaxial digital (75 Ω S/PDIF)~67 pF/mFixed by the 75 Ω geometry. Not relevant here.
USB cableN/A (digital)Irrelevant - bit-stream regenerated by receiver.

When cable capacitance stops being theoretical

Every cable stores a little charge between its conductors, and that capacitance forms a low-pass filter with whatever resistance is driving it. The higher the source impedance and the longer the cable, the lower the frequency at which the top end starts to roll away.

At line level from a modern low-impedance output this is almost always inaudible: the roll-off sits far above the audible band. The interesting case is a phono cartridge, where the source impedance is high and inductive, and where the cartridge maker specifies a total capacitance the cable is part of.

That is why this tool has two modes. At line level it shows you how much headroom you have; in phono mode it checks whether your tonearm cable, interconnect and phono stage together land inside the cartridge's recommended loading - the one case in domestic audio where cable capacitance genuinely changes frequency response.

A moving coil through a step-up transformer is the third case, and phono mode has a switch for it. The transformer multiplies the phono stage's input capacitance by the square of its ratio before the cartridge sees it, so a harmless 100 pF becomes tens of nanofarads, and the cartridge's own source impedance drives that load as a simple RC filter.

Cable capacitance result with the -3 dB corner frequency and the total capacitive load on the source

Worked example

A 1.5 m interconnect at 100 pF per meter, from a 220 ohm solid-state preamp output into a 220 pF input.

Total loading 370 pF and a −3 dB corner at 2.0 MHz.

The corner frequency is the whole answer, and 2.0 MHz sits a hundred times above anything audible. Capacitance only starts to matter when a high source impedance meets a long, high-capacitance cable, which is why the tool asks what the source is before it asks about the cable.

The ideas behind the controls

Capacitance per meter
The specification that matters. Multiply by length to get the total the source actually sees.
Source impedance
The output resistance driving the cable. It is the other half of the filter, which is why the same cable is harmless at line level and decisive on phono.
Cartridge loading
The resistance and capacitance a moving-magnet cartridge is designed to see. Miss it and the treble develops a peak or a slump.
−3 dB point
The frequency at which the roll-off has cost 3 dB. Useful as a landmark, not as the point where the effect begins.
Reflected capacitance
The phono stage's input capacitance as a moving-coil cartridge sees it through a step-up transformer: multiplied by the square of the turns ratio.

Cable capacitance and treble roll-off FAQ

  1. Why does cable capacitance matter for phono cartridges?

    Moving-magnet (MM) phono cartridges are spec'd for a specific load capacitance (typically 100-300 pF including the phono cable). The cartridge coil and that capacitance form a resonance; high-capacitance RCA runs drag the resonant peak down into the top of the audible band, and above the peak the response falls away. The calculator's phono mode adds your cable and phono-stage capacitance together and compares the total with the figure your cartridge's maker publishes.

  2. What is a safe cable capacitance for line-level RCA runs?

    At line level, capacitance almost never bites. A typical solid-state output of 100-600 ohms driving a few hundred picofarads puts the −3 dB corner up in the megahertz, roughly a hundred times above the top of the audio band. The corner only falls anywhere near music when the source impedance is high: a passive attenuator, a tube stage without a cathode follower, or a very long run off a weak output. Put your own source impedance and cable length into line-level mode and the tool shows where the corner actually lands.

  3. How do I find my cable capacitance specification?

    It is usually printed on the cable jacket or in the manufacturer datasheet, listed as picofarads per foot or per meter. Generic RCA cables are 30-50 pF/ft; high-capacitance cables can be 80+ pF/ft; specialty low-capacitance cables for phono use are 15-25 pF/ft.

  4. Can cable capacitance affect digital signals like USB or HDMI?

    Practically no for properly impedance-matched digital interfaces. The issue is reflection and ringing, not roll-off, and modern receivers re-clock the bitstream. The calculator targets analog applications: line-level cable, where capacitance and source impedance form a low-pass filter, and phono cable, where capacitance is part of the load the cartridge sees.

  5. What is the right cable capacitance for an Ortofon 2M Blue?

    Ortofon recommends 150-300 pF total loading for the 2M series. Subtract your phono stage's input capacitance (often 100-220 pF) and the remaining 50-200 pF is your cable budget. A 1 m run at 50 pF/m is comfortable; a 1.5 m run at 100 pF/m gets close to the upper limit. The calculator's phono mode does that subtraction for you and shows how much budget is left.

  6. What is reflected capacitance (and reflected impedance) in a step-up transformer?

    A step-up transformer with a turns ratio of 1:N raises the voltage N times and transforms impedance by N squared between its windings. Anything connected to the secondary appears at the primary transformed by that factor: a resistance looks N squared times smaller, and a capacitance, whose impedance falls as it grows, looks N squared times larger. That is reflected impedance, and reflected capacitance is its capacitive part. A phono stage with 100 pF of input capacitance behind a 1:20 transformer shows the cartridge 100 x 400 = 40,000 pF, or 40 nF.

  7. Why does reflected capacitance matter for a moving-coil cartridge?

    Because it is the load the cartridge actually drives. Straight into an MC phono stage, a low-output moving coil sees a few hundred picofarads and capacitance is a non-issue. Through a step-up, the phono stage's capacitance arrives multiplied by the square of the ratio, often tens of nanofarads, and it forms a low-pass filter with the cartridge's own source impedance. With a 2 to 10 ohm cartridge the corner lands far above the audio band; with a higher-impedance moving coil (40 ohms and up) into a high ratio it can reach the top octave, where it trims treble and softens transients. Switch the step-up on in phono mode to see where yours lands.

  8. Are source impedance and a step-up transformer's primary impedance the same thing?

    No, and they answer different questions. Source impedance (also listed as internal impedance or coil resistance) is the cartridge's own output resistance: it is what drives the capacitance, so it is the R in the calculator's corner-frequency formula. A step-up transformer's recommended primary impedance, or the load the phono stage presents reflected through the transformer, is about how the cartridge is damped and how the transformer's own resonances behave. That matters for the sound too, but it is a separate calculation this tool does not make, so pick the transformer's ratio by the maker's cartridge-impedance guidance and use this tool for the capacitance question.

  9. How do I read the calculator's step-up transformer results?

    Turn on the step-up switch in phono mode, pick your transformer or type its ratio, and enter the cartridge's source impedance. The reflected capacitance is the phono stage's input capacitance times the ratio squared; the load on the cartridge adds the tonearm cable; the cutoff frequency is where the cartridge's source impedance and that load have cost 3 dB; and the levels at 10, 15 and 20 kHz say what that costs inside the audio band. A corner well above 20 kHz (a fraction of a dB at 20 kHz) is inaudible. The model is single-pole: it leaves out the transformer's leakage inductance and interwinding capacitance, so read it as the capacitive part of the story rather than the transformer's full response.

  10. My phono stage hums and the treble is rolled off, is it the cable?

    Hum is grounding, not capacitance. Check the ground wire to the phono stage chassis and the tonearm grounding scheme. If the calculator's phono mode puts your total loading well past the figure your cartridge maker publishes, capacitance is a real suspect: swap to a lower-pF/m cable or use a phono stage with selectable input capacitance (Pro-Ject Phono Box S2 and similar). Both fixes are usually free or under $200.