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Cable Capacitance High-Frequency Rolloff

Calculate the high-frequency rolloff (-3 dB point) caused by cable capacitance interacting with source impedance.

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Formula

fc=1/(2π×Zs×Ctotal)f_c = 1 / (2π × Z_s × C_total)
Z_s— Source impedance (Ω)
C_total— Total cable capacitance (F)

How It Works

This calculator determines the high-frequency rolloff caused by cable capacitance with high-impedance audio sources such as passive guitar pickups and piezo transducers. Guitar players, audio technicians, and equipment designers use it to predict treble loss and select appropriate cables or buffering solutions. Cable capacitance (typically 50-150 pF/m) forms an RC low-pass filter with the source impedance: fc = 1/(2*pi*R*C). A 6 m cable at 100 pF/m (600 pF total) with a 250 kohm guitar pickup creates a 1.06 kHz cutoff - severely rolling off presence and brilliance. Per pickup manufacturer measurements (Seymour Duncan, DiMarzio), passive single-coil pickups have 100-250 kohm impedance at resonance; humbuckers have 200-500 kohm. The resonant peak created by pickup inductance and cable capacitance is a defining characteristic of electric guitar tone, typically occurring at 2-5 kHz. Active pickups (EMG, Fishman) have <1 kohm output impedance, making them essentially immune to cable capacitance effects.

Worked Example

Problem

Calculate high-frequency rolloff for a vintage-style Stratocaster pickup (L = 3.3 H, R = 6.2 kohm DC, approximately 180 kohm at resonance) through a 7 m standard guitar cable at 120 pF/m.

Solution
  1. Total cable capacitance: C = 7 * 120 = 840 pF
  2. Source impedance at resonance: Z_source approximately equals 180 kohm (pickup at resonant peak)
  3. RC cutoff: fc = 1/(2*pi*180000*840e-12) = 1.05 kHz
Frequency response analysis:
  • At 1.05 kHz: -3 dB (half power point)
  • At 2.1 kHz (one octave above): -7 dB
  • At 5 kHz: -13 dB (significant treble loss)
  • At 10 kHz: -19 dB (presence severely attenuated)
Resonant peak with cable capacitance:
  • Pickup inductance: L = 3.3 H
  • Total capacitance: C_cable + C_tone_pot approximately equals 840 + 22 = 862 pF (with 22 nF tone cap)
  • Wait - tone cap in parallel adds 22000 pF, not 22 pF. With tone at max, C_total approximately equals 840 pF cable only.
  • Resonant frequency: f_res = 1/(2*pi*sqrt(LC)) = 1/(2*pi*sqrt(3.3*840e-12)) = 3.02 kHz
  • This resonant peak gives the 'quack' characteristic of Strats; higher cable capacitance lowers the peak.
Mitigation:
  • Low-capacitance cable (60 pF/m): fc rises to 1.77 kHz (69% improvement)
  • Buffer pedal (1 kohm output): fc rises to 188 kHz (essentially flat to 20 kHz)

Practical Tips

  • ✓Use a buffer pedal (JHS Little Black Buffer, TC Electronic BonaFide) at the guitar output for cable runs over 5 m. Buffers convert high-impedance pickup output (100-500 kohm) to low impedance (100-1000 ohm), raising fc to >100 kHz. Unity-gain JFET buffers add zero noise and draw 1-5 mA from battery - essentially permanent treble preservation.
  • ✓Select instrument cables with capacitance below 80 pF/m for unbuffered passive guitars. Premium cables (Mogami 2524: 50 pF/m, Canare GS-6: 70 pF/m, Evidence Audio Lyric: 39 pF/m) provide measurable treble improvement versus standard 100-150 pF/m cables. The difference is most audible on single-coil pickups at full tone settings.
  • ✓Some players intentionally use cable capacitance to shape tone - longer cables (100-120 pF/m * 10 m = 1000+ pF) create a warmer, vintage sound by lowering the resonant peak from 4 kHz to 2-3 kHz. Eric Johnson famously uses specific cable lengths to achieve his tone. Experiment with 3 m vs 6 m cables to hear the difference before investing in 'upgrades'.
  • ✓For piezo pickups (acoustic guitars, violin), output impedance can exceed 1 Mohm, making cable capacitance effects severe. Preamps with >10 Mohm input impedance (FET-based, like LR Baggs Para DI) are essential - passive DI boxes with 1 Mohm input cause significant treble loss even with short cables.

Common Mistakes

  • ✗Assuming problem only occurs with long cables - even a 3 m cable at 100 pF/m = 300 pF with 400 kohm humbucker creates fc = 1.33 kHz. Short cables still cause significant rolloff with high-impedance passive sources. The combination of source impedance AND cable capacitance determines rolloff, not cable length alone.
  • ✗Ignoring the guitar's tone circuit - the tone potentiometer and capacitor (typically 250k/500k pot with 22-47 nF cap) form a parallel RC network that interacts with cable capacitance. With tone pot at maximum (bypassed), only cable capacitance matters. With tone rolled off, the tone cap dominates. This is why tone knob sweep range changes with different cables.
  • ✗Thinking balanced cables have zero capacitance - balanced (XLR, TRS) cables also have capacitance (30-80 pF/m typical), but their source impedance is low (150-600 ohms for mic/line). The resulting fc is in the MHz range (completely inaudible). Balanced cable capacitance only matters for AES/EBU digital audio at 3+ MHz signaling rates.
  • ✗Blaming cable capacitance for all tone loss - degraded solder joints, corroded connectors, and dirty contacts cause 10-20x more tone loss than cable capacitance differences. A corroded 1/4-inch jack can add 10+ kohm series resistance, creating its own rolloff. Clean and maintain connections before upgrading cables.

Frequently Asked Questions

Audio interfaces typically have 1 Mohm+ input impedance (instrument inputs) with very short internal wiring. Combined with a 1 m cable, total capacitance might be 100-150 pF versus 600+ pF for a 6 m run to an amp. This raises fc from 1-2 kHz to 5-10 kHz, preserving significantly more treble. Additionally, the interface input presents a pure resistive load without the complex impedance of a tube amp input, eliminating resonance interactions that can color tone.
Negligibly - microphone outputs have 50-200 ohm source impedance. Even 50 m of cable at 100 pF/m = 5000 pF yields fc = 1/(2*pi*200*5e-9) = 159 kHz - far above audibility. The -3 dB point is 8 octaves above 20 kHz. Balanced cable capacitance becomes relevant only for digital audio (AES/EBU at 3.072 MHz) where 100 m runs can cause waveform degradation. For analog mic and line level, cable capacitance is not a practical concern per AES48 guidelines.
Active pickups (EMG, Fishman Fluence, Seymour Duncan Blackouts) include built-in preamps with 10-100 ohm output impedance - 1000-10,000x lower than passive pickups. This pushes the RC rolloff frequency from 1-5 kHz into the MHz range, making cable capacitance completely inaudible regardless of cable length or type. The trade-off is battery dependency (typically 9V, lasting 1000-3000 hours) and different tonal character (often described as 'clinical' or 'modern' versus the 'organic' impedance interaction of passive pickups).

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