RAVENFORGE

Signal & Transduction Research Log

A04 — Electrical Loading of Passive Pickups

From pickup Z(f) to the instrument output — how resistive/capacitive loading and control topology shape the loaded response

1. Research question — why does the same pickup not have one fixed “tone”?

A03 described a passive magnetic pickup not as an ideal tone source, but as an induced source combined with a frequency-dependent terminal impedance Zpu(f). In a real instrument, however, the pickup output node is simultaneously connected to the volume pot, tone branch, cable, and the input of the next device. The in-use frequency response is therefore a property of the pickup plus its electrical boundary conditions, not of the pickup alone.

Paiva & Penttinen’s AES 131st Convention paper measured and modeled instrument cables and reported that cable capacitance changes the frequency response of a magnetic-pickup system; among the measured cables, the largest resonant-frequency difference was 1.4 kHz. That 1.4 kHz figure is not a universal upper bound: it belongs to the cable set and pickup/load conditions used in that study [1,3].

Figure 1. System boundary between the A03 pickup model and A04 external loading.

Engineering schematic. A04 ends at passive/equipment loading; buffer isolation is handled in A05.

2. Reference circuit — define the simplest full-volume case first

For the A04 reference case, the tone branch is omitted and the volume control is at full. The induced source Es drives the output node through the coil series impedance Zs=Rs+sLs. Pickup equivalent winding capacitance Cp and the external load shunt that node. When the external load can be simplified to Rload and Cext, use C_T=Cp+Cext and R_L=Rpot∥Rin. This capacitance sum is valid only when those capacitances are effectively connected from the same electrical node to ground.

H(s) = Vout/Es = 1 / [ 1 + (Rs + sL) (1/RL + sCT) ]

= 1 / [ 1 + Rs/RL + s(L/RL + Rs·CT) + s²L·CT ]

This is a loaded output-voltage transfer function, not the pickup’s intrinsic resonance. Once a tone network, a partial-volume wiper, or multiple pickups are introduced, the system should not be collapsed into one scalar R_L/C_T unless the topology genuinely permits it; use nodal analysis or circuit simulation instead.

3. Resistive loading — 250 kΩ / 500 kΩ / 1 MΩ are damping conditions

Even at full volume, the pot’s end-to-end resistance remains a resistive load from the pickup node to ground and combines in parallel with the downstream input resistance. For the same pickup and capacitance, a larger R_L generally reduces damping and preserves a higher resonant peak. But “500 kΩ is always brighter” is incomplete: peak position, peak height, and perception also depend on pickup R/L/C, cable/input capacitance, tone network, and volume state.

Nominal pot value may also differ substantially from the measured value. For example, Bourns specifies ±10% standard resistance tolerance for its Model 82 guitar pot and ±20% for Model 95 [6,7]. For repeatable prototype work, the log should therefore record measured end-to-end resistance and taper/position rather than only “500 kΩ.”

Figure 2. Conceptual comparison of 250 kΩ / 500 kΩ / 1 MΩ resistive loading.

Illustrative calculation, not RavenForge pickup measurement. The pot values in the legend combine in parallel with fixed Rin=1 MΩ.

4. Capacitive loading — manage cable by total capacitance, not length alone

An instrument cable is a distributed structure, but at typical instrument-cable lengths and audio frequencies its electrical length is very small, so a lumped shunt-capacitance approximation is practical. For the same construction, total capacitance scales approximately with capacitance per unit length times length, while connectors, patches, and device input capacitance add to the assembled path. Measured total pF is therefore a more direct loading descriptor than “3 m cable.”

Paiva & Penttinen found that, under their measurement and simulation conditions, cable capacitance dominated the audible-band loading effect while cable inductance and series/parallel resistance were negligible [1]. This does not mean low-capacitance cable is universally superior. Lower capacitance tends to move the loaded resonance upward for a given pickup, making it a voicing choice rather than a quality score. As one official example, Canare specifies 160 pF/m nominal capacitance for GS-6 [8].

Figure 3. Conceptual effect of external capacitance on the loaded response.

Illustrative calculation varying only Cext for the same pickup/load. The pF values are test points, not cable quality grades.

5. Input impedance — 1 MΩ is not “unloaded”

Amp, pedal, DI, and interface inputs have finite resistance and parasitic/input capacitance. Input resistance loads the pickup node resistively, while input capacitance can add to cable capacitance at the same node. A “1 MΩ input” may be high impedance, but it is not infinity; even a no-cable bench measurement still contains probe and fixture capacitance.

RavenForge therefore uses specified high-impedance / specified-capacitance fixture rather than “unloaded.” In product comparisons, record Rin, Cin, and cable/patch configuration when possible instead of relying on an input label alone. The mechanism by which a buffer isolates downstream cable capacitance is treated formally in A05.

6. Volume control — lowering the knob changes level and topology

When a conventional passive volume pot moves away from full, input-to-wiper resistance Ra and wiper-to-ground resistance Rb appear. The downstream cable/input is connected at the wiper, so it forms a parallel or compound network with Rb, while Ra becomes a series path between pickup and output. The result is not only attenuation: wiper source impedance and cable capacitance can also reshape the frequency response.

Audio/log taper defines the resistance-versus-rotation law; it does not change the circuit topology. A treble-bleed network is a separate frequency-dependent bypass intended to compensate some state-dependent loss, not a universal restoration of the full-volume response. “Volume down = same tone, lower level” is generally false for an unbuffered high-impedance passive system.

Figure 4. Different load states produced by a volume pot at full and partial settings.

Simplified topology. The actual response must include the tone branch and downstream input.

7. Tone control — at “10,” a standard tone branch does not completely disappear

A common passive tone control is a variable resistance Rt(θ) in series with capacitor Ct to ground. The branch may be written Ztone=Rt(θ)+1/(sCt), but the complete response must be solved with the pickup, volume, cable, and input network. With a standard pot, fully open still leaves a finite Rt, so the branch remains weakly coupled. A no-load tone pot is a distinct topology designed to physically disconnect the branch at its end position.

Values such as 22 nF or 47 nF are not universal answers for particular pickup types. The same capacitor behaves differently with different pickup inductance and load resistance. “Modern” and “50s” wiring likewise differ in which volume-pot node the tone branch uses; their volume-dependent interaction should be treated as a topology question, not as evidence that one is universally better.

Figure 5. Conceptual knob states of a standard passive tone branch.

The Rt–Ct branch is part of the complete guitar network. A no-load pot is treated as a separate disconnect topology.

8. Multiple pickups — separate spatial mixing from electrical interaction

Selecting two passive pickups simultaneously combines the spatial sampling/phase effects discussed in A02 and also connects their source impedances and control networks. A parallel pickup combination is not merely an arithmetic sum of two voltages; in series/parallel switching and independent V/V wiring, each pickup can become part of the other pickup’s load. Each selector state should therefore be logged and measured as a separate loaded state.

9. Evidence matrix — what can A04 actually claim?

10. C02 — Pickup Loading Measurement protocol

The literature and circuit results become a RavenForge specification only after effect size is measured on the actual pickup/load combination. C02 should hold pickup baseline and excitation fixed, substitute controlled Rload/Cload conditions, and compare Hloaded(f). Do not invent a universal “significant” threshold in advance; estimate repeatability and measurement uncertainty first, then compare measured effect size with the voicing target.

For every state, record measured pot R, total C, fixture Rin/Cin, and wiring state. If a peak is well defined, extract fpeak, peak gain, and bandwidth/Q; otherwise retain the full complex H(f) and fitting residuals.

Figure 6. RavenForge loaded-response validation workflow leading to C02.

Engineering validation loop. An effect-size threshold is not fixed before measurement uncertainty and the model target are known.

11. RavenForge application — derive the load from target response, not convention

EDDA’s passive baseline should not begin with a recipe such as “vintage means 250 kΩ.” Define pickup Z(f), the target loaded response, and a reference cable/input condition, then select the pot/capacitor topology. Heritage is a decision about which historical system behavior to preserve, not a requirement to copy nominal part values.

If EMBLA/ASKR place an onboard buffer or preamp after a passive pickup, the input condition seen directly by the pickup can remain A04 metadata, while buffer output impedance and downstream cable isolation belong to A05. Current powered EMG/Fishman architectures should not be directly ranked with the traditional passive-loading model used here.

12. Current conclusion — passive-pickup output is a pickup-plus-load system property

A04 rejects the use of 250 kΩ, 500 kΩ, 1 MΩ, cable length, or “high-Z” as self-contained tone adjectives. In the reference topology, resistive loading governs damping and peak height, while capacitive loading combines with inductance to move the resonance. At partial volume, with tone networks, or with multiple pickups, the topology itself changes and must be modeled or measured in its actual wiring state.

13. Next research — what moves to C01 / C02 / A05

References

[1] Cauduro Dias De Paiva, R.; Penttinen, H. “Cable Matters: Instrument Cables Affect the Frequency Response of Electric Guitars.” 131st Convention of the Audio Engineering Society, Paper 8466, pp. 1–10 (2011). Aalto research portal classifies the conference publication as scientific / peer-review.

[2] Paiva, R. C. D.; Pakarinen, J.; Välimäki, V. “Acoustics and Modeling of Pickups.” Journal of the Audio Engineering Society 60(10), 768–782 (2012).

[3] Paiva, R. C. D. Circuit Modeling Studies Related to Guitars and Audio Processing. Aalto University doctoral dissertation (2013). Includes Publication II: Paiva & Penttinen (2011). ISBN 978-952-60-5388-2.

[4] Lemme, H. Electric Guitar: Sound Secrets and Technology. Elektor International Media, 2012. Technical monograph; not treated as peer-reviewed journal evidence.

[5] Lemme, H. E. W. “The Secrets of Electric Guitar Pickups.” Technical article / measurement notes, BuildYourGuitar resource; accessed 2026-09. Used for circuit/measurement practice, not as peer-reviewed evidence.

[6] Bourns. Model 82 Vintage Guitar Potentiometer product data: 250 kΩ / 500 kΩ range, standard resistance tolerance ±10%. Official manufacturer datasheet/product page.

[7] Bourns. Model 95 Premium Guitar Potentiometer datasheet: 25 kΩ, 50 kΩ, 250 kΩ, 300 kΩ, 500 kΩ values; total resistance tolerance ±20%. Official manufacturer datasheet.

[8] Canare. GS-6 Guitar / Instrument Cable official specifications: nominal capacitance 160 pF/m. Used only as an example of manufacturer-declared capacitance-per-length, not a universal cable value.

[R1] RavenForge / Signal & Transduction Research Log A03. “Magnetic Pickups as RLC Systems.” Figure-corrected revision, 2026.09.09.

[R2] RavenForge / Signal & Transduction Research Log A02. “Pickup Position and Harmonic Sampling.” Verified revision, 2026.09.06.

[R3] RavenForge / Design Methodology Research Log B02. “Voicing Before Specification.” 2026.09.08.