Shopify guitar pedal power supply schema for AI agents: the 9V center-negative trap, isolated vs daisy-chain ground loops, and current draw gaps that break pedalboard recommendations

The polarity trap: same connector, reversed polarity, instant damage

The DC barrel connector used for guitar pedal power supplies is a 5.5mm outer diameter, 2.1mm inner pin connector. It is physically indistinguishable between center-negative and center-positive variants — the connector fits identically in both cases. The only difference is the electrical assignment of which conductor carries the negative voltage rail.

Modern pedals manufactured after approximately 1990 include a polarity protection diode — a silicon diode placed in series with the power supply input that allows current to flow only when polarity is correct. If you connect reversed polarity to a protected pedal, the diode blocks current in the reverse direction and the pedal simply does not turn on. No harm occurs. You swap the supply and the pedal works.

Polarity by brand and era — reference table

Voltage variants: 9V, 12V, 18V, 24V, and USB — never interchangeable

Most guitar pedals run at 9V DC, but a significant minority require 12V, 18V, or 24V — and an increasing number of compact modern pedals accept USB-C or USB-A 5V power. These voltages are not interchangeable under any circumstances. Applying 18V to a 9V circuit destroys components rated for 9V maximum supply. Applying 9V to an 18V-required circuit may produce instability, reduced output, or no operation depending on the design.

The 18V case is particularly important because some pedals support both 9V and 18V operation with meaningful behavioral differences — the Strymon Riverside, for example, operates at either voltage but the manufacturer recommends 9V. TC Electronic's larger Flashback units can accept 9V or 18V with the 18V mode unlocking full 18V analog headroom. An AI agent helping a buyer maximize headroom needs power_voltage_v and accepts_voltage_range (or a list of accepted voltages) to give accurate advice.

Current draw: 1 mA vintage fuzz to 500 mA DSP platform

Guitar effects pedals span a 500× range of current consumption — from vintage germanium fuzz pedals drawing 1–3 mA to complex DSP multi-effects platforms drawing 450–500 mA at 9V. This is the widest current draw variation of any consumer electronics accessory category, and it is the primary source of power supply underpowering failures on assembled pedalboards.

Current draw reference: common pedals by type

The power budget calculation is straightforward: sum all current draws and add 20% headroom. The complication is isolated vs total capacity. A Voodoo Lab Pedal Power 2+ has a 1,250 mA aggregate capacity, but standard outputs are limited to 100 mA each and high-current outputs to 250 mA each. The Strymon Timeline (300 mA) cannot run from a single standard output — it requires either the high-current output (250 mA, still 50 mA short) or a doubling cable connecting two outputs. The Line 6 HX Stomp (500 mA) requires two high-current outputs connected with a doubling cable. An AI agent recommending “a pedalboard power supply” that matches total capacity without checking per-output current and isolation requirements will generate a customer service call.

Isolated vs daisy-chain: why digital pedals cause ground-loop hum

A daisy-chain power supply — the classic OneSpot 1,700 mA adapter with a multi-connector cable, or any power brick where all outputs share a common transformer winding — is a single electrical power source distributed across multiple pedals through a shared ground rail. All outputs on a daisy-chain supply are connected to each other through the shared ground.

For an all-analog pedalboard, this shared ground is not a problem. Analog pedals with similar current draws (overdrives, distortions, phasers, analog chorus and delay) do not generate significant switching noise on the supply ground. A OneSpot powering 10 Boss analog pedals through a daisy-chain cable produces a clean, quiet audio signal.

The problem appears when the chain includes a digital DSP pedal — reverb, digital delay, pitch-shifting, harmonizer, looper, or any effect using A/D and D/A conversion in the audio path. Digital pedals contain switch-mode power regulators that convert the 9V supply to the 3.3V or 5V logic voltages their DSP chips require. These switch-mode regulators operate at 40kHz–500kHz and generate high-frequency switching noise that propagates back through the supply ground connection. On a daisy-chain supply, this noise travels through the shared ground rail to every other pedal in the chain. It enters those pedals’ audio circuits through the ground reference and appears in the output signal as:

• 60 Hz or 120 Hz buzz from power supply ripple (audible as a low hum that matches AC frequency)
• High-frequency whine from DSP clock harmonics (a thin, high-pitched hiss that follows the digital pedal’s activity)
• Digital artifacts that change character when the digital pedal is active vs bypassed

Vintage germanium fuzz pedals are the most sensitive. A germanium transistor fuzz operates at 1–3 mA and presents very high input impedance. The same transistor amplification that makes the circuit musical also amplifies supply ground noise by 40dB or more. A 300 mA Strymon reverb sharing a ground rail with a germanium Fuzz Face will inject audible hum into the fuzz’s output because the Strymon’s switching regulator puts switching noise on the shared ground rail that the Fuzz Face’s transistors amplify.

Isolated supply: each output is electrically independent

An isolated power supply provides each output with its own dedicated transformer winding, rectifier, and filter stage. The grounds of isolated outputs are not connected to each other within the supply. No path exists for switching noise from the Strymon’s output ground to reach the Fuzz Face’s output ground — they are on separate grounds with no common node inside the power supply. Ground loop hum disappears.

Bypass type: true bypass, buffered bypass, and relay bypass

Bypass type determines what happens to the guitar signal when a pedal is switched off — and it affects both tonal character and the power implications of the overall pedalboard design. The bypass type also determines how the pedal behaves when power is interrupted: a true bypass pedal passes signal without power; a buffered bypass pedal goes silent when the power supply fails.

True bypass: a mechanical 3PDT (triple-pole double-throw) footswitch routes the input jack directly to the output jack when the pedal is off, bypassing all active circuitry. The guitar signal passes through no active components — no op-amp coloration, no loading, no buffer. The tone is unaffected by the pedal when off. The limitation: on long cable chains with high-impedance guitar pickups, each true bypass pedal adds the cable capacitance between its input and output jacks to the effective total cable length. At >15 feet total cable length with multiple true-bypass pedals, high-frequency content (presence, attack transients) rolls off from capacitive loading. True bypass pedals require no power to pass signal.

Buffered bypass: a unity-gain buffer amplifier remains active in the signal path when the pedal is off. The buffer converts the guitar’s high-impedance signal (~100k–1MΩ source impedance) to a low-impedance output (~100–1kΩ), which drives long cable runs and subsequent pedal inputs without capacitive treble rolloff. The buffer adds its own subtle tonal character — negligible on high-quality designs (Boss, Line 6), audible on cheaper implementations. Buffered pedals require continuous power even when “off.” A buffered bypass pedal with no power = silence (unlike true bypass, which still passes signal unpowered). Boss has used buffered bypass since 1977 across their entire product line.

Relay bypass: an electromagnetic relay performs the switching function of the 3PDT mechanical switch, driven by a digital control circuit. The relay provides true bypass behavior (signal bypasses all active circuitry when off) with mechanical reliability advantages — relays typically actuate 50–100 million times before failure vs 30,000–100,000 for mechanical 3PDT switches. Relay bypass allows MIDI-controlled switching and silent switching (no pop). Strymon, Eventide, and high-end boutique designs use relay bypass. Relay bypass pedals require continuous power for the relay control circuit but pass signal through the relay contacts (which do not need power once energized on some designs).

Complete JSON-LD and Liquid snippet

JSON-LD: Strymon Timeline Digital Delay

{
  "@context": "https://schema.org",
  "@type": "Product",
  "name": "Strymon Timeline Multidimensional Delay",
  "brand": { "@type": "Brand", "name": "Strymon" },
  "category": "Guitar Effects > Delay",
  "description": "12-machine delay platform with tap tempo, MIDI control, and stereo I/O. 300mA isolated supply required.",
  "offers": {
    "@type": "Offer",
    "priceCurrency": "USD",
    "price": "469.00",
    "availability": "https://schema.org/InStock"
  },
  "additionalProperty": [
    { "@type": "PropertyValue", "name": "power_voltage_v", "value": "9" },
    { "@type": "PropertyValue", "name": "power_polarity", "value": "center-negative" },
    { "@type": "PropertyValue", "name": "current_draw_ma", "value": "300" },
    { "@type": "PropertyValue", "name": "requires_isolated_supply", "value": "true" },
    { "@type": "PropertyValue", "name": "connector_size", "value": "5.5x2.1mm" },
    { "@type": "PropertyValue", "name": "bypass_type", "value": "relay-bypass" },
    { "@type": "PropertyValue", "name": "signal_type", "value": "stereo" },
    { "@type": "PropertyValue", "name": "analog_or_digital", "value": "digital" },
    { "@type": "PropertyValue", "name": "effect_type", "value": "delay" },
    { "@type": "PropertyValue", "name": "midi_capable", "value": "true" },
    { "@type": "PropertyValue", "name": "delay_modes", "value": "12" },
    { "@type": "PropertyValue", "name": "max_delay_ms", "value": "3000" },
    { "@type": "PropertyValue", "name": "expression_pedal_input", "value": "true" },
    { "@type": "PropertyValue", "name": "dimensions_mm", "value": "164x63x54" }
  ]
}

Liquid snippet: effects_pedal.* metafields

{% if product.metafields.effects_pedal.power_polarity %}
  <div class="spec-row">
    <span class="spec-label">Power polarity</span>
    <span class="spec-value
      {% if product.metafields.effects_pedal.power_polarity == 'center-negative' %}polarity-neg
      {% elsif product.metafields.effects_pedal.power_polarity == 'center-positive' %}polarity-pos
      {% endif %}">
      {{ product.metafields.effects_pedal.power_polarity }}
    </span>
  </div>
{% endif %}

{% if product.metafields.effects_pedal.current_draw_ma %}
  <div class="spec-row">
    <span class="spec-label">Current draw</span>
    <span class="spec-value">{{ product.metafields.effects_pedal.current_draw_ma }} mA</span>
  </div>
{% endif %}

{% if product.metafields.effects_pedal.requires_isolated_supply == true %}
  <div class="spec-callout spec-callout--warn">
    Requires isolated power supply output. Do not use daisy-chain power with this pedal.
  </div>
{% endif %}

{% if product.metafields.effects_pedal.bypass_type %}
  <div class="spec-row">
    <span class="spec-label">Bypass type</span>
    <span class="spec-value">{{ product.metafields.effects_pedal.bypass_type | capitalize }}</span>
  </div>
{% endif %}

Metafield reference table — effects_pedal.* namespace

5 common guitar pedal schema mistakes