Amplifier Dummy Load#

A dummy load replaces the speaker during bench testing. It presents the same impedance as a speaker without producing sound, without risking a good driver, and without the mechanical and acoustic variables that make real speakers unreliable as test loads. When verifying an amplifier after repair — or characterizing one during design — a dummy load is essential.

Why Not Just Use a Speaker?#

• Volume — An amplifier at full power into a speaker on the bench is dangerously loud
• Speaker protection — If the amp has a fault (DC offset, oscillation, shorted output), it will damage or destroy the speaker
• Consistency — A speaker’s impedance varies with frequency; the nominal “8 Ω” or “4 Ω” is only accurate at one frequency. A resistive dummy load is a known, flat impedance — measurements against it are repeatable
• Safety — No moving cone means no acoustic feedback, no vibrating objects, no accidentally shorting exposed terminals

Design Rules#

Rule 1: Match the Impedance#

The load impedance must match what the amplifier expects to drive.

Why it matters: The amplifier’s output stage is designed for a specific impedance. Testing into the wrong impedance gives misleading power readings and may stress the output devices differently than real operation.

• Too high impedance: The amp delivers less current and less power than rated. Won’t reveal thermal or current-handling issues.
• Too low impedance: The amp delivers more current than rated. May trigger protection circuits, cause distortion, or damage output devices. Some amplifiers are rated for 2 Ω; many are not.

Rule 2: Size for the Power#

The load must absorb the amplifier’s full continuous output without overheating.

Calculate the requirement:

• RMS power rating of the amplifier per channel
• Multiply by the number of channels being loaded simultaneously
• Add margin — resistors derate at high temperature

Example: A 100 W/channel stereo amplifier needs at least 100 W per load resistor, and you need two of them. A 200 W rating per resistor provides margin.

Undersized loads fail: A resistor run beyond its power rating heats up, its resistance changes, and eventually it fails open. Wirewound resistors may survive briefly; film resistors may burn.

Rule 3: Keep It Resistive#

At audio frequencies (20 Hz – 20 kHz), a purely resistive load is ideal. Inductance or capacitance changes the phase relationship between voltage and current, which can affect stability measurements and doesn’t represent a real speaker load.

Wirewound resistors: Most high-power resistors are wirewound. Standard wirewound construction is slightly inductive, but the inductance is negligible at audio frequencies. For most amplifier testing, this is fine.

Non-inductive wirewound: If you need to test at higher frequencies or want to eliminate any inductance, use non-inductive wirewound resistors (bifilar wound or Ayrton-Perry wound). These cost more but are purely resistive to much higher frequencies.

Reactive loads (advanced): Real speakers have complex impedance — resistive at some frequencies, inductive at others, with resonant peaks. Some amplifier test standards specify a reactive dummy load that mimics this behavior. For repair verification and basic testing, a resistive load is sufficient.

Rule 4: Manage the Heat#

Dummy loads convert electrical power to heat. A 200 W load at full power dissipates 200 W of heat — equivalent to several light bulbs or a small space heater.

Thermal management options:

Thermal compound: Between the resistor body and the heatsink, use thermal paste or pads — same as for mounting a transistor. Air gaps kill thermal transfer.

Rule 5: Make Solid Connections#

High current through loose connections causes arcing, heating, and unreliable measurements.

Connection requirements:

• Heavy gauge wire (12 AWG or heavier for high-power loads)
• Proper terminals — binding posts, banana jacks, or heavy spade lugs
• Tightened connections — inspect and re-tighten periodically
• Short leads — long leads add resistance and inductance

Build Approaches#

Wirewound Power Resistors on a Heatsink#

The most common DIY approach.

Parts:

• Wirewound power resistors — rated for the required impedance and power
• Aluminum heatsink or thick aluminum plate (¼" / 6 mm minimum)
• Thermal paste or thermal pads
• Binding posts or banana jacks
• Optional: 12 V fan for active cooling

Combining resistors: Use series and parallel combinations to achieve the target impedance and power.

Construction tips:

• Bolt resistors flat against the heatsink, with thermal paste in between
• Keep internal wiring short and heavy gauge
• Add a small fan (12 V PC fan) for sustained tests
• Label the impedance clearly

Commercial Dummy Loads#

Purpose-built audio dummy loads are available from test equipment suppliers. Common ratings are 8 Ω for home audio; 4 Ω and 2 Ω models exist for pro and car audio.

Advantages:

• Properly rated and tested
• Often include a scope tap (attenuated output for safe oscilloscope connection)
• Some include multiple impedance settings

Disadvantages:

• More expensive than DIY
• May not match the exact impedance/power combination needed

The Halogen Bulb Trick#

A halogen bulb (car headlight, work light) works as a rough dummy load in a pinch. A 55 W H7 bulb presents roughly 2–3 Ω when hot.

Limitations:

• Resistance changes dramatically with temperature (cold filament is much lower resistance)
• Not suitable for accurate measurements
• Provides a visual power indicator (brightness)

Use only for quick smoke tests when a proper load isn’t available.

Using the Dummy Load#

Measuring Output Power#

1. Connect scope or True RMS DMM across the load
2. Drive the amplifier with a sine wave at the frequency of interest (typically 1 kHz)
3. Read the RMS voltage (V_rms) across the load
4. Calculate: Power = V_rms² / R_load

Example: 28.3 V_rms across 8 Ω = 100 W

Checking for Clipping#

• Scope across the load while increasing drive level
• A clean sine should remain sinusoidal
• Flat tops and bottoms indicate clipping — the amp is at maximum clean output
• Note the voltage at clipping onset; calculate the corresponding power

Checking for DC Offset#

• DMM on DC voltage across the load with no signal input
• Should read < 50 mV for most amplifiers
• Significant DC offset (hundreds of mV or volts) indicates a fault — do not connect a speaker

A dummy load tolerates DC offset that would destroy a speaker voice coil. This is why you test with the dummy load first.

Checking for Oscillation#

• Scope across the load with and without signal
• Look for high-frequency hash, ringing on edges, or sustained oscillation
• Oscillation may appear only under load, only at certain power levels, or only with certain input conditions

Multi-Channel and Bridged Testing#

For multi-channel amplifiers (stereo, 4-channel, etc.), load all channels being tested simultaneously.

Why all channels matter:

• The power supply is shared — loading one channel affects the rails
• Thermal behavior differs under full load vs. single-channel
• Channel crosstalk and power supply sag only appear with realistic loading

Bridged mode: Bridging combines two channels to drive one load at higher voltage. The load sees doubled voltage, so power is roughly 4× per-channel power into double the impedance.

• If each channel is rated 100 W into 4 Ω, bridged mode delivers roughly 400 W into 8 Ω
• The dummy load must be rated accordingly

Safety#

• Burns: Resistors at full dissipation cause burns on contact. Don’t touch during or immediately after testing.
• Fire hazard: Keep flammable materials away. A resistor run past its rating can glow red and ignite nearby materials.
• Ventilation: Hundreds of watts in an enclosed space heats up fast. Test in a ventilated area.
• Secure connections: Loose connections at high current arc and melt. Tighten terminals and inspect before each session.
• Don’t leave unattended: Stay at the bench during sustained full-power tests, especially with DIY builds.
• Guitar amps: Tube amplifiers with output transformers can be damaged by operating without a load. Always connect the dummy load before powering on. Some solid-state amps are also not safe to run unloaded.

In Practice#

• A repaired amplifier should be tested into a dummy load before connecting a speaker — the load survives faults that would destroy drivers
• Power measurements on a dummy load are higher than real-world speaker power because speaker impedance rises at low and high frequencies
• If the load resistance drifts upward during testing, it’s overheating — add more thermal capacity or reduce duty cycle
• For quick checks, one dummy load and channel switching is fine; for full characterization, load all channels simultaneously