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ampMeasure controls QuantAsylum QA402 and QA403 audio analyzers natively on macOS. This page covers setup, the five measurement modes, and answers to common questions. For hardware questions (connectors, specifications, firmware), see the QA40x wiki.

Getting started

1. Connect the QA403 to your Mac via USB and launch ampMeasure.

2. The app probes the device automatically: it reads the factory calibration page and exercises every input/output range — you'll hear the relays click. That's normal and takes a few seconds.

3. The status in the title bar shows "QA403 — ready" when connected.

No analyzer yet? Click Demo Mode on the start screen — a complete simulated QA403 lets you explore every mode. Demo readings are synthetic and clearly labeled.

Loopback check: connect the outputs directly to the inputs with BNC cables (L+→L+, L−→L−, and the same for the right channel — the QA403 is fully differential) and run the Loopback Test. All 32 range combinations should PASS within ±0.1 dB. This confirms your unit and cabling in under a minute.

The five modes

Analyzer

Live spectrum and oscilloscope with a signal generator.

• Signal: a single sine (set frequency and level; it's automatically snapped to an FFT bin for exact readings), or IMD twin-tone presets — SMPTE (60 Hz + 7 kHz), DIN (250 Hz + 8 kHz), CCIF (19 + 20 kHz).

• Outputs L/R: drive one channel only — useful for crosstalk checks and bridged amplifiers.

• Readouts: level, THD, THD+N, SNR, per-harmonic table (H2–H10), and for IMD the individual intermodulation products.

• Averages: coherent averaging (4/16/64) lowers the noise floor ~3 dB per doubling — use it when hunting low-level harmonics.

• Weighting: A-weighting applies to THD+N, SNR and noise readouts (marked "(A)"), never to THD or the fundamental.

• Important — input range: the composite peak of two IMD tones is 6 dB higher than a single tone at the same level. For CCIF at −10 dBV per tone, set Input FS to 6 or 12 dBV. If you see INPUT OVERLOAD in red, raise the input range — the analyzer otherwise engages its internal protection (audible relay clicking, and levels drop by exactly 24 dB).

Response

Frequency response by two methods:

• Log Chirp — fast, one acquisition, thousands of points, accurate phase (enable "Remove delay from phase" to subtract the analyzer's own ~1 ms loop delay). Best for response shape and phase.

• Stepped Sine — slower, one tone per point, but measures THD at every frequency (third plot panel). Best for distortion-vs-frequency.

The sample rate is chosen automatically from your stop frequency — sweeps beyond ~23 kHz run at 96 or 192 kHz as needed. Y Range presets (±1 dB, ±10 dB, …) keep plots comparable between runs; sessions can be saved and reloaded.

Power Sweep

The classic amplifier plot: THD+N versus output power into a stated load.

1. Connect the amplifier input to the analyzer output, the amplifier output to a dummy load (never a speaker), and the analyzer inputs across the load. For bridged/Class-D amps use both differential inputs — don't ground either speaker terminal.

2. Set the load impedance, start/stop generator levels and the abort limit (default: stop above 10 % THD+N).

3. Run. The input range rises automatically as the amp's output grows. Results show P @ 1 % THD+N (rated power) and P max.

The plot reads left to right: falling noise-limited region → flat region (the amp's real distortion) → sharp rise at clipping.

Probe

Shows the device's factory calibration page (per-range corrections for ADC and DAC) and a log of the connection sequence. Mostly diagnostic — if measurements ever look implausible, this is where to check that calibration was read ("Calibration page valid").

Loopback Test

Automated level-accuracy self-test across all 32 output/input range combinations. Use it to verify a new setup, cable, or unit.

Plots and workflow

• Hover shows frequency and level; near a peak the readout snaps to the true maximum.

• Click places cursor A, second click cursor B — the readout shows the delta. Double-click clears cursors.

• Hold (Analyzer) freezes the current trace as a colored overlay for before/after comparisons.

• Export CSV/PNG from the toolbar; the PNG matches the current theme — switch to Light (⌘,) for printable white-background plots.

• Appearance (System/Light/Dark) is in Settings (⌘,).

Troubleshooting

"No QA40x device found" — check the USB cable (data-capable, not charge-only), try another port. The QA403 needs no driver.

"Another process already has it open" — only one app can use the analyzer at a time. Quit other QA40x software (including one under Mono/Wine).

Relays click periodically and levels drop by 24 dB — the input is overloaded and the analyzer's protection is cycling. Raise Input FS. Remember: two-tone signals peak 6 dB higher than single tones.

"The device is in bootloader mode" — unplug and reconnect the analyzer.

First reading after switching ranges looks a few dB off — relay contacts settle thermally; the second reading is trustworthy. The app already discards known-bad first captures where possible.

Readings show "UNCALIBRATED" warning in the Probe log — the device's calibration page couldn't be read; measurements use unity factors. Reconnect; if it persists, contact support.