Guide
Measure impedance with REW – the ZMA file for your crossover
A crossover does not work into 8 Ω but into the driver's real impedance: the resonance peak in the bass, the rise from the voice-coil inductance in the treble. With REW, an audio interface and a resistor you measure this curve yourself and save it as a ZMA file.
What you need
- REW (Room EQ Wizard)
- Audio interface with two inputs
- Sense resistor (e.g. 100 Ω)
- Multimeter
Step by step
- 1
Build the jig with a sense resistor
A non-inductive sense resistor goes in series with the driver. REW suggests 100 Ω when driving from a headphone output; measure its exact value with a multimeter. One interface input measures the voltage before the resistor, the other the voltage across the driver – the diagram in the REW help shows which input goes where.
- 2
Match and protect the inputs
Both inputs need the same gain: use the signal generator at 1 kHz to match the levels within 1 dB, without clipping. When driving from a power amplifier, a divider (about 20 dB) with zener diodes protects the inputs. 100–200 mV across the driver is enough.
- 3
Calibrate
In the impedance settings run “Open circuit cal” (leads open), “Short circuit cal” (leads shorted) and “Reference cal” (a known resistor connected) in turn. Repeat after changing the leads or the sample rate.
- 4
Measure the impedance
In the measurement dialog choose “Impedance”, enter the exact sense resistor value, set start and end frequency (e.g. 10 Hz to 20 kHz), level at most −3 dBFS, length at least 256k, then “Start”. Measure the driver in its final cabinet – the box resonance and port tuning change the curve.
- 5
Export as text
“File → Export → Export measurement as text” writes a text file with frequency, impedance and phase. Check the header lines that the second column is ohms, and rename the file, e.g. woofer.zma – Crover also recognises it as .txt from its values.
- 6
Load it into Crover
Drag the file onto the driver in the schematic, or choose “Add measurement data…” on the driver. With a ZMA the simulation uses the real load instead of a nominal resistor – the low-pass inductor, notch filter and Zobel then act as they do in the speaker.
Why impedance matters for the crossover
Passive filters are calculated for a load impedance. If a woofer's impedance doubles above 1 kHz, a low-pass filter's crossover frequency moves up noticeably – the formula with 8 Ω is off. That is what Zobel networks and notch filters are for, and that is what a simulation with a ZMA shows.
The system impedance (what the amplifier sees) only follows from crossover and drivers together. With ZMA files for every way, Crover calculates it too, including its minimum – important for valve and class-D amplifiers.
Continue in Crover
How Crover reads FRD and ZMA files and what the import checks is in the manual: File formats and import.