Dual Channel Measurement Jig

Here's a schematic for a simple dual channel measurement jig, which can be used with ARTA, REW, or SoundEasy. This jig is made with USB audio interfaces in mind, so the use of 1/4" TRS connectors is recommended for this purpose. Main reason is that the 48V phantom power on audio interfaces is only present on the XLR pins, so use of TRS will ensure that 48V will only ever be present when a mic is connected, not when using this jig even if you leave phantom power turned on.

R1 should be at least a 5W resistor, and the rest can be small 1/4W resistors. the 1K resistor R5 ensures that the common ground of both the audio interface and amplifier are at the same potential. A spreadsheet it attached to aid in part value selection for the probe voltage divider resistors R2/R3/R4/R5.

You will notice that this jig doesn't include a "calibration" switch. To calibrate, simply disconnect the speaker from the jig.

To switch from impedance measurements to SPL measurements, simply flick the switch, disconnect the impedance probe and plug in your mic. Reference probe remains connected at all times.

WARNING: Do NOT use this jig with internally bridged amplifiers, such as many low cost "chip" amps. Amplifier must not be bridged so that the negative speaker terminal is a neutral common.


Speaker acoustic measurement configuration.


Speaker impedance measurement configuration.





A photo of the internal wiring. It may not look very pretty, but it does the trick perfectly well.


[Edit Feb 2025: Updated with photo of ugly internal wiring. Added spreadsheet attachment to aid in voltage divider value selection.]