Field Report: Mooretronix Stereo Tracer

Jan 1, 2004 12:00 PM, By Tom Atkins

When Bob Moore of Mooretronix contacted me about his latest product for testing analog audio lines called Stereo Tracer, my first thought was that everything we need to check analog audio pairs has already been manufactured. Quickly, I was put back on track and started to realize the power of this device.

Stereo Tracer is a device that connects to a dual-trace, 10MHz (or better) oscilloscope with an external trigger input and will verify amplitude, phase, and channel position of balanced audio lines by the video signature waveforms on the oscilloscope.

Operation is quite simple. Connect the oscilloscope inputs and external trigger to stereo tracer's front panel BNC connectors, plug in the wall wart power supply, set the scope to display both traces, adjust the inputs of the scope to measure 2V peak per division and set the sweep to 1ms and use the external trigger function of the scope. You are now ready to test balanced audio cable via the front panel mounted XLR connectors. A 4dB signal is sent to the male XLR connectors to the cable pairs under test. The other end of the cable pairs are then connected to the female XLR connectors, which are the return input to the Stereo Tracer. The correctness of the stereo audio path can be viewed on the oscilloscope. The hardest part of using this unit is getting used to the signature waveforms. Realistically, it took about 60 seconds to get used to them.

It was time to put the product to a real world test. We first started simple. We found a couple of XLR microphone cables about 10 feet long buried in a box of junk. Stereo Tracer quickly verified that one cable was in phase while the other was not. On removal of the XLR connector's covers, we found one cable was wired with pin 2 hot while the other was wired with pin 3 hot. Not bad for the first test, but I was insistent on finding the true power of this testing device. So we connected the unit in the terminal room and decided to check the integrity of the connection to and from the audio console in the production room with the audio console in circuit. My victim to test was the 8x2 stereo selector switch on the console. Testing was going smoothly until we got to input number six. Stereo tracer detected that left channel audio was appearing on the left and right console outputs while the right channel audio being sent to the console was not detected. While investigating of the mechanical switcher in the audio console, we noticed that when a repair was made the right channel cable pair to the switcher input was not connected and a solder bridge was causing the left channel input to appear on the left and right channel output. In my endeavor to test things to their limit, I decided to try and test CAT-5 UTP. After making the appropriate RJ-45 to XLR conversion connectors, we connected the device to some of the runs in the building. This unit verified the connections flawlessly. One other use we accidentally stumbled across is that the Stereo Tracer will balance the inputs of the oscilloscope. We found this arrangement useful when trying to test pieces of equipment that do not test well with an unbalanced input.

The unit attached to an oscilloscope and ready for use. The circuit board that drives the Stereo Tracer.

Built in a rugged metal enclosure, the unit can be rack-mounted with optional rack ears or can be secured to the top of most oscilloscopes by the optional nylon straps. The send outputs are rated at 700ohms balanced while the inputs are a 40kohm bridging. Frequency response is within +/-0.5dB from 20Hz to 100kHz. To me, the unit is well worth the investment to turn an oscilloscope into a powerful audio cable-pair testing tool.