2-channel loop-back for sound cards with one input only

If your sound card only has one input line, a device has to be used for summing the signals as shown in the figure below. We see that the signal going to the Right input consists of both the microphone signal and the loop-back signal.

To sum these two signals, an audio mixer or so-called combiner may be used. A passive device (consisting of 3 resistors) can also be used but it may give impedance problems. Contact us for purchasing such a device.

Figure 6 Schematic diagram for two-channel measurement using synchronization loop-back for one single input.