In order to work SW needs a connection between the output and the input of the sound card, and the device to test (DUT). There are three types of connections depending on what we want to measure:

LOOP CABLE  is used for channel calibration and for volumes setup

IMPEDANCE CABLE is used to measure drivers impedance, to calibrate SW and to measure passive components

ACOUSTIC MEASURE CABLE is used to measure frequency response


I suggest to use bi-polar shielded cable, 2 x 0.75 mm, well made connectors and keep short the cables that are to be connected to the sound card, Line In and Line Out, while there is no length problem with the DUT cable. Be careful when soldering the minijack: remember that SW use the left channel as REFERENCE and the right one as DATA.

In the following pictures the red cable is right channel; the white is the left channel; the black is the ground while the light blue squared cable is the one connected to the DUT.










As you can see my JIG is very easy to build and we can use it as LOOP or IMPEDANCE CABLE just changing a piece of cable with a resistor.


 Using the green cable we get a loop cable.


This cable is used for impedance measurement and is obtained connecting the reference resistors (11.15 ohm)

This is my JIG in Toto: the 2 blue cables with minijack are connected to the sound card (LINE IN and LINE OUT) while the light blue cable with two black faston is where the device under test (DUT) is connected 


This  JIG doesn't have any voltage divider resistor so be careful: if you have an amplified sound card you can fry it, during the volumes setup: the modern sound cards are just pre-amplified and, usually, supports up to  1.5-2.0 volts input, so there's no risk of damaging them; while old SC (Sound Blaster AWE) are amplified so check it up before connecting and be very careful during the volumes adjusting.

Pay attention at the quality of your pre-amplified sound card: being such it means that it is intended to work with high impedance loads and not with the typical low impedance of the drivers that we want to measure. This can induce the SC amplification transistor or IC to work badly, introducing distortions that would alter the measurements. In these cases you need to add a 1000 Ohm resistor in series with the  LINE OUT or you can use an external amplifier with the knowledge of the risk of being able to burn the SC if a voltage  divider resistor is not used. An example of voltage divider using two resistors (1k and 200) per channel is the following:



Personally, if I have to use the amplifier for the impedance measurements, I don't use any voltage divider simply because we have to work on small signals, thus 0.3-0.8 V are expected to come out of the ampli. Of course I pay a lot of attention to the volume knob! When using an amplifier, just connect the SC Line Out to the ampli Input, and the ampli Output as in the previous pic. To prevent ground loop, you can disconnect the sound card Line In ground, but only after you measured, with the ampli turned off, that a short exists between the ampli Input ground and its Output negative.

If your SC has a front module, I suggest you not to connect the LINE IN and OUT through it but using the back connectors cause usually the front panel connections are noisier.

 Let's go to the volumes setup.