Since products may use balanced connections, unbalanced or a mix of both, it may on occasion become necessary to connect balanced inputs to unbalanced outputs, or vice versa. However, one needs to pay close attention to how this is implemented, otherwise several problems may arise.
Although it rather a straight forward affair to connect a balanced XLR to an unbalance Jack via using an appropriate XLR to Jack lead, certain pin wiring is required other wise one may loose up to 6 dB of signal, which is quite a significant drop in signal level. Note that a drop of 3 dB equates to half volume, whereas 6 dB amounts to a ¼ volume (dB scale is logarithmic).
A classic example of this point can be demonstrated by connecting the balanced output of a Mixer (say a Mackie 1202 VLZ) to a power amplifier that only has unbalanced Jack inputs via using a balanced XLR to TRS Jack lead. In this example, pin 3 (negative signal) may be left floating with no connection at the input stage of the amp and the result can lead to around 6 dB loss of signal, which is pretty severe. The outcome is that overall system will never produce the level of output that is capable of producing, which may leave the user somewhat short of performance. By simply ensuring that pin 3 is shorted to ground, at either within the XLR or Jack, will remedy this issue.
Another example that I have encountered is when feeding say the balanced output of a channel strip (e.g. Focusrite Voice Master Pro) to the input of a PCI soundcard (e.g. an EMU 1212M). Note that some soundcards are capable of accepting either balanced (+4dBu) or unbalanced (-10dBU) signals. However, as the rating-per-signal suggests, balanced signals are significantly stronger, or hotter, than unbalanced. To compound this issue, generally soundcards use software switching via a software control panel to adjust input sensitivities.
Therefore distortion can easily occur, if say connecting the balanced output of a microphone pre-amp or channel strip (Focusrite Voice Master or Trak Master Pro) into a soundcard capable of accepting a balanced signal (EMU 1212M) unless the appropriate levels are set. Note that in this example the input sensitivity needs to be set to +4dBu, which gives the input of the soundcard additional headroom to accept the stronger balanced signal. If set to –10dBu the balanced signal will simply overload the input of the soundcard, generating severe distortion even when using low output from the channel strip.