Not getting any signal into your Apollo interface? The problem may be related to how you're connecting your input device and how you have the Apollo's corresponding input configured. See below for an explanation of the physical input connections on Apollo and how to configure Apollo for each input type. 

Connecting via XLR

When connecting via XLR you will always need to set the corresponding Apollo input to "MIC". When a particular input on Apollo is set to MIC, Apollo takes signal from the XLR connection corresponding to that channel. If you have your input set to MIC but are connecting via 1/4" TRS, or if you have your input set to LINE but are connecting via XLR, that would cause no input to show on Apollo.

The image below shows two Console input channels set to MIC in order to accept signal from the XLR connections:

Console input channels set to MIC (for XLR input)

Connecting via 1/4" TRS 

When connecting via 1/4" TRS you will always need to set the corresponding Apollo input to "LINE". When a particular input is set to LINE, Apollo takes signal from the 1/4" TRS connection corresponding to that channel. If you have your input set to LINE but are connecting via XLR, or if you have your input set to MIC but are connecting via 1/4" TRS, that would cause no input to show on Apollo.

1/4" TRS should be used for all line level signals when possible. 1/4" TS connections should be avoided for line level signals.

The image below shows two Console input channels set to LINE in order to accept signal from the 1/4" TRS connections: 

Console input channels set to LINE (for 1/4" TRS input)

Connecting via 1/4" TS

1/4" TS is generally only used for Hi-Z connections such as electric guitar or bass. On all UA Audio Interface models (except Apollo 16) the Hi-Z input(s) on the front of the device are intended for use with 1/4" TS connections. 

When you plug in a 1/4" TS cable to Apollo's Hi-Z input, that input channel automatically switches to Hi-Z and cannot be switched to MIC or LINE until the 1/4" TS cable is removed from the Hi-Z jack. While a 1/4" TS cable is plugged into a Hi-Z input, the corresponding XLR and 1/4" TRS connections for that input channel cannot be used. 

The image below shows a Console input channel in Hi-Z mode:

Console input channel set to Hi-Z (for 1/4" TS input via front panel)

Understanding Combo Jacks

Most UA Audio Interface models (except Apollo FireWire and Apollo 16) use combo jacks for their XLR and 1/4" TRS connections. These combo jacks combine an XLR and 1/4" TRS connection into a single jack to save space. These combo jacks may look a little bit different than your standard XLR or 1/4" TRS connections however they function just the same - when connecting via XLR set the corresponding input channel to MIC, and when connecting via 1/4" TRS set the corresponding input channel to LINE. Combo jacks accept all standard XLR and 1/4" TRS connectors.