Scarlett 18i8 (1st and 2nd gen), Scarlett 18i20 (1st and 2nd gen)
Clarett 2Pre, 4Pre, 8Pre (USB and Thunderbolt)
Saffire Pro 24, Pro 24 DSP, Pro 26, Pro 40
Liquid Saffire 56
If your external preamp unit (for example Scarlett Octopre) has an optical output that uses the ADAT format, you can connect it to the optical input on your Focusrite interface in order to record more channels into the recording software (DAW) on your computer. Some larger Focusrite interfaces also have an optical output, which can be connected to your external preamp unit's optical input, should it have one. Using ADAT, this will allow you to pass audio from your computer to any analogue outputs on the external preamp unit.
Connecting the external preamp to the Focusrite interface in order to record audio into the computer
You will need a TOSLink optical cable which can be purchased from music and hi-fi shops. This should be connected from the optical output on the external preamp unit to the optical input on the Focusrite interface. The ADAT digital format can carry up to 8 channels of audio and this depends on the sample rate you wish to use;
44.1/48kHz - 8 channels
88.2/96kHz - 4 channels
176.4/192kHz - 2 channels (Only on Clarett Thunderbolt, and Liquid Saffire 56)
In order to pass audio clearly without clicks and pops the two devices will need to be digitally synced. As well as passing audio, optical ADAT connections can carry digital clock. In this example, the simplest way of syncing the devices is to set the sync source of the Focusrite interface to ADAT, meaning the device receives its digital clock from the optical input. This can be set in the Mix Control or Focusrite Control software, depending on which Focusrite interface you have. The external preamp unit's sync/clock source should be set to Internal in order to pass it's clock to the Focusrite interface. In order to pass audio, both devices need to be set to the same sample rate.
The order the ADAT channels show up your DAW will depend on the Focusrite interface you have. Channel ordering charts can be found in the interfaces User Guide which can be downloaded from here.
Connecting the Focusrite interface to the external preamp unit in order to pass audio from the computer to the external preamp
The process for doing this is very similar to the above, except the optical cable would be connected from the optical output of the Focusrite interface to the optical input of the external preamp unit. In this example, the sync source of the Focusrite interface should be set to Internal and the sync source of the external preamp set to ADAT.
Using Mix Control or Focusrite Control, you can route separate Playback channels of your DAW to the individual channels within the ADAT output.
Example of using Focusrite Control to route channels from the computer to the ADAT outputs of the interface
Example of using Mix Control to route channels from the computer to the ADAT outputs of the interface
Note: If you connect two optical cables between the two units (Unit 1 output to Unit 2 input, Unit 2 output to Unit 1 input), it does not matter which one is set to Internal sync source and which one is set to ADAT sync source, as long as one is set to ADAT and the other Internal.
If you have a Clarett 8PreX or Liquid Saffire 56, it is possible to connect 2 ADAT devices to expand your setup further. For more information on this, please see this article.