The OTB16 is a 16 channel 1U mixer with 16 balanced inputs and a Tonelux discrete summing bus. 

Each channel has a level control and a panner, and each pair of channels has a button that slaves the right channel level control to the left channel level control.  Each channel also has a dual color LED, which shows signal level in green and then switches to red at +14dBu.  The access to the OTB16 is through 2 eight channel Dsub inputs at a level of +4dBu nominal.

The summing of the 16 channels is through the famous Tonelux TX-240 and TX-260 discrete op-amps, and then throught the main output transformer, which can be adjusted by the MASTER fader pot, which comes out of a pair of XLR connectors.  Additionally there is a pre-master fader output for monitoring or for expansion, which is balanced but transformerless, and 2 additional balanced EXT inputs to the MIX bus.

The additional pre fader output can be plugged into the EXT input to extend the number of channels with a second or third OTB16.  120 0r 230 volts AC powered 50/60 Hz.