Now I wanna find a separate audio card/mixer which will allow to use digital input audio from Yamaha pianos, synth and output it to a selected audio interface (mixer or audiocard).

Will Steinberg audio card or Yamaha AG06 mixer work like I need?

The only thing that will accept digital signal from any of these keyboards is the computer (or tablet), “host”. When working with a Yamaha keyboard or synth that uses the “Yamaha Steinberg USB Driver”, the audio connection is digital via USB.

When, for example, you connect the MOXF to your computer via USB, and you install the “Yamaha Steinberg USB Driver”, you can use the MOXF as both a MIDI interface and an Audio Interface, in addition to being the multi-timbral synthesizer. Don’t confuse this — it acts exactly like you had a small, separate Interface... only it’s built-in to the MOXF.

The MOXF has a 4-in/2-out Audio Interface, plus it has facilities for five MIDI Ins and Outs... all via that single USB connection.
This means your computer uses the YSUSB Driver to ‘see’ the two Stereo USB (1/2, 3/4) audio pairs; and the 5 ports of MIDI communication. The computer ‘sees’ the MOXF as 4 audio inputs coming in, and the computer will output a stereo (2) audio mix out intended for the (analog) speaker outputs.

If you purchased an audio interface separately you would need to connect the MOXF to it via many physical ports and cables. The MOXF can route any of its Parts including both the 16 synth Parts and the A/D Input Part, to either USB 1/2 or USB 3/4 as you may desire to record to your DAW.

The 5 USB-MIDI Ports (the equivalent to 10 cables, one each for In and Out).
Port 1 - musical performing (Notes, controllers, tempo...)
Port 2 - Remote Control Surface communications (DAW Remote mode)
Port 3 - 5-pin MIDI jack connection to external device (back panel jacks)
Port 4 - MOXF VSTi graphic interface (Yamaha MOXF6/MOXF8 Editor VST)
Port 5 - Remote VSTi Control communications (Custom templates for controlling plug-in synths)
USB-MIDI communication is handled separately but requires the same multi-purpose YSUSB Driver.

The USB Audio Out from the MOXF is deliverable only directly to the computer Host (not to any other audio interface) — in order to connect the MOXF to your computer without using the USB (digital) Outputs, you must use the standard 1/4” Stereo Output (analog)... in this case your computer would use the recommended ASIO Driver for the device acting as audio interface. The audio interface must do the heavy-lifting (the number crunching that converts analog-to-digital,and digital-to-analog).

An external audio interface would take analog input from the MOXF and convert it to USB digital audio which is then handled by the computer (using the driver of the external device that does the conversion)!

When you have an audio interface on a device like a P125, notice an important difference, it is able to output its own audio only via USB, it cannot take external audio, and convert it to a digital USB signal, like the MOXF with it’s A/D In. In an A/D In an expensive analog-to-digital conversion occurs...

When you have an audio interface on a device like the THR, you have yet another type of device, but a similar scenario — the role of the audio interface is to convert analog signal to digital for the computer, and then take digital audio in from the computer and convert it back to analog so it can connect to speakers. But the Audio Interface is the IN/OUT for analog-to-digital, and for digital-to-analog! Don’t let the fact that it’s built into some of these product confuse you...the role would be the same if it were a standalone small device (only the cabling requirements would change!)

You cannot interconnect these particular audio devices digitally to each other. They each have a peer-to-peer connection to the host (computer).

Summary and Take away
You can assemble audio in your computer DAW application from various sources — not all things have to or are recorded simultaneously. Although the computer likes to deal with one interface at a time, nothing prevents you from using each device separately on the same project. Just keep a consistent Sample Rate for the entire session... so that you don’t have to do more work when finally mixing it down.

Audio devices can playback audio recorded through a different interface. If you recorded your P125 tracks using the P125’s usb audio capability, you could still later overdub tracks using the MOXF as the audio interface. And you could use yet an entirely different audio interface to overdub your vocals.

You no more have to limit yourself to one audio interface per session, than you’d have to limit yourself to a single microphone in an analog session. Simply connect and record through the device you desire. The limit is the computer can see one set of incoming audio ports at a time (on a Mac you can build an aggregate audio device which ‘fools’ the computer into seeing one composite set of audio inputs).

Let’s say you want to overdub vocals to your session, you could use whatever microphone and audio interface you desire... even though your other audio tracks were recorded via other audio interfaces.

Say you got one of the new Steinberg audio interfaces with the Rupert Neve transformers because you want to use a large diaphragm condenser mic for the vocals. You would configure your DAW to use the Steinberg device as your audio interface... (it uses the same driver as the other Yamaha, but you would select it as your current I/O ‘Device’; all your pre-recorded tracks would playback through the Steinberg I/O while it provides the phantom power to your condenser and you overdub your vocals to the DAW.