The optical input does not alter the signal, it is lowered at the other end.

No, it seems to be lowered in the Adat sending device.
Or the analog output and the digital output are not matched levels (wich is normal)

In the digital domain, the digital audio data is untouched and not changed in terms of level.
Except, you would lower the digital volume setting on one of the devices in the chain.
Different volumes can also arise by the choice of different preamp gains or on line-level inputs,
depending on the choice of reference level.
But no matter which ref level you use, if the signal is at 0dB it will be AD converted with the same volume.
Only SNR might be different depending on the hardware, see manual.

I use BFP's TS input to enter +4db and gain at 0, but the optical input is smaller. The gain of the preamplifier before input is the same...

ADAT only transfers already digitized audio signals. With AES, SPDIF, MADI, and similar formats, there is no "bigger or smaller" in terms of level—unless you adjust the faders of the ADAT ports in TotalMix FX. Ensure all ADAT faders are set to 0 dB.

These digital ports transmit signals already converted from analog to digital. In the digital domain, there’s no waveform anymore—just digital values, including those representing signal levels.

No, the optical input performs no conversion. It transfers an already digitized audio signal as a stream of zeros and ones, unchanged. In TotalMix FX, you can adjust the signal level for a submix, but if all ADAT-related faders are set to 0 dB, the transport through the digital port remains "transparent"—or "bit-perfect." It doesn’t alter the A/D-converted signal, regardless of where the conversion occurred, whether via a preamp, converter, or simply playing back a file through an audio player.

Other example: The RME reference converters (ADI-2 DAC/Pro *, ADI-2/4 Pro FS) include a built-in Bit Test to verify bit-perfect transport (unaltered transmission of digitized audio data) from end to end. This test covers the path from an audio player to the DSP of the reference converter, whether it’s connected directly to a recording interface or through a device chain (e.g., PC → USB → UFX III → Octamic XTC → AES → ADI-2 Pro FS). Using a prepared test file (WAV format), you can confirm bit-perfect audio transport—and it works.

I’m explaining this to clarify that ADAT and similar digital formats don’t alter the signal level, provided no changes are made in TotalMix FX.

I see, thank you! So, is it a problem on the preamplifier side?

The difference of the levels between optical-digital and analog was about how much ?

Sorry, but for me, the description of the setup A/B and the respective settings is unclear.

A clear drawing of how the devices are connected and how the ports or mixer settings are end-to-end would be helpful. It's just guesswork and nobody knows exactly what you're doing. It should be clear by now that this is normally not a problem in the digital domain (and you have checked the faders of ADAT inputs and outputs?!=.

So once that has been clarified, you should turn to the source and then you should also go into what kind of signals you are using. Is it really clear, a sine wave signal, or are you comparing it “by feel”?

My point is that if you don't describe exactly what you're doing, we'll be left guessing for a long time.

Yes, it takes work to describe something like this exactly, but it also takes a lot of useless work to have to guess wildly.

About 7db, if it is an optical input, it will go down. However, if I raise the gain in daw after being recorded by daw, the volume will be almost the same, so I would like to respond with that. Thank you!

[EDIT: I had forgotten that the switch on the bottom of the BBF Pro is only for adjusting the reference level of outputs, not inputs.]

What is the setup? Are you connecting the same SSL preamp
1. via line level inputs of BBF Pro and
2. via ADAT ?

As you still do not deliver this needed information, I assume the following.
BTW, it didn't take long to make such a little ASCII drawing, it makes things much easier.
You know this term? "One picture tells more than 1000 words.."

PC
|
| USB
|                                                            (1) AD conversion
BBF Pro FS---ADAT IN<------ADAT OUT---SSL Pure Drive Quad---Mic In1------Mic1
     (2b) \                                                /  (2a) line level analog domain
              +-----Line IN<------Line Out---+

(1) here you get the AD converted signal from SSL Pure drive via ADAT

In the analog domain:

(2a) The analog signal from the mic is being sent at a specific output level via the SSL's analog output:
- Does the SSL have adjustable reference levels for its output signals?
- A higher reference level will result in a louder signal.

(2b) The Babyface Pro FS offers two types of line-level input:
     - AN 1/2 IN: balanced
     - AN 3/4 IN: unbalanced
       Assuming you’re using a balanced connection (SSL -> BBF Pro AN 1/2 IN):

       Check the BBF Pro FS manual (ch. 9.6 and tech specs 27.1).
       It shows that the sensitivity of the analog inputs is adjustable,
       with maximum input levels ranging from -57 dBu to +19 dBu
       depending on the gain settings and whether PAD is enabled:
                      Maximum input level XLR, Gain 0 dB: +8 dBu, PAD +19 dBu
                      Maximum input level XLR, Gain 65 dB: -57 dBu, PAD -46 dBu

       To handle "hot" signals with the highest headroom, set gain to "0" and enable "PAD" (+19 dBu).
       With gain = 0 and PAD disabled, the sensitivity increases, and the max input level drops to +8 dBu.
       If you should raise the gain, sensitivity would increase more, and max input level would drop further.

       Depending on how "hot" the SSL output signal is, you’ll need to find an appropriate balance between:
       - SSL mic gain
       - BBF Pro FS input sensitivity (combining gain and PAD settings).

Comparison of Signal Paths

2a (via ADAT): Signal level depends only on the SSL mic preamp's gain.
2b (via analog): Signal level depends on:
    - The SSL's line-level output settings (if adjustable—check the manual).
    - The BBF Pro's analog input settings (gain and PAD).

This level matching is fully normal in the analog domain. So it is also normal that you have little differences. If those differences disturb you for whatever reason, try to compensate it with the gain knob / PAD on the BBF Pro FS analog input.

It’s pointless to directly compare the analog and digital paths as if something were "wrong." They inherently involve different processing.

To clarify further, a detailed overview of your setup and signal flows would help. I’ve attempted to interpret your description to move things forward, but some assumptions were necessary.

In conclusion, this is not a problem. You just need to acknowledge that analog signal paths inherently introduce variability in signal strength compared to the ADAT path.

Addition to SSL

The SSLs inputs seem to support up to +24 dBu line level.
For the outputs it is not clear, also not switchable, possibly also up to +24 dBu.
So you might need to use the following settings on the BBF Pro FS to support such hot signals, when using AN 1/2 IN: gain 0, PAD enabled. Try it out, you will see what's right or wrong in the TotalMix FX metering of BBF Pro's inputs. Start with this highest insensitivity on the BBF Pro FS to have the highest headroom / to support hot signals.