here are your audio interface output specs
Analogue Audio Outputs 1-2 (¼" TRS)
Electronically Balanced Outputs
Maximum Output Level (0dBFS): +9 dBu
THD+N: 0.0025% (0 dBFS input, 20Hz/22kHz bandpass filter)
Analogue Audio Outputs 1-4 (RCA phono)
Maximum output level (0dBFS): -3.5 dBu
THD+N: 0.03% (0 dBFS input, 20Hz/20kHz bandpass filter)
Frequency Response: 20Hz-20KHz +/- 0.1 dB
SNR (A-weighted): 103 dB
Dynamic Range: 103 dB
Maximum Output into 32R: +3 dBu (-1.4 dBV)
Power into 32R: 24 mW
Output Impedance: < 7 Ohms
Load Impedance: > 24 Ohms
As you can see, to get the hottest out signal you should use TRS analog out, not headphone, or RCA.
so the official anwser is right now that a common audio interface dont have enough gain to trigger the mb, and i have buy a preamp? wtf?
In best conditions you audio output is +9dBu, for example my Motu 828 is +18dBu, the other cheap alesis MultiMix4 I have is +19dBu
That is a huge difference..... both are trigging fine
On the other side I just tried an M-audio firewire audiophile that outputs 9dBu, using this one it was indeed more difficult (but still possible) to make it trig.
So the official answer is that some
common audio interfaces may need a preamp to properly trig the gate from audio input.
I know it not what your wanted to ear, but I have not solution on this point, except, as I told earlier changing a resistor.
This function has probably not been tested enough before validation of the design... sorry about that.
one last thing you should know, the ability to trig is depending on the audio signal you are using, indeed before the trig circuit, the audio is averaged (to have a clean trig, without bounces). So that means that you should not use "short" signals, such as a very dry snare or metronome click, or claps. The longest the input signal is, the better you will get a proper trig. For example a long decay, well compressed kick will do the job much better than a rim-shot.
I hope it will help anyway.