My question is really strange, so please read with attention.

I need to record an audio signal from my mixer and going from my speaker. Does it exists any "audiocard" that record automatically in a usb key signal incoming if it's connected?
I am thinking about DJM 900 recording system...it can be the same trick...

 

Can you explain it better? Maybe with a drawing or a picture.

 

A lot of inexpensive portable recorders have line inputs

 

Yes, a recorder! Any cheap example with line input?

 

https://www.zoom-na.com/products/field-video-recording/field-recording/zoom-h1-handy-recorder

 

Depends on your mixer as not all digital recorders can handle the mixers output without an attenuater.

 

Digital Dj mixer Pionner dj 700 or 850

 

Both have record out RCA outputs, which stay at a contant level no matter what you do with your master sliders, but I would check on the Pioneer Dj forums to see what other Djs are using, as some recorders can't handle the output and will clip.

 

Yep, Zoom is about the cheapest & ok-quality you can get.
But @The Teknomage is right, the output of a Pioneer is too hot for a consumer-type recorder, the recording will sound clipped, distorted when the level goes past 0 (and every DJ does).
So you need an attenuator that takes a few dB of the signal, like these:
http://www.canford.co.uk/Products/20-901_CANFORD-IN-LINE-RCA-PHONO-ATTENUATOR-Phono-in-out (2x, for left & right)
or this box: https://www.bhphotovideo.com/c/product/331962-REG/Rolls_MX42_MX42_Mini_Stereo_Mixer.html
You could also use these in combination with an audiointerface/soundcard, and skip the Zoom recorder..