How can I make an input observable?

You can wrap them in a form and listen to changes

this.myForm = fb.group({  
  'sku':  ['', Validators.required]  
});

this.sku = this.myForm.controls['sku'];

this.sku.valueChanges.subscribe(  
  (value: string) => {  
    console.log('sku changed to: ', value);  
  }
);

this.myForm.valueChanges.subscribe(  
  (value: string) => {  
    console.log('form changed to: ', value);  
  }
);

http://blog.ng-book.com/the-ultimate-guide-to-forms-in-angular-2/

or

@Component({
   ...,
   template: '<input (change)="onChange($event.target.value)">'
})
class MyComponent {
  this.inputChange =new Subject();

  onChange(e) {
    this.inputChange.next(e);
  }
}

See also this issue open https://github.com/angular/angular/issues/4062