My FFT (actually just DFT) algorithm is currently very naive and uses floating point math, therefore it is very slow. It is ok as long as it is detecting only one frequency at a time, but getting a whole spectrum would be too slow.
But there is no modification necessary: you can use the frequency response application as a very slow spectrum analyzer by connecting only the input. The phase curve will be meaningless but the amplitude should display whatever you feed into channel A.