I try to set a very basic 4 stages phaser (two 2-pole all-pass filters), with a particular feedback path where only one peak is raised at center frequency when the feedback is above 0.
The following amplitude responses describe what I try to get: in the first measure (light green) we get two notches (no feedback). In the second measure (purple) we get two notches and a single peak at center frequency (feedback).
My chain is very simple: the input is sent into two 2-pole all-pass filters having the same center frequency, the output is then mixed 50/50 with the input. I've also added a feedback path, where the output of the last all-pass filter is feed back to the input of the first one (input + state * feedbackAmount).
Instead of getting a single peak I get two (as described in the following picture). I've also tested negative feedback it does not help. Any idea?
Edit1: My signal flow leading to two peaks when the feedback is close to 1:
Edit2: My implementation may be wrong indeed, to be sure I've also run a test in Ableton Live 8. Noise straight to the phaser.