A top-selling model of the original 9-Series, the FL-9's wide operation range and full frequency response allow it to work with any input signal to create a diverse assortment of modulation and time-delay effects. Improved construction including through-hole, glass reinforced epoxy PCB's, snap-in PCB connectors, and true bypass switching guarantee trouble free performance while the combination of one Panasonic MN3207 BBD IC and one Signetics NE570 compander provides warm yet crystal-clear flanging with zero clock noise. Rate, Depth and Regeneration controls allow the user to dial in everything from shimmering chorus and vibrato to hollow, metallic overtones to thick, jet-like flanging sweeps.

In addition to these standard controls the FL-9 features a Delay Time knob that allows for a vast array of all-new spatial effects. The Delay Time control exhibits an inverse relation between the delay time and the frequency shift of the flange - as the delay time increases, the frequency that is shifted decreases. This inverse relation creates an amazingly realistic, three-dimensional flanging effect. In addition, the Delay Time control is interactive with the Rate and Depth controls, so that if you increase the Depth, the Delay Time range increases, and if you increase the Speed, the Delay Time changes faster.

Like other time-delay models in the Maxon line, the FL-9 uses an analog comb filtering circuit to provide its lush effect. A comb filter reintroduces the delayed signal against the original signal, generating "double level" peaks and "cancellation" notches at certain frequency points that create the effect. If there is a downside to comb filtering, it is that the peaks can cause the output level of the unit to be louder than when bypassed. In an effort to eliminate this problem, the FL-9 has been carefully calibrated to provide unity gain when engaged. In addition, the FL-9's performance is kept exceptionally quiet thanks to a Signetics #NE570 compander and true bypass switching using a 4PDT mechanical switch.