hello I have an old f1500B velodyne servo Control subwoofer and I want to know what the piazo is with its four-pin connector connected ton thé amplifier. . Without any signal to its nput, the speaker emits a strong and regular high-pitched sound. I unplugged the piazo's four-pin connector to the amp and the subwoofer no longer emits the high-pitched sound. With a signal at its input, the subwoofer works normally because it reproduces the low frequencies. So, what is the piazo for if the speaker reproduces the low frequencies nornally without the piazo being connected to the amplifier? thank you very much in advance for your technical information.