Imagine a capacitor and a resistor connected in series (one form of an "RC" circuit). This is a Zobel network, I assume named after some dude named Zobel. Hey, I'm not an historian. It is placed in parallel with the speaker terminals (from + to -) and what it does is to remove the inductive impedance rise at high frequencies. All dynamic drivers have an inductive component due to the fact that a voice coil is exactly the shape of an inductor. This causes the electrical phase to become increasingly inductive at high frequencies, which can drive some amps into oscillation. Not normally a problem, but the Zobel network, if designed properly, will virtually flatten the impedance and phase at high frequencies and eliminate any possibility of the problem developing.
What the heck are "zobel" circuits?
I recently bought a pair of Classe 15 Amps from a gentlemen in Toronto. He was a great guy, but a bit of a fanatic. He heavily modified these amps - one mod was to get rid of the 'zobel' circuits at the speaker binding posts. As a safety prcaution, he gave me a pair of 'outboard' circuits to be attached via Banana plugs in the back of my amps. He explained that they prevented runaway high impedance that could oscillate the amps. I noticed that Alpha-Core speakers are now offering what the call an RC circuit built into their speaker cables, that I believe effective obtains the same results. Can anybody further enlighten me?