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A Zobel by any other name...

Posted by Jon Risch (B) on March 8, 2009 at 10:51:37

In Reply to: Another speaker filter comment and question posted by Awe-d-o-file on March 7, 2009 at 07:20:07:

The so-called speaker filters are essentially extended range Zobel networks.

Zobel networks are frequently used at the output of amplifiers to help stabilize the output stage, so that non-resistive loads can be handled well.
However, the presence of a Zobel network on the output of an amplifier will not always provide "unconditional stability", or the ability to drive any combination of a reactive load without problems. In many cases, an amplifier will be stable into 'typical' loads, using zip cords, and will work fine under these conditions.

Speaker cables that are less inductive/more capacitive than zip cord can cause an amplifier that is less than "unconditionally stable" to oscillate, or exhibit other unstable behavior, such as extended clipping behavior (the signal 'sticking' to the rail voltage for a time much longer than the nominal over-excursion of the signal past the rail voltage level), bursts of oscillation at high power peaks or with clipping or near clipping, increased distortion as operating bias points shift or control loop servos lose proper control.

Adding a Zobel at the far end of the speaker cable can often cause the amplifiers phase margin (the reason for the instability) to shift to a more benign range, and avoid the worst of these symptoms or behavior.

Interestingly, what we commonly refer to as a Zobel network, is really a misnomer, but common usage has prevailed over the years and almost any RC network can and has been referred to as a Zobel network, even though a true Zobel network has a very specific use and range of values.

Jon Risch

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