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"Low frequency ringing"...

...I've noticed this phenomenon in a number of vintage amp designs, notably the various Williamson implementations. It manifests as an intermittent to continuous low frequency "bouncing" of the output at a couple of Hz with an amplitude of maybe 1/2V P-P. Easily noticible as woofer pumping. It can be induced by power line perturbations, large signal transients or even just physically pushing on the woofer which injects a low frequency signal into the output that the feedback circuit attempts to correct. While the PS may indeed be a factor, I was not able to significantly reduce the ringing by adding practically large amounts of decoupling capacitance betw stages. The cure for the designs I've worked with involved modifying the time constants of the coupling of two cascaded stages within the global feedback loop.

Typically, the ratio of time constants of successive stages in the unstable vintage designs was about 5:1. A rule of thumb even back then was that the ratio should be a minimum of 10:1. I've found that 20:1 to 25:1 to be most effective w/o significant affect on LF response or distortion. Some stable designs of the era achieved the staggering of time constants by using large coupling caps at g1 of the finals and small caps at g1 of the drivers. I like to reverse this with relatively small caps at the finals and large caps at the drivers. This seems to reduce the effects of output stage blocking under heavy drive conditions.

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