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Old 13th Oct 2017, 11:07 am   #118
Radio Wrangler
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

Quote:
Originally Posted by Craig Sawyers View Post
speaker cables - for which I use Kimber. This is a woven construction, with very low inductance and very high capacitance - so it inherently acts like an RF filter - although another way of skinning the RF cat is just to use a Zobel at the speaker end.
We don't often disagree, Craig, but I think you've missed something.

The full Zobel network on the output of an amplifier consists of two sections. One is a series R-C pair which shunt the output of the amplifier. Together these provide a nice, resistive load to the amplifier at frequencies above audio, without burdening it significantly in the audio range. This is nice but isn't terribly important with well designed amplifiers.

The second part of the full Zobel network is a parallel indictor-resistor in series with the output (after the Zobel shunt). The job of this part is to prevent capacitive loads (exactly like those woven speaker cables) from creating excess phase shift in the feedback loop by the load capacitance and the output impedance of the output transistors creating an additional pole in the loop. The amplifier is isolated at RF from the cable/crossover capacitance and it sees the series resistor instead. This takes the cable/crossover C out of the loop.

Some high faluting amps of the nineteen eighties eschewed the series path Zobel network (presumably on religious grounds) and were renowned for taking off at RF when confronted with high-C interwoven cables. Smoke and large bills followed.

So high-C speaker cable isn't an alternative to the full Zobel, it is something which needs the full Zobel.

The business of running feedback amplifiers and feedback controlled voltage regulators into capacitive loads is a classic problem in design. The maths of loop stability scare a lot of people, but the problem is real.

David
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