Quote:
Originally Posted by bikerhifinut
merely a Zobel network across the output
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The bit that goes ACROSS the output is only half the proper zobel network. The full thing also has a bit that goes in series with the output.
The series bit disconnects the output from the amplifier at high frequencies, while the bit across the output (and upstream of the series bit) comes in at high frequencies and connects a little dummy load. The dummy load isn't needed per-se, but in the crossover region it helps damp a resonance.
It is the disconnection business which protects the amplifier from seeing excessively capacitive loads at high frequencies.
Capacitive loads are a classic problem for feedback-controlled voltage amplifiers.
So it's chiefly the section in series with the output which does the hard work.
You see some amplifiers with the series part missed out (oops, someone didn't understand what it did)
You see some amplifiers with the shunt element after the series (Double oops!)
And then there are some amplifiers where the designer wrote up how the Zobel diplexer was simply wrong and shouldn't be included, for the best sound. These amplifiers, as you mentioned, have a reputation for self-destruction if they see a load which looks capacitive at high frequencies. Certain boutique cables for example.
Relatively few degree level electronics designers are aware just what this little network is for. You also need to have stayed awake, alert, and following through several mind numbingly boring lectures on control theory and stability analysis... or you need to have taught yourself once you realised there was something you were missing.
Zobel was a very smart cookie into network analysis and filter design at Bell Labs he was one of the people who laid down some of the foundations of what Antol Zverez at Westinghouse later pulled together. There isn't one Zobel network, he did loads!
David