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Old 24th January 2002, 07:19 PM   #11
Super is offline Super  United States
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Jam: I did have a capacitance problem and thats why I used the zobel. Sorry for the misunderstanding
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Old 24th January 2002, 07:55 PM   #12
Geoff is offline Geoff  United Kingdom
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Jam

It's not only modern equipment, none of the amplifiers (either commercial or DIY) that I have used during the past 30 years has had an output inductor.

My understanding is that provision of a suitable termination load for a cable, by fitting a Zobel at each end, ameliorates the effects of the low impedance of some of the more esoteric cables which can cause amplifier instability. It should also reduce the amount of RFI picked up by the cable and transmitted back into the feedback network.

However, the original question related to the effects of a high capacitance cable on the Leach amp. I am not familiar with his 'superamp', but if it follows his 'Low TIM' design there is an output inductor fitted as standard. In which case, cable capacitance shouldn't be a problem.

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Old 24th January 2002, 11:15 PM   #13
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Amplifiers using global feedback do not like capacitive loads because they increase the overall phase shift of the amp and reduce phase margin. In extreme cases the amp will burst into high frequency oscillation and sometimes self-destruct. 20 years ago transistor technology was much more primitive than today so the phase margins were much lower. Today, with faster devices and progress in circuit design most can get away with either no series output component at all or just a resistor. Naim amps put a 0.2-ohm WW resistor in series with the speaker output, although even they get very upset with capacitve loads.

If you have the guts and the equipment to measure stability it is a good test to attach a capacitor box to your amps speaker terminals and feed it a low amplitude square wave whilst dialing up various capacitor values (say 1nF to 10uF). Look for signs of instability then compensate in the amp itself or with a series resistor and/or inductor located right at the amp pcb. The zobel should preceed this and should also be located on the amp pcb with its own dedicated ground return to the ground star point. If you cannot make it stable with reasonable R/L values then you may need to reduce the feedback gain or reduce the frequency of the 1st open loop pole, etc. By the way, amps are often unstable into capacitive loads due to poor grounding alone so you need to ensure that your amp uses a star ground point and there are no disparate currents sharing common ground wires.

I've never heard of putting a second zobel at the speaker idea. I'm not sure how this would help.

By the way, if you use an output inductor you should also use a resistor in parallel with it as has been suggested earlier. Otherwise your inductor and the speaker cable capacitance form a series resonant circuit that will create a low Z at the resonant frequency, depending upon the Z of the speaker at that frequency.
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Old 24th January 2002, 11:54 PM   #14
jam is offline jam  United States
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Geoff,

I should have said most high-end amplifiers, most Japanese equiment still use inductors at the output.
What you say is true but this effect usually occurs when a certain amount of capacitance is presented to the load, sometimes a more capacative cable will not affect the amplifier.

Check http://passlabs.com/articles.htm

Jam
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Old 25th January 2002, 07:24 PM   #15
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I recommend the link Jam lists, as it goes into detail
about the resonance function that occurs when you
are driving a transmission line, which is what a cable is
at high frequencies. All cables do this unless terminated
in their characteristic impedance at the speaker end.

The reason some of the low inductance cables are
problematic is that the resonance can be low enough to
be inside the bandpass of the amplifier's feedback loop,
and oscillation occurs.

When oscillation occurs, all sorts of effects show up, often
as subtle as simply bad sound, but occasionally resulting
in smoke coming from the amp.

At this late date, I find it amazing that this stuff is not
better understood by the audiphile population.
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Old 25th January 2002, 09:19 PM   #16
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Um...I know that many hereabouts regard me as being, well, sanity-challenged, but can anyone explain to me why it's worth pursuing high reactance (capacitance, inductance, or both) cables when there are so many cable designs that don't require hanging such an albatross around your neck?
I've never heard such a cable that I didn't feel was masking or offsetting problems elsewhere in the system.

Grey
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Old 25th January 2002, 09:23 PM   #17
rtate is offline rtate  Canada
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where can i find out about these other cables.
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