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Old 23rd January 2002, 11:16 PM   #1
rtate is offline rtate  Canada
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Default leach superamp and capacitive speaker cables

I have recently found a speaker cable recipe that,acording to all reviews is outstanding. the cables are very lo inductance but at the price of high capacitance, in the order of 4000pf for and 8 ft length!!
My question is this,Has any one out there experimented with exotic(high capacitance) cables with the leach superamp. I am aware that there is a zobel network on the binding posts but I am not sure if it is enough. The cables are moderatly priced except that i have to by a 1000ft box , and are very labour intensive to make. If its going to be a problem for my mono block amps i would rather save the time and cash and heartache!!
thanks...
P.S. the leach amps are fabulous!!!
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Old 23rd January 2002, 11:25 PM   #2
jam is offline jam  United States
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You could add a Zobel at the speaker end.
10 ohms in series with 0.47uf, across the speaker leads, should take care of it.

Jam
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Old 23rd January 2002, 11:41 PM   #3
rtate is offline rtate  Canada
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Default speaker cables

Thanks for the fast response jam.
Do you mean as well as the 10 ohm .1 mfd at the amp.
How will this affect the sound?
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Old 24th January 2002, 01:34 AM   #4
Geoff is offline Geoff  United Kingdom
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A Zobel network on the output of an amplifier is fitted to ensure stability into an inductive load (ie a typical speaker). It will do nothing for a capacitive load. Stability into a capacitive load is normally achieved by fitting a series inductor in the output feed, with a parallel resistor to control the Q of the circuit.

Geoff
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Old 24th January 2002, 01:48 AM   #5
jam is offline jam  United States
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Geoff,

I beg to differ, under certain conditions the capacitance of a speaker cable can cause an amplifier to go into oscillation and terminating the cable will stop the oscillation.
See Nelson Pass's article on the subject, available on the Pass Labs web site.

Rtate,
I have found it not to affect the sound of the amplifier unless it was controlling some instability.

Regards,
Jam
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Old 24th January 2002, 01:52 AM   #6
Super is offline Super  United States
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With my last set of cables, I tried both methods, and didn't perceive any audible difference between the two.
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Old 24th January 2002, 02:00 AM   #7
rtate is offline rtate  Canada
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jam, you did'nt adress my other question of wheather to use a zobel at both ends of the cable ie. amp and speaker or just at one end.
thanks ..
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Old 24th January 2002, 02:05 AM   #8
jam is offline jam  United States
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Sorry I ment both end's.

Super,

If you do not have a stability problem you should not hear any difference.

Jam

P.S. This should also not be a problem with amplifiers that do not use global feedback.
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Old 24th January 2002, 02:06 AM   #9
Geoff is offline Geoff  United Kingdom
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Jam

I agree that a highly capacitive cable can cause oscillation, but adding additional capacitance across the output of the amp, in the form of a Zobel, is not the way to cure it. Either extra compensation within the amp is required to control the Nyquist stability or an output inductor is fitted. I quote Doug Self on this aspect " The most effective precaution against this form of instabitity (capacitive loading) is a small air-cored inductor in series with the amplifier output. This isolates the amplifier from the shunt capacitance, without causing significant losses at audio frequencies".

Geoff
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Old 24th January 2002, 02:15 AM   #10
jam is offline jam  United States
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Geoff,

Most manufacturers of modern amplification have deleted the series inductor at the output because it decreases the damping factor at high frequencies and hurts the sound.

Jam
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