what to consider when you have 500ft of speaker cable how to deal cable capacitance?

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rhythmsandy

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Joined 2012
how to deal with long length cables as much as 500ft will the zobel and output inductor of 10uH be sufficient to handle this?

My problem is im thinking to use two pole compensation and hence amp will be considerably faster in transients so using a long cable will make it unstable?

how to handle this situation..
 
Ian Finch

Ian Finch

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I would have thought that asking and visiting people, organizations, churches etc. that use large PA/sound systems and trolling through the equipment catalogues would soon show you how it's done these days - often wireless too.

It seems odd though, building high power amplifiers to audiophile recipes before you even know how or even if they can used.
 
Elvee

Elvee

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Joined 2006
rhythmsandy said:
how to deal with long length cables as much as 500ft will the zobel and output inductor of 10uH be sufficient to handle this?..
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This will result in a capacitance in the 15~30nF region, and an inductance of 100 to 200µH, nothing to worry about for the amplifier especially if one takes into account the fact that these are distributed, not lumped components.

On the other end, it will severely impact the frequency response: the roll-off frequency could be as low as 5KHz depending on the cable.
If it is just for musak/PA applications, it is unimportant, but if you need quality, that will certainly pose a problem
 
G

godfrey

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Joined 2009
The easiest way to deal with the capacitance of a long cable is to put an appropriate Zobel network at the loudspeaker end of the cable. Something like 100 Ohms and 10nF. The idea is for the resistor to (more or less) match the characteristic impedance of the cable.
 
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rhythmsandy

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Joined 2012
Elvee said:
This will result in a capacitance in the 15~30nF region, and an inductance of 100 to 200µH, nothing to worry about for the amplifier especially if one takes into account the fact that these are distributed, not lumped components.

On the other end, it will severely impact the frequency response: the roll-off frequency could be as low as 5KHz depending on the cable.
If it is just for musak/PA applications, it is unimportant, but if you need quality, that will certainly pose a problem
Click to expand...

how about extending the frequency using two pole compensation? like 150Khz even then will it attenuate inside the audible spectrum?
 
W

wg_ski

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Joined 2007
Two pole compensation in the amp won't help. What you're doing is adding a 6dB/oct low pass to your tweeter - independent of what the amp is doing. If you're stuck with that filter response you're better off overcompensating the amp because you won't need 20k. And any method of stepping the voltgae up and back down again is going to have frequency response limitations as well.
 
D

DF96

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myweb.tiscali.co.uk/g8hqp/
rhythmsandy said:
how about using a pi network?
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No, a 'Zobel' is correct. The idea is to provide a rough termination at RF frequencies so the cable looks roughly resistive to the amp where instability could be a problem. The speaker itself is likely to present a high impedance here.

At audio frequencies the 'Zobel' does nothing, and the cable looks like a resistor plus inductor because it is loaded with a much lower impedance than its own (frequency dependent, reactive) characteristic impedance.

Note that this trick is not needed for typical short domestic speaker cables, unless they are unusual - such as 'audiophile' stuff with high capacitance.
 
Elvee

Elvee

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Joined 2006
Here is a crude sim of what you can expect for a typical cable:
 

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J

jcx

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Joined 2003
jcx said:
John Allen found potentially audible effects with 150' runs in theater installations - used solid twisted "star quad" made from standard building electrical wire:

http://www.hps4000.com/pages/spksamps/speaker_wire.pdf

"star quad" is fine, coax would be even better if you could get heavy enough AWG to keep the resistance down - but with coax cable C may become a stability problem for some amps

Allen's "poor man's" star quad 3 phase + bond of solid copper @ 14 AWG or heavier is most readily available

not that copper is that cheap

EQ certainly seems practical and likely cheaper although multiband will be needed to come close to the proximity loss component sqrt(f) dependency
Click to expand...
 
indianajo

indianajo

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Joined 2010
Older professional organ installations in churches (Allen) etc use a 4 wire cable, 2 twisted ones for audio, 2 straight ones for a 24 VDC signal to turn the amp on via relay. there is AC main at the speaker, as is the amplifier, with a built in turnon relay.
Hammond used "tone cabinets", a powered speaker. The cable had 7 VAC driving the sound out in a balanced opposite phase signal, and 120 VAC on straight wires. The amp in the speaker had a differential input to null out the AC noise. The advantage of this setup was the amp was always on the same side of the 120 VAC transformer as the organ. This eliminated potential hum when differential inputs were tube based which could have variable and unbalanced gain over the life of the tube (unlike IC op amps). My Hammond 10-82TC tone cabinet came with a 100' cable weighing about 20 lb. That includes stereo, two audio signals out of phase for the "dual vibrato" leslie simulator. My 10' 10 ga cables for 8 ohm speakers weigh more than that, have more copper. With the speaker amp driven by the organ AC, the switch in the organ turned the speaker amp on or off.
 
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