Capacitance and shielded cables

Discussion in 'General Electronics Chat' started by nbaptista, Aug 21, 2014.

  1. nbaptista

    Thread Starter New Member

    Aug 21, 2014
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    I have a shielded LappKabel cable . I´m doing a DIY power cord from the LappKabel ,to connect my stereo amplifier but I have a doubt. The shield raises the cable capacitance, which some suggest is not good...I need to connect shield of shielded cable to earth ground, but if I would prefer not to connect it, som doesn´t the capacitance raise?I prefer a cable with lower capacitance!Or even if the shield is not connected to the earth ground, doing the fact that the cable is shielded the capacitance raises, no matter if the shield is connected to the earth ground or not?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If your preamp cant drive a few hundred pF of cable capacitance, get a proper preamp.
     
  3. nbaptista

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    Aug 21, 2014
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    It is an integrated stereo amplifier, a Densen B 110. Could you explain better?
     
  4. to3metalcan

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    Jul 20, 2014
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    What Mike means is, the output impedance of your preamp should be low enough that cable capacitance is pretty irrelevant unless you're running cables 20+ feet long.

    Disconnecting the shield might lower cable capacitance, but then there's no point in having a shielded cable...ordinary lamp-cord (or just a bent coat-hanger ;)) would work just as well. The bigger mistake here is in thinking that all capacitance is a bad thing...a few pF of capacitance will keep ultrasonic and radio from being picked up at your power amp. It's very unlikely that even a cheap cable has enough capacitance to impact the audible range over a short distance.
     
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  5. hexreader

    Active Member

    Apr 16, 2011
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    You say you are making a power cord (other responders seem to have missed that).

    I assume that you mean the mains power (120V AC or 240V AC) ?

    Capacitance will not be an issue on a power cord.

    A shielded cable must have the shield connected a one end (not both) otherwise it becomes a radio aerial and has the opposite effect of that intended.

    I think it unlikely that a shielded power cord will give any measurable improvement in sound quality, but fit it if you want.
     
  6. nbaptista

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    Aug 21, 2014
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    But do I have RF and EMI enough on an áudio instalation that justifies the shield??
     
  7. nbaptista

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    Aug 21, 2014
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    Yes, that´s right!
     
  8. hexreader

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    Apr 16, 2011
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    Then capacitance is of no consequence whatsoever. If anything, a bit of capacitance is probably a good thing.

    I think you are wasting your time and money, but it is your time and money to waste.
     
  9. #12

    Expert

    Nov 30, 2010
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    We can't tell from here. The point is, cable capacitance has been understood since before your amplifiers were made. The bandwidth limits of cable capacitance compared to load resistance can be calculated. If it was done wrong, fix the driving impedance and the load impedance. If it was done well, quit fighting it. Tens of millions of amplifiers use shielded cable because it works well and properly when designed correctly.

    Edit: A power cord? A shielded power cord? Go with hexreader. The capacitance is of no consequence.
     
  10. nbaptista

    Thread Starter New Member

    Aug 21, 2014
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    But not connecting the shield to the earth ground lowers the capacitance,no?
     
  11. Alec_t

    AAC Fanatic!

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    More important than capacitance is the voltage rating of the cable. Is it rated to be used at mains voltage?
     
  12. nbaptista

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    Aug 21, 2014
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    I think so, is a LappKabel 3*1,5 mm2
     
  13. kubeek

    AAC Fanatic!

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    That the capacitance if of no consequence means, as Dave would put it, that it doesn't bloody well matter.
    As other have said, for a power supply cable you dont need to care about it being shielded or not, and even less about some tiny capacitance. Do you realize that all the wires coming into your house are completely unshielded and with buttloads of capacitance? The filtering and regulation inside your audio equipment is about a billion times more important than anything that is happening on the power plug side.
     
  14. MikeML

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    The line output of all stereo equipment I have ever seen uses an audio opamp with feedback around it. This makes the output impedance a fraction of an Ohm. The input impedance of the amplifier at the other end of the cable is typically ~1K.

    The current driving limit of such pre-amplifiers is typically about +-30mA, so if you connect some capacitance to it, you need to ask at what frequency does the current that flows into the cable capacitance overwhelm the ability of the driver? The normal 1K load uses about 1mA, so 29mA is left to charge/discharge the cable capacitance.

    The highest frequency you should care about is 20kHz, so
    Z=E/I=1/2∏fC=1/29m, so C = 29m/2*∏*20000 = 0.23uF

    Which means that your preamp should drive 230000pF of cable capacitance. Now most cable I know about typically has a capacitance of ~50pF per foot, so you should be able to drive ~4000 feet of cable...
     
  15. nbaptista

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    Aug 21, 2014
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    But is effect(a radio aerial) worst than a unshielded cable?I think the unshielded cable also becomes a radio aerial!
     
  16. hexreader

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    Apr 16, 2011
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    @MikeML

    Please wake up :eek:

    We are talking about a mains lead ! :)
     
  17. hexreader

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    Apr 16, 2011
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    Yes.

    A cable with disconnected shield is worse than an equivalent cable with no shield.

    ..... but on a mains cable - nobody will ever know the difference, as the effect is insignificant. Just find any old kettle lead to get mains power to your amplifier and concentrate on enjoying the music.
     
  18. to3metalcan

    Member

    Jul 20, 2014
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    Okay, I thought you meant a cable to your power amp...if this is the power cable that connects to the wall outlet, it doesn't need to be shielded, but any capacitance is 100% good. There is no advantage to reducing it...the first thing your power cable is going to meet when it reaches your amp is probably a capacitor, and it's there to shunt emi and interference from appliances and digital devices before rectifying AC to DC. Any reduction in capacitance will, at best, net you MORE interference.
     
  19. nbaptista

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    Aug 21, 2014
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    But why the effect is insignificant? The technician who´s making the power cord to me says that is pointless to connect the shield to the earth ground, that´s why I´m confused!
     
  20. hexreader

    Active Member

    Apr 16, 2011
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    How much are you paying the technician to make this power cord for you?

    Is he a qualified to work on mains cables and will he be complying with all local standards that might apply?

    Is he an expert audio designer with a degree and many years of relevant experience in a research laboratory?

    I sense that he has no idea of what he is talking about and just wants your money.

    ..... sorry, but I am tired of telling you over and over - do whatever you want - it will make no difference, other than maybe replacing a safe cable with a dangerous one.

    And if you don't believe me - try with and without shield connected and see which you like best.

    Why not wrap tin-foil around your existing cable -it will have same effect as shielded cable with the shield unconnected - prove to yourself that it makes no difference, and then you can re-use the tin foil to recover it's cost.
     
    Last edited: Aug 21, 2014
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