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Wire shielding techniques

Discussion in 'Electronic Basics' started by Matt Distefano, Jul 26, 2003.

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  1. I'm just starting my first somewhat major electronics project, building a
    pair of amplifiers up from a Rod Elliott (http://sound.au.com) design, using
    a PCB supplied by the same. I'll also be constructing a simple passive
    volume control (just a pot in a box, basically) to place between these amps
    and my CD source.

    So I'm wondering about the internal wiring for these units. Specifically,
    shielding of the wire. I'll probably be using 22AWG hookup wire for the
    internals on the volume control unit and the inputs on the amps, and
    something larger for the wiring between the amp's outputs and its speaker
    terminals. For the volume control unit, I can see the benefits of shielding,
    so it's more an issue of how to shield than whether to shield, but for the
    power amp, is there much point to shielding the few wires I'll have? Most of
    the amp is built onto the PCB, with the only actual wires being the ones
    connecting between that PCB and the power supply, input, and outputs. The
    input wires will probably be less than 2 inches long, and even if I shield
    them, the traces on the PCB are still exposed, the back end of the RCA jack
    is exposed, etc. It seems like there're enough unshielded parts anyway that
    another inch or two of wire isn't going to make that much difference, all
    things considered. Would I be wrong to think this? Also, as far as the
    actual shielding goes, would it be better to look for pre-made shielded
    wire, or just use regular unshielded hookup wire and wrap the insultation in
    aluminum foil, grounding it on one end?

    Thanks in advance,
    Matt
     
  2. In most cases you won't need wire shielding. You have complete control
    of the environment inside the amp. Keep sources of noise far from
    sensitive wires. You already have to do this on the PCB so it comes
    naturally in the hookups too. A grounded chassis near the circuits
    keeps stray signals from going very far through the air.

    Please don't use aluminum foil! That's so unprofessional looking, and a
    fire hazard when the electrical tape holding it together rots off. You
    can buy braided shielding at electronics stores if you really need it.
    Push the ends in and it expands, pull the ends out and it shrinks. Heat
    shrink tubing can hold it in place.



    Tell me what you think of this output circuit? It's the power end of an
    amplifier like in the circuits you were looking at.
    http://www.pixelmemory.us/Photos/Nerd/AutoBiasAmp.png

    I built this as a cheap and compact way to solve a thermal runaway
    problem I was having. One pair of output transistors has a regulated
    bias while another pair is under biased. The result is that the
    distortion is soft enough to be taken care of by NFB at any temperature.
    There are no hot emmitter coupling resistors like in a typical output
    stage.

    The four transistors in the center are generic low power, low voltage
    transistors. The two resistors to their bases are ~5 Ohms. The first
    follower transistors are medium power and may be darlingtons if desired.
    The final follower transistors are high power. The voltage dividers on
    the final transistors' bases are ~2 Ohm resistors. The optional pot
    raises the bias if needed.
     
  3. Did you mention what you were using for case(s)?
    If it is metalic/conductive then it is quite likely that
    you don't need to shield the input wire. Of course
    the power and output wires do not need shielding.

    You could just wire it with regular "hookup" wire and
    see how you like it. Simple enough to replace with
    a short piece of shielded wire if it "hums", etc.
     
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