Wire shielding techniques

[Matt Distefano]

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

 

[Kevin McMurtrie]

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

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.

 

[Richard Crowley]

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.

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.