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PostPosted: 22 Sep 2018, 20:14 
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ILoveHiFi wrote:
In theory twisted pairs only reduce noise when both noise signal appears on both +- cable, in audio systems most are not balanced system. -signal is grounded, noise never appears on ground cable to noise directly appears on positive wire.
This is wrong. Both twisted pairs and shields have their place; you just have to understand what each is for.

Twisted pair wire gives good immunity to induced currents due to magnetic fields. It also reduces magnetic field intensity around the wires due to the reciprocity of the field equations. This is why high current conductors should be twisted and why unbalanced signal leads should be twisted as well. The former greatly reduces the presence of magnetic induction within the chassis and the later provides signal path immunity from residual magnetic fields. This is how one controls mains frequency and twice mains frequency hum in an amplifier chassis. High currents produce larger magnetic fields; for example, filament circuits. Most field induced magnetic coupling is at lower frequency due to the currents required to generate large fields.

Shields give good immunity from electrostatically coupled noise and signals. Large changes in voltage produce large changes in electric fields and cause coupling issues. For example, the large voltage swings on the primary side wiring of an output transformer can generate significant alternating electric fields. If signal leads are in close proximity to such fields, they will couple back into the signal side and cause unwanted feedback issues. But low voltages don't produce much in the way of electric fields. This is why there is almost never a need for shields in a SS amp; the voltages are generally too low to cause such coupling.

At higher frequency, the use of shields protects from free-space radiation via TEM mode wave propagation by providing a ground plane reference for wave termination. The shield can carry the termination current but there is no induced field within the shield to couple to the center conductors.

And this is why we use twisted-shielded pair wiring in circuits requiring high noise immunity. Both physical arrangements are required to reduce the chance of unwanted coupling into the circuit.

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PostPosted: 22 Sep 2018, 22:32 
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I used to be using twisted pairs for input wires and output wires for pre amps, since its easier to wire up compared to sheilded wires.
I've found out they've been causing noise and reducing sound quality.
Both solid state pre amps and valve pre amps.

The twised pairs are picking up noise.
My practicall testing shows results that dosen't match up to what you are saying.

I think your theory is correct.
I did learn about how summ of magntic feilds arround a wire is equal to total current passing through a wire, don't remeber learning about electrostatically coupled noise though.

However because the ground wire of the twisted pair is ground so that means electrostatic noise dosen't appear in the gound wire. Left with noise appearing on positive wire of input.
Voltage cannot appear or be induced in any electircal systems ground.
This is how a sheilded wire works, the ground wrapped arround the postive wire prevents any noise from penatrating into the postive wire.

I had tried putting a twisted pair close to a transfomer 50hz of my pre amps, picking up more humm than a shielded wire.
If it was immune to induced currents due to twisting then no noise.

My explantion on why the current noise are not canceling out.
Ground is like a end point of a system.
Ohms Law V=IR.
Becasue ground is like 0R so any induced current into 0R causes 0 voltatage on ground wire to cancel with noise on postive wire.

For the noise signal due to induced currents to cancel out the noise on postive wire, induced noise must both appear on positive and negative wire of inputs.
Because voltage cannot be induced in any systems ground, therefore noise will only appear on postive wire leaving you with noise.


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PostPosted: 23 Sep 2018, 01:50 
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There is a Stereophile reviewer who doesn't use shielded inter-connects. Just one I know of.

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PostPosted: 30 Sep 2018, 00:40 
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Joined: 20 Sep 2008, 19:49
Posts: 171
Location: Montreal, Canada
Dan wrote:
Here is the working circuit, and the amp running,
Not quite finished yet but very close, I caught a little blue glow on the kt88s too :)

-Dan


Hi Dan,

This is a very clean and great looking amp. Congrats ! :thumbsup:

BR
Eric

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DIY Spkrs : Cyburgs Needle | MLTL (FE206E) | CSS EL70 BR | Open Baffle (Alpha 15A + FE206En) (in process...)
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PostPosted: 05 Oct 2018, 21:27 
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Great work... .... Id like to know how it sounds?


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PostPosted: 07 Oct 2018, 03:07 
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Joined: 01 Jun 2013, 09:05
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Quote:
noise never appears on ground cable to noise directly appears on positive wire


:D so you don't have any computers in your house, you have no mains filters using X and Y caps.
The mains ground contains all sorts of noise as does mains supply including RFI and harmonic distortion.
If the equipment is built for minimum pick up then you will get issues from other sources. So you can inject problems from grounding via capacitance coupling.

There is more to this twisted pair vs shield and one of the issues is capacitance loading but let’s not go there.
In theory capacitance dielectric has an effect and as such any close proximity between conductors will have a capacitance effect. (the wire insulation is the dielectric).
This can also be between chassis and conductor. If your shield is grounded it is part of the chassis. The point of noise cancelling is to ensure the signal is not corrupted this can also be via modulation. (ground current), (magnetisation of nonferrous metal via current) which is easy to understand when you realise copper is nonmagnetic. Eddy currents is another.

This is really not fair on the build thread, so I'll leave it because you can talk all day about these kind of issues.
I will add one more thought, if shielded wire has no issues then why is there an adjustment on an oscilloscope probe?
On low level signals shield is required but its down to cable drive and signal level/frequency. :D But what about dielectric or wire conductor :eek: Not skin effect again. :D
All of the above can "appear" to create frequency selective problems but its just a thought. Here is another a frequency sweep is not the same as a burst of white noise or a complex signal. Dielectric absorption is another but the argument is it doesn't effect music frequencies. One more thought, what is the miller effect, what amount of capacitance are we talking about and does it effect frequency response. Then look at cable capacitance and triboelectric effect, but wait there is more..yawn sorry.

Regards
M. Gregg

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