Unshielded cable inside Amps / FX

[BobbyKa]

Hello!
I'm new to these Topics and got a very simpel question.

I'm going to do a very simpel Mod inside my Amp.
For this, I need a cable to get my unprocessed, unbalanced guitar-signal from the Inputsocket to a Switch, and from there to the preamp.
The first cable would be max 40cm long, the second one max 10cm.

Is it better to use a shielded Instrument cable? (for example https://www.thomann.de/gb/sommer_cable_the_spirit.htm)
Or can I also just use two wires without expecting any influence on the Signal? (for example https://www.elektroheld.de/sonstiges/la ... gKbsfD_BwE)

Are there more/less concerns depending on the fact that it is inside a preamp or an amp? Digital or analog?

If the second option is ok, are there any suggestions which brand/model of wire to use?

Thank you for your help!

[Racing]

One of the most widely used shielded wire is dubbed RG-174.
It is a coax wire and available at basically every parts store there is.

[BobbyKa]

Thank you for your help.

Sorry for this noob question:
So only the Tip from the jack gets directly connected, the shield is only connected on one side?

I'll tell you my wiring:
My Preamp only got an front Input. I want to install an aditional rear Input. When front Input is in use, rear Input should be deactivated.
I want to do this mod plug and play without soldering the original wiring.
I want to install a new Frontinputsocket, from there a short cable with monojack goes to the original inputsocket that now is inside the chasis.
The new front Input socket is switched (Neutrik NMJ4HC-S). from the switched contacts a cable goes to the rear inputsocket (also NMJ4HC-S. Of course I wouldn't need a switched one here).
For testing I used the linked sommer cable instrument cable. Shield connected on all sides.
It works as it should, but when there are no cables connected to front and rear Input I get a lot of humming noise.
It disapears when I shield both switched contacts of the rear Input to the chasis (when front or rear Input is used, this shielding is disconnected)
So this method of shielding the switched contacts of the rear Input is best choice (I don't think so) or what should I do? Removing shield of which end of which cable?

[Racing]

A shield connected to ground at both ends isn´t a shield.
It is a ground.
A perfect way to create groundloops in turn.

[BobbyKa]

Attached my "shematic".
I know that this shematic isn't drawn correctly, but I hope it helps to understand what I want to do.
The wires that go to 3 (original Preamp socket) got a mono jack. forgot to draw this.
In red, you see the "solution" that seems to work. But I don't think that it is the right solution...

If you also think there is a better solution, please tell me which wire (S, SN) at which point (Socket 1, 2, 3 or Jack between 1 and 3) should I disconnect.

thank you for your help!

[BobbyKa]

Not realy a lot to see cause already heat shrinked.
But mabe it halps to see what I want.

[ajaxlepinski]

A shield connected to ground at both ends isn´t a shield.

No, but the outer mesh wiring of a coaxial cable is referred to as the "shield".

Don't be a douche.

Of course, I'm new at this stuff and I'm just curious.... please correct me if I'm wrong...
When the woven wire, in a coax cable (aka, "shield") is connected, at one end, to a ground, it functions as a "shield" that protects the core wire from noise.
If you connect both ends of the woven wire to a ground, it no longer protects the core wire from noise and therefore, stops functioning as a "shield".
Is this the case?

[BobbyKa]

OP, I'm looking at that pic and I'm more confused than ever about what the heck you are doin' there...

It realy is very simpel.
Maybe it is my bad english or my lack of professional knowledge that confuses you.

I'll try it again:
I got a guitarpreamp (rackmounted) that has only has a front input.
I want it to have a front input (for use with guitar cable) and a rear input (for signal from the wireless guitar system).
I want the rear input to be automaticaly disconnected when I plug in a cable in the front input.

The original frontinputjack is a stereo jack mounted on a small circuit board.
I don't want to solder on this circuit board because I'm not realy sure what might happen and because I want a fully removeable plug'n'play solution.
So I had the Idea to put the original frontinput jack INSIDE the preamp and put the rear input and a new front input before it.

In the picture you see 3 parts.
#1 is the new front input. It is a switched mono jack (neutrik NMJ4HC-S).
From its two contacts (Tip (T) and Sleeve (S)) a short cable goes to a 6,3mm Plug (= #3). This Plug will go into the original (=old) Front Input.

The switched contacts (SN + TN) of the new front input jack (#1) are connected via the longer cable with the Contacts T + S of the rear Input (= #2).
The Rear Input Jack (#2) is also a switched mono jack (neutrik NMJ4HC-S). That's the reason why you see two blank contacts there (=TN + SN).

My problem is, that I get a loud humming noise when the plug (#3) is connected to the old front input but NO cable is connected to the new front input and no cable is connected to the rear input.
It seems I created an unwanted groundloop (On all ends of the cables Shield (S) is connected!).
I need someone to tell me at which position(s) (#1, #2 and/or #3) I should remove the cable from the shield (S or SN) contact to remove the ground loop. Also I need Information If i just should remove the wire there or if i should connect it directly to the chasis.

In my "shematic" you see an aditional red drawn part. This is a "soltion" I found. I just connected the switched SN and TN from the rear input (#2) to the chasis. This way everything works as it shoud. But I guess this shortcut is not a propper solution.

I hope this explanation is a bit better....
Thank you

[ajaxlepinski]

It looks like you're trying to create an AB type circuit.

In this diagram, the Input would be your #1 tip/sleeve male jack and you need a switch.

[BobbyKa]

Thanks for this option. But is this method realy neccessary?
I thought I already got a working method and all parts without the need of a seperate switch. All I need is to find out where to open the ground loop.

Or is this REALY not possible?

[Racing]

A shield connected to ground at both ends isn´t a shield.

No, but the outer mesh wiring of a coaxial cable is referred to as the "shield".

Don't be a douche.

Of course, I'm new at this stuff and I'm just curious.... please correct me if I'm wrong...
When the woven wire, in a coax cable (aka, "shield") is connected, at one end, to a ground, it functions as a "shield" that protects the core wire from noise.
If you connect both ends of the woven wire to a ground, it no longer protects the core wire from noise and therefore, stops functioning as a "shield".
Is this the case?

It will at that instance become a ground. As Albert Ross points out this creates an issue as we ,due the Murphys law,will see ever so slight creepage across the wire. It is a surefire way of messing this up.

However.
For the OP.
Instrument wire is rarely wowen. Coax wire is. Only real "issue" to keep in mind when using coax is that it will produce very small amounts of capacitance (as will most wire).Using wiring as heavy gauge as instrument wire within an apparatus is frowned upon. It is simply bad practice.
Reason i recommend RG-174 is that it is widespread professional product,you can basically pick it up anywhere,it is of the correct proportions for the job and thus will get the job done.

Does that mean that the signal from a guitar isn´t shielded? In theory,yes. The bare wire of an instrument cable indeed carries ground (for the circuit to become complete) and as such the instrument wire is NOT a shield. Will,in practice,the ground surrounding the signal protect the signal no matter? Yes. To an extent. Point here being that the currents at hand are very very small.

In short a shield that conducts CURRENT is a no-no. In fact a shield that CAN conduct current is a no-no,as a shield that carries current is not a shield anymore.

The use of "breaker jacks" a´la Cliff jack will indeed solve the puzzle,when conducted with the correct materials.
The Cliff jacks carries "breakers" for each prong. These breakers can be put to several uses,one of being to send signal up the road.
It is however common practice to make the entry jack hit ground via its breakers when no instrument wire is present. This will deduce hum when in idle.
It is likely that the STOCK entry jack does exactly this and therefore this needs to be looked into. All due respect vs the OP but this much NEEDS to be checked on the stock PCB.
From thereon out though,carry that signal where you see fit but...

1/Use the correct grade shielded wire.
2/Hook the shields up to the chassis where there´s no real danger of currents being otherwise present.
3/Use a separate wire for jacks ground and head this ground wire to the EXACT SAME POINT that the stock jack grounds.