Questions on FET Preamp Cable

[Ucho]

I'm going to build JD Tillman's FET Preamp Cable

I'd like to have some clarification about connections of plugs and cable.

First: do I need a 'normal' mono audio cable (the one we use to connect guitar to amp), a stereo one, or what?

About the XLR plug...

Looking at the schematic (the phone plug part) it seems that the ground from the circuit portion inside the plug is connected, by the cable groud screen, to pins 1 and 3 of XLR plug. What about the ground pin of the XLR plug? Should i connect it to pins 1 and 3? Should I leave it unconnected?

Looking at the pic(a portion of the 'phone plug' schematic), what is that wire I 'highlited' in red?


Same question about the 'box' schematic: pins 1 and 3 from XLR plug are connected to ground, but there isn't info about XLR ground pin.

NOTE: About the picture I posted: it's a fragment from the schematic on Tillman site, modified to add 'highligth'. It's ok if I post it or should I remove it?

Thanks.

[anti-idiot]

that cable is ground. u need mono if the battery is inside the guitar (plus a stereo jack to turn it on).stereo if u r gonna phantom power the preamps (XLR or Stereo).

[Ucho]

that cable is ground. u need mono if the battery is inside the guitar (plus a stereo jack to turn it on).stereo if u r gonna phantom power the preamps (XLR or Stereo).

So, since i'm going to use the box for power (battery or DC), I need a stereo cable.
Then I should connect ground from the circuit (inside the plug) to one of the two cores of the stereo cable, and that core to pins 1 & 3 of the XLR.
The ground of the stereo cable should then be connected to XLR plug ground.

Is it right?

[slacker]

The part of the image that you've highlighted is the cable screen. It looks like he's using 2 core cable, so you've got a signal lead, a ground lead and screen.

[Ucho]

The part of the image that you've highlighted is the cable screen. It looks like he's using 2 core cable, so you've got a signal lead, a ground lead and screen.

Thanks. So, a 2 core cable is what i called a stero cable (2 cores + screen) ?

The screen should be soldered to the XLR ground pin?

[Ucho]

If i'm understanding it right, the cable screen should be soldered, with circuit ground, to pins 1 and 3.

So, what's the matter with the second core, if it's soldered to the screen?
Couldn't we just use one core for signal and the screen for ground?

[Gilles C]

You can use a standard guitar cable and just install the XLR plug. That's what I did for mine. You just connect the shield to both pin 1 & 3 of the XLR plug.

That cable between the guitar and the box doesn't need a microphone cable (shield + 2 cores) because it is not a balanced line. The power to drive the preamp shares the same wire as the audio signal, and the shield is used for the ground.

If you were to use a microphone cable, the ground wire would need to be the second wire of the cable, and the shield would only be used as that, a shield. Please note that it is also important to connect the shield only at the destination side of the cable, pins 1 & 3 of the XLR plug in this case.

Gilles

[Ucho]

Thanks Gilles.
I'm going to use a standard cable and connect its shield to pin 1&3 of XLR cable.

Please note that it is also important to connect the shield only at the destination side of the cable, pins 1 & 3 of the XLR plug in this case.

This is true only if I use microphone cable, or even if i use a standard one? The other end of the shield should be connected to the plg circuit ground, or not?


Still.... what about XLR plug ground pin?

[Gilles C]

It is only if you use a microphone cable.

With a standard guitar cable, you need to connect the shield to pin 1 & 3 of the XLR plug.

Gilles

[Ucho]

Thanks again gilles

Only a couple of questions (sorry).

The other end of shield should be soldered to the circuit (the one inside the plug) ground, right?

On my XLR plug i have a fourth pin, the ground one, mechanically connected to the metal 'body' of the plug. Should i solder it to pins 1&3 or leave it unconnected?

[Gilles C]

Thanks again gilles

Only a couple of questions (sorry).

The other end of shield should be soldered to the circuit (the one inside the plug) ground, right?

On my XLR plug i have a fourth pin, the ground one, mechanically connected to the metal 'body' of the plug. Should i solder it to pins 1&3 or leave it unconnected?

No problem...

Right. as I said, the shield is the ground wire for the circuit. So you connect the shield to the ground of the FET circuit, and the middle wire of the cable to the output of the FET circuit.

Do you use a plastic or a metal box?

The fourth pin is the metal body of the plug and can be left un-connected. It will connect to the metal body of the XLR jack of the Power Box when you connect everything. It is the Chassis connection. But if you use a plastic box, I would suggest you to connect the body of the metal jack with the grounds of the 1/4" jacks of the outputs.

There is some nice info on how the pros do it on this site

http://colomar.com/Shavano/pa_page.html

Like this

http://colomar.com/Shavano/xlr_cable.html

And this to show how to connect the Ground to the Chassis.

http://colomar.com/Shavano/snake_cable_diy.html

Gilles

[Ucho]

Do you use a plastic or a metal box?

The fourth pin is the metal body of the plug and can be left un-connected. It will connect to the metal body of the XLR jack of the Power Box when you connect evrything. But if you use a plastic box, I would suggest you to connect the body of the metal jack with the grounds of the 1/4" jacks of the outputs.

I use a metal box, so i can leave it unconnected.

Thank you again Gilles, you really helped me. Now I can go on ordering the rest of material and starting the build.