Ampeg - Adding a temp' rear input

[Walker]

[b]Ampeg - Adding a temp' rear input[/b]

I want to add a rear input to my Ampeg SVP Pro, but without butchering it.

If I tack two wires (7/0.2), to the original input socket, and run them as shown in my diagram to a new socket that I'll hot melt onto the side or back - will that cause any problems?

Will the 7/0.2 wire be OK or should I use something else?

Will it add noise or cause interfence?

Cheers!

Chris

[MarkBassChat]

The input jack is shorted to the ground when there is not plug inserted. So a parallel input will be also shorted and the preamp will not work unless you insert a "dummy" jack into the front socket. Why do you want to do this? Maybe an angled jack will help.

Mark

[bigjohn]

Nice pre-amp you've got there.

I've seen people use an angled jack and put the cable through the rack mount hole.

[Walker]

Thanks for your comments.

Everything else on my rack has rear inputs and I feed my signal to my pre' from wireless. I thought it would be a simple and clean way as I have zero space to pass a cable through.

And I have the parts in my parts box.

So if I did as my diagram and had a dummy jack in the front, it would work OK?

[rmorris]

all good advice. You should use a shielded cable for any extra wiring. Guage isn't important (as there's very little current involved).

[MarkBassChat]

Yes, it will work. You can put a piece of e.g. paper between contacts of the input jack. I will work in the same way as a dummy jack.

Mark

[Walker]

Thanks for the info. I've been doing a bit of research, as I'm trying to understand this a bit better, is it this?: "[i]You create a dummy jack by shorting the center conductor and shield together.[/i]"

Also, and sorry for the ignorance, I've had a look online at sheilded cable and it seems to have core wire(s) and a metalic sheath. Does the sheath get soldered too?

Thanks again for the advice.

[MarkBassChat]

No, it's exactly the oposite: the center conductor cannot be shorted with shield. As I mentioned earlier, you can also put a piece of paper between contacts of the input jack. Have you read it?

Mark

[Walker]

Thanks Mark, yes, I did read it but didn't understand the bit about the paper. Would that be on the 'new' input jack or the front one? What is the paper doing?

[rmorris]

[quote name='Walker' timestamp='1353316568' post='1873441']
Thanks Mark, yes, I did read it but didn't understand the bit about the paper. Would that be on the 'new' input jack or the front one? What is the paper doing?
[/quote]

On the shielded wire - you can use single core shielded . Signal is soldered to centre conductor. Shield /0V connection soldered to the cable screen.

The paper would go on the existing front panel jack - simply stops the signal being grounded which is what usually happens with no input due to the spring action of the jack.

[MarkBassChat]

I'm talking about a jack socket. if you look at any jack socket (even at the one provided by yourself), there is a conductor which connects to the tip of the jack plug. On your photo it's just to the right of the orange line. When there is no plug in the socket this conductor is shorted to the ground with another conductor which is below this one (it's not visible on the photo). What you need is to make that these two conductors are not shorthed together. You can achieve it but putting any isolator (like paper, wood, fabric) between those two conductors. In the evening I can make a photo but it is so simple (to do, not to explain). In this case you don't need any jack plug in the front of the preamp.

Mark

[Walker]

Thanks both. I've got it! Totally understand.

Thanks for your patience!

[flyfisher]

I'd caution about using paper to keep the jack socket contacts open.

Functionally, it will work fine, but I'd be concerned about the paper becoming dislodged at some time due to vibration etc.

Murphy's Law will ensure this happens mid-song in a live gig.

Paper may also interfere with the socket's normal operation if you ever want to use the front socket as normal.

Just a thought.

[MarkBassChat]

Use a drop of a super-glue in this case.

[rmorris]

It's probably best to use a jack plug to keep the contacts open I reckon. After all it's what the socket is designed for and there's no problem if you want to use it as normal.

[Walker]

Thanks for your help with this. I'm still struggling to understand the shielded cable thing... Sorry!

So shielded cable is the same as instrument cable that I make my patch leads from?

So I could use a length of instrument cable, solder the core to the tip connection of the original socket and the tip connection of the new rear socket, and the outer braid to the sleeve connections on both sockets?

Have I understood that correctly?

[flyfisher]

Yes, you've got it.

Where are you fixing the new rear socket?

While I understand the good reasons for wanting to do this, I think I'd try for something a little less invasive. You've got a nice unit there and it would be a shame to devalue it by messing around inside it.

As previously mentioned, a right-angled jack in the front socket would be very unobtrusive and if you used a short patch lead together with a jack-to-jack adapter at the rear then there would be no soldering involved either.

Just an idea though. It's your gear and what you're proposing will work perfectly well.

[Walker]

Flyfisher - yes, I think you are right. I've got a cunning plan! Will post a photo when sorted.

[Walker]

Here's the solution I came up with

[Walker]

I did the original planned mod today (put a rear input on), and it works without have to put a blank jack in front socket.

Is that bad? Does that mean there is something wrong?