General cable connection

[Sweep]

Hi. Sorry to ask a question in my first post here, but I've hit a problem.

As a quick introduction: I play many instruments, with synthesizer as my main one. I took up bass guitar recently, but as I've played classical and electric guitar for years I'm finding it pretty easy to make the transition. I made a note of this forum when I bought the bass not long ago.

My question is something I thought I'd be able to solve quickly using Google, but I've had no success.

I recently acquired a test tone oscillator from a lab, which I want to be able to connect into my recording setup. I was advised by the local electronics shop to separate the two leads inside a standard guitar cable and connect one to the output on the oscillator and the other to the oscillator's earth terminal.

That should be simple enough, and I have suitable connectors for that. The problem is that I need to know which of the two cables in the lead goes to which terminal. You'd expect some website would say which is the earth, but I've spent hours trawling through sites that call the two cables the `top cable' and the `bottom cable' or simply say throw away a lead that needs work and buy a new one, so I've got nowhere.

I don't want to just try it without knowing as - obviously - having the wrong wire to earth doesn't sound like a very good idea. Any advice would be much appreciated.

Edited November 27, 2010 by Sweep

[Stewart]

The outer wrap (rather than the inner core) is your earth connection

[Sweep]

Right, thanks for that. And thanks especially for the quick response.

I'm really not sure how to do this, now. I think I'll talk to someone with electronics knowledge and show them the connections on the oscillator before doing anything.

What you've said is helpful, but I think someone needs to look at this directly.

[flyfisher]

I'm not entirely sure of the problem here, but it seems like it's being over-complicated.

A cable does not have an "earth connection" as such. Your oscillator has an earth connection (which you seem to recognise) and so does your recording set up. You could use two bits of single-core wire to connect the equipment and it would work (electrically).

But Stewart is correct that you should use the outer wrap of a screened cable for the earth connection because this will provide shielding against external interference getting into the cable.

Unless I've misunderstood the question here.

Perhaps it would if you could identify the types of connectors involved. Or better still, post a photo.

[Sweep]

Thanks. You're right - this does seem to be more complicated than it should be.

I suspected the information I'd been given in the electronics shop wasn't reliable, and after the first reply here I was sure about that.

The oscillator itself has a proper mains plug on it, British type (I'm in Britain and the oscillator is from a lab here), so it's properly earthed through that.

There are three connectors on the front of the oscillator, marked TTL, Common (which is the earth as it has an earth sign on it), and Output.

The guy in the electronics shop sold me two jacks that will fit two of the connectors, and told me to take the two wires in the lead and attach one to the output and one to the earth. I'm now inclined to think one goes to the output and the other goes to one of the others, but not necessarily the earth.

If this isn't any more familiar to you than it is to me I think I'll do a Google search again to see what the TTL socket is, and ask around a bit more. I've also emailed a friend who's a musician with some electronics background - she built filters and so forth as part of her music tech college course.

[Stewart]

I think you'll find that the TTL output is fixed at 5v (peak) for use with logic circuits, and is probably always a square wave even if the oscillator can produce other waveforms - the other output is probably variable.

To use either output it you connect the cable outer screen/braid conductor of the cable to the common/earth on the oscillator.

The inner core conductor of the cable is connected to either TTL or Output (this ends up going to the tip of the jack plug at the other end)

5v (TTL output) is a very high level signal for most musical/audio equipment, so I'd recommend the other output (assuming it has a level control and that you check you're not putting out enough voltage to damage the input of your gear).

But try searching for the exact model of oscillator and you'll probably find a manual which is the only way you'll know for sure.

[Sweep]

That's great - thanks. It's starting to make sense now. I'm used to just plugging in audio equipment, as you'll have gathered. Bands like Hawkwind used to use audio oscillators and it seemed possibly interesting to see what might be involved in using one, hence getting this thing when it appeared.

Foolishly I hadn't thought about output levels, though I suppose I was right to proceed with caution. It sounds like it'd be advisable to borrow a voltmeter, or, as you suggest, get a manual. I've now managed to locate one, at a price.

So I think I know where I'm going with this now. I'm very grateful for all the advice. Thanks.

[flyfisher]

Yep, I'd echo everything Stewart has said. TTL is definitely a 'logic' output and will be swing from about zero (ground) to 4.5 volts. You almost certainly don't want this output (unless you're designing and developing digitlal logic circuits ) because not only will the voltage swing probably be too high for your audio equipment but the square wave nature of the signal will sound pretty bad (although maybe you want that!).

An ordinary voltmeter won't be much help because it almost certainly won't be able to respond to the oscillating nature of the output or TTL waveform. Any reading you get will be an 'averaged' voltage rather than the peak. It will give you some information but you need to understand what's really happening to make any sense of it and even then it won't tell you all you'd like to know.

The ideal test instrument would be an oscilloscope. This would show you the precise shape of the output waveform, its voltage and its frequency, but I appreciate such things are not exactly standard-issue home items. I only mentioned it in case you have access to one through school, uni or work perhaps.

Otherwise, I agree with Stewart about trying to track down a manual.

[Sweep]

Thanks again. This is one of those times when I find out how little I know. I expected it to be a simple matter of making up a cable. I'm very grateful for all the advice with this.

I hadn't thought of the nature of the signal when using a voltmeter, either, though on reflection I should have done. I've done stuff with semi-modular synthesizers.

I think I'll get a manual - at least I'll learn a few things, in addition to what I've learned already from posting here.

Maybe my next post here should be about playing bass. I'm working on something with a main part played on bass, so that should be on my website fairly soon. There's just the mix to sort out now. And I'll report back on this oscillator if I manage to do something musical with it.