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  1. #1

    Wiring up a custom guitar lead - with a 9v DC feed

    Hi there, I've been looking round the forum but not found anything which answers my query... so I shall post my position here so that you guys may be able to advise what action I should take. Apologies that I am not an expert on such matters but thats why I am here! To learn!

    I have a need to construct a cable that does two things -

    a) act as a standard shielded cable
    b) carry a 9v DC feed to plug into the bass guitar

    It's worth noting that the 9v DC feed is not for an active circuit on the bass but for a sound to light sound system (flashing LEDs inlaid in the neck). The 9v feed is to power the circuit for the LEDs and this circuit is responsible for switching the LEDs on and off electronically. It is worth noting that due to the switching nature of this circuit, the cavity for this circuit (and the active preamp) have been extensively screened to avoid noise that may be generated and introducted into the audio signal path.

    You may ask why I don't run the LEDs off some onboard batteries... well, they don't last too long... hence the idea of tapping the power off a transformer and connecting to the bass via the cable that leads to the instrument.

    I am expecting the cable solution to have two ends, one that plugs into the bass guitar and then the amp. The other would lead from the 9v source and the other end would plug into the cavity on the back of the case where the circuit for the LEDs is held. I want to have a professional finish so have been thinking about pvc sheathing/braids (expando) etc but ultimately think that if I could get an off the shelf cable, this would be even better.

    The bit that I don't understand is the screening of the cable. So a standard guitar cable is shielded. If I run a 9v DC cable directly alongside the audio cable, would I be introducing a source of interference?

    Maybe if I was to get a shielded mic cable, could I use two of the connectors for the guitar and 2 for the 9v feed. This would allow for a really neat solution to be assembled... but what worries me is putting the audio and 9v cable directly together.... WITHIN the shielding. Is that a bad move?

    As you can see, I'm out of my depth here in terms of my understanding - I am hoping that you guys can set me straight!

    I hope that I have explained the problem clearly and if you guys can suggest something better than I have already thought of... then go ahead.

    One thing I don't want to do is put the 9v down a stereo cable. (Can't do that anyway due to the switching mechanism on the bass as the preamp circuit is turned on by shorting the connections on the stereo jack socket...

    Thanks in advance!

  2. #2
    Putting the DC and audio together within the shield is asking for trouble. It will work fine if the DC supply is very well filtered, but if not, you're going to get hum in the audio. And the hum that comes out of a badly filtered DC supply is 120Hz, not 60Hz, so it will be within the frequency range of a lead/rhythm guitar and amp. (Low E is about 80Hz.)

    What you want is a shielded wire for audio and a pair outside the shield for DC. The DC wires should be twisted around each other, not just spiral around the audio cable. You may discover that what's available is two pairs individually shielded, which will work. But the DC should be connected to the twisted pair and have no connection to the audio circuit, even the ground.

    Do you need a coiled cord? How long does it have to be? It's easy to find cable that works right, but hard to find it in flexible rubber jacketed, the way you want a guitar cord to be. The cables used for handheld push-talk mikes are perfect, but they're usually shorter than musicians like.

    What kind of connectors are you going to use? In theory, a 3-circuit phone connector should work, but in the real world, you'd almost be guaranteed to couple hum and clicking and popping into the audio due to the common ground. You may have to do something custom.

    Next: What's going to switch the LEDs on and off? That's a potential source of clicking in the audio. If you wire it up and have that problem, come back here and we'll work on it.

  3. #3
    My thinking is correct then - I figured that DC within the shield would be a bad idea!

    OK, just to clarify some of the points. There will be two sockets on the guitar; one explicitly for the audio and one for the 9v feed. I defo want to get away from the common ground to avoid the potential clicking as you state. I know that in reality, the circuit is quiet - it's all shielded and when using onboard batties to power the LEDs, there are no problems whatsover. This is all about saving batteries as the circuit literally eats them. You are right though, without the proper shielding, the switching of the LEDs can be noisy... especially if you are do dimming effects with them! Careful consideration of the placement of the circuit and liberal coatings of shielding paint have seen me OK.

    The lead will effectively be a Y lead at both ends. All I want is a neat solution for having a single, pro looking cable carrying both the audio and power, whilst retaining the flexibility and length (5m) of a standard guitar lead. The easy option is to cable tie a 9v cable to the guitar cable itself, but that's going to look pretty shoddy, especially if any photographs/video are taken with the guitar in action. The one jack socket has to be a standard guitar jack -



    - as the guitar has to be still be usable without the custom power/audio cable. The other half of the Y end, I'm probably looking to use a standard barrel connection - something like this -


    This has lead me on to thinking about using some PVC sleeving.



    I've found some high quality guitar leads (George L .155) which ordinarily are used as interconnects on pedal boards as opposed to use for connection to a guitar. Due to the handy slinky diameter of the George L cable, would putting this (shielded cable) in a PVC sleeve alongside a seperate thin diameter twisted cable carrying the 9v feed be a better bet? The appropriate sized PVC sleeve then would make the overall cable looks like the diameter of a standard guitar lead and hopefully maintain the flexibility of a standard guitar lead. Is this a better route to take? Is this going to look good? I'm thinking of drilling a small hole in the rubber boot to let the DC cable out to go around to the rear cavity of the guitar.

    Karl - have you got a link to some cable that is the two shielded pairs that you mentioned?

  4. #4
    No link, but any cable company makes the stuff. I like your solution better. The 2-pair shielded stuff is usually for permanent installation and not very flexible.

    How many LEDS times how much current? Make sure your DC wires are heavy enough to prevent major voltage drop. Shouldn't have to be very heavy. Putting the twisted pair outside the jacket of the shielded wire is, maybe, a little better...crosstalk varies with the inverse square of distance, or something like that.

    I wouldn't think you'd have to drill a hole in the rubber boot. Bring all the wires into the phone plug, then fold the DC wires back and bring them out alongside the PVC sleeve.

  5. #5
    Quite a few LEDs, 17 on the front and then all the side light LEDs, although they are run from a couple of single LEDs, with optical fibres going to the edge of the fingerboard. I run two 9 volt batteries in parallel two cater for the draw of the circuit and the LEDs. I may get the meter out and check the voltage drop through the final cable. I may be able to offset any problems by changing the output from the trasformer on the transmission side of the lead! I'll be using the thickest and most flexible DC cable I can find and twist it up (that the sleeve will allow for along for) along side the George L lead... I don't want any fatigued cable breaking in there!

    Looking at the boot on the jack, I may be in luck in getting everything in and out of it. I'll get some more sleeving or heatshrink to house the DC connection when it leaves the safety of the main sleeve!

    Hopefully, the effects of cross talk won't come into play to an audible level!

    Thanks for the advice!

  6. #6
    Senior Member SC358's Avatar
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    You mention about using twisted pair for the DC, Karl, but from the descriptions in this thread, maybe a co-axial stereo RCA-RCA (or phono-phono) could be used and modified instead. One for audio and the other for the 9V. The cables would be bonded by their jackets.

    I usually don't think about using twisted pair cables in conjuction with DC
    . Is there an advantage of using 9V on a twisted pair?
    Last edited by SC358; 22nd Mar 2010 at 05:11. Reason: Grammatical
    SC358
    Relationships are based on compromises - behavior accepted is behavior repeated.

  7. #7
    The advantage is that it's balanced. There are two kinds of interference radiation, electrostatic and electromagnetic. Static radiation would be taken care of by the audio wire's shield. Magnetic will be canceled if the DC is carried on a twisted pair. We usually think of balanced lines as being relatively immune to received radiation, but they also transmit almost no radiation.

    It doesn't matter whether the DC line is really balanced or if one side is grounded somewhere. As long as there aren't two grounds, from a magnetic (current) standpoint it's balanced. Current out equals current back, and the fields generated by the two wires cancel. Ground-fault interrupter outlets work on the same principle.

    So is there really a problem with DC radiating interference? Not if it's really well filtered, but maybe it won't be. Dunno what Flump is using for a power supply, so I thought it best to be cautious.

    In such a case, a single shielded wire for DC is actually not as good. However, I bet it would be good enough. If he were to use a dual coax cable it would probably work fine. Then he could just put a little heat shrink on it where it splits, to keep it from splitting further.

  8. #8
    OK, with regards to the filtering, the DC power adapter is going to be fed from the back of a Furman power conditioner... now I don't know if thats the filtering you are on about...?

    Am I right in thinking that the DC being carried on two sleeved (not shielded) cables wrapped around each other will do the business?

  9. #9

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