DIY Instrument Lead Mini-Tutorial
Hi Everyone,
Since I'm making some XLR to XLR leads for a good mate of mine and I'm going to be doing some repair work on some of my own leads today, I thought that it would be a good excuse for me to do a mini-tutorial thread about making your own DIY instrument leads, and show you how I go about making them.
I'm going to be documenting the process with some pics as well as a write up.
To start the ball rolling, here's a quick re-cap of the four steps for preparing the leads prior to soldering the plugs onto them, they are:
1, Cut.....Cut the cable to the required length.
2, Strip...Strip the plastic insulation off the cable and the inner-conductors.
3, Twist...Twist the individual strands of shielding braid together so that it forms one neat bundle, do the same for the strands of the inner conductors.
And finally....
4, Tin.....Tin the neat bundles of strands by coating them with solder, they should look shiny once you're done.
I will be going into each step in more detail and adding pics soon so stay tuned....
Unbalanced/balanced cables Pt 1
Quote:
Originally Posted by
DrNomis_44
I tend to make most of my DIY Instrument leads about 5 metres long, although I have made a couple of 10 metre ones in the past, I think 5 metres is about optimum, but there's nothing stopping you from making them longer than that, although you do start running into loss of high-end issues with longer lead-runs, that's one of the reasons why balanced XLR leads started being used in live gigging situations, the balanced XLR leads also reduce a lot of the hum-pickup that can occur with un-balanced instrument leads.
You are mixing up some very different issues there, Doc.
XLR (or TRS) balanced leads have a similar capacitance level to unbalanced guitar leads - often a bit more - as they are all made of very similar materials and the extra core and insulation increases the capacitance.
In the case of guitars and basses with passive pickups and tone controls, it's the high impedance of the instrument output that makes the effect of the cable capacitance more prominent, as the capacitance forms a low pass filter network along with the output impedance that has a lower and lower cut-off frequency the more capacitance there is i.e. the longer the cable, the more high frequency loss there will be.
This can partly be countered by using cable with extra low capacitance per unit length for longer runs. Some of these cables have half the capacitance per unit length of standard cables, so a 10m long low-capacitance cable will sound similar to a 5m long standard cable in terms of HF loss. A lot of guitars can sound over-bright if you use the very low capacitance cable for short leads, especially when going straight from the guitar into an amp. Of course this may benefit some guitars, whilst on others they may just sound a bit harsh. So it can be used as a creative 'effect'. You just need to understand what's going on.
Pedalboards and cables need a mention, especially if you have a board filled with 'true bypass' pedals. In this instance, with all the pedals in bypass mode, whilst no pedal will affect the guitars tone per se, the guitar cable is now the length of the cable from the guitar to the board, the length of all the patch leads, plus the cable from the board to the amp, which if those lengths are long, is going to affect the straight guitar sound considerably.
Which is why it's a good idea to have at least one 'always buffered' pedal in the chain. A good buffer should be transparent (though not all pedals have good transparent buffers - especially some of the older ones). The audio bandwidth of standard op-amp circuits is way higher than our hearing ability, so you should only be able to hear the effect of a good buffer in a good way. It will provide a high input impedance (normally 1 Meg ohm) for the guitar to 'see', which keeps the guitar's tone normal, and will provide a low output impedance for the signal to the amp (or other pedals) which will remove most of the capacitive effect of the output cable to the amp as the low pass frequency of the cable 'filter' is now above the normal audio bandwidth. So a buffered pedal can drive a far longer output cable than you can get away with with all true-bypass pedals on your board.
For a buffered pedal design, look no further than Boss, as their pedals all have buffers and and have no 'true bypass' switching. Alternatively it's now fairly common for people to have an 'always on' booster pedal like the Xotic EP Booster or TC Spark at or near the start of the FX chain, set for maybe a very slight signal boost. This can both add a 'nice' small EQ tweak to the sound and also act as a signal buffer.
You'll have noticed that most guitars and basses don't have balanced XLR output connections on them (though some with internal pre-amps do and the Les Paul 'Recording' style guitars had transformer balanced low impedance outputs, designed to be plugged directly into a mixing desk). Balanced outputs are generally the province of 'pro audio' equipment i.e. the kit (apart from instruments) that you were once only likely to find in a recording studio or in stage PA equipment (but can now be found in home-studios).
Balanced audio connections are normally more than just 'balanced' (which refers to balanced impedance along the two signal lines and equal impedance to ground), they also operate using opposite polarity signals for better noise rejection.
A guitar lead is an 'unbalanced' connection, not because it uses two wires, but mainly because the shield is directly connected to ground, whilst the signal input has around a 1meg ohm resistance path to ground. There will be a slight connection impedance mismatch because the signal cable impedance will be different to that of the braided outer 'shield' cable.
Balanced/unbalanced cables Pt 2
A microphone output, once you rise above the cheap 'toy' plastic types and ignoring valve mics, will (these days) normally have a 3-pin XLR connector providing a balanced output signal with a differential output on pins 2 and 3, with pin 1 used for the shield connection. It will feed into a microphone preamp (that may be built into a mixing desk, digital audio interface or a stand-alone mic preamp etc.) that feeds the signals into a differential amplifier.
The mic output signals that have opposite polarity and amplitude to each other. If you fed those signals into a summing amplifier, they would cancel out and you'd hear nothing as a result. That's why they are fed into a differential amplifier, which outputs the difference in the signals, so you get a signal that has twice the amplitude of a single signal. The benefit of this is that any noise picked up by both signal cores won't have any reversed polarity to it, so when it hits the differential amplifier, the noise signals (theoretically) cancel out, and the signal comes out noise free.
Obviously the two signal cores can't occupy the exact same space, so there will be some difference in the noise signals each cable picks up. The nearer the cable is to a source of noise, the greater the difference in the electromagnetic noise field experienced by each core. You only get perfect noise cancellation if the noise levels are exactly the same phase and amplitude, so you can still get some noise in the signal, but at a very much lower amplitude than otherwise.
To increase the ability of the cable to reject noise, the two signal cores are twisted together (the tighter the twist the better), so that there's a better chance of both cables picking up the same amount of noise i.e. with parallel cables, one core is always likely to be nearer the noise source than the other, and so the noise amplitude on each cable is always going to be slightly different. Twisting the signal cores averages out the distance of each cable from a noise source, so it's only when the cable is very near to the noise source that they won't pick up the same amount of noise.
Think of the noise signal as ripples spreading out from the point a stone was dropped in the middle of a lake. Near the noise source/stone impact point itself, you've got very small radius circular waves that are quickly expanding into larger radius waves. If you floated a straight piece of wood in the water on a line perpendicular to the circle radius, the centre of the ripple would noticeably hit one part of the wood first then spread out and hit the rest. But move away 100m and the height of the ripples is now much less and the radius of the ripple waves is now so great that it appears almost flat. The same piece of wood set perpendicular to the radius will now experience the ripple hitting all the wood at almost the same time.
Replace the wood with our twisted pair, and the stone for a noise source. Very near the noise, even though the cable is twisted, one core will be that bit nearer and pick up the noise before the other core does, so there will be a very small amplitude and phase difference in the noise signals picked up.
To increase the noise resistance further, an overall braid shield is added to the cable, which captures a lot of the noise signal and takes it to ground. The better quality cables have a lot more wires/copper in the shield than cheap cables (as copper costs a lot these days). Also the tighter the twist in the signal cables, the more copper you need per unit length of cable. So there's a reason why some cables cost a lot more than others.
Cables for installation that aren't planned to be moved at all after installation will typically have a couple of wraps of aluminised mylar foil sheath with a drain wire around the signal cores instead of a copper braid. This provides more comprehensive protection as there are none of the gaps in the shield that a braid gives you (which let in very short wavelength noise). However, these cables are less flexible than braided shield cables and moving them can cause audible noise as the layers of foil move across each other and cause slight capacitance changes and some static to be produced.
For really noisy environments there's 'star quad' cable, which has two twisted pairs that are also twisted around each other. Each pair carries the same signal i.e. two cores are connected to pin 2 and two are connected to pin 3 on the XLR, but the extra cores and twisting helps the noise levels average out a lot more. Typically used for outside broadcast/live situations where there's a lot of generator power and limited cable route choice so you can't run the audio cables away from power cables etc. The downside is that these cables have a lot more capacitance than a standard mic cable, so whilst you could run a standard 'mic' lead around 100m before the sound starts to suffer, you are at least halving that with star quad.
And what is good for mic cable is also good for connecting other items of audio equipment together. The main benefit of using balanced connections between equipment, especially in the digital age, is that it allows you to break the ground connection provided by the shield at one end of the cable to cure any ground loops whilst the shield still provides its extra noise protection. With unbalanced connections, you can't break the shield, because it’s also acting as a signal carrier.
You don't need an XLR to have a balanced cable you can do exactly the same with TRS jacks. And just because there is an XLR or TRS connection doesn't always mean that the connection is balanced and described as above. A TRS cable used for a stereo signal isn't a balanced cable, even though it is probably exactly the same as a mic cable in every other sense.
But finally, back to guitar leads. The most important cable is the one that you plug into your guitar, so don't go cheap on that one. You don't need to spend a lot to get a good cable, you just need to spend enough, with good jacks and a reputable make of cable in-between them. Don't believe the marketing hype of the mega-expensive brands. I have an £80 (Aus$140) 10 foot boutique brand cable that was included in the case candy when I bought my DG Strat, and it sounds no different to my £20 (Aus$35) cables that have the benefit of Neutrik Silentjacks.
