Balancing act

Back to the music recording advice and an explanation of why it is sometimes necessary to have a soldering iron at a gig.

There are two ways to send an analogue signal down a cable: the standard (and common on consumer equipment) unbalanced audio and the more "professional" balanced audio. An unbalanced signal is very easy to understand. One line is the ground and the other is the so called 'hot' signal. A balanced cable requires three lines - one for ground, one for the 'hot' or '+' signal and one for an inverted, 'cold' or '-' signal. This makes more sense in the diagram below...

So, what is the point of a balanced cable? Basically for very long lengths of cable (over 20m is probably noticeable) it helps to reduce noise. The "balancing" refers to the fact that the impedances at each end of the cable are the same i.e. balanced. Any noise that is picked up along the cable should produce equal signals on both the hot and cold lines. Then a differential amplifier in your mixing desk or soundcard is used to compare the two lines and since the noise appears the same on both, it is not amplified. Another advantage of this system is that there is very little electromagnetic field around the cable as the two signals have opposite polarities and so crosstalk between different cables is minimised.

The problems come when you have a mixture of balanced and unbalanced inputs and outputs in your setup. Remember that a line needs balanced connectors at both ends for the noise cancellation discussed above so ideally you should use transformer adaptors to keep any long cables balanced. However most people find it necessary at some point to connect equipment of different types without worrying about having it balanced... and it is a real pain! Firstly there are three kinds of balanced output:

1. Impedance balanced - only the hot signal is driven and the cold signal is at ground (this works despite what I said above since noise is still removed by the differential amplifier but crosstalk is not reduced).

2. Transformer balanced - an output transformer is used to provide the opposite polarity signals with the centre 'tapped' to provide the ground.

3. Active balanced - hot and cold lines are both actively driven but with opposite polarity signals (this is what much modern equipment now consists of).

So now that that is clear (!) I have compiled a nice list of how everything should be connected (I've used both XLR type plugs and TRS jacks to demonstrate how you might need to use both, however they are of course interchangeable). Also note that often 2- or 3-conductor cable can be used. The advantage of using three is that the outer screen can be used as a shield, connected only at one end. Try to be consistent with which end you connect the shield so that they are all grounded to one point and there are no 'earth loops' which will pick up noise.


And that is why a soldering iron and multimeter can come in handy... two cables may look the same from outside but be wired up completely differently.

So... not an entirely successful day. Being the absolute idiot that I am, I managed to mistake 4.7k resistors for 47k ones. My power supply was working nicely giving me both a positive and negative output, then I noticed a few things (in this order):

1. The negative voltage was about 0.5V too large.

2. The negative supply suddenly became quite noisy.

3. There was a burning smell.

4. The negative supply was no longer negative but positive.

Conclusion: The circuit will almost certainly work if I use the correct resistors.

Future work: Get a new LT1054.

A quick note to say shiiiiiit. Don't need the LM2930 regulator in my power supply as it's already regulated. Oh well it is a nice design and can still be used minus a couple of components!

Power to the Crunchie Boost

So after much insecurity over whether my design would work I think I now have something that in theory converts a 9V unregulated supply (as provided by the standard Boss PSU) into +8V and -8.5V (see above). The LT1054 is a combined charge-pump converter and regulator to provide the negative supply and the LM2930 is just a regulator for the positive voltage. Both were found after trawling through the RS catalogue for far too long and reading far too many datasheets! The LT1054 is pretty cool though and if it works I'll be impressed - inverting a voltage with essentially some clever switching and a fat capacitor seems like it could be dodgy to me.

Well I've ordered the bits and will test it out soon, so in the meantime I'll pray to the electronics gods.

Fight Like Apes

Having recovered from exposure to excessive heat and slightly excessive alcohol consumption I am in a fit state to update my blog. Last night I went to Cargo to see Fight Like Apes... and was impressed. The first time I listened to them I thought of the Ting Tings but without the crapness. However I think that was just the one song (Jake Summers) and maybe because of the vocals. Really they are quite hard to describe musically - some kind of punky indie rock/pop but given that they have no guitars (apart from the bass) and everything is done on synths that description doesn't really work. So I'll go with their own description of "karate rock" which kind of sums up their slightly insane energy and is catchier than electro-punk-pop rock. Also they sing about everything from fish and chips to the feelings of chimps towards bats, with a slight angry edge, which is good by me.

Anyway in between being sprayed with wine from MayKay and not being hit by the big musical stick thing I took some photos. The big musical stick things are clearly visible before being used to hit the big musical board thing (you'll be pleased to know that the crowd got a go with them later in the evening). In summary - good gig. I've been in need of one.