Thursday, 27 June 2013

To Crimp or Not to Crimp....

When building a mixer, there is a need for many audio connections to be made between the active modules of the mixer and other components. For example, all the mic and line inputs need to be connected from rear panel mounted XLR connectors to their individual channel modules which in turn need to be connected to and from channel faders. The current design of the EZTubeMixer uses a motherboard into which the channel modules are plugged. This motherboard provides all the common connections between channel modules such as power and audio buses which saves a lot of tedious error prone wiring. However, connections like mic and line inputs, faders and direct out still have to be hand wired on a per channel basis to the back of the motherboard.

There are two major problems with this. The first is that access to the motherboard is not always  easy so wiring to the motherboard in-situ is not always possible. To overcome this the motherboard can be loomed before it is fitted, with the free end of the loom being wired to XLRs and faders once the motherboard is in place. That's fine but often the XLRs and faders are not in a place that is easily accessible for soldering. The second problem, is that once all this wiring is complete, it is now very hard to access it to make modifications or correct errors.

The answer, of course, is to use connectors for these signals on the rear of the motherboard. XLRs can then be wired to flying leads with connectors attached . The whole seemly than then be offered up and plugged into the motherboard. Similarly a complete fader assembly can be built with flying leads that simply plug into the motherboard. Soldering can be done of the bench where access is not a problem and the assemblies can easily be disconnected for fault finding or to make modifications.

The only question remaining is what connectors to use to fit on the rear of the motherboard and on the ends of the flying leads? We need a PCB connector to fit onto the motherboard and a mating half that can take a screened lead. It looks like the only viable solution is to use 0.1 inch pitch crimp connectors. The Molex KK range is typical. It has two and three pin headers that can be soldered directly to the motherboard and crimp contact free receptacles (sockets) that plug into them. They can be polarised and they are not expensive. The only problem is the free sockets use crimp contacts and I have never quite got on with crimp connections. It would be nice if there were solder connection versions of the free sockets but I have not been able to find any.

As crimps can potentially solve so many mixer construction problems I thought I ought to give them a another chance. So I bought myself a proper hand crimp tool, some two and three way Molex KK range housings and a packet of crimp contacts. I sat down and tried to come up with a reliable way of connecting a twin screened mic cable to a three way 0.1 inch pitch free crimp socket. Crimping screened cables had been a problem in the past for me because the screen ended up being a much greater diameter than the two signal wires. Fortunately Van Damme does the 'Install' range of twin screened cables that has a foil sheath and a multi-strand drain wire that is little bigger than the two signal wires. The picture below shows the results of my initial efforts in crimping this cable to a three way Molex.

The unsheathed connection was my third attempt. As it has no insulation, it is hard to judge the correct depth to insert the drain wire into the crimp and it is all too easy to push it in too far. This not only leads to a bent connection because one wire is shorter than the others (as you can see in the picture) but sometimes can prevent the crimp contact engaging properly in the plastic housing. I was also unsure how much insulation to strip off the signal leads. Too much and you get the same problem as with the drain wire, too little and you get no electrical connection. That's why it took three goes to get it to work. The green sheathed one was my fourth attempt. It is still bent because I have not got the drain wire length right yet, but signal leads went in fine now knew how much insulation to strip off.

The two brown sheathed connections were my final two attempts. By now I have just about got the hang of it. The connections are straight because the drain wire length is right.

The thing with the blue handles is the crimp tool. It works well but working with crimps is fiddly and you never seem to have enough hands. I found the best way was to put the crimp contact in the tool and start the ratchet action. This holds the crimp in the tool so you can now hold the tool in one hand and with the other offer up the wire (which in the meantime I was holding in my teeth).

It is still a bit of a clumsy process. Holding the fairly heavy crimp tool in one hand and offering up the wire with the other is not easy. I think I might try clamping one handle of the crimp tool in a vice to hold it steady. The I only have one wobbling hand to worry about.

Overall I think with a bit more practice I could become competent enough to make reliable connections with it. Thanks to Holger  ( )  whose motherboard design using Molex KK connectors got me thinking about this again. His two module motherboard is a really neat idea. I might just have to do my own version.

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