Making the Connection

The other day I borrowed some microphones, as my band had a last minute gig crop up and we had helpfully lent our mikes to a local school for their play. So far — so good. It was only when we unpacked them at the venue that we realised we had a problem — they all had XLR connectors whereas our PA system exclusively used jacks. So how do you linkup balanced line XLR-connected mikes to mono jack plugs? As we were frantically soldering odd jack plugs onto the leads the bass player muttered curses and complained that he was sick enough of doing similar things with phono and DIN plugs for home recording without having the same hassle on stage.

The next day whilst we were removing the jacks and replacing the XLRs we discussed what we would need to make sure we never got caught again. Adding in the various recording requirements, we came up with the list below:-

Balanced line <-> XLR ¼" stereo jack
Balanced line XLR <-> ¼" mono jack
Balanced line XLR <-> two phono
Balanced line XLR <-> mono phono
Four phonos <-> 5 pin DIN
3.5mm stereo jack <-> ¼" stereo jack
3.5mm mono jack <-> ¼" mono jack
2.5mm mono jack <-> 3.5mm mono
2.5mm mono jack <-> ¼" mono jack
One phono <-> Two phono

(The double headed arrow indicates that the conversion should be available in either direction).

The circuit I came up with covers our initial list and a few other combinations (such as 5 pin DIN to various jacks) that came 'free' with the design.

The idea is to build into a metal box as many sockets of different design as possible, inter wiring them so that they can be patched in any configuration. In order to achieve all the possible combinations we also need to add four switches. (Strictly speaking you could get away with three switches but one of them would need to be a four pole double throw type. As these are very difficult to obtain I've opted for two DPDT switches mounted close together so they can be operated simultaneously.)

Options

Interconnections between XLR, the three sizes of stereo jack and dual phono sockets are straightforward — just plug in, in any combination, and they're connected. Similarly conversion between the mono jack sockets simply requires the relevant plugs to be inserted. In the unswitched mode the stereo sockets will act as in-line couplers (mono) with the mono sockets so, for example, a mono ¼" jack can be put in the ¼" stereo socket and another into the mono socket allowing you to extend a lead. For recording buffs I've also included a 5-pin (180°) DIN socket and four phono sockets which are permanently connected to pins 1, 3, 4 & 5 with pin 2 as the common (earth) — the standard record/playback configuration.

Switching

The switching extends the scope of the unit allowing stereo/mono conversion, coupling of balanced line inputs to mono output and switching of all connectors into either record or playback mode with respect to the 5 pin DIN/phono sockets.

The switch functions are as follows:

Switch 1 — Converts balanced line to mono by connecting one line to ground.

Switch 2 — Short circuits the right and left stereo inputs to give mono output. Will also split a mono input into twin mono output — a so called Y patch.

N.B. Switches 1 and 2 cannot be used together as this would short all inputs to ground. I suppose you could use them as a system OFF switch if you were that way inclined.)

Switch 3 — (two switches operated together) switches either record or playback mode of the 5 pin DIN into the main socket field. If phono sockets are used it will select either bank A or B into the main socket field.

Alternating the switches allows the left channels of A and B to be connected together, as are the corresponding right channels.

Although the design is very straightforward, it does require a lot of interwiring. To some extent this can be reduced by using skeleton jack sockets and a metal case, the ground connection being automatically made as you tighten the socket into the box. As the use of XLR connectors seems to vary from manufacturer to manufacturer, I've included both male and female types so that you never get caught out.

Parts List

1 off 3 pin XLR type chassis plug
1 off 3 pin XLR type chassis socket
1 off 5 pin DIN (180°) chassis socket
4 off phono chassis sockets
1 off ¼" stereo jack socket*
1 off 3.5mm stereo jack socket*
1 off 2.5mm stereo jack socket*
1 of ¼" mono jack socket*
1 off 3.5mm mono jack socket*
1 off 2.5mm mono jack socket*
2 off SPST switch
2 off DPDT switch
1 off metal case (eg aluminium box)
Hookup wire

** N. B. All the sockets should be of the skeleton, unswitched type.

These are all fairly standard bits and should be readily available from any decent electronics component shop/supplier. If you find you cannot get skeleton jack sockets, fear not, simply use plastic ones and connect the ground pin (the jack plug barrel contact) to the metal case. As a metal case is used you should be able to get away with using single core wire to hook it up provided you keep the leads as short as possible, though screened cable will give greater protection against left/right crosstalk in recording applications.

Planning the box

In Figure One I've given a wiring layout for the box. Obviously the layout of components is going to depend on the size and shape of the box and your own particular preferences. I would recommend that you leave a little room for later additions as Sod's Law states that sooner or later you'll come across a combination I've left out and you'll want to add a few car radio aerial sockets(?) or something. The main difficulty is in cutting the holes for the XLRs and the DIN. Unfortunately I have no magic solution to the problem. The ideal answer is to use a hole saw but, sadly, these seem difficult to find these days. I suspect that the largest hole you can drill plus a lot of work with a rat tail file is the only viable way. The switches introduce another dilemma. DPDT slide switches are dirt cheap and could be used for all the switching needs but they require a rectangular slot to be cut in the box. Well, you pays your money... etc. I confess that I copped out and bought the more expensive toggle switches 'cos I can just about manage to drill a hole (but never in exactly the right place!).

So there we are. OK I confess that my band's problem would have needed several boxes to get us out of the fix but you must admit it's a dead handy item to have kicking around in the bottom of the lead bag.