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Get it out of your System (Part 1)

(The best that is)

If you're planning to wire up a small studio, then this short series is for you. Part one looks at cable harnesses and associated problems.


In this two part series we will be examine the problems encountered when installing and operating a typical 16-track budget studio.


When a product is reviewed in H&SR, it is normally carted off to the long suffering editor's home studio to be field tested, so this year, said editor, being of sound mind, decided to upgrade to 16-track and in the best public interest decided to use a very popular combination of mixer and recorder so that any problems subsequently encountered could be solved and written about.

The system in question comprises the ubiquitous Fostex B16 and the Seck 18:8:2 mixer, both of which were supplied by Turnkey, though I must point out in the interest of impartiality that I did have to pay for them. These were to be connected up using patchbays and tielines in a single room home studio situation and there would of course be a rack of effects, a monitoring system and a 2-track mastering machine.

During the course of wiring up the system, there were one or two problems to do with earthing and high frequency instability that had to be overcome so these will be covered in the section on wiring up the cable harnesses, a job that was done entirely using West Penn foil screened twin core cable, again obtainable from Turnkey and relatively inexpensive if you buy a whole drum.

Figure 1.


Getting into Harness



Referring to Figure 1, you can see that the wiring is divided into five sections, A to E. (No harness as such is needed for the feed to the monitor amp as this is only a single pair of screened cables.) This seemed a practical way to operate and it does mean that parts of the harness can be modified or extended without having to break down the whole thing and start from scratch. In order to keep track of the cables within any particular harness, small self-adhesive numbered stickers were stuck to the plugs at either end of a run and these are easily obtainable from most stationery shops. As twin cored cable was used throughout, the black core was wired to the screen when balanced operation was not required.

Harness A



This harness connects the multitrack ins and outs to the mixer so there are 32 runs of cable terminating in jack plugs at the mixer end and phonos at the recorder end. Though the mixer is designed for balanced operation, the B16 is not and so standard mono plugs and leads were made up. In order to minimise the effect of earth loops, it was found to advantageous to remove the screens from all the cables at the recorder end apart from channel one's input and channel 16's output. Channel 16 output should then have its screen connected via a 100R resistor which, if of the ⅛ Watt type, will fit inside the cover of the jack plug at the mixer end which is easier than trying to fit a resistor into the phono plug at the recorder end. Though the system is fairly quiet even with all the screens connected, the improvement in background hum level is well worth the extra effort.

It must be said at this point that if this earthing system is adopted, you will have to earth the mixer, but remove the earth from the B16 or the resulting hum will be fearsome to behold. As long as the system is left permanently connected, the B16 will be safely earthed via the mixer but it's advisable to build in an extra safety feature for equipment that requires a ground lift. One way of doing this is to build a plug board that incorporates 22R resistors (at least 10W wire wound) in series with the earth conductor of each socket. In the event of a failure or short, this resistor will pass enough current to allow your earth leakage trip to turn off the current, but if you are not protected by some kind of circuit breaker, you really should fit one before proceeding. For details on this subject, see Ben Duncan's highly informative and detailed series of articles on studio mains supplies which started in HSR June 85.

It might seem obvious to point out that the harness should be made long enough but bends take up room and you may want to move things around in the near future. Also avoid having signal harnesses running near mains cables and if they must cross, make sure they do so at right angles to avoid induced hum.


Harness B



This is simply a pair of ins and a pair of outs to the master machine, in my case a faithful old A77 which has worked for nearly ten years with little sign of head wear. You might think that this cable run would present no problems, and so it seemed at first, but there was one odd effect that caused me to scratch my head at first.

The problem was only evident when the Revox was powered down and the mixer gain was turned up; there was this awful hum. It turned out to be high frequency instability in the mixer which was far too high in pitch to hear but it was obviously giving the power supply a hard time and hence the hum. Inspection with a scope proved this to be true so a solution had to be found. The answer was to simply connect a 330R resistor in series with each of the 2-track outs from the desk and these could again be made to fit into the Jack plug cover of the appropriate lead. A twist of insulation tape is sufficient to ensure that they don't short out. This problem seems to be a function of the design of the Seck mixer, but if you have similar problems with other desks, it's worth checking out.

Harness C



This one was a lot of fun!

Again it looked simple: just a send and return from each of the first 16-channel insert points to a semi-normalised patch bay. Even better, the engineers at Turnkey, who were very helpful throughout this installation, assured me that I could use a single piece of twin cored West Penn cable to carry both the send and return providing that the run wasn't too long. As the effects rack was to be located right next to the desk, this seemed to be great news, but the gremlins were waiting.

Figure 2.

The cables consisted of a stereo jack at one end to fit the insert points and a pair of phonos at the other end to connect into the back of the patch bay. Figure 2 shows how the phono end is arranged. The screen and one of the centre conductors is fixed into one of the phono plugs in the normal way but the other conductor is brought out in a loop some three or four inches long before being fixed to another phono plug. This one has no earth but this is no problem as the input and output earths are common on the patchbay, in our case the HSR design featured in the Sept 84 issue.

However, having wired up all the cables there was a problem. It transpired that the more insert cables were plugged in, the more hum became apparent and this without the patchbay being earthed or plugged into any other equipment. At first I thought that there was an obscure earth loop problem but I was wrong; it was more high frequency instability caused by the capacitive load of the cable. It turned out that the op-amp driving the insert point did not have a large enough series output resistor which meant that it would oscillate at the slightest provocation so the answer was (you've guessed it) to wire a 330R resistor in series with the send line inside the jack plug. You can tell if you do have an instability problem by turning up the monitors with no input to the desk. When you turn up the channel gains, the background noise should be clearly audible and as the gain is further increased, the noise tends to take on a whistly characteristic usually followed by the onset of hum as the power supply starts to struggle. Apparently this shortcoming is being rectified on newer versions of the Seck 18:8:2, but if you already have one, these simple mods should get you out of trouble.

One comment on the layout of this mixer is that you really do need to use a patchbay system because once the tape returns are plugged into the desk, it's very awkward to get at any of the other sockets because the cable harness is in the way.

Harness D



This cable run services the eight group insert points and also provides access to the auxiliary send and return points. The insert points were wired in the same way as the channel inserts using a single twin cored cable and the resistors were fitted into the send lines as a matter of course.

The auxiliaries have separate sockets for send and return so these were given separate cables but, just for old times' sake, 330R resistors were fitted in all the aux send lines as a pre-emptive measure against any more instability problems.

Wall Box E



By the time that you have got the previous harnesses installed, it will have become apparent that there is no way that you are going to be able to plug mic and line inputs into the desk without a struggle, so a wall box is the only practical answer. I used one of Turnkey's pre-punched affairs for convenience but you can make your own quite easily using a di-cast box or similar if you are really tight for cash.

As the Seck desk offers balanced inputs on both mic and line, you may as well use stereo jack sockets and plugs to keep the line input balanced as far as the wallbox. Most of the time you will be connecting unbalanced equipment but it's nice to have the option.

Depending on your studio layout, you might need more than one wallbox to give you the necessary flexibility and so you have a choice of either splitting the inputs into two groups, or duplicating them and paralleling the two boxes together. You will therefore have a choice as to where you plug in. Again keep this run well away from mains leads as mic inputs are particularly susceptible to pick-up.

Tidying Up



Possibly the best way of keeping the cable runs tidy is to use plastic tie-wraps spaced every six inches or so but don't fasten these too tightly or you may damage the cable. Tie up the cable runs separately rather than all in one huge bunch and don't forget to stick labels on the plugs to avoid getting into a mess. Also don't let the harnesses hang on their plugs; tie them to hooks on the wall or to the frames of racks to take the strain off the connectors.

That's all we've got room for this month but next month we'll be looking at the wiring of the effects rack, the monitoring system and a few operating idiosyncracies.


Series - "Get it Out of your System"

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Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Mar 1986

Donated & scanned by: Mike Gorman

Topic:

Home Studio


Series:

Get it Out of your System

Part 1 (Viewing) | Part 2


Feature by Paul White

Previous article in this issue:

> Making the Most of...

Next article in this issue:

> A Better Mousetrap?


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