Last month we dealt with the basic cable harnesses. This concluding installment finishes off the wiring and examines any operational oddities.

By now, you should have a working system with two of the three patchbays wired up and the dreaded hum should be absent. If it isn't, try temporarily removing the earth from your monitor amp and if this cures it, try fitting a groundlift resistor of around 22R (10W wire wound) in series with the earth lead as described last month. The other option which is in theory preferable from the safety point of view is to leave the earth on the power amp but break the screens on the signal cables. This may either be a complete break or you could put in a series resistor of 22R (¼ Watt) or thereabouts to see which gives you the quietest results.

On The Rack

I opted to mount the patchbays in the same rack as the effects units to minimise wiring and a suggested patchbay system is outlined in Figure 1. All the effects plug into the patchbay by means of jack to phono leads which have not been tied into a harness purely for the sake of flexibility. Effects patching is then carried out using simple jack to jack leads between the effects bay and the two mixer bays. In a more flexible system, you might want to bring the tape machine ins and outs to a patchbay or include a cassette machine in the system and this is easily catered for by returning the cassette deck through the unused tape inputs on channels 17 and 18. At a pinch, you can even use the phones output to feed your cassette machine to save tying up a patchbay location and all you need for this is a stereo jack to pair-of-phonos lead.

If at any point you do set up an earth loop, simply unplug the last piece of gear you connected and isolate the problem before using one of the previous two methods to abolish it.

Many pieces of rack equipment are designed for use without an earth conductor and many of those that do have an earth lead have a built-in ground lift but there will always be one that gives you a problem. The same solutions apply as to the monitor amp and this means fitting a ground-lift resistor in series with the mains earth or breaking the screens and inserting a resistor where the effect unit joins the patchbay (see Figure 2). In extreme cases, you may have to isolate the chassis of the offending effects unit from the metal frame of the rack unless you do choose the resistor in the mains earth option. Again, the method of breaking the coax screen is to be recommended from the safety point of view if it will do the job.

To recap then, if you put a ground lift resistor in series with the mains earth of a piece of equipment, it should be of the wire wound type and rated at at least 10 Watts. If on the other hand you can do the job by inserting a resistor in series with a signal ground, so much the better (this may be a small ¼ Watt device and its value should not be too critical), anything between 22R and 220R should do the job.

On the subject of earth loops, this system gets particularly upset when earthed instruments are plugged into it as you immediately set up another earth loop, so why not make up a few jack to jack adaptors as shown in Figure 3? This incorporates the standard 22R resistor in series with the screen connection and should solve the majority of problems without you having to pull the earths off things, a practice which is frowned on and can be dangerous in the case of an electrical fault.

Operation

Having got everything working with no hum, it's time to check out the system. One thing that's quite normal with the B16 is that there's a noticeable amount of low frequency noise present on the output when the mixer gain is wound up and this is largely due to the Dolby C system which removes a lot of the high frequency noise leaving this characteristic residual noise. This is at a very low level and is quite inaudible at normal operating levels but it can make you worry if you come across it during the course of de-bugging the system and are not familiar with it.

Shortcomings

The system as it now stands is excellent value for money and is capable of truly first class results, the in-line monitoring and the sheer number of auxiliaries available at mixdown being most welcome. Even so, there are still a few niggling problems about which little can be done.

One omission on the Seck mixer is that there is no insert point on the stereo output which is bad news if you want to put an enhancer, limiter or equaliser across the entire mix. Of course you can route all the channels via a pair of subgroups on mixdown and patch your effect into the subgroup insert points, but that limits any further subgrouping activities to zero.

The only other practical option is to bring your mixer's stereo master output via a patchbay and then monitor the 2-track machine's input rather than the mixer's output when mixing down, and in this way you can patch in the effect on the patchbay.

Another annoying problem, and one that I haven't yet fully solved, is that of having no channel mute buttons. You can't really use the routing buttons because they don't mute the aux sends and if you try to use the mic/line switch, it introduces a noticeable click onto the master output as does operating the PFL buttons, which is rather naughty. If anyone has found a satisfactory way around these problems, please let us know and we will pass it on; all I can think of is the possibility of fitting a toggle switch to each channel of the insert patchbay to mute the signal there but it's hardly convenient.

One further annoyance that you are bound to come across is that of otherwise admirable outboard equipment that simply does not have the output level necessary to drive the aux returns or insert points of this desk which operate at 4dBm. The booster amp described in the December 85 issue of H&SR should do the trick if you're handy with a soldering iron, though you can use the mic inputs on channels 17 and 18 as returns for low line level effects if you only need two.

Trailer

Though this has been by no means an exhaustive article, it has been an exhausting one and I hope that it has covered a number of important points that will be of interest to owners of this popular 16-track package. Even if you run a different system, this article's theory would still be valid and should help you to sort out some of those patching and cabling problems.

If anyone has anything to add to this series, please get in touch because it's reader feedback that lets us know just what is going on in the world and in particular, which problems are most common.