Brian Milliken

Hi can you say what the large aerial is for please, and any info on it performance.

High

Hi There and greetings from Scotland There is a thread on the METACORDER Forum regarding High Freq Whistles, I have eMailed Jim at BLACKBOXVIDEO http://www.blackboxvideo.com/ . He has kindly eMailed me the following info, it is of interest to anyone who is powering multiple toys from one source. Hello Brian Thanks for your Email - good to hear from you. It is true that the problems of whistles when using equipment powered from switch mode supplies is caused by "hash" from DC-DC converters. This is an increasing problem as more and more equipment uses switch mode power supplies for light weight and improved efficiency. Put simply, to achieve this increase in efficiency a switch mode supply switches power on to the load in short bursts. These short bursts are typically stored in an inductor and the bursts smoothed out to a nearly steady flow by capacitors. To keep the components small the frequency of the "bursts of power" can be as high as 1 MHz. This is in the RF spectrum and because these are square waves they have many harmonics too. With only one switch mode supply in the system there is unlikely to be a problem as the manufacturer of the equipment will have designed it such that it does not cause any interference with the equipment it supplies. However, two systems from different manufacturers connected together will use different switching frequencies in their power supplies. If these frequencies are very different there is unlikely to be a problem. If the frequencies are quite close they will beat together to produce sum and difference frequencies which may well be in the audio band, hence the audible whistle. Now imagine half a dozen different pieces of equipment all with switch mode supplies and all on slightly different frequencies. The numbers of combinations of beat frequencies is going to be enormous! It is impossible to completely remove all the noise (hash) from these supplies. The noise is often quoted as a percentage of output power, so power hungry devices like computers are usually the worst offenders. Also computers are digital so are not generally affected as much by this noise as audio gear which will have at least some analogue circuitry. Small amounts of this noise will appear on all input and output connections of equipment supplied by a switch mode supply, not just the power supply. Ground loop problems can make matters worse especially in unbalanced signal connections. Problems can often be cured by unplugging unbalanced phono returns or feeds to unbalanced devices. As far as our BIG One is concerned all the six outputs are individually filtered and this gives about 80dB of rejection between sockets which is much better than most other systems on the market. As well as each XLR being well filtered each XLR socket is wired individually back to the power output capacitor to prevent ground loop problems. Daisy chaining power out sockets is a major cause of ground loop problems. Using XLR splitters for high power noisy devices is a sure fire way to trouble. Some times there is no solution to these problems other than to power incompatible devices separately. But remember the problem may not just be down to power supplies it can also be caused by in/out coupling, unbalance or external ground loops. You rarely get problems with well designed equipment like SQN mixers where the power supply floats and ins and outs are all balanced. Semi pro equipment often has poor susceptibility to RF, one power leg tied to ground, poor common mode noise rejection, etc which causes no problems on its own but gives rise to many when connected to similar Semi pro gear. With regard to using additional external filters these can sometimes help but to be effective filters have to be designed for a specific interfering frequency so just applying a general purpose "filter" is a very hit and miss affair. Also it may not just be power supply filtering that is required, signal in and outs can also be part of the problem. Hope this has been of some help, it is a subject you could fill many books with and one that sadly is likely to be come increasingly troublesome. Perhaps you'd be kind enough to give me the link to the forum - I'd be interested to go and have a look at this and other topics being discussed. Take care, and best regards Jim

If you have the chance please post your test results, I am sure it is of general interest. Regards Brian

I was wondering if you have heard the Aaton minima, it is quite noisey, what kind of doco is it, will you be far away enough from the Camera? Regards Brian Milliken AMPS