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Hugh Frequency Whistles/DC + Blackboxvideo


Brian Milliken
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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

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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.

Thank you for posting the informative notes regarding switching power supplies, DC to DC converters, common power source problems and so forth. I was aware of many of these potential problems and have in fact experienced this noise issue several times with various bits of equipment. Almost everything I use is powered by one centralized DC power source, an battery power supply that uses a simple and elegant design utilizing a regulated DC power supply floating across the batteries. This regulated DC power supply is a switching power supply (rather than the linear DC supplies I had used in the past) so it has the potential to cause these problems. Also, as noted in the post above, problems can occur even when only a battery is used when equipment that is DC powered is connected to this common power supply. All of the DC output connection points on mhy cart are "home runs" to the Cart Power Supply, and this helps a lot to avoid problems. There have still been problems at times necessitating the use of 2 powering schemes to solve, and even then, as the equipment is connected to other equipment, problems can develop. The only one I was never able to solve was with a Lectrosonic IFB transmitter which always exhibited a whine in the audio whenever connected to a common power source AND to the audio source needed to feed it signal.

Regards,  Jeff Wexler

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