Re: [ARSCLIST] Effect of vibrations on audio tape

[Ron Fial <ron@xxxxxxxx>]

Some suggestions for UCSB:

Try to determine if the problem is local (grounding, shielding, equipment oscillating above audio frequencies, etc.) or external (strong radio waves, electromagnetic interference)

The time of day (24 hrs vs 9AM-5PM vs only at night) you experience the interference is a good clue.  Look around your area for any rotating lighted business signs.  These can have bad slip-rings that radiate interference.

Sometimes the problem is due to a building grounding problem.  Make sure the main electrical panel for the building has a substantial earth ground (need electrician to check earth ground).  Often these degrade with time.  Make sure that nearby sub-panels do not have the neutral (usually white) connected to the ground (usually green).  This is a common error, is against the electrical code, is dangerous, and the ground loops formed are interference transmitting antennas.   Make sure all your wall plugs have the safety ground connected, and that hot and neutral are not reversed.

Move any computer and monitor away from the turntable and preamp. Computers radiate lots of  cyclical noises in the radio spectrum.

Try taking the turntable, preamp, etc. to another room or even out to a quiet valley in the country to see if the interference is gone (take some good wine and some cheese and crackers).  It may be an instability in the equipment, like defective bypass caps in the preamplifier, or ultrasonic oscillations in the preamp. Check your audio with a scope, your ears can't hear that kind of problem.  Some amps and panels will make funny noises if there is excessive common-mode noise on the balanced cables.  Try grounding the shield of the balanced cables at only one, then only the other end and compare with grounded at both ends.  There is equipment that will actively amplify, invert, and subtract out the common noise real noisy environments, but that mostly needed for balanced audio lines in video studios, radio transmitter rooms and power stations (I design that stuff for a living).  Another trick is to run the balanced audio through 1:1 600 Ohm Z transformers, to break any ground!
  loops
.

If you think the electrostatic noise is getting into the pickup, build a small wood frame around the turntable and cover it with copper screening (overlap and solder the edges).  You now have a Faraday cage. ground the shields of the wires that must enter to the copper at only one point.  If this works, try an AC isolation transformer between the wall outlet and the table, get one with a faraday (copper) shield between windings.

You may be close to a paging company transmitter, these turn on and off for only short periods to send out pages, and they use very high power.  But the signal is usually random on and offs, not cyclic.

Call the Electrical Engineering department and ask if they can bring over a spectrum analyzer and a small antenna.  Scanning the electromagnetic spectrum up to a few hundred megahertz may show you the frequency of radio interference and the signal should be going up and down at the rate of the interference you are experiencing.  This should only take a few minutes.  The center frequency, modulation-type and spread will give an knowledgeable electronics/radio person a good idea of the possible sources.  For instance if the signal is narrow band and in the Amateur radio bands, it may be a nearby beacon transmitter.  If you are near a medical clinic, they have powerful 10 MHz transmitters to heat muscles for therapy.  The  possibilities are many.  The right spectrum analyzer can also scan the radar frequencies.

Contact the local ham radio club.  If they have members that do 'transmitter hunts', then those people will know a lot about localizing interference, and have directional antennas and receivers specifically for tracking signals.  But first determine with the spectrum analyzer that it probably is radio interference.  

The local power company often has an interference meter ( a noise receiver tuned to about 120 MHz) used to track down power poles with arcing electrical insulators.  You might give them a call and ask them to check around the building.

An announced power failure gives you a good opportunity to divide and conquer.  If you know all the electricity is going off tonight for 15 minutes, you can bring in batteries and inverters, and run you equipment when everything else in the vicinity is off.  If the interference is gone, then you know it is from a nearby intentional radiator (radio transmitter) or un-intentional radiator(noisy electric motor) nearby.

I would be glad to answer any questions you have about strategy, etc.
   
   Ron Fial   (503) 607-1940

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At 02:45 PM 10/27/2006, you wrote:
>Speaking of interference, our campus is about a mile from the Santa Barbara airport. We get interference from the airport radar (we think). It's cyclical and we've ruled out just about everything else (elevators, fluorescent lights). It's only picked up by things that are microphonic like phono cartridges and doesn't affect the shielded balanced lines. If you listen very, very carefully to our cylinders you might be able to hear it.
>
>Jim Wheeler once suggested that we line the studio with that copper sheeting with holes in it like the door of a microwave oven. We never did this, but I'm open to suggestions.
>
>David Seubert
>UCSB
>>.....snip