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This may not strictly be a question for this forum so I shall be posting it elsewhere, however I hope someone may be able to help......I have a Yamaha Dolby Digital receiver connected to five Mission speakers. I have bypassed the internal amps on the Yamaha and connected Rotel THX 200 watt power amplifiers......real cool! Everything was fine and dandy until after saving for an age I bought a Toshiba MT1 projector. When powered up the vicious hum that sounded through the speakers sent me running from the room!


Everything has been unplugged individually and it all comes back to the projector. I even ran an extension lead to a neighbours so that I could power the projector from a 'clean' power source, no dice...but big buzz and hum...I sent the Rotels back to the manufacturer for checking and all was fine. Naturally being a long time reader of this forum, when I got them back I took them to a friends and checked them again...no problem there but oh brother big noise back at home in concert with the projector.

Does anyone have any ideas that might help? Heres hoping

Parmenion
 

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First a quesiton, then a couple suggestions...


Have you tried the projector in someone elses setup? You mentioned that you tried the speakers in someone elses setup, So I'd give that a try frist.


I'd try plugging the power for the projector in, but not the signal cable. Try a new cable between the projector and the amp, and also, if you're using SVideo, try composite, or vice-versa.


Finally, I'd give the folks at Toshiba a call. It sounds like you might have just gotten a bum unit.


John.
 

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JZink, thanks for your reply. I just tried a yellow composite video lead and the problem remains......Also I noticed that when the unit is in standby.....ie with power going to it but not with the lamp lit, there is no noise, however as soon as the button is pressed and the bulb lights up, the noise through the speakers starts........Hope this provides more food for thought.


parmenion
 

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Is it a hum or a buzz? Do you hear the noise when the video cable to the projector is disconnected from the rest of the system?


Vern
 

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Vern,


It sounds like you are experiencing a ground loop in your system causing the hum in the audio. Have you tried connecting all of the components in the system to the same a/c circuit? By doing so you will eliminate the possibility of using two circuits which may be on a different phase from one another and hence the ground loop and hum problem. The other possible source which you havent' checked for is too see if the a/c outlets themselves are properly grounded.
 

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Ghibliss:


I am not the one with the problem, parmenion is.


I doubt it's a ground loop if it only shows up when the lamp is on, but since parmenion seems to be MIA here we don't know.


Vern
 

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Discussion Starter #7
Thanks all for your replies. Sorry I couldnt get back sooner but shift work rules..........


I do not believe the sockets (outlets) are grounded. I will have to get a good electrician to check. At the same time I will have him check if the projector and the amps are on the same circuit.


The thing that really puzzles me is why the buzz ( it is a buzz) doesnt appear with power going to the projector...it only appears when the lamp is powered up!


Weird.....


Parmenion
 

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Parmenion: You never did answer the question about whether disconnecting the video cable with the lamp on has an effect on the buzz.


Vern
 

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parmenion, the key to your problem is the comment "I do not believe the sockets (outlets) are grounded. ". This fact alone explains your problem. More detail:


Your Toshiba projector uses a UHP lamp in it's light engine. Without getting too technical, a UHP lamp is a form of arc lamp with an electronic regulator that ensures reliable startup, a long electrode life, and a fairly consistent light level over the rated life of the lamp. However, an electrical arc also generates massive amounts of electrical noise across a wide band of frequencies including audio and radio frequencies. The common way of dealing with this is to incorporate electrically conductive shielding (metal or conductive paint) as part of the projector design, then ground the shielding. Without such a ground, the metallic lamp enclosure or the conductive painted shield inside the projector case can become an antenna, radiating the electrical noise into your audio gear.


Another possible problem is you mentioned bypassing the internal amplifiers of your receiver in favor of external power amps. Hopefully, you did not run shielded wiring from outside the receiver inside the receiver chassis - if you did that, you also created pretty good receiving antennas for conducting the radiated electrical noise into the sensitive low-level audio circuits.


Frankly, your biggest problem may be one you didn't mention - the electrical ground on audio gear serves as a life/safety ground, protecting the user from electrical faults in the electronic gear - using ungrounded equipment is not safe. Have your electrician ground your receptacles, and then power all your equipment from a common circuit, and use grounded AC power cords everywhere. If you still have noise, come back and tell us - but be safe first.


Gary
 

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I completely agree with Gary on this one. If your equipment is not properly grounded, you will definately have noise issues!


I don't know about over in london, but here in the states you can go down to your local hardware store and get a circuit tester. They cost about $5USD and have a diagnostic lights on them that tell you if you have the circuit wired properly, including if it is properly grounded.


John
 

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Discussion Starter #11
Thank you all for your replies................


I think, from your concern for my safety, that I have worked out that 'grounded' is what I know as 'earthed'...........!


Rest assured all outlets are earthed.


Removing the video lead whilst the projector is running does remove the buzzing sound...........also the picture!!!


I have not broken open my amplifier to add external power amps. The Rotels have been connected via the correct pre outs......


If the projector lamp is generating this audio 'pollution' how on earth is it cured in other peoples systems? Am I the only one to have suffered this problem?


Parmenion
 

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Glad you are safe. It sounds like you have another common problem we call "Ground Loop" (somehow "Earth Loop" doesn't sound correct). This problem is caused by a different AC (mains) branch circuit powering the different equipment. Multiple receptacles are commonly attached to a single such branch circuit and protected by a single circuit breaker. Electronic equipment such as computers - that is to say gear with "switching" or "non-linear" power supplies - induces a small voltage in the ground (earth) conductor, which you will hear as a power line buzz - in your case, 50Hz. When you interconnect projector and audio gear in different locations and powered by different branch circuits, the small induced voltage gets injected into the audio input - this can be difficult to understand because sometimes the offending equipment is in an adjacent room. Possible ways to cure this:


1) Use a 1:1 AC isolation transformer to power the projector or audio gear.


2) Use a purpose built device we call a "Ground Isolator" (I have no idea what it is called in the UK) inline with your video cable.


3) Rearrange the equipment power cords to power everything from the same branch circuit.


4) In older structures, the ground(earth) conductor may be the metallic cable sheathing or ridgid conduit containing the seperate wire conductors. This type of mains power is very susceptible to the above problem, and also unsafe, because the metallic sheath may over time develope a high electrical resistance and render the safety aspect questionable. I have found such problems in early 20th Century "Brownstone" buildings, and the best way to fix such a problem is new wiring with integral copper "ground" conductors.


Gary
 

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parmenion,


Often the "cable" television feed has an earthing or "grounding" that is at odds with the rest of the electrical system wiring creating a ground loop and attendant hum/buzz.


This may be the problem. Verify by leaving the video cable connected to your PJ but disconnecting the "cable tv" connection to your dvd/tuner/tv/vcr/? what ever is feeding to your PJ.


In my system, for example, Cable connect feeds vcr and through that the tv. DVD player feeds tv and PJ. Well almost. When I connect dvd to pj I disconnect the cable feed to the vcr cause of hmm/buzz. I don't watch "TV" on the PJ and don't watch DVD on TV and PJ at the same time.


Have no time yet to call cable company or contact http://www.jensentransformers.com/apps_wp.html see http://www.avsforum.com/avs-vb/showt...=hum+AND+cable


May be this helps.


EDIT - spelling
 

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Just out of curiosity, have you tried using only the Yamaha's internal amps and bypassed the Rotel's? And have tried newer (better sheilded) cables. I had a similar problem with my receiver and when I switched to professional grade cables, the hum disappeared.
 

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Discussion Starter #15
Again, thank you for a wealth of information.



As a matter of fact I have unplugged the Rotel power amps and have had to return to the internal amps on the Yamaha.


The result..................No buzz or hum.


I realise that this solves the problem, however, I now have a fortune invested in some Very heavy paperweights!:)


Just as important as the money is the difference in sound quality. The Rotels (difficult to describe in words!) had a sort of "grip" on the sound........everything had better definition, more impact......

I would love to be able to solve the problem so that I could re introduce these beasts.


I can feel a solution is there .........I just cant put my fingers around its neck!

Parmenion


ps. I will try the various suggestions at the earliest opportunity.
 

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Have you tried professional grade cables on all equipment. It made a hell of a difference on my setup.
 
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