quote: Brad Miller

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Tomorrow stick a business card in between the film and red light source before you thread up the first show.

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Brad, where does he stick the card if he hasn't threaded up yet?
 

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Posted by Steve Guttag (Member # 268) on 10-16-2004, 07:56 AM:
 
The key is to find out if the noise is generated by the film (poor lateral alignment). So before loading the film, Brad is having him block the LED to see if the noise is there because if it is, it isn't poor film alignment.

If the noise is there only with the film running and blocking the LED with the business card kills the noise...it indicates a poor alignment.

Is the buzz channel specific?

I would recommend using Dolby-A or Mono at first when trying to check the problem since mono has no noise reduction and A has much less than SR but will let you see a channel specific noise.

Steve
 

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Posted by Edward Jurich (Member # 1787) on 10-16-2004, 09:06 AM:
 
I had a buzz problem here in the analog (analog is all we run) sound. The buzz was much worse when the processor was switched to mono, this is because the Dolby noise reduction will mask the buzz but then the buzz modulates with audio. Anyway, the problem turned out to be the house light dimmers (SCR or TRIAC solid state). The solution was to disconnect the shield wire at both ends from the cell to the processor. You can get away with no shield connected if your processor has a balanced input. And no, bypassing the SCR/TRIAC's or changing ground circuits did not help.
 

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Posted by Mark Gulbrandsen (Member # 72) on 10-16-2004, 09:44 AM:
 

quote: Wolf King Morrow

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The problem is starting to slightly creep up in a couple of other auditoriums, so I'm assuming this is maybe a common part wearing out.

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quote: Edward Jurich

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The solution was to disconnect the shield wire at both ends from the cell to the processor. You can get away with no shield connected if your processor has a balanced input.

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quote: Wolff King Morrow

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So I tried replacing the pre-amp board mounted behind the solar-cell, but it had no effect on the buzzing problem.

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quote: Steve Guttag

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The key is to find out if the noise is generated by the film (poor lateral alignment). So before loading the film, Brad is having him block the LED to see if the noise is there because if it is, it isn't poor film alignment.

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I know, I was just being a smart ass Brad said to stick the card between the film and the light source before he threaded up! If he hasn't threaded yet, the film is still on the platter or reels.

Rick
 

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Posted by Wolff King Morrow (Member # 2226) on 10-17-2004, 01:34 AM:
 
Hey guys!

I got to the bottom of the problem, which turns out has been mentioned here before. It ended up being the House & Stage light switch box on the wall causing the buzzing sound. The moment I turned the lights off while troubleshooting, the buzzing stopped. So I turned the lights back on and the buzz came back.

I guess this means a tech visit is in order because this sort of problem requires some serious re-wiring I bet.

BTW I checked all our lateral guide rollers and they are all in working order.
 

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Posted by Brad Miller (Member # 2) on 10-17-2004, 02:24 AM:
 
Out of curiosity, what kind of dimmers do you have?
 

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Posted by Wolff King Morrow (Member # 2226) on 10-17-2004, 04:16 AM:
 
Something like: "Dual Auditorium Light Dimmers" according to the manuals. I'll have to check the exact name and make when I go back to work on Tuesday.
 

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Posted by Gordon McLeod (Member # 33) on 10-17-2004, 01:04 PM:
 
In a perfect world lamp dimmers should not be wired onto the same pahse as lowlevel components of the soundsystem (ideally soundsystems should have a deditcate technical power transformer and isolated grounding system)
In this case one should check that the dimmer isn't on the same phase and if it is a problem that has sudenly surfaced check neutral connections and ground connections also check that the filter choke in the dimmer hasn't failed and shorted out
 

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Posted by Wolff King Morrow (Member # 2226) on 10-17-2004, 03:53 PM:
 
Well the problem isn't really all that sudden. It has been bothering me for several months now. I kept replacing parts trying to chase it down, but only just last night did I finally figure out what it was. I set up an appointment with our company tech to have him address the issue.
 

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Posted by Darryl Spicer (Member # 711) on 10-17-2004, 06:52 PM:
 
Does that pre amp board have a ground connection on it. If it does try running a wire from the ground point to the frame of the soundhead. I have had the exact problem related to the house dimmers and got rid of it by grounding to the frame from the ground on a line amp for the Jax light. Now I know yours is different than a Jax light system but this may be something to look into and it may save a hell of a lot of time by preventing re-wiring.
 

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Posted by Edward Jurich (Member # 1787) on 10-17-2004, 10:33 PM:
 

quote: Steve Guttag

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Whom ever told you that just because you have balanced inputs negates the need for a proper shield doesn't have a clue as to what they are talking about. What you just gave was very bad advice/information.

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AH, but then you must not have any broadcast experience A true balanced input will cancel any signal present on both wires, both wires being hot in a balanced system. Example, 30 years ago high quality audio was fed over "unshielded" pairs of telephone wires over "miles" from a radio station studio to a transmitter site. These lines had to met the spec of 50HZ to 15,000HZ with noise 55db down from 0db.
The solution is in fact temporary but yet, not an unsound (no pun intended) solution.
 

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Posted by Steve Guttag (Member # 268) on 10-18-2004, 01:43 AM:
 
Actually I do have broadcast experience and a vast knowledge of grounding.

In the case of broadcast...it is imperitive for the shield to be connected at both ends...this leads to a problem with audio since connecting shields at both ends often leads to a ground loop situation that causes audible hum. Often a solution to that situation has a capacitor in line with the shield at one end such that at high-frequencies both ends are terminated but at low-frequencies only one end is terminated.

As to balanced inputs...yes, one does take advantage of CMRR (Common mode rejection ratio) and as such if both wires are afflicted identically with noise, then, depending on how good the circuit really is, it should be supressed. Twisting the pair of wires goes a long way to increasing the likelihood that both wires are equally afflicted.

However, this does not negate the shield and to say it isn't needed is flat out wrong. Especially when one considers the path the cell wire often must take to get to the preamp...in a console there is nasty power all around and igniters...in the booth it may be traveling many feet.

I don't have a problem with the advice of trying to lift the shield to see if the noise goes away (indicating a grounding issue)...I do have a problem with dipensing advice that just is plain wrong. Getting away with something doesn't make it right nor good to advice others with.

Steve

[ 10-18-2004, 10:51 AM: Message edited by: Steve Guttag ]
 

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Posted by Edward Jurich (Member # 1787) on 10-18-2004, 10:13 AM:
 
In my case, I need to run new conduit because the current system, about 2 years old, was run with the audio line conduit near the AC and dimmer circuits. So whenever I get the time I'll be re-doing the audio runs.

However, getting back to shield/no shield, the SCR or TRIAC type dimmers tend to generate noise in the form of RF. So, if your audio lines are too close to the RF noise field, like in my case, the shield wire actually picks up the noise and feeds it into the audio ground system.

I once had to get our FM out of a church PA system. They used an Altec amplifier with balanced inputs. They had installed the amplifier at the back of the church and ran the mike cables through the ceiling the entire length of the church. The ONLY way to get rid of the RF was to lift the shield wire off every mike connector at the amplifier. No grounding scheme worked, I even tried going to a water pipe.

Here's one you'll like. An Am station with a newsroom studio that I just could not seem to get RF out of the console. Studios were located at the transmitter site. This was an old RCA tube console. One day I decided remodel the studio and tear the old soundproofing tiles off the walls. It was the old tin tiles with holes and insulation inside. When I pulled the tin tiles off I discovered that someone had run ground straps behind all the tin I guess to try and make a big shielded studio. What they did was create a RF nightmare. Since the tin tiles just pushed into steel rails, over the years the tin and steel corroded creating thousands of diode junctions that detected the AM RF and fed right into the console ground system. Once all the tile was off the wall the RF problem was gone.

As a closing note, I've found that good engineering practice does not always work and what's right is what works.
 

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Posted by Steve Guttag (Member # 268) on 10-18-2004, 10:54 AM:
 
I have yet to find when good engineering practice did not work for noise elimination and the "what works is what is right" attitude tends to beget unstable systems.

Sorry, I don't buy it.

Steve
 

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Posted by Gordon McLeod (Member # 33) on 10-18-2004, 12:46 PM:
 
"I have yet to find when good engineering practice did not work for noise elimination and the "what works is what is right" attitude tends to beget unstable systems."
Also it not only make unstable but in some case dangerous situations athat also may contravene other laws
I have never found an instance where unsheilding improvement occurs that didn't indicate a bad grounding scheme to start with
again this is a case where a little knowledge could be dangerous if missapplied
if in doubt get back to basics first
 

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Posted by Edward Jurich (Member # 1787) on 10-18-2004, 11:28 PM:
 
Oh well, I suspect the original problem in this thread was figured out anyway and my unshielded pair has been working fine (and stable ) for several weeks now.
 

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Posted by Daryl C. W. O'Shea (Member # 1303) on 10-18-2004, 11:33 PM:
 
Maybe if you sheilded those wires you'd know that less than three days have passed. Weeks generally have at least 7 days in them, each!

Can't say it's hard to agree with Steve and Gordon.
 

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Posted by Mark Gulbrandsen (Member # 72) on 10-19-2004, 03:32 AM:
 

quote: Gord Mcleod

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Also it not only make unstable but in some case dangerous situations athat also may contravene other laws

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And here's proof he should have really been a trial attorney instead of a poet...... .

Mark
 

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Posted by John Hawkinson (Member # 1135) on 10-19-2004, 08:02 PM:
 
Looking back over this thread, I see Gordon said, "lamp dimmers should not be wired onto the same pahse as lowlevel components of the soundsystem."

I always get confused when this comes up. Can someone explain this to me, either theoretically or practically?

Is the above just shorthand for "on the same phase in the same panel," i.e. "with a conductor in common, such that a large load on the dimmers would result in a voltage drop to the lowlevel components"? Or is it something else?

I've heard people assert that worrying about phasing in common of devices is an urban legend...

--jhawk
 

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Posted by Gordon McLeod (Member # 33) on 10-19-2004, 09:10 PM:
 
In a perfect world sound systems would have there own tranfsformer to provide isolatedd clean power
dimmers should at least be on a different phase thow than sensitive devices
 

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Posted by Mark Gulbrandsen (Member # 72) on 10-19-2004, 10:11 PM:
 
Jhawk,
The reason is because dimmers typically are very noisy devices. The noise emitted from them can be spread both though proximity coupling and back through the AC line. Hence the need a seperate phase for the dimmers and located some distance from the nearest audio component in the system. Dimmers in a console with an integral sound system are sometimes problematic in relation to the dimmers. Seems like no matter what type of device they use to handle the load, SCR, Triac, SSR, ETC there are alot of by products generated. The quietest way to dim lights is with the old Variac type dimmer. There have been magnetic type dimmers but they are obsolete due to the economical and flexible nature of solid state dimmers.

Mark @ CLACO
 

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Posted by Edward Jurich (Member # 1787) on 10-20-2004, 09:32 AM:
 
The best thing is to have dimmers mounted as far away in the building from the booth as possible. Here for some reason, they could have done that but put them in the booth. But then for some reason they did not provide any ventilation in the booth and it gets hotter than h'll after a couple hours. OK, so they didn't know what they were doing
 

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Posted by John Pytlak (Member # 331) on 10-20-2004, 11:10 AM:
 
Light Dimmer Circuits:

http://www.epanorama.net/documents/lights/lightdimmer.html

quote:

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The modern thyristor (Triac or SCR) dimmer has one fairly severe drawback in its performance in that it dims by switching on the current to the load part-way through each mains cycle. Cutting the leading smooth-part off a mains cycle produces a current with a very rapid turn-on time which generates both mains distortions and EMI. Chokes are included in dimmers to slow down the rapid switch-on (rise time) of the chopped current. The longer the rise time the less EMI and mains distortion produced.

Turn on of the triac in the middle of the phase causes fast voltage and current changes. A typical thyristor/triac starts to fully conduct at around 1 microsecond time after triggering, so the current change is very fast if it not limited in any way. Those fast voltage and current changes cause high frequency interference going to mains wiring unless there are suitable radio frequency interference (RFI) filter built into the circuit. The corners in the waveform effectively consist of 50/60Hz plus varying amounts of other frequencies that are multiples of 50/60Hz. In some cases the interference goes up to 1..10Mhz frequencies and even higher. The wiring in your house acts as an antenna and essentially broadcasts it into the air. Cheap bad quality light dimmers don't have adequate filtering and they cause easily lots of radio interference.

Dimmer circuits typically use coils that limit limit the rate of rise of current to that value which would result in acceptable EMI. Typical filtering in light dimmers causes the current rise time (current rises from 10% to 90%) to be in range of 30..50 microseconds. This gives acceptable results in typical dimmer applications in home (typically this limitation is made using 40..100 uH coil).

If the dimmers are used in places where dimmer is a serious problem for sensitive sound equipments (theatres, TV-studios, rock concerts etc.) a slower current rise time would be preferred. Typically the current rise time in light dimmer packs made for stage applications have a current rise speed of around 100..350 microseconds. If noise is a big problem (TV studios etc.), even slower current rise times are sometimes asked. Those current rise times up to 1 millisecond can be achieved with special dimmers or suitable extra coil fitted in series with the dimmer.

The coil itself does not typically solve the whole problem because of the self-capacitance of the inductor: they typically resonate below 200 kHz and look like capacitors to disturbances above the resonance frequency. That's why there must be also capacitors to suppress the interference at higher frequencies.

If your dimmer circuit cause interference, you can try to filter out the interference by adding a small capacitor (typically 22nF to 47 nF) in parallel with the dimmer circuit as near as possible to the electronics inside the circuit as possible. Keep in mind to use a capacitor which is rated for this kind of applications (use capacitors marked with X). Keep in mind that the filter capacitor and it's wiring make a resonance circuit with certain resonance frequency (typically around 3.6 MHz with 0.1 uF capacitor). The capacitor does not work well as filter with the frequencies higher than the resonance frequency of the circuit.

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quote: Edward Jurich

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The solution was to disconnect the shield wire at both ends from the cell to the processor.

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