[MLStrand56]

I’m designing a custom TV cabinet with built in Center Ch. Speakers under the TV. Since this cabinet is for a CRT TV, I either have to use shielded speakers (hard to find) or shield the inside of the enclosure [12” H x 36” W x 24” D] (easy but Expensive!).

I’ve already investigated using Mu-metal for magnetic shielding, but that would cost $400+ just for the Mu-metal & therefore out of my price range.

I want to use the following speakers:
1 x 8” Shielded Dual Voice Coil Woofer
2 x 6” Shielded Midranges
2 x Shielded Dome Tweeters.

These will be wired as a Stereo pr. (with 2 x 3-way X-over) & have a Stereo/Mono option at the input cup. Reason for Stereo pr. wiring is that my center Ch. Amp is a stereo Yamaha MX-1000U (330 W RMS/Ch, Class A). So my center Ch. Speakers have to be able to handle a LOT power.

Please don’t tell me how Center Ch. Sound is mono, I already know that. I also know about the freq. range of the center Ch. Signal. Also please don’t start telling me to buy a new LCD TV, unless you’re willing to pay for it. My small pension won’t allow for a new flat screen TV.

I’ve already found some Excellent shielded & high power tweeters at Parts Express ( http://www.parts-exp...number=264-1012 ) but they no longer carry shielded midranges or woofers (DVC or other).

So does anybody know a good source for GOOD High Power (min. 100 Watt’ish) shielded speakers, OR a good way to shield the inside of the speaker enclosure?

Thanks

MLStrand56

[mjd420nova]

Even shielded speakers in close proximity of the huge magnetic field from a CRT will emit a HUM. I've done a few extra steps to add more shielding inside the speaker case. Lines with foil and grounded will help to kill the hum. I like to use spray on adhesive to tack the foil in place. All foils should be connected together with short wires and soldered. Grounding to the case may not be the cure as many CRTs unless very old will have plastic cabinets. A seperate ground would have to be provided. This may not cure it entirely as there is no way to shield the face of the speaker and this would allow the magnetic field to enter the cabinet from the front. This in no way can have as much effect as some distance would. Maybe rethinking the placement of the speakers would negate the need for shielding.

[MLStrand56]

mjd420nova, on 24 January 2012 - 01:42 PM, said:

Even shielded speakers in close proximity of the huge magnetic field from a CRT will emit a HUM. I've done a few extra steps to add more shielding inside the speaker case. Lines with foil and grounded will help to kill the hum. I like to use spray on adhesive to tack the foil in place. All foils should be connected together with short wires and soldered. Grounding to the case may not be the cure as many CRTs unless very old will have plastic cabinets. A seperate ground would have to be provided. This may not cure it entirely as there is no way to shield the face of the speaker and this would allow the magnetic field to enter the cabinet from the front. This in no way can have as much effect as some distance would. Maybe rethinking the placement of the speakers would negate the need for shielding.

My current design places the speakers within their own enclosed space directly below the TV. Using double 3/4" plywood under the TV. Since the TV space is 48" wide, I will need to use locally avail. angle steel to prevent bowing from the weight of the TV.

Since the speakers are designed to be Center Ch. I'm kinda limited on how far from the TV they can be, & still sound like the sound is coming From the TV. I'm still open to suggestions though.

Yesterday I actually went to a Hi-End (for Philippines) TV store & measured 46" HDTV's (130,000 Peso / $3,095) . With that information I decided that I need to build the cabinet big enough to (later) accomodate a 46" HDTV. So the cabinet has grown from 36" to 48" wide. That gives me a couple of inches to play with on either side. So now the center speaker enclosure has grown to 48" wide, x 12" high x 24" deep. That's plenty big enough to house an 8" DVC woofer + 2x 8" Midrange + 2x Tweeters.

I've also found a Good speaker manufacturer (HiVi, http://www.swanspeak...ucts.aspx?cid=9 ) that still manufacturers shielded speakers. For extra piece of mind, I'm also considering a sheet of 1/8" to 1/4" sheet steel, inside the top of the speaker enclosure. I've read that it will help because the magnetic lines of force, are automatically directed through the steel & not radiated up through the 1.5" plywood & into the TV. Any ideas on that?

As to grounding, that's a Major Headache in Philippines. There is NO AC GND anywhere. That's why everything gives me a shock. There isn't even an enforced electric code here. Electricity is 220V, 60Hz, Ungrounded, on a old USA style 2-prong plug. Now that could be 220V single phase, OR I've also seen 290V on the hot lead & 70V on the 'neutral' lead!!!! I will have to install my own GND system, when I build my own house. However your idea to GND the shield is well founded. I hadn't thought of that. I think I'll include a GND lug @ the back of the speaker enclosure & tie it to the steel plate shield. At least that way I can experiment with Grounded vs. UNgrounded shield.

I'm using Google SketchUp to design the cabinet. It worked GREAT for my custom Super Speakers, so I thought I'd use it again.

[mjd420nova]

Things can get real dangerous quickly if speaker phasing and polarity are not adhered to without a grounded system. It would be possible to get full voltage potentials between components and ground loops will burn up speakers in a flash and chaos would insue with amplifiers and such burning out. Fuses could prevent serious damage but loops are often ahead of any protection and wall fuses or breakers don't react quick enough to prevent smoke signals and lightning.

[MLStrand56]

mjd420nova, on 26 January 2012 - 08:23 AM, said:

Things can get real dangerous quickly if speaker phasing and polarity are not adhered to without a grounded system. It would be possible to get full voltage potentials between components and ground loops will burn up speakers in a flash and chaos would insue with amplifiers and such burning out. Fuses could prevent serious damage but loops are often ahead of any protection and wall fuses or breakers don't react quick enough to prevent smoke signals and lightning.

This isn't my 1st. trip around the block. I've been doing Hi-End systems for 30+ yrs, so I know what the RED terminal is for. I've fought Gnd Loops in Belgium, Germany, Japan, USA, & now Philippines. In most cases, it's faster to just put down my own Gnd rod.

The harder prob. is getting 30 amp service to the Power Amp cabinet. 20 amp AC will run my PA's, but ONLY if they are turned on seperately. Turning all my PA's on at once usually creates a Surge Current that blows a 20 amp ckt. breaker.

MLStrand56