Need to shield my Seas 27TBFC/G - should I use a bucking magnet?

owdi · 19th August 2006, 07:11 AM

If I place the Seas 27TBFC/G tweeter within 12 inches of my CRT monitor, it starts distorting the picture. I want to add enough shielding to be able to place the tweeter within 6 inches of the monitor.

From doing searches, the options seem to be bucking magnets and iron cups. A few posts mention either of these methods can change the behavior of the driver, but no details about how.

Parts Express is having a buyout on bucking magnets, so this seems like a good choice. However, I have no idea which one to use. Should it be the same size as the 80mm magnet on the Seas tweeter, or smaller? Any ideas on how it will change the behavior of the tweeter?

Dan

sreten · 19th August 2006, 02:47 PM

Hi,

Approximately 50% to 70% of the size of the main magnet. But
note this is by weight, not linear dimensions, a magnet with 90%
dimensions of another is 70% smaller, 80% is 50% smaller.

Adding it will increase BL by ~ 7%, increasing output by ~ 0.5dB.

/sreten.

BobEllis · 19th August 2006, 03:40 PM

The C in 27TBFC/G means chamber. It means that your bucking magnet might not be quite as effective and will likely have less impact on output. Removing the chamber will drastically change the response - don't do it.

A 27TBFC/G messes up my 19" monitor at about 8" above it, while putting two back to back I could move in to around 4". I have 27TDFC in a metal cup that seems quite effective - 2" from a 32" TV and no interference. The metal cup must be thick to be effective. I bought mine from zalytron for $15 - designed to fit Focal drivers, it was called a "Silent Shield" IIRC. Not sure if he still has them.

sreten · 19th August 2006, 04:12 PM

Quote:

Originally posted by BobEllis
The C in 27TBFC/G means chamber

Hi,

Didn't even think to check this, good spot BE.

At distance you need a smaller magnet to cancel the
residual magnetic field but you also get more leakage.

You could try a steel plate in the side of the box.

/sreten.

owdi · 19th August 2006, 05:50 PM

I have to order some other parts from PE, so I will try the bucking magnet first.

The magnet on the 27TBFC/G is 72mm diameter and about 14mm tall. The cup is 20mm tall, so I'm going to try the 60mm x 32mm x 8mm and 60mm x 25mm x 8mm magnet from PE. At less than a dollar each they are very economical.

Dan

19th August 2006, 07:11 AM	#1
owdi diyAudio Member Join Date: Sep 2003 Location: Bellevue, WA	Need to shield my Seas 27TBFC/G - should I use a bucking magnet? If I place the Seas 27TBFC/G tweeter within 12 inches of my CRT monitor, it starts distorting the picture. I want to add enough shielding to be able to place the tweeter within 6 inches of the monitor. From doing searches, the options seem to be bucking magnets and iron cups. A few posts mention either of these methods can change the behavior of the driver, but no details about how. Parts Express is having a buyout on bucking magnets, so this seems like a good choice. However, I have no idea which one to use. Should it be the same size as the 80mm magnet on the Seas tweeter, or smaller? Any ideas on how it will change the behavior of the tweeter? Dan

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19th August 2006, 02:47 PM	#2
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	Hi, Approximately 50% to 70% of the size of the main magnet. But note this is by weight, not linear dimensions, a magnet with 90% dimensions of another is 70% smaller, 80% is 50% smaller. Adding it will increase BL by ~ 7%, increasing output by ~ 0.5dB. /sreten.

19th August 2006, 03:40 PM	#3
BobEllis diyAudio Member Join Date: Sep 2003 Location: Upstate NY	The C in 27TBFC/G means chamber. It means that your bucking magnet might not be quite as effective and will likely have less impact on output. Removing the chamber will drastically change the response - don't do it. A 27TBFC/G messes up my 19" monitor at about 8" above it, while putting two back to back I could move in to around 4". I have 27TDFC in a metal cup that seems quite effective - 2" from a 32" TV and no interference. The metal cup must be thick to be effective. I bought mine from zalytron for $15 - designed to fit Focal drivers, it was called a "Silent Shield" IIRC. Not sure if he still has them.

19th August 2006, 05:50 PM	#5
owdi diyAudio Member Join Date: Sep 2003 Location: Bellevue, WA	I have to order some other parts from PE, so I will try the bucking magnet first. The magnet on the 27TBFC/G is 72mm diameter and about 14mm tall. The cup is 20mm tall, so I'm going to try the 60mm x 32mm x 8mm and 60mm x 25mm x 8mm magnet from PE. At less than a dollar each they are very economical. Dan

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