On the road to recovery, Sub problem almost fixed.

SharkyRivethead · « **on:** 9 Dec 2009, 01:36 am »

I had asked before about changing an Ohm rating for a sub. To my dismay, Boston Acustics have no specs on their own stuff. It's unlikely that they will tell me who manufactures their speakers. Could I hook up a multi meter to my sub speaker to find it's nom Ohm?
I pulled the number 300-BFREEDOM-250 off the sub, but can't find anything on the web for it. After this I will stop pestering you folks. =)

richidoo · « **Reply #1 on:** 9 Dec 2009, 02:47 am »

Hey Sharky! You're not pestering, that's what the lab is for! You are most welcome here.

I assume you are wondering if the amp in the BA box can handle the load of the new driver, right?

To find out for sure, you have to measure the technical specs of the old driver, then make sure the new driver's specs are similar or better (easier on the amp.) To do this you need a tool like Woofer Tester.

If the new driver has similar or lower Fs, similar or higher sensitivity, and similar or higher impedance in the frequencies you will be using (<200Hz) then you are safe to swap in the new driver.

The real question is how stout is the amp? 200W is not very much for a subwoofer that will play loud and very clear, but a HTIB is going for big rumble, not precision, so it is probably using an easy to drive but sloppy driver. You can suspect that a HTIB amp rated for 200W is really more like 100W continuous duty. So you need a very easy load driver.

Kevin Haskins · « **Reply #2 on:** 9 Dec 2009, 03:04 am »

Quote from: SharkyRivethead on 9 Dec 2009, 01:36 am

I had asked before about changing an Ohm rating for a sub. To my dismay, Boston Acustics have no specs on their own stuff. It's unlikely that they will tell me who manufactures their speakers. Could I hook up a multi meter to my sub speaker to find it's nom Ohm?
I pulled the number 300-BFREEDOM-250 off the sub, but can't find anything on the web for it. After this I will stop pestering you folks. =)

If you have the driver removed, take a VM and measure the resistance across the voice coil. That gives you a DCR (DC resistance) and from that you can get a fairly good idea of what nominal impedance it would be rated at.

Typically at DC, the resistance is slightly lower than the nominal impedance rating. So a 8-ohm nominal transducer would have a DCR of around 6.5 Ohms. This only works for transducers, not loudspeakers with networks connected to them.

richidoo · « **Reply #3 on:** 9 Dec 2009, 03:07 am »

The driver you want to use
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=295-414
Has 88dB sensitivity 4ohms and Qts of .44. Is this going into a ported box, or is it sealed?

It's a complicated process to know how well the amp will handle it, depends on how precise you want it to sound and how loud you want it to play, and how big is the box, and.... on and on. It's kind of expensive driver so I hate to say go for it and then you start blowing fuses everytime you turn up the volume because the driver need more power to play loud enough.

This kit has your desired driver, mated with a 240W amp. It's tough to know how true the 200W rating is on your BA amp. But this makes me a little more at ease to try the Titanic.

This woofer is cheaper, and easier load for the amp. But it may not have the same detail or control as the titanic under optimum conditions (enough power, correctly sized box) But it will probably be better than the original BA driver. I am just an amateur speaker dork, so no guarantees on my advice.

Actually changing the impedance of the Titanic driver is not practical. Better to choose a driver with the correct impedance.

SharkyRivethead · « **Reply #4 on:** 9 Dec 2009, 11:41 pm »

Thanks Richidoo, thanks Kevin.

Great minds think alike. I just bought that MKIII sub from parts express yesterday. I had a good laugh because I know when my friends talk about their swell HTIB, I kinda roll my eyes. But here is a link to the actual product...
http://www.bostonacoustics.com/Avidea-770-P269.aspx

The sub amp is very well made. I took a look at it and there is nothing cheap about it construction nor the driver. Dual magnet, rubber surround, etc. I'm not as knowledgable as some folks. But can spot a well made speaker or amp. In any case I put the BA driver in another sub box where I had just replaced the amp with this
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-805
I wanted to test the amp with a better driver. The original was a crappy KLH driver. I know I should put the BA back with it's original stuff....but then what kind of audio tweeker would I be? Besides, like I mentioned before. The BA amp has no controls and I feel as though the amp is to much. Because when there is a paticular low LFE signal the driver really rattles. I thought if I added something beefier (driver) that it might help this problem. Is this the wrong way of going at it?
Also. the BA box is vented. Dual ports, venting downwards.

richidoo · « **Reply #5 on:** 9 Dec 2009, 11:53 pm »

The amp is not too big. Watts describe power, not volume (voltage.) You control the voltage of the signal to the sub so you control the volume not the amp. For a given volume level, a more powerful amp can deliver more current into the driver to control it and avoid clipping. You want a reserve of power to handle dynamic peaks when the volume is loud. The driver can handle 400W continuous. Since it seems like a decent amp you should have a nice combo. If you're eager beaver you can use a simulator like WinISD to simulate the new driver in your box/ports. Maybe the box volume or ports can be tweeked a little to optimize with the new driver. There might have been something wrong with the driver that rattled, or loose mounting screws.

SharkyRivethead · « **Reply #6 on:** 10 Dec 2009, 01:08 am »

When you say tweek the ports, are you saying make them bigger? I know I never really understood their purpose as to speakers that are not ported. Till I took the feet off my sub and had it resting on the floor. (I wanted to make it fit somewhere.) I understood very quickly why it was ported. =)
I thought that it might be a good matching. I guess then it comes back to the Ohms. I was looking at the BA sub and it does have a safety reset switch. Nice to know in case I do something wrong.
I do hope it works, because I like the way I have my stuff set up right now. I don't know if I mentioned this, when I bought the setup it was the speakers only. It was a discontinued item. I bought the 6.1 speakers without the dvd system. I only paid 200.00 for the 6.1. I gave away the sat speakers because they have been replaced with some B&Ws....paid 10.00 at a garage sale. (I'm real good like that) yeah, I could not pass up on the deal for the 6.1 but in the end I kept the sub.

SharkyRivethead · « **Reply #7 on:** 6 Feb 2010, 05:43 pm »

Just an update...

I took the sub and amp completely apart. I ran a DMM with an open Ohm reading across the subs driver of the Boston's subwoofer. It comes in at 6 Ohms. Then I checked the other woofer, the one that was a cause for all this. It came in at 4 Ohms. So, I did blow something on the amp. It was an IC TDA7293 (monolithic integrated circuit) It was completely blown, fried, roasted....you get the point.

Because the subs amp is multi channel. Being a HTIB setup type. There are 8 of these IC's on the board. 6 for the 6 channels running through the amp and 2 for the sub. (1 for the pos lead and 1 for the neg lead) Since I'm not running any other speaker through the subs amp. I was wonder if I could just rob the board of one of the other TDA7293 IC's?

Other than this isolated problem. There was no other visible problem with the amp. I checked the resistors with a DMM, followed the trace lines, spot checked the caps and solder points. Everything looks good up to this point. I'm most likely going to buy the IC from mouser or digi-key. I just wanted to throw the idea out there.

http://www.mouser.com/ProductDetail/STMicroelectronics/TDA7293V/?qs=uepKQD%252bspxBrLGmEx5TjhQ%3d%3d

(edit)
Since there are 2 of these for the sub portion on the amp. Does the 2 add up to 8 ohms and then they break it down to 6 ohms via the board? I don't imagine it would be that simple. But it's also got me thinking about the 6 that are not being used on the board.