AB805 in-wall problems

larrymcg

I am using two AB805s as front speakers as part of a 5.1 system. A couple years ago the left front AB805 developed a rattle and I tried replacing the main drivers but that led to other problems. There is a long history at
http://www.polkaudio.com/forums/showthread.php?t=25387
The summary is that I acquired two MW7008 drivers but then discovered that my AB805s have MW7006 drivers (contrary to Polk documentation). When I tried the MW7008s (which look identical to the MW7006s) I got lots of distortion but no rattle. I finally settled by reinstalling the MW7006 drivers and disconnecting the driver that was rattling.

After two years of looking on eBay I finally got two, never installed, AB805s! I replaced just the left (bad) AB805 with a new one. A couple days later it started having lots of distortion. It sounded like the bottom driver was distorting. I reversed the left and right leads from the amp and the distortion stayed in the left speaker. I did lots of experimenting and what I accidentally discovered is that if I momentarily pushed lightly on the cone of the lower driver then it would stop distorting but some hours later it would start up again. I did the same trick a few times to convince myself that it was really happening.

Remembering that I had two extra MW7008 drivers, and verifying that the new AB805 truly had the MW7008s, I replaced just the bottom driver. So far it is working just fine (a few days use).

I guess I find it absolutely amazing that the replacement AB805 went bad a few days after being installed. If I listened to loud music I could understand but I listen to jazz and classical at fairly low levels and, of course, watch DVDs. Some DVDs have a pretty wide dynamic range so some loud sounds do happen.

My amp (receiver) is a Yamaha RX-V793 purchased (1998) when I bought the original AB805s. It's a 5.1 receiver with 80W per channel, Dolby digital, etc. I'm sure I'm not driving it into clipping. In any case I would expect that to kill the tweeter, not the main drivers.

Is there something else I should be looking for to make sure the speaker doesn't go bad again?

--Larry

wallstreet

I've got ab755's serving as the rear surrounds in my 7.1 setup. As is, they only get surround sound use as I never use them for music. I've never had any problems with the quality of the speaker. Then again, these aren't top flite in walls either. I picked mine because it's timbre matched, inwall, and it's all I could find. In regards to the amp, if your running anywhere near 12:00 on your volume control your too high. Make sure you're front speakers are set to small too.

larrymcg

I use ab755s for the rear surrounds too. The receiver volume is typically no more than 1/4 the way up, typically less.
I've been re-reading the receiver speaker setup info. It has speaker-level connections for the fronts, rears and one or two center speakers. It also has line outs (to go to an amplifier) for fronts, rears, center and subwoofer. Note there is no dedicated speaker level output for the subwoofer. If you set up for small fronts and small rears, all the <90Hz output goes to the line-level subwoofer output. So "small" is not a good choice for my setup because the passive Polk subwoofer is wired in parallel with the front speakers. Thus the fronts and the subwoofer get the full range signals. The passive subwoofer is from a Polk 3-way system and expects to get a full range signal. "Small" is probably OK for the rears and that's how I've had them set.

Also, the second set of speakers, the B speakers, consist of a Polk 3-way with two small free standing satellite speakers and a passive subwoofer (wired in parallel with the satellites). Since the B outputs are simply left and right and seem to get the same signal sent to the main front speakers, the only way to get a full signal to the B speakers is to have a full signal going to the main fronts. This also means "effects" need to be turned off to get a full signal to the B speakers. But that's OK since I only need effects when listening to TV or DVDs or VHS tapes, in which case no one is listening to the B speakers.

--Larry

larrymcg

Additional thoughs on my previous post.....

The subwoofer in the 5.1 system is a RM1000 connected in the usual way (in parallel) to the front speakers (ab805s). So the full range sound goes to both the subwoofer and the fronts.

Would it make sense to put high-pass crossovers in front of the ab805s to limit the low frequency stuff from getting to them? The idea being to protect them from low frequency stress. However, they are claimed to be full range speakers and should be able to handle the full signal.

If so, how would I find appropriate crossovers? It would be fairly easy to put in passive (mono) crossovers right at the rear of each ab805, if such were available. It would be difficult to wire a single stereo crossover between the subwoofer and the ab805s (wires in walls, etc.).

I previously mentioned the B speakers -- they are a RM3000 system including a RM1000 subwoofer. It's my understanding that those satellite speakers have crossovers built into them so they are OK.

--Larry

wallstreet

Honestly I think your speaker is broken. 1/4 volume shouldn't cause rattles. You can go to great lengths to try to deny this fact, but in the end, the speaker will still rattle.

If it were me, I'd dump the 805's and purchase a higher quality inwall and buy an active subwoofer so you can use the LFE output.

larrymcg

wallstreet,
I replaced the rattling 805 with a brand new 805 and it developed distortion problems after a few days use. I've now replaced a driver in the new 805 and it is doing fine so far. That's why I was considering a high-pass crossover to protect the 805s.

The problem with using an active subwoofer is that then the B set of speakers would get no low frequency signals.

I've also decided that putting speakers into the wall is not such a good idea because you can't find replacements that will fit the cutout. I suppose you could always get even bigger ones and make the cutout bigger.

New speakers, of better quality than the 805s, are quite expensive. As I recall the pair of 805s were more than $300. Not cheapies by any means.

--Larry