DB840 sub in LSI15's replace the stock 8"driver!!!!!

aggie113 · March 2019

No clearance issues with the MM842 either. Just need to figure which is better.... wish I had better ears

Did have to trim the spades off the wiring to use the spring connectors on the MM, was a tight fit getting them connected outside the box. Wasn't able to rotate to have the labeling match up, if anyone cares about that stuff.

mdschlayer · March 2019

Good to hear that the dimensional clearance is not an issue. After the new woofers settle in, let us know which you prefer and sounds better.

VR3 · March 2019

You guys can always mount the magnet on the outside!!

)))))

aggie113 · March 2019

I just went with two MM's. Have to return the other through Amazon so just figured I'd go with the higher end sub...

critical_gp · September 2021

Bringing this post back from the dead...

What specs are we looking at to make sure that these subs will work besides the physical dimensions.

Would these fit?

http://www.loudspeakerdatabase.com/Dayton/DCS205-4

Hansvelton · October 2021

VR3 wrote: »

You guys can always mount the magnet on the outside!! )))))

Actually, although I know you are kidding, that would improve deep bass to some extent.

The added "Volume" cabinet wise, with the area of the speaker and magnet reversed would help.
God only knows why Polk had to do with such a tiny bass enclosure.

I calculated it to be only about .7 cu ft. Most woofers this size, use at least 1.00-1.5 cuft.

I know the answer was a narrow enclosure, but not only did we lose deep bass, but it is not all that efficient either.

caardvark · January 2

I'm late to the party, being a new LSi15 owner. I appreciate the wealth of knowledge in this forum, that came in handy when researching various Polk options and whether to purchase the LSi's!

I don't dislike the low-end of the LSi15's at all, but I have a subwoofer handling much of that duty, so upgrading the 8" woofers for more of that mid-pass bunch that people have described here makes sense. My uncertainty is which drivers to select, with the DB840 being long gone...

It appears that either the DB842 ($70 ea) or MM842 ($104 ea) will fit, and both have the same VAS requirement. I figure it's worth paying a bit more for the better MM's, but I did read at least one person expressing the concern that they might not match the rest of the LSi as well as the DB, which is what Polk recommended as a replacement in the first place.

DB vs MM: 3-7/8" vs 4-1/16" top mount depth, 250W vs 300W, 84 dB vs 85 dB, 7" vs 7-1/8" cutout diameter, same 0.66-1.2 cu ft for both (target = 0.54).

The specs mostly favor the DB's. The MM's are a little deeper and a much wider magnet and basket, so I'm surprised they have the same VAS. Since both are a little over the ideal 0.54 cu ft, I'm thinking the more svelte DB's might suffer less reduced performance by the tiny enclosures?

Looking at other brands, there's a pretty limited selection of shallow-mount SVC's, so there's not much incentive to risk mixing tonal qualities.

BTW, reading through this and other old threads, I only ran across one other mention of woofer grills like mine, so I might as well add this to increase the odds of other people finding it: Mine are not the type that pry out of the wood trim. They are screwed into the back of it, so the wood must be removed from the tower. I find that rather difficult to do, as it is on too tight for plastic pry tools or putty knives. Screwdrivers were the only thing I could get enough leverage with. Fortunately, the slight marring of the finish that resulted gets covered back up by the wood.

txcoastal1 · January 2

@aggie113

caardvark · January 2

BTW, how do you follow a thread here to receive response notifications? I figured it must be automatic for anything you reply to, since I can't find the option anywhere, but I checked back in and see I didn't receive anything for the previous comment.

Edit: I clicked on the Bookmark icon alongside the thread title ... we'll see if that does anything.

Viking64 · January 2

As Maxwell Smart says: "I missed it by THAT much."

caardvark · January 2

Yep, that did the trick. I couldn't think what else the bookmark icon might do, once I saw it hiding up there (I expected a mouseover explanation of its purpose), so figured it was worth a shot.

VR3 · January 3

I mean... If you're single with no kids... Magnets on the outside could be an awesome look, just make sure you reverse the polarity!

Funny responses are 2 years apart... That's carrier pigeon speed

caardvark · January 3

VR3 wrote: »

I mean... If you're single with no kids... Magnets on the outside could be an awesome look, just make sure you reverse the polarity!

I've seen that proposed previously in one of the woofer upgrade threads, but assuming it's not purely a joke, is it really comparable push-pull sound quality? Low frequencies aren't totally non-directional, so I wouldn't think so. It would also be tougher to seal, as most mounting flanges aren't flat on top.

BTW, I might be contacting you at some point about crossovers...

VR3 · January 3

Absolutely... As long as it sealed to the enclosure the preformance would be the same... The trick would be routing the wiring while maintaining the seal.

Again just need to reverse the polarity of the wiring

gp4jesus · January 3

Read from the “OP to today” this am. My 2 cents because, unless I missed it, no one else speculated.

I believe the replacement driver’s bass “extension” is due to its lack of “boom.” The original was masking the same under use.
Original & replacement measure the same.

Another case against putting too much “weight” on “measurements” differing from performance.

Haven’t seen any db/mm842 observations save for @mm840lsi15. I wish he’d chime in w/more details.

caardvark · January 3

gp4jesus wrote: »

I believe the replacement driver’s bass “extension” is due to its lack of “boom.” The original was masking the same under use.
Original & replacement measure the same.

...which is my concern with stepping up to the MM842 and overpowering the balance of the lovely mids and highs. MM840 reviews go back to at least '09, so it was presumably available when the DB840 was determined by Polk's engineers to be the ideal upgrade. Implying they had that option on the table and decided against it.

On the other hand, maybe the MM makes the LSi15 a better all-around speaker if room EQ is available to account for the extra low end impact, but Polk didn't want to recommend something that had any such dependencies.

la2vegas · January 3

Luckily I still have a few DB-40's stashed away.

skrol · January 5

I attached a spreadsheet with small parameters or various Polk Audio car subs. Maybe you can match something fairly close to the db840. There was some conflicting info from various sources on the old Polk Audio site. I tried to note them in the spreadsheet.

caardvark · January 5

skrol wrote: »

I attached a spreadsheet with small parameters or various Polk Audio car subs. Maybe you can match something fairly close to the db840. There was some conflicting info from various sources on the old Polk Audio site. I tried to note them in the spreadsheet.

Thanks. I think I saw your posts elsewhere with that breakdown, along with the inconsistent data sources.

I went ahead and ordered two DB842's yesterday, and will report back with impressions next week. (The MM842 was just too much of an unknown, from an engineering intent standpoint.) I lowered my sub's crossover from 80 to 60 Hz, to lean more heavily on the LSi15's musicality. That left it feeling a little thin in the mid to upper bass range, so it should provide a good before & after test with the new woofers.

caardvark · January 9

After waiting for the mail to arrive quite late, I open the box to find that Amazon sent a DB842 and a MM842 instead of two DB842's. What are the odds, given the above dilemma of which of the two to go with?!

Now I have to decide which one to install... Or do I install both and answer the head-to-head question we're "all" wondering, then hope there's no hassle returning the one I choose against, seeing as it was their fault sending me a mismatch Qty 2? Either way, it'll sort of suck going back to, or staying, stock on one tower while waiting a week plus for the replacement.

caardvark · January 9

Well, I decided to open up the DB842 and install that one, then make up my mind about the MM. I quickly ran into a problem... The stock woofer and cutout measure 8-3/16", but the DB is 8-3/8". That's the dimensions of the metal basket, not a rubber surround like people have discussed with the old DB840.

I don't see any way the DB842 can be made to fit without modifying the cabinet.

txcoastal1 · January 9

Well we know why we didn’t get a response before

Before opening the MM may want to see if Polk can verify or send you the dimensions for the driver

Sorry to hear

caardvark · January 9

txcoastal1 wrote: »

Well we know why we didn’t get a response before

Yeah, so much for trusting things you read on the internet.

caardvark · January 9

I came up with one semi-plausible explanation for the discrepancy: As mentioned a few posts up, my grills are the apparently less common type that screw onto the back of the wood side trims, rather than being pressed in from the outside. It seems unlikely that that setup would correspond to a different size cutout (and presumably different size woofer, unless it's a loose fit in a bigger cutout on others), but it would be interesting to see cutout measurements from people with the regular grills.

txcoastal1 · January 9

Open a new thread, may get some attention
Interesting

caardvark · January 10

txcoastal1 wrote: »

Open a new thread, may get some attention

Will do. In the meantime, I took @VR3 's advice and tried out an inverted woofer mount. I had some old Sounstream SS8's lying around that just happen to be 4 ohm SVC, the exact same diameter as the stock driver, and 4 of their 8 holes line up. They just needed a little outward ovaling of the holes to hit the cabinet's holes.

Since I had the stock woofer out already, and had taken the pedestal off to make it easier to push out, it was trivial to extend the wires out through the port for a trial. I didn't want to conclude the day empty handed.

The improvement is quite pronounced in the chest slam region, even though it isn't sealed at all. (There's actually quite a bit less air coming out of its port than there is on the other tower with stock woofer. The rest is probably leaking around the thin, raised lip that has no gasket.) It's also a bit small of a cabinet for the SS8, even without their driver displacing VAS. I have to think an inverted mount of a properly sized driver, and a good seal, would be quite a winner! I also think the stock woofer must have been a standard O.D. for its day, given that the Soundstream is an exact match.

Also, here's a direct overhead shot (as opposed to a misleading angled perspective) of how minimal the overlap is between the inner cutout (baffle) and port. It would take a plumb bob to get an actual measurement, as I'm guessing it's less than 1/8". So, any deep magnet'd driver will drop in just fine with no overlap. VAS is the only issue there.

caardvark · January 10

Spinoff thread: https://forum.polkaudio.com/discussion/199706/lsi15-woofer-sizing/p1

caardvark · January 13

Here's a better indication of what the inverted sub mount looks like on the Right speaker, with wood trim in place (the Left's wood trim is sun faded and needs some stain applied):

I'm liking that approach, but the SS8 subs aren't particularly high power handling, so they're probably not a long-term solution. So, I decided to keep the MM842 and figure out how to make it work... (The DB842 appeared to have been an Open Box item, sold as New -- no tape on the outer box, tape removed and set aside on the inner packaging -- so I felt that one should be returned, regardless. They managed to screw up both halves of that order.) Its beefy all-black basket will also look a lot better inverted.

I measured the depth of the baffle and it's exactly 0.25", which makes it easy to match without planing. As soon as the weather clears, I'll go look for supplies to make a trim ring that depth. My router circle jig should be able to get the inner and outer diameters right.

caardvark · January 13

Whoops, I forgot the co-reason for posting an update. The day after deciding to keep the MM842 and opening the box to verify it is 8-3/8" OD (same as the DB842), I heard back from Polk regarding the dimensions. Maybe it's my fault for expecting some basic level of technical proficiency, but I was pretty disappointed to see their response was 7.2", highlighting the inner cutout (basket clearance) value on the spec sheet. You'd think anyone in the biz would know that the outer diameter can't possibly be less than the 8" driver size.

pitdogg2 · January 13

Maybe ask for Kim Jasper, he's an original and knows his stuff.

DB840 sub in LSI15's replace the stock 8"driver!!!!!

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