Polk LF14 Conversion to Car Subwoofer

The Raven

Perhaps this is considered heresy, but I thought I would share the photo story of our project to convert an old Polk LF14 bass module to a subwoofer for a 1989 Ford Ranger pickup.

http://picasaweb.google.com/boatguy3800/Subwoofer?authkey=Gv1sRgCP6q5NDaycfhxAE#

comfortablycurt

Wow. I've never seen/heard of anyone doing that before.


I don't know if I'd consider it heresy. A little bit probably.

It seems to me that you could have gotten much better results using a couple of 8" subs...or possibly something like a Bazooka Tube...but I'm not a car audio guy.



On another note...what the hell is this LF14? I'm looking at the XO's...it plainly says "14" on the sticker...and it also has two pairs of binding posts and a couple switches on the back. Is this a Monitor 14? I've heard these words mentioned around the web before...but I've never actually seen one. Looks like it's basically an RTA12, with a non-attached HF module.


I hate to say it...but I think you've destroyed an extremely rare part of Polk history here. Hopefully you didn't scrap the cabinets/XO's. I'm talking like seriously...extremely rare. I always thought that the concept of a Monitor 14 was a misprint on polksda.com. Does this mean that there is also a Monitor 15T?

skrol

I agree. It is a waste of a rather rare Polk speaker and I think they could have achieved better results with equipment suited for cars. It will probably work until he hits a good bump and the magnets shift and jam the cones. Personally I think it is a waste.

The Raven

The LF14 was sold as a low frequency module intended to pair with a smaller set of bookshelf speakers -- one of the earliest sub-sat designs I am aware of. It has a built in low-pass x-over but was not designed to provide any high-pass signal to the satelites. You just run the sats on speaker A from your stereo and the LF14 from the speaker B outputs. It has four binding posts to take in a stereo input. It could also be modified to operate in a system with a pair of LF14's. In that case one would be left and the other right. A switch setting on the back and bridging two of the binding posts achieves that. The other switch is for "efficiency", changing the point of x-over. There is an old scan of a manual floating about somewhere on these forums.

The changes we made are completely reversible -- assuming the drivers aren't damaged as skrol points out -- something I'll admit I didn't consider. I did research these forums during the design phase and concluded nobody seemed to value the LF14s much. They don't sell for much, when up for sale on ebay.

I've been a faithful Polk consumer for over 25 years and am still operating my RTA12Cs, but I value them primarly for their utilitiarian use. This particular unit had little value to me as it was.

comfortablycurt

All the rareness/heresy aside...how does it sound? These vintage Polk's are capable of some pretty prodigious bass output...so I'd imagine it sounds pretty decent. That looks pretty similar to the low frequency section of my SDA 2A's...which are capable of some awesome bass output.



As Skrol mentioned...these are NOT the ideal drivers for this application. The vintage Polk MW's have a long history of having the magnets shift/fall off during shipping/transport. If you do plan on keeping them in your truck...you are going to at least want to take the MW's out of the enclosure and JB Weld the magnets to the spiders...or else eventually you'll likely end up with a couple of junk drivers. This is a worthwhile mod/security measure to perform on any vintage Polk speakers...including your RTA12's.



I know very little about these, aside from simply hearing them mentioned a couple times. Do you happen to know the LF14's rated frequency range? What about the crossover frequency on the low pass filter? It looks, essentially, like a Monitor 11 or SDA 2 with a shortened cabinet, and no tweeter. Or, if you prefer, an RTA12 with no HF module.

Do you know what the model numbers of the midwoofers and passive radiator are? 12" passive radiator, yes?

Do you happen to know the rough dimensions of the cabinet? It looks like about 14" wide, 28" high and 12" deep...or somewhere in that area anyway. Am I close?

I'd love to pick up one(or a pair) of these eventually...very cool Polk history here. If they can dig down low enough...it would be cool to use one(or a pair) as a "sub" with some smaller Monitors or SDA's...I think a pair of these would look very nice with my Monitor 7A's on top of them...:cool:

skrol

Raven
Welcome to Club Polk. You may be right about the value. If they are of no use to you in their original form, too bulky to store and no one wants them, why not utilize them in another application. How do they sound?

Strongly consider securing the magnets with some JB Weld though. I had older (1992) Polk Mobile Monitor magnets shift after going over a large bump. I just picked up some JB Weld to do my RTA11TL's and Monitor 5A's. If you search the forum you will find instructions.

Stan

leroyjr1

I have a LF14 bass module and I thought the performance was a joke. It doesn't even compare to Polks entry level sub the psw10. It's basically is RTA 12 without the tweeter and a different crossover.

Nice job on the conversion.


Curt, mine came with MW6500 mids and I think it's a 12 PR.



.

skrol

Curt
I couldn't find specs but did find a manual for the LF14.
Stan

RuSsMaN

That's cool!

Looks like the 'grill' is too close to the drivers, especially the PR.

comfortablycurt

leroyjr1 wrote: »

I have a LF14 bass module and I thought the performance was a joke. It doesn't even compare to Polks entry level sub the psw10. It's basically is RTA 12 without the tweeter and a different crossover.

Nice job on the conversion.


Curt, mine came with MW6500 mids and I think it's a 12 PR.



.

Really? That bad? I can't speak from personal experience, but I almost find it hard to believe that they're that bad. This is essentially the same low frequency section as my SDA 2A's, which are capable of some bass output that's almost scary at times. They also use the same MW's as my 7A's(6500), and the 7's are capable of some pretty good bass output on their own. These LF14's each have two MW6500's, compared to my 7's single 6500...plus a 12" PR, as opposed to the 7's 10" PR. Do you think one or two LF14's would work well with a pair of Monitor 7's? As I said, the 7's have good bass output on their own, but they definitely leave a lot to be desired compared to my SDA's. The OP mentions that a 30hz test tone came through loud and clear from the enclosure in the truck. I'd imagine their LF output in their original cabinets would have to be fairly close to that.

Like I said...I've never heard them, but I can't help but find it hard to believe that a PSW10 would sound better...the vintage Polks have some of the tightest, cleanest bass I've ever heard. I'd love to get my ears on one.

If you're ever interested in selling your LF14, let me know, Leroy. I may be interested.:)

skrol wrote: »

Curt
I couldn't find specs but did find a manual for the LF14.
Stan

Thanks for the link...some interesting reading there.

These LF14 modules are really grabbing my attention...

I'm planning on eventually putting together a secondary vintage rig with some vintage Marantz separates, utilizing my Monitor 7A's...I think a pair of these would go great with that rig!



All the sound quality aside...this is some pretty cool Polk history!!

I'm guessing that these LF14's are the "Monitor 14" that's mentioned on this page...

http://www.polksda.com/list.shtml

???

Anyone have any idea if the "Monitor 15T" that's mentioned there ever existed?

If the Monitor 12 had a 12" PR, and four MW's(reminiscent of the SDA 1C, minus one tweeter)...would this mean that the Monitor 15 would have had a 15" PR with eight MW's!?(ala SDA SRS, with no SDA effect)

The Raven

...and I'm leaving now for the store to pick up some JB Weld.

It is hard to tell the grill clearance from the photos. From a side view when driven hard, there seemed to be plenty of clearance for the drivers and PR. Something I am keeping an eye on.

The original LF14 dimensions are noted on the enclosure drawing in the link (16x28x11 outside dimensions).

I don't know the driver numbers but I'll note them when we do the JB Weld. The hole we cut for the PR was 11 inches in diameter.

How does it sound? I'm very happy with the sound. As I mention in one of the photos the 30 HZ test signal was reproduced "loud and clear". I haven't listened much with my choice of music but my son is very happy (and suprised) w/ the bass reproduction. We kept the enclosure volume as close as we could to the LF14. Even though the shape of the enclosure is different, my hope was to benefit from the enclosure/driver design that Polk did.

One of the shortcomings of the LF14 was lack of a high-pass filter to pass the highs w/out the lows to the sat speakers. At the time the LF14 was being sold, home theatre surround systems weren't popular. The car amp we use works around that shortcoming.

skrol

comfortablycurt wrote: »

...I'm guessing that these LF14's are the "Monitor 14" that's mentioned on this page...

http://www.polksda.com/list.shtml

???

Anyone have any idea if the "Monitor 15T" that's mentioned there ever existed?

If the Monitor 12 had a 12" PR, and four MW's(reminiscent of the SDA 1C, minus one tweeter)...would this mean that the Monitor 15 would have had a 15" PR with eight MW's!?(ala SDA SRS, with no SDA effect)

Curt
I am guessing that the Monitor 15T (and possibly the Monitor 14) became known to us as the RTA15TL as they were released in 1991 also. Or it was branded that way special for the military. I happen to know the guy who did much of the design work on the RTA15TL. He filled me in on some of the marketing twists that went into the name.

Sorry, I know I posted this before but it seems fitting to the discussion.
Stan


"...I guess marketing changed the name later to TL. ... Original RTA8 and RTA11 had different tweeter ... new tweeter SL3000 came up just before I started design on RTA15T. So RTA15T started with tri-laminate tweeter ... but 8 & 11 went from T to TL when they were upgraded. Your assumption is correct RTA15T and RTA15TL are the same.

RTA15 was originally being scheduled and introduced as RTA14, I worked on it as RTA14 ... we had some literature printed ... when one day national sales manager came and said stop, stop, can't call it RTA14 ... he learned from Asian distributor that number "four" means in Chinese "death" and that we wouldn't sell any speaker ... (after RTA11 we didn't want to call it 12 because of Monitor 12, didn't want to call it 13 because of western superstition, couldn't call it 14 because of Chinese superstition and the next number was 15)"...

The Raven

The only markings on the drivers from the LF14 is an "M14" on the basket. There is a label on the back of the driver but it doesn't have any writing except a quality control signature.

Reading all the posts about JB Weld, it looks like there are two potential areas to secure.

1) Around the top and bottom of the magnet to secure the magnet to the front and rear plates.

2) Between the front plate and the basket where they meet (if the basket and front plate are not riveted together).

When I shine a flash light through the spider, I see four round shiny metal things that look like the head of a flat head screw. Does that mean that the front plate and basket are riveted together? I would say yes, except the slots that look like the slot in a flat screw head thows me. Are they machine screws or rivets?

The Raven

Grills were to close. When we removed the drivers to JB Weld them, we had to change out the grill clips and put in ones with longer spacers for more clearance.

mhardy6647

The LF14 came a long time after the M&K "Goliath" subwoofer paired with the Visonik "David" minispeakers. The earliest sort of purpose-built subwoofer I know of was the Heathkit SS-1B module, which enhanced the LF and HF of the simple two-way shelf-ported SS-1 bookshelf speaker. The SS-1 was quite similar to the Jensen "Duette" speaker of the same era (mid-1950s).

http://www.heathkit-museum.com/hifi/hvmss-1b.shtml
http://www.heathkit-museum.com/hifi/hvmss-1.shtml



The Acoustic Research AR-1W (just the acoustic suspension woofer portion of the seminal AR-1 two-way sealed box system) was available by the late 1950s and would still make a decent "subwoofer" for hifi 2-channel systems.

The LF14 is indeed probably not too competitive with modern subwoofers (active nor passive).
http://www.polkaudio.com/forums/showthread.php?t=19965&highlight=LF14