Need Assistance with Polk SDA-SRS 3.1TL

decato

Hi everyone,
I bought a pair of SRS 3.1TLs in December and finally got around to trying them out. It turns out I have two unmatched speakers. One has a serial number on the back of 1099 without many mentioning that it is a 3.1.TL. That's only the beginning of the differences.

One has a 2.0 ohm resistor and 25 watt resistor in series with dimensional array, as seen in this thread: http://www.polkaudio.com/forums/showthread.php?124267-Should-I-remove-resistor-3.1tl
Interestingly, the 18.0 mH inductor is made out of 20 gauge wire.

The other channel matches the schematic. As expected, the 18.0 mH inductor is made out of 22 gauge wire, and there is no resistor in series with driver. Using spare parts I have, I can find a way to make both channels the same, so I'm not too concerned there.

The problem is the 12" passive radiator is also a prototype. It is marked BD1212-A, and the date of assembly predates the schematic for the 3.1TL. It does does not have "Polk Audio" stamped on the rubber surround, the basket is slightly different (smaller hole, different side supports), and tapping the cone produces a different sound than when tapping the other channel's final revision of BD1212-B.

Question: Does anyone have a Polk 12" paper cone passive radiator (or pair) they are willing to sell? The only other speaker that has a similar passive radiator is the Monitor 12 Series 2, but it uses SW123 whereas the 3.1TL uses SW122. I don't recall the differences being documented. If you have any spare parts lying around or can point me to some information about the different passive radiators' specifications, I would be very grateful.

It's a shame that Polk released unmatched pairs to dealerships. I highly doubt the previous owner noticed, so I cannot fault him. I myself did not know to inspect at this level of granularity.

Thanks,
Brian

Mike Reeter

Very seldom do Serial #'s run consecutively, I wouldn't know if the interior differences are due to Polk at the time of manufacture, or someone else at a later date trying to repair something, that had no idea as to what they were doing. Just a guess on my part.

Good luck on finding a replacement PR, those things can be very difficult to come by.

decato

I took a few pictures of the different inductors in case anyone else is in the same position.

The left channel had BE0032-A, 18 mH, 20 gauge. This channel also had the 2.0 ohm, 25 watt resistor in series with the dimensional array.
The right channel had BE0033-A, 18 mH, 22 gauge. Notice how much less space is taken up as the wire is wound around the bobbin.

The date on BE0032-A is 3/90. However, the 3.1TL schematic is from 4/90.

I think I understand what the engineers at Polk were doing. The single dimensional driver exhibits long cone excursions with bass frequencies. In order to attenuate the bass so that the excursions are approximately equal to those exhibited by the four stereo drivers, a 2.0 ohm resistor was put in series. However, the resistor affects all frequencies, not just bass. Moreover, the added resistance shifts the crossover frequency.

Once Polk engineers finalized the design, they settled on an inductor with 22 gauge wire instead of 20. The thinner wire offers more resistance, eliminating the need for the separate 2.0 ohm resistor, and affects only the low frequencies. Less components, thinner wire, and no series resistor = a simpler and less expensive design.

Now, it turns out I do not have the necessary spare parts to complete the crossover. I think I'll have to send the bobbin out to Madisound and have them create a custom one for me, if that's at all possible. It's virtually impossible to find an 18 mH inductor with 22 gauge wire. No one wants insertion loss and possible saturation in a low-pass design (unless the bass driver is outnumbered 4-to-1)!

Please correct me if you have additional information.

F1nut

Only you Brian and just the man up for the challenge.

brgman

This is quite a catch decato.What an odd duck.
I will check as I may have the standard SW-122 around.
If memory serves me I picked up a pair a couple years ago knowing they are rare.If I have 2 you can have one.I doubt I'll ever need 2 spares.
At least until I get rid of one of course.

decato

brgman wrote: »

I will check as I may have the standard SW-122 around.

Thank you so much! If you find it and are willing to part with it, let me know your price.

Jhayman

Hi Decato, I recently picked up a pr of 3.1TL's but mine have 12" subwoofers not passive radiators..
Now I'm extremely new into the SDA SRS world but according to the spec sheets they use 12" subwoofers only the 1.3 and 2.3's used passive Rads..

decato

F1nut wrote: »

Only you Brian and just the man up for the challenge.

As I get older, I just want things to be easy. Apparently my Staples "Easy Button" does not work!

decato

Jhayman wrote: »

Now I'm extremely new into the SDA SRS world but according to the spec sheets they use 12" subwoofers only the 1.3 and 2.3's used passive Rads..

All Polk SDA speakers used passive radiators. They were called "fluid-coupled subwoofers" on the brochures, where the fluid was air inside a sealed cabinet. The 3.1TL was the only one to use a convex paper cone. All other SDAs used a planar styrofoam disc.

agfrost

decato wrote: »

All Polk SDA speakers used passive radiators. They were called "fluid-coupled subwoofers" on the brochures, where the fluid was air inside a sealed cabinet. The 3.1TL was the only one to use a convex paper cone. All other SDAs used a planar styrofoam disc.

This. No active subwoofers in the Polk SDAs.

decato

Well, Adam from Madisound was less than helpful. He said, "We don't make our own coils and, to be honest, I don't care about that stuff [Polk SDAs and my project]." So, screw Madisound.

Does anyone know where I can get a custom inductor made?

Thanks!

fjross

Yes, Trey @ V3 Mods should be able to help you out.

Jhayman

Ah see I learn something new everyday...Thank You

decato wrote: »

All Polk SDA speakers used passive radiators. They were called "fluid-coupled subwoofers" on the brochures, where the fluid was air inside a sealed cabinet. The 3.1TL was the only one to use a convex paper cone. All other SDAs used a planar styrofoam disc.

brgman

decato wrote: »

Well, Adam from Madisound was less than helpful. He said, "We don't make our own coils and, to be honest, I don't care about that stuff [Polk SDAs and my project]." So, screw Madisound.

Does anyone know where I can get a custom inductor made?

Thanks!

Are you just looking to get the inductor to be standard spec?
Or are you tweaking white paper spec on these 3.1's?

decato

brgman wrote: »

Are you just looking to get the inductor to be standard spec?
Or are you tweaking white paper spec on these 3.1's?

I will follow the standard spec for the inductor -- 18 mH, 22 gauge wire. I might tweak other parts of the design, but this part is pretty much etched in stone.

PolkieMan

Go ahead and get Tony's boards and do the whole crossover right and both the same IMHO :-)

brgman

decato wrote: »

I will follow the standard spec for the inductor -- 18 mH, 22 gauge wire. I might tweak other parts of the design, but this part is pretty much etched in stone.

do you need a stock inductor?

decato

brgman wrote: »

do you need a stock inductor?

Hi Mike,
Yes! Do you have one?

westmassguy

The 18mh Inductor is part of the "Full Compliment Sub Bass Drive Circuit". That inductor should be changed to a lower resistance Inductor, if your amp can handle the load.

decato

westmassguy wrote: »

That inductor should be changed to a lower resistance Inductor, if your amp can handle the load.

Yes, in any of the SDA models where the number of stereo drivers is equal to the number of dimensional drivers. I have made that modification to the 1.2TL. Making that modification to the 3.1TL will put a lot of stress on the single dimensional driver. I will not make this modification to my speakers.

westmassguy

Fair enough

brgman

decato wrote: »

Hi Mike,
Yes! Do you have one?

Pretty sure i do.I got a set of 3.1tl xovers to mod differently than the Solens xovers i have in but after further investigation and how dang great they sound already i will not be making the investment.

decato

OK, let me know about the inductor and passive radiator. I'm hopeful you can help me out.

stubby

I have a PR if brgman can't locate his.

stubby

Toolfan66

I have to agree about Madisound being less then helpful, I avoid them when I can..

decato

stubby wrote: »

I have a PR if brgman can't locate his.

Thanks, I appreciate your help. I'll follow up with you shortly, but I think brgman will be able to complete the transaction.

stubby

Sounds good - what ever it takes to get a great pair of speakers back on their feet!

decato

stubby wrote: »

I have a PR if brgman can't locate his.

I wish to thank stubby for selling me his spare passive radiator. Without his help, I would not be able to get my 3.1TLs back up and running. I am just waiting for my custom inductor to arrive in the mail, and I should have them playing again this weekend.

As a side note, 3.1TL owners might want to check the integrity of the internal bracing. All three supports than run from front to back had detached from the front baffle. Tapping on the cabinet produced a hollow thud sound. I spent some time with my hot glue gun, and the cabinet feels much more solid.

stubby

Always glad to help out a fellow Polkie.

decato

I also want to publicly thank Chad from Meniscus Audio in Michigan for his help with my project. I sent him the bobbin, and he removed all the 20 gauge wire and replaced it with 22 gauge wire, matching the resistance and inductance called for in the schematic. The price was also very reasonable. So, I'm up and running!

Now it's time to make a few modifications...