Challenge: Upgrade/Expand a Polk Center channel?

[digitalman]

I have Polk RT1000P mains and a CS245 center which is too small really with only 5.5 drivers. I want a much larger center soundstage. I should have the CS1000P center which is 4 x 6.5 drivers.
The CS1000 is tough to find and fairly old now. It is also a little deeper then i would like. I also looked at the CS350 with its 4 x 4.5 drivers. i think i want something more like that, wider with more drivers, but still an old used speaker.

So... would it be possible to expand my center with a new enclosure and 2 more 5.5 drivers and either modify or replace or add to my existing crossover in my CS245? i have found the drivers already and am looking for advice if this is feasable. I would basically end up with a CS350 with 1 inch larger drivers. my goal would be to have the similiar 10 inch depth and it can be any width needed. Also would my denon 887 model 100watts be enough power?

http://www.polkaudio.com/homeaudio/products/cs350/

http://www.polkaudio.com/homeaudio/products/cs245i/

CS245 doesnt seem to be on there but the 245i is basically the same thing.

I am new to this forum but after much reading, you guys are just amazing and can probably design this with your eyes closed

thanks

[---k---]

If some of the guys that do the cross-overs had measurements of the drivers in the box you intended to use, then yes they could probably come up with a design for you. But without good measurements, about all they could give you is a generic cross-over that wouldn't sound good. Several people have come here looking to modify retail speakers, it just isn't as easy as it sounds - especially for them to help a person without any experience.

Might I suggest rather than trying to mod your existing speaker, wander over to the Mission Accomplished forum and take a look at CJD's RS150 Center Channel:
https://www.htguide.com/forum/showthread.php?t=19887 It might not match perfectly, and is probably a lot bigger and deeper than you want, but you'll be able to build it for around ~$150, and it will blow that Polk away. You'll cry when you hear them side by side.

If not that one, maybe try building augerpro's Bose Buster shown in this thread:

https://www.htguide.com/forum/showthread.php?t=24504

Those are very small (4.25"w x 9"h x 8"d), very cheap (@Augerpro said ~$400-$450 for all 5!), and likely still better than the Polk. There is a center channel design further into the thread.

[ThomasW]

I am new to this forum but after much reading, you guys are just amazing and can probably design this with your eyes closed

Unfortunately we can't walk on water....

Would it be possible? Yes pretty much anything is technically possible. Is it a practical idea? No not really, you're talking about designing and building an entirely new speaker. The cost to do would be $pendy.

You're better off to find a used speaker like you want or follow ---K--'s suggestion. Also look at the other center channel speakers in the Missions Accomplished section of the forum.

[digitalman]

ok

I understand what your saying. I have looked at that same center here already but it is much too deep. i have been researching online with different companies trying to find wider thinner speakers with more drivers, thinking they will sound better. What do you think the difference in sound would be from the polk cs350 center versus the DIY center rs150mtm ?

Is there a good wide WMTMW center available i could build? 10-12 inches would be an ideal depth with my projection screen. Or is the MTM center really the best thing there is?

theres a CS350 on ebay for like $250 right now but i think its just more then i want to spend when my labor is free, the matching polk drivers i found are $40 shipped, it's basically a kit to build my same speaker. i have often considered just getting another one and running dual centers side by side. again i dont really know if this is a good idea.

Bottom line is i'm poor and looking for the cheapest solution to give me a bigger sounding center.

thanks

[Dennis H]

You can build the enclosure for any of these wider and shallower. Just keep the same height and internal volume. Height could change a bit but it needs to be pretty close for the baffle step compensation built into the crossover to work right.

[---k---]

An MTM is acutally the worse configuration to use for a center channel. The reason it is so popular is based on the size/layout works well.

I haven't heard the Polks, so I can't say. I'm guessing you'll hear a big differance. I've had CJD's MTMs in my house at the same time as some Paradigm Studio 40s($1400), DynAudio 42 ($800), and Ascend 340SEs. They were clearly superior in my mind, so I have no doubts it will be better than the Polk.

Jon is in the process of designing a WMTW center channel to match the NatP and Modula projects. But, I think this will still be bigger than you desire, and probably cost more.

Better speakers usually come at a price. Size is ofter the price you pay. Just look at how big the RS-3way center WMTW center chanel is. It is HUGE.

You also need to be into tools and woodworking to do DIY. If you aren't wanting to get dirty, experiment a lot, likely swear a little when you mess up a few times, and then feel really good about yourself when you're done, then DIY may not be for you.

[digitalman]

width

OK, that's the best news i have heard all day! i can easily make it a few inches wider and compensate the depth to be exactly the same volume. so which one is the best model to build?

https://web.archive.org/web/20071102080300/http://www.eldamar.net/audio/rs150mtm/

https://www.htguide.com/forum/showthread.php?t=15323&highlight=center

the Dayton RS WMTW Center should be the best one right? it's 16" deep so i could make it 12" deep and calculate the exact width it needs to be to maintain the same volume.

[digitalman]

i have no problem with building the speakers. i am a computer geek and very much into figuring things out. i have installed several car speaker systems in the past and built several subwoofer enclosures myself from scratch. i have read a couple old books on loudspeaker design and know a bit. i have a good friend that is a nasa engineer and can help me out with any technical issues i might encounter.

[ThomasW]

Chris's design will certainly sound better, but I doubt that it will be significantly louder given the rather limited Xmax of the RS150.

If you have a digital camera? If so pop open your speaker and take a picture of the crossover (the greater the detail the better). Post it so we can see what's in it.

If Polk does what I think they do, you might be able to squeak by with the added Polk woofers and just a couple crossover components.

No promises though..

[digitalman]

cool, i will do that tonight, i just moved and bought a new denon reciever yesterday (no interest until 2010 from circuit city!) it will last me for a lil while, i really needed hdmi upconversion for my sources. i'm not REALLY that poor, just cant justify spending too much money on a center for a minimal improvement. my system is all tore apart, i plan on hanging my electric screen and projector this weekend. i have certainly envisioned myself building the best speakers i can from this site down the road....

i didnt realize the 3way was actually using 8's and is 12.5" tall, i might be able to fit that if the polk expansion doesnt work out.

[cjd]

I may be working up a 2.5way variant as well with four RS150's...

[kingpin]

I may be working up a 2.5way variant as well with four RS150's...

Darn you. :B
I keep getting drawn back to this place.
Have to make more money so I can afford to build a center speaker and feed myself as well. :W

Mike

P.S. You guys haven't got rid of me yet. I visit everyday to see what the masters of speaker design are up to. :T

[---k---]

Chris's design will certainly sound better

In the context of the thread, that could get misinterpreted by a newbie. You just meant that Chris's will sound better than the Polk center and made no judgment between it and the RS WMTW center. Right??

[digitalman]

cs245 pix

OK here's the pix, hope this works...

external dimensions are 16.75"w x 6.5"h x 9"d port is little over 1.25" across.

caps are 10,20,46MFD - 50V

the additional woofer specs (and i -think- my woofers are the same)

5.25" Polly Cone Shielded Woofer
Impedance 4 ohms
Weight 3 pounds each
Revc - 3.4679 ohms
Fs - 56 Hz.
Zmax - 68 ohms
Qes - 0.4969
Qms - 9.2893
Qts - 4717
Le - 0.3484 mH (at 1 kHz.)
Vas - 22537.9746
BL - 0.1401 N/A
Sensitivity - 90 dB
Frequency response 40 Hz. to 7 kHz.

the kit comes with the tweeter also:

1" Silk Dome Soft Tweeter
8 ohms
Power 75 watts
Sensitivity - 89 dB
Frequency response 1.8 kHz. to 20 kHz.
Weight 1.5 pounds each

I don't know if its better or worse then my existing one, and assuming i would never want to have 2 tweeters?

thanks

[ThomasW]

Can you read the values of the inductors (coils) ?

The only part of the circuit we're interested in is the woofer section.

Green and red are the + wires to the woofers, white is -
The woofers are wired in series, which is why W2+ (red) shares the circuit with W1- (white)

See if you can make a drawing of the crossover circuit based on the wires, components and circuit board traces.

For example.....

[Bent]

Unfortunately we can't walk on water....

I can...
But only in the winter.

[digitalman]

crossover

Can you read the values of the inductors (coils) ?

The only part of the circuit we're interested in is the woofer section.

Green and red are the + wires to the woofers, white is -
The woofers are wired in series, which is why W2+ (red) shares the circuit with W1- (white)

See if you can make a drawing of the crossover circuit based on the wires, components and circuit board traces.

For example.....

will this suffice?

[digitalman]

So is that schematic good enough to figure it out? i need to know if this is possble so i can go ahead and order the extra drivers and get them shipped.

thanks

[joecarrow]

The schematic looks to be quite clear; I don't know who is helping with this, but it should be good. Can you verify that the component values "as built" match the schematic?

[ThomasW]

It looks a little strange.

First might recheck the PCB since I'd expect an L-pad on the tweeter instead of a single resistor in the woofer and one on the tweeter.

Compare your drawing to the individual generic circuits below. I think your drawing has the woofer and tweeter circuits reversed.

2nd order high pass



2nd order low pass



3rd order high pass

https://web.archive.org/web/20200628173552/http://www.carstereo.com/help/Articles.cfm?id=9

https://web.archive.org/web/20200628172854/http://www.carstereo.com/help/Articles.cfm?id=10

[Dennis H]

The layout on the diagram is a bit unusual but, tracing it out, it looks like a 2nd order 2.5-way (electrical) to me. The resistor on the woofer circuit is a zobel. Actually, a 2.5-way makes a lot of sense for a horizontal center as it eliminates some W-W lobing problems.

But back to the original post, adding woofers to this 2.5-way is unlikely to work well. You'll create as many problems as you fix. Just cross the stock speaker a bit higher to the sub and you'll get the same SPL advantage as adding woofers. Otherwise start over with a brand new design.

[digitalman]

hrmm, well that answer wasnt good news for me i assumed adding the 2 extra speakers would give me a much bigger sound comparable to the CS350 with its 4x 4" woofers. as far as the schematic, i'm no engineer but it does seem to be right. nothing magical or complicated about it, you can see all the paths to each component in the first pix i posted and looking at it again it all seems correct. that schematic was created by nasa engineer out at kennedy space center, he measured the coils with an inductance meter. it should all be correct.

[digitalman]

i found this on the polk site, its the schematic for the CS350

http://www.polkaudio.com/forums/showthread.php?t=38755

It's the 4th post down for the CS series.

I want to make essentially the same speaker with 1 inch larger drivers and i'm assuming a slightly larger enclosure to handle the airspace needs of them.

[ThomasW]

We admire your tenacity, but aren't sure what you hope to obtain with that information. Since the 350 uses different drivers, it's not like it could be copied and used with your drivers.

[digitalman]

well i was just using it as a reference for the design i was trying to duplicate. the cs350 with larger drivers. but anyways, thanks for your help but i am going to abandon the idea. i found a used CS400 online and am just going to buy that instead. its not as big of a center as i wanted, but it will do for a good deal.

thanks for you help. i think i will still build the big dayton WTMW center and MTMWW mains in the next year or so, they look like great speakers. i can just start buying parts over time and build em once i get them.