My Center Channel Project

westmassguy

This has been a journey, more than just a project. The question is asked frequently, "Which Center Speaker matches the Vintage Polk Models". Polk has recommendations, and I believe some of them now qualify as vintage too. This all started when I bought my Monitor 10s. The center I was using in no way matched the timber of the 10s. I've since used CRS+ and now use my SDA 2A-TLs as my fronts, for 2 channel and Home Theater use. I initially built a proto type center, for experimentation, never intending for it to be permanent. I used many different combinations of Polk drivers, Crossovers etc. It's been vented, PR'd, and actually sounded quite acceptable, even though it was undersized and aesthetically challenged.

Since I've now completed all the upgrades to my 2As, and have re-configured my 5.1 to a 7.1 system, using my Modified Monitor 5s and 4.6s, it was time to get serious and build a proper Center, that will match the 2A-TLs, and look professional.

westmassguy

I knew from all the experimentation, that a Monitor 10 B crossover would be used, since it's designed for a single tweeter, dual woofer and PR configuration. I have the T/S parameters for most of the vintage woofers, the only item missing, is the SPL rating for the drivers, which will come into play later. The main problem, is the 10 uses 8 ohm nominal drivers. All the 8 ohm drivers have relatively high VAS figures. I could tune a smaller vented enclosure, but my cut-off would be somewhere in the 70 Hz range, which is not what I was looking for. I decided, that the only way to make this work, was to use the MW6502 4 ohm drivers, which have half the VAS of their 8 ohm brothers, and modify the 10 crossover. I also wanted to use two 6.5" PRs from a pair of 5JR+s I'd picked up off eBay. Instead of fabricating my own enclosure, I decided to find a suitable knock-down, CNC cut enclosure. As it happened, Parts Express was having a sale, and I found the perfect candidate. It's billed as a tower/large center, so I ordered it. The internal volume turned out to be almost exactly twice the size of two Monitor 5JR+s. This meant I could theoretically use 2 MW650s, plus the two 6.5 " PRs. When I began assembling the cabinet, I realized the CNC cuts, left a lot to be desired. Some fill, very careful Belt Sanding (Yes I do know how to use a belt sander), and some final block sanding were required to smooth things out. The front baffle was pre-cut before assembly. After assembly, the cabinet was internally sealed, and Hurricane Nuts installed.

My next challenge, was to modify the Monitor 10 Crossover for use with 4 ohm drivers. The 10 uses two 8 ohm drivers in Parallel, for 4 ohm nominal resistance. The two 6502s would be wired in Series, for an 8 ohm nominal resistance. In order to keep the exact crossover frequency and slope, I would have to double the mh value for the inductor, and halve the uf value for the shunt capacitor.
I found some suitable temporary parts in my ever growing parts bin, and installed a 18 gauge 3mh inductor, and made up the 17 uf cap. The hi-pass section had already been upgraded with Sonicaps and Mills resistors, and had the TL mod dove to it. I made the modifications, and put everything together for a dry run.

After firing it up, It sounded quite good, except for a rather hot top end. The 10 crossover was designed for drivers with higher output, and a much larger enclosure. I decided there were two ways to balance the Tweeter and Woofer output. I could change the value of the .5 ohm resistor I installed in place of the Polyswitch. By raising the value, I could reduce the output of the hi-pass without adversely affecting the voicing of the tweeter. The other option is to install a much lower resistance Inductor, to increase the output of the two woofers. I found a 4ohm, 25 watt L-Pad, and temporarily installed it in place of the .5 ohm resistor. I used the common and 4 ohm leg, and preset it to .5 ohms. I fired up the speaker, and slowly increased the resistance until my ears told me it was in balance with the Woofers. The value turned out to be approximately 7.5 ohms. I wanted to test my other theory, and found a 15 gauge, 3.3 MH P-Core Inductor in my parts bin. I re-installed the .5 ohm resistor. I unwound the Inductor, until I reached the correct 3.1 mh rating on my LCR Meter. I disconnected the other inductor, and installed the P-Core. Woofer output increased, and to my ears, everything was balanced. I don't like P-Core Inductors, since they saturate easily, and have other distortion components. I decided to order a new Erse Hi-Q, Laminated Steel Inductor, which I've used in the past with great results. I also ordered a 17 uf Cap from Sonic to complete the permanent modifications to the crossover.

westmassguy

After I received the new parts, everything was installed on the crossover. The Inductor was remote mounted since it's rather large. I then began the final finish on the cabinet. I'm a bit of a traditionalist, so American Walnut is my Veneer of choice. The top, front and sides were veneered, and the rear and bottom were primed and given 3 coats of Satin Enamel. The veneer was stained and 3 coats of Matte Finish applied. The front grill was made from 1/2" MDF, with beveled edges. I choose the same Black Architectural Grill Cloth I used on my 2As.

I think she looks pretty damn nice. More importantly, she sounds awesome. Exact timber match to my 2As. It sounds so good, I actually turned it upright, and ran it as a stereo pair with one of my Monitor 7s. There was no perceptible difference other than deep base between the two.

pitdogg2

I take back ALL those nice things I've said about you.....YOU SUCK!!!
just kidding that is VERY nice. I'll take 65 as my KARMA number:redface:

westmassguy

pitdogg2 wrote: »

I take back ALL those nice things I've said about you.....YOU SUCK!!!
just kidding that is VERY nice. I'll take 65 as my KARMA number:redface:

Thanks pitdog, it was an adventure, but very satisfying.
I could have taking the easy route, and used some 5Jr+s or the new 4.6s I just got, but where's the fun in that?
I actually tried the 4.6s end to end wired in series. It sounded great, except it was too hot on axis. I would have had to reduce the tweeter output on both. Two tweeters within inches of each other is not good for this application.

pitdogg2

yep with them that close you'd need sunglasses for your ears:eek:
completely understand

monepolk

Very nice work. Now enjoy the fruit of you labor.

westmassguy

monepolk wrote: »

Very nice work. Now enjoy the fruit of you labor.

Thanks, I have been

GospelTruth

Very nice work. I'm sure you put in a ton of time to get these where you want them and they look fantastic. Looks like you might need a new stand though as your TV and center are fighting for space.

westmassguy

GospelTruth wrote: »

Very nice work. I'm sure you put in a ton of time to get these where you want them and they look fantastic. Looks like you might need a new stand though as your TV and center are fighting for space.

Thanks, that stand was actually my late grandmothers dry sink which I re-purposed, so a new stand is definitely required.
And cable management is another area that needs attention desperately.

ZLTFUL

Hmmm...been looking for a reason to use my RTA15TLs in the HT... :twisted:

Beautiful work man.

zane77

Great job! That looks really sweet, bet it sounds as good as it looks.

westmassguy

ZLTFUL wrote: »

Hmmm...been looking for a reason to use my RTA15TLs in the HT... :twisted:

Beautiful work man.

Thanks zltful

westmassguy

zane77 wrote: »

Great job! That looks really sweet, bet it sounds as good as it looks.

Thanks, it does. Crystal clear, and plenty of bass too.

chumlie

Beautiful work & great engineering. Well done. Congrats !!!

westmassguy

chumlie wrote: »

Beautiful work & great engineering. Well done. Congrats !!!

Thanks chumlie

Conradicles

Fantastic job! Somebody should start building these and selling them.:cool:

westmassguy

Conradicles wrote: »

Fantastic job! Somebody should start building these and selling them.:cool:

Thanks Conradicles

drumminman

Really nice work WMG. Impressive how you did all the Xover work. Wish I had the knowledge/equipment/skills as I'm a DIYer at heart.

How do make the radiused edges with the veneer?

westmassguy

drumminman wrote: »

Really nice work WMG. Impressive how you did all the Xover work. Wish I had the knowledge/equipment/skills as I'm a DIYer at heart.

How do make the radiused edges with the veneer?

Thanks drumminman. The Veneer I used is 10 mil PSA backed. The edges are 3/4" radius, and are easy to wrap. It's when you get down to 1/2" or 3/8" radius, that you run into problems. What I do in that case, is wet the veneer with distilled water, exactly where the radius occurs. It may take a 5-10 minutes, and several applications to saturate the wood, then very slowly bend and press at the same time. The water softens the wood fibers and minimizes cracking or splitting. I use distilled water, since tap water has dissolved minerals which can react badly with certain wood species, and leave stains. I also primed the entire cabinet with 2 coats of clear lacquer so the PSA would adhere fully. It also made painting the rear and bottom with Enamel a lot smoother.

sk88

Very impressed with your work and knowledge/skills. I wonder if decoupling would be needed between your center and the cabinet it sits on?

westmassguy

sk88 wrote: »

Very impressed with your work and knowledge/skills. I wonder if decoupling would be needed between your center and the cabinet it sits on?

Thanks. It's sitting on 1/8" thick, 1 1/2" round foam disks. Just to keep it from sliding around.

nooshinjohn

I want one too... any chance blueprints are available?

westmassguy

nooshinjohn wrote: »

I want one too... any chance blueprints are available?

Blue prints no, but I will make up a parts list with suppliers.

sk88

westmassguy wrote: »

It's sitting on 1/8" thick, 1 1/2" round foam disks.

I don't know for sure, but I worry for you if 1/8" is enough to avoid resonance from the cabinet since the center has such good bass. I'd say 1.5" would be better if there is room. Please let us know if you hear resonance or not. I am just thinking that a temporary way to compare is if you can find some dense packing foam (that won't compress much) to put underneath the center and see if it makes a difference to how the bass sound.

westmassguy

sk88 wrote: »

I don't know for sure, but I worry for you if 1/8" is enough to avoid resonance from the cabinet since the center has such good bass. I'd say 1.5" would be better if there is room. Please let us know if you hear resonance or not. I am just thinking that a temporary way to compare is if you can find some dense packing foam (that won't compress much) to put underneath the center and see if it makes a difference to how the bass sound.

So far I've not noticed anything unusual regarding resonances. If I raise it any higher, it will block my screen.

rooftop59

Dude that does look awesome! I wonder if putting that much work into your speakers makes you hold onto them longer...hmm...

westmassguy

Here's a list for those that would like to know the parts and suppliers I used:
Cabinet: http://www.parts-express.com/pe/showdetl.cfm?partnumber=300-7066
Veneer: http://www.ovisonline.com/Wood-Veneer-Walnut-Quartered-2x8-PSA-Backed-P1538C128.aspx
Hurricane Nuts: http://www.parts-express.com/pe/showdetl.cfm?partnumber=081-1080
Machine Screws: #8-32 X 1 1/4" Your choice, some like cap screws, I prefer black oxide coated, stainless, pan head, Phillips. Both types should be available locally, or many sellers online.
Crossover: Monitor 10 B (eBay) with SL3000/TL Mod done (see schematics section). Hi-pass, Sonicaps, Mills Resistors, Jantzen 18 gauge, .27mh inductor (Parts Express) and 3.0 ohm resistor (not 2.7 ohm) for hi-pass shunt. Lo-pass, Sonicap 17uf, and Erse Super-Q 16 gauge, Laminated Inductor (Parts Express or direct from Erse), 3.3mh, unwound to 3.1mh. The Erse inductor replaces the original 1.55mh.
Woofers: Two MW6502s (eBay)
Tweeter: Your choice based on how you modify/recap the crossover
Passive Radiators: Two 6.5" from Monitor 5JR+, # SW6528 found on eBay
Other: The crossover wiring harness must be modified from parallel to series connections for the woofers.
The Replacement lo-pass inductor can be Air-Core, from your favorite vendor, however as stated above, the lower the DCR, the better to increase the output of the dual MW6502s. The alternate method, is to increase the value of the .5 ohm resistor used to replace the Polyswitch. I found 7.5 ohms to be optimum to my ears, but this can be adjusted.
You could also roll your own crossover, point to point. The Monitor 10 isn't that complex, and you can use better quality binding posts etc.

westmassguy

rooftop59 wrote: »

Dude that does look awesome! I wonder if putting that much work into your speakers makes you hold onto them longer...hmm...

Thanks. I do tend to hold onto my pet projects. I doubt anyone but a fellow Polkie would be interested in it, if I ever decided to sell it.

Conradicles

westmassguy wrote: »

Here's a list for those that would like to know the parts and suppliers I used:
Cabinet: http://www.parts-express.com/pe/showdetl.cfm?partnumber=300-7066
Veneer: http://www.ovisonline.com/Wood-Veneer-Walnut-Quartered-2x8-PSA-Backed-P1538C128.aspx
Hurricane Nuts: http://www.parts-express.com/pe/showdetl.cfm?partnumber=081-1080
Machine Screws: #8-32 X 1 1/4" Your choice, some like cap screws, I prefer black oxide coated, stainless, pan head, Phillips. Both types should be available locally, or many sellers online.
Crossover: Monitor 10 B (eBay) with SL3000/TL Mod done (see schematics section). Hi-pass, Sonicaps, Mills Resistors, Jantzen 18 gauge, .27mh inductor (Parts Express) and 3.0 ohm resistor (not 2.7 ohm) for hi-pass shunt. Lo-pass, Sonicap 17uf, and Erse Super-Q 16 gauge, Laminated Inductor (Parts Express or direct from Erse), 3.3mh, unwound to 3.1mh. The Erse inductor replaces the original 1.55mh.
Woofers: Two MW6502s (eBay)
Tweeter: Your choice based on how you modify/recap the crossover
Passive Radiators: Two 6.5" from Monitor 5JR+, # SW6528 found on eBay
Other: The crossover wiring harness must be modified from parallel to series connections for the woofers.
The Replacement lo-pass inductor can be Air-Core, from your favorite vendor, however as stated above, the lower the DCR, the better to increase the output of the dual MW6502s. The alternate method, is to increase the value of the .5 ohm resistor used to replace the Polyswitch. I found 7.5 ohms to be optimum to my ears, but this can be adjusted.
You could also roll your own crossover, point to point. The Monitor 10 isn't that complex, and you can use better quality binding posts etc.

Thank you! Also note from what I have heard from a few veteran members, it may be a good idea to raise the tweeter up a bit so that it is not in the center line of the woofers.

westmassguy

Conradicles wrote: »

Thank you! Also note from what I have heard from a few veteran members, it may be a good idea to raise the tweeter up a bit so that it is not in the center line of the woofers.

Well it's a little late for that. What's the reason given? I would think all the drivers acoustic centers should be inline, but I'm by no means an expert.