My Center Channel Project

ken brydson

westmassguy wrote: »

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.

Could have fooled me. VERY nice work!!

Face

westmassguy wrote: »

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.

Actually, it depends. There is no simple answer.

westmassguy

ken brydson wrote: »

Could have fooled me. VERY nice work!!

Thanks Ken

westmassguy

Face wrote: »

Actually, it depends. There is no simple answer.

If you could elaborate a bit, it might help the next guy that builds one. I've seen center speakers with the tweeters raised, but also with everything inline. I could raise it up a bit, however the lower screw holes would then be exposed, and the plastic trim might interfere with the grill cutout. The Tweeter is surface mounted. The lip on the back side of the plastic face plate was ground off, so it sits about an 1/8" high.

westmassguy

westmassguy wrote: »

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.

I just realized there's a huge error/typo. The value of the resistor should have read .75 ohms, not 7.5 ohms

Face

westmassguy wrote: »

If you could elaborate a bit, it might help the next guy that builds one. I've seen center speakers with the tweeters raised, but also with everything inline. I could raise it up a bit, however the lower screw holes would then be exposed, and the plastic trim might interfere with the grill cutout. The Tweeter is surface mounted. The lip on the back side of the plastic face plate was ground off, so it sits about an 1/8" high.

Without modeling it in a design program or preferably taking actual measurements, I can't give a definite answer.

westmassguy

Face wrote: »

Without modeling it in a design program or preferably taking actual measurements, I can't give a definite answer.

I understand. I don't have modeling programs, and the only measuring instruments I have are my trusty Rat Shack Digital SPL Meter and my 56 year old ears.

PolkieMan

Well I think you did a perfect job and if I did not go out and buy a CS6i (kick myself now) I would be gathering everything I need right now to do this.

westmassguy

PolkieMan wrote: »

Well I think you did a perfect job and if I did not go out and buy a CS6i (kick myself now) I would be gathering everything I need right now to do this.

Thanks PolkieMan

westmassguy

I thought I'd update this thread since I've made some changes to the original design. In my original calculations, and after testing different combination of inductors and resistor values, I determined that in order to balance the tweeter and woofer outputs, I needed to either change the .5 ohm resistor which replaced the Polyswitch, and use a higher resistance inductor in the Lo-Pass, or leave the .5 ohm, and switch to a lower resistance inductor in the Lo-Pass. I opted for the Erse SuperQ low resistance inductor as the solution. Having over 6 months to evaluate the results. I decided to re-visit the crossover modifications. She sounded good, but I felt there was something missing. In addition, I wanted to upgrade the binding posts, and replace the factory 18 gauge wire, which I'd modified, with something a little more substantial. Believe it or not, even the short leads coming off the old binding posts were 18 gauge as well.
I should preface this be saying all my new calculations were based strictly on the known DRC values for the drivers and inductors, not the nominal Impedance. The Monitor 10 used two 6503 drivers with a DCR of 6.54 ohms. Wired in parallel, that equals approximately 3.27 ohms The original 18 gauge, 1.55mh inductor measured approximately .6 ohms. The total DCR for the Lo-Pass was about 3.87, call it 3.9 ohms. When I switched to the 6502s wired in series, my DCR for the woofers was 7.04, call it 7 ohms. The Erse 16 gauge was .18 ohms when unwound to the correct 3.1mh. So my total DCR for the woofers and Inductor was 7.22 ohms. I felt his was a bit lower, than it should have been. I ordered an 18 gauge Jantzen 3.4mh Perfect Lay Air Core Inductor, and unwound it to exactly 3.1mh. The resultant DCR was exactly 1 ohm. Added to the 7.04 ohms for the two drivers put me precisely at 8 ohms. I knew from my previous experimentations, that a .75 ohm resistor would be needed in place of the .5 used to replace the Polyswitch, so I installed that **** well. After completing the other upgrades, I re-assembled everything and gave it a test run. Perfection is how I would describe it. Nothing was missing. Bass was strong, and perfectly balanced with the tweeter now. My feeling is, the lower overall DCR of the original, modified low pass resulted in a slightly lower crossover point for the woofers, even though their output had increased. This is what was "missing". I will say the overall SPL decreased slightly, and was expected. I recalibrated my system, and it's even better than before. Timbre matching is perfect with my 2ATLs.

Zentish

I keep coming back to this thread to try and understand what you did here (which looks pretty great) and I have a simple question:

I'd always thought that laying a stereo speaker on its side mess up with the dispersion and phase. Now maybe things are different with a single center speaker, and I see that various POLK speakers have the tweeters mounted different ways.

Why did you choose to mount the tweeter 90 degrees from the "normal" Monitor 10 tweeter alignment?

westmassguy

Zentish wrote: »

I keep coming back to this thread to try and understand what you did here (which looks pretty great) and I have a simple question:

I'd always thought that laying a stereo speaker on its side mess up with the dispersion and phase. Now maybe things are different with a single center speaker, and I see that various POLK speakers have the tweeters mounted different ways.

Why did you choose to mount the tweeter 90 degrees from the "normal" Monitor 10 tweeter alignment?

I'm not following you. How is the tweeter 90 degrees? On a normal Monitor 10 the tweeter is above the woofers. If I did that, my center would be twice as tall.

Face

Zentish wrote: »

I'd always thought that laying a stereo speaker on its side mess up with the dispersion and phase. Now maybe things are different with a single center speaker, and I see that various POLK speakers have the tweeters mounted different ways.

It does cause lobing.

westmassguy

Well if there's lobing, my lobes aren't hearing it

Zentish

westmassguy wrote: »

I'm not following you. How is the tweeter 90 degrees? On a normal Monitor 10 the tweeter is above the woofers. If I did that, my center would be twice as tall.

I was referring to the tweeter's vertical or horizontal alignment, not the relationship with the other drivers.

Most of the vintage POLK tweeters I've observed are rectangular and mounted with wider length horizontal. My 5B's, the 7's and the 10's are all that way.

Your kit has the tweeter mounted vertically.

I was wondering why that was - what advantages there might be to that arrangement.

westmassguy

Zentish wrote: »

I was referring to the tweeter's vertical or horizontal alignment, not the relationship with the other drivers.

Most of the vintage POLK tweeters I've observed are rectangular and mounted with wider length horizontal. My 5B's, the 7's and the 10's are all that way.

Your kit has the tweeter mounted vertically.

I was wondering why that was - what advantages there might be to that arrangement.

I understand now what you meant. It was done for symmetry, nothing more. The Tweeter itself radiates equally in all directions, which is typical for a soft dome. Vertical or horizontal mounting won't affect the tweeters output. The same tweeters are using in a vertical mount on all the SDAs.

westmassguy

Face wrote: »

It does cause lobing.

Face, I've been reading up on that subject. Apparently in a horizontal MTM arrangement, the tweeter should be above the woofers forming a 45 degree angle from the centerline of the tweeter to the centerline of the woofers to reduce lobing. Unfortunately, due to the size of the RDO-198-1, the spacing between the woofers, and the height of the enclosure, that would be impossible. I could possibly modify the plastic face plate, and raise it up, but 30 degrees would probably be the best I could hope for.

xjghost

This is all really cool and way above my pay grade! I'd love to build a center and have a surround system with my 10's being the mains and some 7's for rears with something like this for my center. Dream on!

Zentish

westmassguy wrote: »

I understand now what you meant. It was done for symmetry, nothing more. The Tweeter itself radiates equally in all directions, which is typical for a soft dome. Vertical or horizontal mounting won't affect the tweeters output. The same tweeters are using in a vertical mount on all the SDAs.

That makes sense - thanks for the elaboration.

I've been considering swapping my M5B mains with some 11T's, and then thinking that the 5's would make a nice center, either one of them standing up under the panel (it will just fit) or - and this is why I was asking about the tweeter alignment and such - laying both 5B's down, tweeter in and PR out as a double center.

westmassguy

Zentish wrote: »

That makes sense - thanks for the elaboration.

I've been considering swapping my M5B mains with some 11T's, and then thinking that the 5's would make a nice center, either one of them standing up under the panel (it will just fit) or - and this is why I was asking about the tweeter alignment and such - laying both 5B's down, tweeter in and PR out as a double center.

If the two 5s are laid next to each other. It will be very "hot" sounding on axis, with the two tweeters next to each other.
I tried it with a pair of 4.6s

mikemokr

Might it be better then to use just a single 5(JR in my case) as a center?

westmassguy

mikemokr wrote: »

Might it be better then to use just a single 5(JR in my case) as a center?

Yes, others have done that successfully.

xjghost

mikemokr wrote: »

Might it be better then to use just a single 5(JR in my case) as a center?

I like this idea. Never thought about that.....

mikemokr

As I run through different permutations of future upgrades to my main and secondary systems, it occurs to me I could end up with two different 5JRs - one with TL mod and RDO-198-1 (I believe I saw someone saying they'd done this with the 5JR or possibly I'm thinking of the TL mod schematic for the 5B) for a center with CRS+ "4.1 TL" and the other with a 194 tweeter for a non-TL system. My 5JRs are stock with SL2000.

westmassguy

Whatever you have up front, should be timbre matched as close as possible.

mikemokr

Yep, that's my plan. Most likely my main HT will end up with 198s in the fronts and center, and a secondary system will start with all SL2000s and end up upgraded to all 194s. I might just have use for a third system at some point and could even do all Peerless across the front there - Denmark in L/R vs US in the center, but I suspect that wouldn't be an issue.

dpowell

mikemokr wrote: »

Might it be better then to use just a single 5(JR in my case) as a center?

I tried this and hated it. The 5jr just wasn't designed for use as a center speaker. It can't handle the load and stay up with the rest of my system. I also found it lacked the detail I wanted in a center speaker.

I would love to hear a custom timbre-matched center speaker but quite honestly, it would truly have to shine to out-perform the LSiM706c.

westmassguy

dpowell wrote: »

I tried this and hated it. The 5jr just wasn't designed for use as a center speaker. It can't handle the load and stay up with the rest of my system. I also found it lacked the detail I wanted in a center speaker.

I would love to hear a custom timbre-matched center speaker but quite honestly, it would truly have to shine to out-perform the LSiM706c.

Mine shines pretty damn good, and matches my SDAs perfectly

Nightfall

Curious, why didn't you mount the tweeter horizontally instead of vertically? Such as the way it is in a standing speaker. Do dome tweeters have a perfect 360 dispersion pattern making orientation negligible? I'm guessing so because a few of the SDA's have their tweeters mounted sideways like that so I'm assuming it doesn't matter, but curious.

westmassguy

Nightfall wrote: »

Curious, why didn't you mount the tweeter horizontally instead of vertically? Such as the way it is in a standing speaker. Do dome tweeters have a perfect 360 dispersion pattern making orientation negligible?

Aesthetics