Center channel 8 ohm shielded

[filo91]

Hi everyone. I'm new and i'm Italian, so sorry if i make some mistake typing.

I would build i center channel for my system. The requirement are (as in the title) 8 ohm impedance and shielded drivers.

I watched some project here and i like the MTM with the dyton rs180 as mid and the scan speak tweeter or seas. The Modula MTM for example. But it has an impedance of 3 ohm (read on a comment).
Other project uses unshielded drivers.
So please, give me some advice. I already built a 3-way front speaker, but it was a closed box and i already had the crossover because it was a system build in a car. So i'm not capable on project the crossover.

Last but not least: maybe is a stupid question, but if i never ask, i will never know. Many times i see crossover project with the mid part separated from the tweeter part. At the end the two cross need to be set in parallel? if yes, what about the impedance?
Thanks.

[cjd]

It's a rare project that ends up MTM and 8ohm nominal impedance, and shielded drivers are becoming a rarity (though Neo motors are inherently capable of being used where a shielded motor is required.

The problem is availability of suitable 4ohm drivers. They exist, to be sure, but there aren't many these days. Dayton offers both a 4ohm and a shielded version of the RS180, but you can't get a shielded 4ohm.

For the record, that 3ohm actual impedance still sneaks into an amp that would be rated "4ohm" - 8ohm speakers often dip to 6ohm.

As for your last question: yes, the crossovers are wired in parallel. Impedance is fine because a crossover causes impedance to increase outside the pass-band (i.e. where the crossover is doing its job) so that the sum in parallel remains largely dominated by the primary driver: i.e. the tweeter impedance is the effective impedance in its pass-band, same with mid etc. You can see this in impedance charts published around when individual driver impedances are included. It's easiest to show with a picture but I can't find one just now!

For your project: are you looking to import? Have you checked what is available to you more easily?

More important: what are your mains? Matching a center channel is somewhat important.

Also, why do you need the center to be shielded?

FWIW, the only center-usable design I know if just now that might meet this criteria is the Ochocinco

[filo91]

Almost all of the drivers are available here, so no problem. The ones of the projects that I saw was available and also the ones of the Ochocinco. The prices are a bit higher but it's fine. For example, the Dayton ND140-4 26$ (read from the topic) and 35€ in Italy.
We also had the Ciare, that is an important Italian brand. It produces very good drivers, do you have it in the US?
Maybe it would be a bit more difficult to find the part of the crossover of the same producer, but i think there is no problem to use different parts if they have the right value.

My mains are the 3-way that i told in the first post, they are old and the drivers brand doesn't exist anymore (and i don't have any specs of the drivers). It is simply impossible to match the center with those. So I start with a good center and maybe one day I'll built two new mains.

The center should go under the tv (plasma panasonic). I don't think it's bad for the tv, but if i put the PS3 near the magnet of the Center, the hard drive would not be happy.
If there is no choice I'll put the PS3 in another place, now it is above the tv, so it should be fine there. So i started thinking the center channel should be shielded, but I prefer have one that sound great that have one shielded.

What does it mean FWIW? anyway I like the Ochocinco, I just think it has cheap drivers, but if they sounds good for a center channel it's ok.

[cjd]

FWIW = "for what it is worth." The Ochocinco is a budget design, no question about it. I didn't really think it was something you'd be particularly interested in - it just happens to be the only design I know of that fits your criteria.

If we leave the thought of a shielded setup behind, it is still tough from the common brands here, though Dayton RS, Scan-Speak, SB Acoustics all offer a fairly good line of 4ohm mid-woofers.

Before we chase that too far - what is the reason for an 8ohm speaker? I assume it is amplifier related; Which model do you have? Most are capable of driving a stable 4ohm speaker as long as you don't crank the volume up super high, so even that may not be hugely critical.

Ciare is tougher to get here but available; it's also only the pro-sound stuff. I've never seen anyone carry anything else from their line. Then too, it is even less likely to find a design using those drivers.

I also must admit I'm not sure what kind of results (or sound) you have from your existing mains; The description sounds like a recipe for something that might please you right now, but be very poor as far as measured response.

I've been keeping an eye out for additional designs that meet your needs, but have not found any yet - this is why I'm trying to understand better what is driving those needs.

[filo91]

The reason for the 8 ohm choice is the amp, as you assumed.
I have a Yamaha RX-V671 since almost a month now, before of that i had a Denon AVR-1311 (entry level).
I make that choice (the yammy 671) because I found it @ €350, and it was the only amp rated for an impedance less than 6 ohm at that price, plus it has really great review with the price of 500€. I didn't found nothing rated for 4 ohm under 800€.
Still, the Yamaha 671 is rated for 4 ohm speaker only for mains. The other channel needs to be minimum 6 ohm.
Since i was using the 4 ohm mains with the Denon (rated 6-16 ohm) and i had a problem just on those channels (a continuous rustle and a boom coming front the front channel at the start) i don't want to repeat that again.
If there is something with an impedance of 6 ohm, i'm fine with it, but i don't wanna risk with 4 ohm.

The mains sounds good to me. At the time it was expensive stuff. Here a picture i already had.

10" Woofer 5" mid.
If there's a way to do a measurement with the microphone of the yamaha (YPAO) and a pc i'll do it. Explain to me.

[cjd]

Why not a TM placed above the TV? I could be wrong, but I'm guessing the space you have to fill isn't huge and I see a chair to the right (I think) which would be solidly in off-axis null problems a traditional MTM brings.

Or a WMTW but...

That amp problem suggests it's not the net impedance, but something more specific (bad crossover part causing a short? brutal phase? Actual impedance being really really low even though it might claim "4ohm" ?) I've done that to an amp with one of the speakers I have here and it has both a brutal phase angle AND a solidly low 2.8ohm minimum; Starts out with oscillation and eventually leads to amp component failure.

Measuring speakers is almost the hardest part, so unless you're really interested in the long road to many more speakers than you ever wanted... the mic *might* work with a computer and some type of RTA software (real-time analysis) to give you a general picture of what the room + speakers measures like. I'll have to see if I can find info on that, or someone else, or (unfortunately) more likely forget soon after I return to work (which I should be doing now!) Something'll work.

C

[filo91]

Forget about the cabinet on witch stands the TV, I had to change it. After I decided the measure of the center i built a new cabinet for the TV and all of the other electronics.

The chair you see on the left is actually a sofa, but i put there because is part of the living room. When I watch movies no one is sitting on there.

Back to the MTM, the Ochocinco is an 8 ohm right? and the drivers are shielded.
Maybe i'm gonna start with that low budget project and then we'll see.