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Old 2nd July 2008, 09:42 PM   #1
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Default Crossover Advice for Home Cinema DIY

Hi,

This is my first post so please be kind

I wish to clone M&K's S-250 THX loudspeaker - they are no longer going to produce it, even though they are operating again.

Look here for a reminder of their construction.

http://www.dreamhomecinema.com/GEN/P..._K%20S250.html

I have heard these in operation, and for the purpose I wish to use them, I found them to be excellent speakers.

So to do initial testing, I purchased enough drivers to build one speaker, which I will try as a center before building the remainder.

Now the box for the original is fairly compact and I would like to maintain the same approximate size and construction if at all possible.

Here is a list of the drivers I bought

2 x Vifa PL14WJ09 8 ohm
2 x Vifa PL18WJ09 8 ohm
2 x Dayton PT2C - 8 8 ohm

I'm going to keep each pair of drivers within the single cabinet sealed from the other pairs and also keep the entire cabinet sealed.

I know the originals were designed to operate with THX specs and not necessarily provide useful output below 80Hz(spec) and 70Hz actual. I'm fine with that.

I'll be driving them with Adcom's 7807 multi-channel amp (300w RMS 8Ohm). Room size is likely to change in the short term, so I'm not going to consider that for now.

I'll be using something like Elemental Designs A7-900 for low frequencies.

I'd actually like some help on box sizes and crossover design.

Suggestions and comments are welcome.

Jason
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Old 3rd July 2008, 05:31 PM   #2
JLC7 is offline JLC7  United States
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Let's get this out of the way first. Are you going to be designing your crossovers from driver/cabinet measurements with a mic and impedance jig?
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Old 3rd July 2008, 06:33 PM   #3
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This is the reason I posted

I take it that's the best way to do it.

I don't have a mic and software (yet) but I will be able to get hold of those.

The question remains for me though, that implies I've built the cabinet first and then making the crossover work with the volumes that already exist - unless I've misunderstood?

Would it give me a better option to work out the volumes that work best first and then refine with further measurements in-situ so-to-speak?

Thanks
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Old 3rd July 2008, 07:41 PM   #4
JLC7 is offline JLC7  United States
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Assuming you're building speakers that you want to use, I think you have in mind a box size you're willing to accept into your home or theater's decor. No point in telling yourself a 4 cubic foot enclosure is ideal if it's too big to hang on the wall or if the significant other says no.

Why don't you take Vifa's cabinet recommendations and start from there? You can model their proposed cabinet volume in winISD. Or you could play with winISD's recommended cabinet volume and take it from there. You don't have to spend too long modeling the volumes because you're not interested in having a lot of low frequency extension anyway. Having a sealed cabinet simplifies the process even more.
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Old 3rd July 2008, 08:49 PM   #5
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I would use tweeters that have a smaller frame than those of the original. THey need to be mounted very close together and have the outer two roll off at around 3000-5000Hz or at a frequency that is lower than the distance of the wavelength of the driver spacing so only one tweeter is operating at higher frequencies. Otherwise there will be sound quality issues.

I can only assume that the reason they used three tweeters was to get a lower crossover point to the mid/bass speakers and have smaller diaphrams driving the acoustic signal down to around 1500Hz . This whole scheme is probably to maintain a wider dispersion for keeping the frequency balance over a broader range of seating positions. And for increasing dynamic range.

I question this approach for a very small room. For a medium sized room with a wide listening area it may have better application.
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Old 3rd July 2008, 11:38 PM   #6
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So you wouldn't recommend a planar array instead of the usual dome tweeters of the originals?

I felt that the wider dispersion of a planar array with their limited vertical transmission was an ideal quality for a home cinema setup.

Would this be a poor choice then?
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Old 4th July 2008, 03:03 AM   #7
pjpoes is offline pjpoes  United States
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I thought I would comment, having made a few M&K clones myself (As a way to use some of the surplus stuff from their liquidation), I did get a good idea what they had designed for the crossovers. I must say, not what I expected from such an esteemed company, but not so terrible either. Though never mentioned, I believe these tweeters were used in a sort of Bessel array, similar to McIntosh. I talked with an M&K engineer once who told me the reason for the three tweeters was for controlled vertical dispersion, and to change the distance at which the highs would switch from radiation in 2pi to 4pi space.

I agree that a tweeter with a small faceplate, so that close placement was possible, is a good idea. A planar tweeter isn't a bad idea, its a fine choice, but....I think they are going to be more limited in output as compared with three tweeters. I mean, you can recreate the look, if thats all you want, but you must have 3 tweeters to achieve the effect they created in that particular design.

I think you are confusing two issues with your comment on the enclosure. First, yes you should always build an enclosure before designing the crossover. You won't know what the frequency response of the drivers are without a test enclosure. If you rely on the manufacturers specs, you will get inaccurrate results, as they were taken on an IEC baffle, and your enclosure will suffer from Baffle Step problems. Also, you don't know how the drivers will interact with the enclosure surface and what sort of baffle diffraction you will have.

If I may discourage you a little, this is a pretty complicated design for your first DIY attempt. I mean, to do this right, you really should read up on line arrays and bessel arrays, to begin to understand the interaction of multiple closely spaced drivers at different frequencies. It's the kind of speaker that could go very wrong very easily, and be quite expensive in the process.
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Old 4th July 2008, 03:37 AM   #8
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I have to agree with Pjpoes,

This speaker will be hard to implement a proper crossover for unless you have a lot of filter design experience. Also, a lot of speakers crammed into a small cabinet is not the best way to get good sound quality. It looks impressive but a simple quality built medium size floor standing speaker will blow it away sonically.

With HT you want dynamic and sensitive speakers. If it was me I would build three Elsinore type floor standing speakers for the front and some scaled down ones for the surrounds. These are very easy to drive speakers and would be perfect for HT in my opinion.

THe three tweeters of the M&K design would have the tendency to reduce vertical dispersion which might improve clarity of voice by reducing sibilant reflected sounds by a small degree. But the vocal primary frequencies are between 80-1200Hz. Which are handles by two mid/bass drivers crammed into a very small space which will not sound the most natural. This is the most important area to get right for center channel speakers. But then you are cramming two 6 inch drivers into this small box also. You can't usually get very good bass this way. If anything you should make the box taller with more internal space and forgo speaker stands. I actually heard those speakers once when I was auditioning audiophile speakers.

I though they were OK but fell way short of the audiophile speakers I was trying to clone. In terms of total sound quality and dynamics you can do a lot better. I would read the Elsinore thread and website before you proceed.
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Old 5th July 2008, 10:19 PM   #9
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Thanks for the replies.

I had a look at the Elsinore's and whilst they seem like impressive speakers, I already have 3 pairs of floorstanders around the house and I don't want to fill the A/V room with even more.

Where I work the audio director in his office has the active versions of the MK S-250's and the SS-250's in a 5.1 setup. I regularly listen to them when we are checking out the audio for the game. The bottom-end is catered with another MK, I don't remember which one though (it has twin 12's, I think in a push-pull configuration). I'm not worried about the lower octaves for now. I have a B&W ASW3000 (15inch driver, 18Hz +/-3db) that I will use until I get around to something different - for the moment it's more than adequate.

I like the compact nature of the MK's, their dynamic range and (to me) seem to have the spatial distribution of sound that puts me in the movie.

I'm not interested in making a speaker stuffed full of drivers for some cool factor (I'm 38 not 12...). Appearance is less important than sound quality and high dynamic range.

Most of the purpose-built LCR's that I see from the better quality manufacturer's tend to go for a multiple tweeter array. Now I understand this is not going to be an easy thing to do, but my work requires me to tackle difficult things that people haven't done before at a cost of around 14millionUSD for about 18months and get it right, so I'm no stranger to challenges. If it takes 2 months instead of 2 weeks to solve the problem, then so be it.

I looked at bessel arrays, but they all seemed to indicate you need at least 5+ drivers, so I'm a little unsure how using 3 tweeters (from the original) fits into that category?

Hezz, you mention that you'd need to roll-off the outer two until only centre is operating at around 3-5KHz otherwise you'd have sound quality issues - is the acoustic foam the use on the tweeters also trying to prevent interference between the waveforms eminating from each driver? In essence narrowing their dispersion?

If you want to know my degree of knowledge that relates to electronics. I have a Btec Nat Dip in Electronics and City Guilds 224 in Electronic Servicing. Those of you from the UK will know at what level that is. Those of you from the US, its probably equivalent to an associate degree in Electronics (its a 2 year full time course).

However, I was 18 when I finished that and I haven't used it much since, so my knowledge is a little rusty. We were never taught anything specific about audio during the course, instead built things like logic probes and signal injectors.

FYI, current room size is 20ftx15ft, though I will be moving in the next 3-4 months.

So my question is this, are there any places online or in literature you can suggest I read that deals with integrating multiple tweeters into a speaker design. Perhaps specifically, triple tweeters. If necessary I can change the two planar drivers to a more traditional 3 tweeter setup. If that becomes a recommendation, then I'd probably go for an all Vifa setup and look for something appropriate from their catalogue.

Thanks
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Old 6th July 2008, 12:16 AM   #10
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Quote:
Originally posted by RedCoat23
Thanks for the replies.

Hezz, you mention that you'd need to roll-off the outer two until only centre is operating at around 3-5KHz otherwise you'd have sound quality issues - is the acoustic foam the use on the tweeters also trying to prevent interference between the waveforms eminating from each driver? In essence narrowing their dispersion?

Thanks
Ya the foam might be there to help reduce a possible problem. There is a problem that is worse than dispersion. IF the tweeters are too far apart there can be some very uneven sound at different places in the sound field. If all three tweeters are operating up to the maximum frequency they must be placed closer together than the wavelength of the highest frequency otherwise their sound field will not acoustically integrate and if there are any small phase variations between the tweeters it will cause comb filtering and some phase cancellation. This will result in a very uneven sounding treble at different places in the room. Probably the foam is there to help reduce this problem. As the wavelengths are short at high frequencies it is difficult to get the tweeters close enough together to solve the problems.

So if the tweeters are say three inches apart a little less than a three inch wavelength is the maximum that all three tweeters can operate without a problem. However, near crossover frequencies there is going to be some phase shift which will cause a problem so the crossover frequency needs to be somewhat lower or at a larger wavelength then the tweeter spacing.

Down around 1500-3000Hz it is often useful to have more surface area than a 1 inch tweeter has to couple with the air or the speaker can sound a little thin. So operating two of the tweeters between say 1500-3000Hz give better air coupling, better dispersion than a larger cone, and better power handling than a single tweeter can do. But as you continue up the frequency spectrum the other problems arise.

You will probably need to talk to someone more knowledgable than I am for more information in this area.
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