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Old 27th December 2006, 06:34 PM   #1
bezeek is offline bezeek  United States
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Default Advanced crossover setup (Newbie Alert)

I've got a 2ch amp that pushes out 350w continuous (600w peak) per ch. at 8ohms. I'm wondering if there's a way I can utilize this to power 2 4ohm speakers and an 8ohm center as if they were 2 8ohm speakers, while still using them in as left, right, and center.

I'm a firm believer that almost anything is possible, so before I give up I'd like to talk to someone with great electrical knowledge. My thoughts are that I could accomplish this with a pair of custom-built crossovers between the amp and the speakers.

My left and right speakers I presume are 4-way, each with a horn, two identical mids, and a sub.

The center has two identical mids and a horn.

My idea is that perhaps a pair of external crossovers (pardon if the terminology should be different) to divide the signal into four parts, then reconnect two of the parts to drive the center speaker, while driving the left and right each with one of the remaining parts. I won't be particularly surprised if this is impossible, but I figure it never hurts to ask.

This rough figure attached may prove helpful to describe what I'm thinking.
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Old 28th December 2006, 08:46 AM   #2
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Default center speaker setups

i've seen center speaker setups over the years, and the simplest one is to connect a 16 ohm speaker between the two "hots", so that the speaker is driven by the difference between the L and R channels. of course this reduces channel separation a bit. another way to do it is to use a center channel amp, with a L-R subtraction device that gets it's signal from both channels' amp inputs, maintaining channel separation. this device is usually just a unity gain diff amp (or "instrumentation amp" in the op amp application guides). using a box such as this also allows you to select L+R for your center channel if you want. if you can find a copy of the APT-Holman preamp schematic, there is a circuit that allows you to have a continuously variable adjustment between subtraction and addition (in the preamp, it was for control from a stereo image, through a regular mono image, to a subtractive mono image. the image control was the basis for many "karaoke" vocal subtractor boxes.
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Old 28th December 2006, 01:58 PM   #3
bezeek is offline bezeek  United States
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Thanks for the reply.

You talk about using a 16ohm speaker for the center, but is the same thing possible with an 8ohm? I'm buying speakers from an old friend of mine, and although he has a selection of over 20 speakers total, I don't think any are 16ohm. I'm on a tight budget, so I can't really afford to buy one new, and I personally find the added harmonics of high-resistance speakers to be distasteful.

A little background may help. The amp I'm using is a Carver M-1.5t, and my goal is to get as much out of it as possible. I'll be buying a 7.1ch receiver to power the arrangement, using the amp for the surround L+R, and (hopefully) an added rear center driven by the amp as well. The rear surround channels will be used to bi-amp the front channels, and if I can find a solution for my question, a front center will be connected as well, resulting in a pseudo-6.1 system.

$100-200 is no problem for me if it means I can accomplish this goal, since I purchased the amp for well under its used cost, but that's the only headroom I've got in my wallet right now.

I assume slight loss of channel separation in the rear L+R won't be too detrimental to the whole experience, but please correct me if I'm wrong there.
If I'm right, could you elaborate on your first solution a bit? I'm a little new to some of the terminology & concepts still.

In the meantime, I'll do a little research on your other suggestions. Thanks again.

- Brian
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Old 29th December 2006, 10:27 PM   #4
Geoff H is offline Geoff H  Australia
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Hello Brian. First of welcome to the forum. Just a few tips to start with. Try not to ask for help from "someone with great electrical knowledge" or similar. It may frighten off modest types who have your answer, or suggestions. Also, it's a good idea to outline what you are trying to achieve in the initial question.

Now that we know you plan to obtain a 7.1 chan receiver, I think it's best to keep the Carver as the front main amp. Once you get the 7.1 on line, you can drive the additional speakers with the extra channels on the 7.1 receiver.

It is possible to derive a centre front channel from the front L&R using transformers. But then you will be putting money towards devices that will become obsolete when the 7.1 goes in. Put that money towards another dual channel amp if the 7.1 doesn't have enough power to match the Carver.

On that, make sure the 7.1 has preamp / line out sockets to bypass the internal front L&R channels. I am not familiar with current multi channel amps.

Your comment "I personally find the added harmonics of high-resistance speakers to be distasteful" is a bit odd to me.

Your reaction to 15 ohm speakers has possibly got more to do with the type of speaker rather than it's impedance.

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Old 1st January 2007, 06:29 PM   #5
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Bezeek, I think I just posted on one of your other threads on this related topic. You definitely can, in theory, do what you're asking with a Hafler circuit (and more simply than what you're thinking), but I also read recently (while re-researching the Hafler circuit this weekend) some warnings about some Carver amps not having a common ground. So, you'll have to determine whether or not that is the case with your amp, then find a diagram that works with that situation.

And you don't NEED a 16ohm speaker as long as your amp can handle low ohms. These weird wiring schemes put a hard load on the amp. So as long as the amp is, say, 2 or 3 ohm stable, you're probably OK. (Most receivers probably aren't.) So, if you used a 16 ohm speaker, the load wouldn't be nearly as bad. If the load is too hard for the amp, it'll overheat, and bad things might happen.

So, while I can't specifically comment on how to wire it in your case, I'm now enjoying 3 speaker channels from my 2ch amp....
(My amp is supposed to be stable down to 1 ohms! So, no worries here. )

And I definitely am not "someone with great electrical knowledge", just enough to be dangerous, thus I hesitate giving out too much specific information on the actual wiring.
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Old 2nd January 2007, 02:40 AM   #6
bezeek is offline bezeek  United States
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To clear something up, the reason I was thinking of using crossovers instead of just using some fancy wiring is that crossovers split the signal at a given point within its range. I've gathered from various articles that center channels are well suited for higher ranges, and I was going to dedicate the higher frequencies to them. They're all going to be relatively close together, so I would like to see how it sounds that way.

My problem is that I'm unable to determine how the crossovers need to be designed, and if arranging them as in the diagram I provided will be safe for the components.

>> The other thread you saw is just regarding a temporary solution for now. It's going to be a little while before I buy the rest of my equipment and take a shot at the wiring. Money is an unfortunate issue at the moment. Thanks though, if I do end up using the Hafler model I'm pretty sure you've given me all the info I'll need.

Geoff H:
>> Thanks for your advice. I only asked for people with good electrical knowledge because I have read a lot of conflicting opinions about using wiring tricks like the Hafler trick, so I was looking to discourage sub-guaranteed answers, but I will keep that (among other things I've learned) in mind for the future.

>> Under normal conditions I would use the amp for the front speakers, and just wire the center through the receiver, but my aim is for 6.1 surround sound, which requires two centers and the front speakers require less power & are the only ones skinny enough to fit on top of either side of my desk. Instead of buying a 6.1 receiver though, I want the two extra outputs (rear L/R) to use to bi-amp my front speakers.

>> As far as my concern of high-impedance speakers goes, I thought it may have just been the speakers I was listening to at first, but I've heard a lot of people with the same opinion. I suppose it's all personal taste though.

Thanks again guys.

- Brian
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Old 2nd January 2007, 12:25 PM   #7
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Instead of doing something strange with the crossover, you could just wire the speaker "normally" with a high-pass crossover, if you wanted to. Although, personally, I would think you'd want it to be full range, if you can get away with it! I think the problem is that most of the bass is going to be in the center channel, so if your center channel is already "smaller" than the mains, you may lose some bass. You can counteract this to some extent with the ol' bass control or an eq.; I don't know if a crossover would "force" the bass back to the other speakers, but maybe someone else can speak on that.

My personal opinion is to just keep things simple.

Thanks though, if I do end up using the Hafler model I'm pretty sure you've given me all the info I'll need.
I'm pretty sure I haven't , but hopefully you can find the rest of what you need and figure out the details. (I'm not even sure of all of the details! )

Good luck!
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Old 2nd January 2007, 04:30 PM   #8
bezeek is offline bezeek  United States
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Well, I was considering lining a high-pass filter up in front of the center, because it's going to be placed in the rear and at night my seat is closer to the rear then the front (this is my bedroom I'm wiring) so I like the higher frequencies (which most often include speech) to have predominance so they're more audible at lower volumes, but not much. However, all the speakers already have their own crossovers, and the center runs from 62Hz-20kHz, so it's not really necessary.

The problem with that though is that crossovers throw more impedance into the mix, and the center channel is already higher impedance then the others.

The crossovers aside, what I really need to know is what impedance will be expected of each output channel of the amp if I wire them both to that center speaker, or at least how to test that safely.
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