MTM Crossover Design to Achieve 8ohm?

[zachary]

Hello everyone, I am suffering from crossover confusion...

I purchased a pair of MTM boxes and have created my short list of drivers, but I want to make sure that the impedence load on my receiver is a nominal 8ohms.

It seems that most MTM designs out there are 4ohm, and I understand why... 2 8ohm drivers wired in parallel = 4ohm load.

Is the solution here to use 2 4ohm drivers and design a crossover that wires the drivers in series?

Are there other alternatives that I am missing?

Frustrating thing is that the boxes are cut to accept a 7" driver and there are not a ton of choices out there (at least compared to 6.5"), and even fewer choices for a 4ohm 7" (Vifa is probably the only one in my price range).

Thanks for your help!!
Zach

[bogoes]

If you are willing to go with 5.25" drivers instead of 6.5/7" you can getter better midrange and just about as much bass. GR Research makes their M-130 in a 16-ohm version that you can parallel for an 8-ohm load. Lots of people have worked with the M130s and loved them, they have a nice response graph, and they only cost $26 a piece.

GR Research M-130/16

Good luck.

[BillFitzmaurice]

Use 4 ohm drivers wired in series, but only if you have to. Is your amp restricted to 8 ohm loads?

[zachary]

Thanks for the responses so far.

In terms of using 16ohm drivers... good idea, but unfortunately the boxes I purchases are already cut for 2 7" drivers and 1 3" tweeter each. I did a brief search and could not find any 16ohm drivers that were 7" (approx 180mm). GR Research at one point offered a 6.5" 16ohm (GR-165), but it seems it is no longer available.

I am currently using a Yamaha RX-V2400 integrated receiver. I called Yamaha and they said that it was rated to handle up to a 4ohm load, but below that it would probably cut out.
The room that these would be used in is fairly large and somewhat sound-proofed, so I often play music/TH relatively loud => more volume, more strain on the amp.

What are the disadvantages/challenges associated with wiring the drivers in series?

Thanks!

[noodle_snacks]

I Really wouldn't worry. My Yamaha RXV-640 (quite a few models below your own) handles my speakers which are 4Ohms extremely well without cutting out etc. There is usually an impedance switch at the back of the yammy amps, set that to the lower if it is the same for the 2400

[RHosch]

Wiring the drivers in series decreases efficiency and increases inductance. The former may not be any better re: your receiver's ability to drive them to the desired SPL, depending on it's voltage/current capabilities. The latter affects high frequency response of the drivers, which may or may not be a problem depending on the specific driver and the XO point/slope chosen.

Given the options (drivers) available, I'd tend to risk the parallel 8 ohm route. The increase in efficiency may be enough to prevent amp shutdown, and the decrease in inductance is generally a 'good thing.' If all else fails, you can always add a used outboard amplifier for a reasonable cost--I'd opt for the better and more future-looking speaker design instead of making compromises based on your current receiver.

[wharfrat]

After finishing building my Shamrock se-1's (a 4ohm mtm with vifa 6.5s) I auditioned the Yamaha 2400 and the Denon 3805. (I ended up buying the Denon because I got a better deal on it, not because I didn't like the Yamaha.) I played the Yamaha at very high volumes - I was testing it, after all - and it had no problems with the 4ohm load. I had it cranked for over two hours; no shutdown. I think the efficiency advatages of a 4ohm mtm are worth it, and that the Yamaha will handle it.

Larry M

[BillFitzmaurice]

Wiring the drivers in series decreases efficiency and increases inductance.

This isn't necessarily so. First, efficiency does not enter the equation. What you probably are referring to is sensitivity, and wiring two drivers in series results in a sensitivity loss of 3dB from the increased impedance, but a gain of 3dB from the doubling of radiating area, so the net result is a wash- no change in sensitivity.

The inductance issue is ameliorated by the fact that a 4 ohm coil has less inductance than an 8 ohm coil, all other factors being equal, and the increased inductance that may result is often actually beneficial, rolling off the high end outside the intended passband at a steeper slope.

If it were me I'd make the decision based on to sub or not to sub. The higher woofer sensitivity at 4 ohms is always helpful in overcoming the baffle step, and if this receiver is typical a second set of remote speakers will be series wired anyway. If you plan on having a sub that will take care of the high power requirements on bass passages; if you don't and want the ability to play loud go with the two 4 ohm drivers in series.

[RHosch]

Ah, yes, I mixed my nomenclature.

And made the common mistake of forgetting about conversions between db/W and db/V RMS sensitivity ratings.

[bogoes]

I have a Yammi receiver as well. You won't have any trouble driving a 4-ohm load with it, and I've never heard of anyone with a Yammi having trouble with a 4-ohm load.