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Old 22nd June 2003, 11:35 PM   #1
Guss is offline Guss  Canada
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Default Lowpass , two 8ohms speaker

The question is:
Does two 8 ohms midbass drivers hook on the same lowpass should be considered as a 4 ohms load? I have to fit my speakers imp. to my amp tap ( 8 ohms ), what should I do?
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Old 23rd June 2003, 01:01 AM   #2
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in parallel you will get 4 ohms and in series you will get 16 ohms...only way to rectify this is to use a resistor in series...drawback is.... it will have to be quite significant in wattage and you are going to waste alot of power from your amplifier
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Old 23rd June 2003, 01:07 AM   #3
Guss is offline Guss  Canada
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And what about inductors value, they wont be the same for a 4 and a 8 ohms load so what to do?
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Old 23rd June 2003, 04:40 AM   #4
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If you are going to modify your crossovers, then cut the value of the inductors in half and double the value of the capacitors.

One thing-in addition to what Joe DIRT said-if you do add the resistor in series with the woofers, then the Qts of your woofers will go high and you will have a pronounced "boom" in the bass range, unless you adjust your whole speaker system to account for it.

In other words, like Joe said, it is possible but definitely not recommended.

Other things: At any given drive level, the two paralleled woofers will play 6 dB higher than if you had just one woofer hooked up. So an 89 dB woofer becomes a 95 dB pair of woofers when the second one is added.
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Old 23rd June 2003, 04:56 AM   #5
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Here is an alternative.

Hook the two 8 ohm midbasses up in series, add an 8 ohm tweeter, and hook it up to your 8 ohm tap.

While the impedance will be doubled, the fact is that two 8 ohm midbasses hooked up in series, with the same voltage level, will play just as loudly as one 8 ohm midbass. So you won't be hurting anything.

Assuming you are using a parallel network, you can double the value of the inductors and cut the value of the capacitors in half.

Although it is somewhat dangerous to give advice on these things without actually being there and testing, I think these things will give you a good chance to accomplish what you are trying to do without getting hurt too bad.
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Old 25th June 2003, 01:10 PM   #6
Roscoe is offline Roscoe  Australia
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I too have the difficulty of having to deal with an amp requiring 8 ohms impedence.
A speaker newbie, I had been looking at building a set of MTM speakers using Vifa tweeter/woofer combo (D19SD/P17SJ) a not expensive yet available set of shielded drivers. Given they are both 8 ohms, I don't think I can manage this. The woofers in series would seem to give a large impedence especially given the frequency/impedence response vifa's website shows. Would that really make much difference? Especially when the impedence for the tweeter will only be 8 ohms??

An inline resistor is clearly not the way to go. I might justl have to give up the MTM thoughts.... unless anyone has any viable options/pointers??




Woohoo.... first post, hopefully not last.
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Old 26th June 2003, 04:47 AM   #7
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Roscoe:

Perhaps I did not explain this well in my last post.

High impedance is not a problem for an amp. If you have two woofers, one 8 ohm that plays 90 dB on 2.8 volts and one 16 ohm that plays 90 dB on 2.8 volts, then an 8 ohm amp has no problem driving either speaker. In fact, the 16 ohm speaker puts less strain on the amp!

The problem for an 8 ohm amp is low impedance. There, your amp will have trouble, because a 4 ohm speaker demands too much current for an 8 ohm amp to provide. A 16 ohm speaker, (or two 8 ohm speakers wired in series to produce a 16 ohm speaker), will not demand that current.

Hooking two 8 ohm speakers in series to get a 16 ohm speaker will yield no problems to an 8 ohm amp. You just have to adjust your crossover to the tweeter.
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Old 26th June 2003, 11:26 AM   #8
Roscoe is offline Roscoe  Australia
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Thanks for the reply kelticwizard. Appreciate it.

I see what you are saying.... 16 ohm not such a bad thing after all.

Would compensating for an 8 ohm tweeter in the crossover be a simple task? I'm just wondering in light of the general avoidance of a resistor in line to increase impedence. I'm slowly learning about crossovers (amongst everything else) but still a huge ways to go...

Cheers
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Old 27th June 2003, 05:28 AM   #9
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Roscoe:

I see you are doing an MTM. I have never done an MTM. However, I can explain a few things.

Do not put a resistor in line with the tweeter. This will
A) Cut down output
B) Change the Qts of the tweeter. If you cross over 2 octaves above the resonance frequency, it probably doesn't make much difference. Still....

Use an 8 ohm tweeter. Your speaker will be 16 ohms in the midbass range, 8 ohms in the tweeter range. If I were an 8 ohm amplifier, I would ask, "Yeah, so what?". Remember, driving a 16 ohm load is easier for an 8 ohm amp.

If you have a crossover handy for an MTM which uses parallel crossover, then you can treat the woofer portion of the crossover different from the tweeter section.

Crossovers are difficult and tweeter spacing from the centers of the woofers are important. What midbasses where you planning on, and what tweeter, if any?

I have never done this, but you can adapt the woofer branch of a parallel crossover by adjusting the value of the inductors upward and the value of the cpapcitors downward.

The best thing to do is to measure. Have any woofers or tweeters in mind?
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Old 27th June 2003, 01:03 PM   #10
Roscoe is offline Roscoe  Australia
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kelticwizard,

A MTM design is the current idea, but by no means guaranteed. I have plenty of research left to do, of which you are helping with and I'm greatful.

The tweeter I am currently considering is a Vifa model (D19SD-05-08 http://www.d-s-t.com/vifa/data/d19sd-05-08a.htm )
And the midbass another Vifa model (P17SJ-00-08 http://www.d-s-t.com/vifa/data/p17sj-00-08a.htm )

Both chosen as they are a name brand, shielded, cheap and I think reasonably available. Again, not final, but a starting point for consideration.

And I was planning to use SpeakerWorkshop to test speakers so all can be optimised.

Cheers
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