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Old 01-25-2007, 07:41 PM   #1
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Speakers & Watts & Amps. Ohm My!

The thread from last week about the PnP rear spkrs got me thinking about replacing my OEM 3.5s. But before I go out and buy 4 spkrs that are wrong for my setup, I thought I better do some research. This has raised some questions and who better to ask than the experts?

1. Bentley says the amp provides 2x40w to the dash spkrs and 2x70w to the doors. Nothing mentioned about the rear spkrs. Does the amp provide pwr to them and if so, how much?

2. The owners manual says the min spkr ohm is 4. Each pair of rears are wired together (in seq?). Do you add the ohms in that scenario? The reason I ask is our local mega store has some expensive Infinity Kappas that he thinks I should buy but they are 2 ohm. If they are in seq and you add, these would be OK. If not, they would not meet the min req. I'm not sure I'm going to spend $200 for 4 spkrs, but other brands also sell 2 ohm spkrs and I would like to know if I should avoid them.

3. The owners manual says the cdr220 radio has 4x18w power. If the amp isn't supplying pwr to the rears, should I buy spkrs that are less than 18w? What happens if I buy a 25w min spkr and the power is 18w?

4. There are 11 diff 3.5" spkrs listed on CarDomain. I figured I would disregard the high and low end models and concentrate on the mid-priced ones. I'm a bit uneasy about buying something as subjective as spkrs off of a list without hearing them. I do plan to call them and see what they suggest, but does anyone have a fav they would suggest?

As always, thanks for you input.

Terry

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Old 01-25-2007, 10:01 PM   #2
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Quote:
Originally Posted by husker boxster
The thread from last week about the PnP rear spkrs got me thinking about replacing my OEM 3.5s. But before I go out and buy 4 spkrs that are wrong for my setup, I thought I better do some research. This has raised some questions and who better to ask than the experts?...
As always, thanks for you input.

Terry
Hi,

You fail to mention model year and whether or not you have the Bose system. Assuming you have the standard system, here goes:
  • 1. For model year '97-'02, in both the Basic setup - M441 (two dash mounted speakers only), and the M490 option (6 speakers, incl. door speakers), the radio's amplifier itself powers the rear speakers while the external Amplifier powers the Dash and Door speakers (M490 only).

    But, for '03 and up m.y., the radio powers the rear speakers for the M441 option, but, the rear speakers plug into the external amplifier if you have the M490 option. All the Door speakers are wired to the Ext. Amplifier.

    2. For speaker wiring, you add the specified speaker impedence if you wire them in Series (so two 4 Ohm speakers wired in series = 8 Ohms total impedence).

    If wired in Parallel, you divide the impedence of a single speaker by the total number of speakers paralleled together (so wiring four 8 Ohm speakers in parallel = 2 Ohms total impedence ). See Diagram below for a visual example.

    3. If you use speakers with a minimum power rating which is higher than the radio output, the speakers will not have a very good volume, or gain, and the voice coils in the speakers themselves will wear out much sooner.

    4. This is always a leap of faith, whether buying off a list or from a Stereo Store. Each speaker will sound differently in a different environment. The store is not your car, and someone else's Accura is not your Boxster. That said, you'll get the best idea by hearing them in person at a stereo shop.

    But, the Speakers are literally the business end of any system - they're what actually makes the sound, so you don't necessarily want to scrimp here, buy the best you can afford. Boston Accoustics, MB Quart, Focal, JL Audio are the high-end products and are usually low-impedence speakers. Find the ones you like at a retailer and sample them in person, then shop them online to get the best price over retail.

Hope this helps...

Happy Motoring!... Jim'99

Last edited by MNBoxster; 01-26-2007 at 11:32 PM.
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Old 01-26-2007, 05:13 AM   #3
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Great explanation MN.
And for cases when you have speakers of differing impedances the formulas for the total impedance is:

serial connection: r = r1 + r2 + r3 +...
parallel connection: 1/r = 1/r1 + 1/r2 + 1/r3 + ...

By combining these two formulas you can calculate the total impedance of any complex circuit of speakers (resistors).

Z.
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Old 01-26-2007, 08:22 PM   #4
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As always, great information. Thanks.

BTW, I have MY01 with a basic M490 system. I added the rear shelf OEMs a yr ago.

Had I been thinking, I could have answered Q1 myself since I wired the OEMs into the radio and not the amp. Guess I'm excited about actually getting some sound out of this location and my excitment must have "impeded" my brain.

But I have 1 more question. The current setup is parallel. I have not seen any 3.5s with more than 4 ohms. With 2 4 ohm spkrs per side, the parallel formula says I have impedence of 2. The manual says a min of 4 is required. If I do a little snipping and soldering, I get 8 if I turn it into series. I'm assuming that's what I should do, but what are the consequences of leaving it at 2?

I'll hang up and listen to your response.

Terry
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Old 01-26-2007, 11:30 PM   #5
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Quote:
Originally Posted by husker boxster
As always, great information. Thanks.

BTW, I have MY01 with a basic M490 system. I added the rear shelf OEMs a yr ago.

Had I been thinking, I could have answered Q1 myself since I wired the OEMs into the radio and not the amp. Guess I'm excited about actually getting some sound out of this location and my excitment must have "impeded" my brain.

But I have 1 more question. The current setup is parallel. I have not seen any 3.5s with more than 4 ohms. With 2 4 ohm spkrs per side, the parallel formula says I have impedence of 2. The manual says a min of 4 is required. If I do a little snipping and soldering, I get 8 if I turn it into series. I'm assuming that's what I should do, but what are the consequences of leaving it at 2?

I'll hang up and listen to your response.

Terry
Hi,

Part of your issue may be that the existing speakers are not identical. They are a mid-range and a Base speaker. In the wiring of speakers that use passive crossovers the impedance is affected only within the frequencies that the speakers share.

In other words, assume you have a midrange that plays from 150Hz up to 2Khz, and want to add a tweeter that plays from 2.5Khz up to 30Khz, and both are 4 ohm and have crossovers on them that only allow them to play within their ranges, then they will only present a 4 ohm load when wired in parallel, since they do not play within each other's ranges. If you were to then add a second identical midrange with an identical crossover in parallel, the impedance would be 2 Ohms from 150Hz to 2Khz and 4 Ohms from 2.5Khz to 30Khz.

Nominal Impedance is the manufacturer's specification given for the speaker. It is meant to represent the average impedance a speaker presents to an amp. One thing you can do is to wire a resistor into the circuit. Go to a high-end custom stereo shop (or even a Radio Shack) and get the proper resistor to drop the load into the required range. Hope this helps...

Happy Motoring!... Jim'99

Last edited by MNBoxster; 01-26-2007 at 11:33 PM.
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Old 01-27-2007, 06:17 AM   #6
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Quote:
Originally Posted by MNBoxster
Hi,
One thing you can do is to wire a resistor into the circuit. Go to a high-end custom stereo shop (or even a Radio Shack) and get the proper resistor to drop the load into the required range. Hope this helps...
Happy Motoring!... Jim'99
Not that simple. For example, if the load you're looking for is 4 ohms, but the speakers are going to give you a 2 ohm load, you'd need a 2 ohm resistor in series with the speakers to bring it back to 4.

However, you've just divided the power between the speakers and the resistor. In simple terms, if you had 40 watts available to drive the speakers, now you're applying 20 watts to the speaker, and 20 watts to the resistor. Fortunately, you haven't halved the sound produced by the speakers, you've reduced it by 3 - 6 dB, depending on how it's measured. You can probably adjust the fader to balance things back out.

The other problem is the resistor. I seriously doubt Radio Shack will have it, as it will have to be a Power Resistor. It will be dissipating, as heat, up to 20 watts peak in my example above. That's a pretty specialized, big, and hot resistor. A little 1/4 watt resistor from RS won't do the job.

Best thing to do is to stay within design limits and avoid bandaids like this.
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Old 01-27-2007, 09:11 AM   #7
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Originally Posted by JackG
Not that simple. For example, if the load you're looking for is 4 ohms, but the speakers are going to give you a 2 ohm load, you'd need a 2 ohm resistor in series with the speakers to bring it back to 4.

However, you've just divided the power between the speakers and the resistor. In simple terms, if you had 40 watts available to drive the speakers, now you're applying 20 watts to the speaker, and 20 watts to the resistor. Fortunately, you haven't halved the sound produced by the speakers, you've reduced it by 3 - 6 dB, depending on how it's measured. You can probably adjust the fader to balance things back out.

The other problem is the resistor. I seriously doubt Radio Shack will have it, as it will have to be a Power Resistor. It will be dissipating, as heat, up to 20 watts peak in my example above. That's a pretty specialized, big, and hot resistor. A little 1/4 watt resistor from RS won't do the job.

Best thing to do is to stay within design limits and avoid bandaids like this.
Hi,

Thanks for the caveats. I knew, but didn't mention, the loss of power and heat generation associated with adding a resistor to the line. There are such resistors though and even radio shack sells them. About $1.25 for a 2-Ω, 20W resistor. But, I wouldn't use them for all the reasons you state, just stating that you could.

The issue about matching impedence is really one of preventing overheating, or local hotspots, in the Amplifier. This was especially true with Tube driven ones. But, modern solid state IC amplifiers do not use matched impedances at all, contrary to myth.

The driver amplifier has a low output impedance such as < 0.1 Ω and the loudspeaker usually has an input impedance of 4, 8, or 16 Ω - many times larger.

This type of connection is impedance bridging, and provides better damping of the loudspeaker cone to minimize distortion, which is why it's used. Old style tube type audio amplifiers, required strict impedance matching for proper, reliable operation.

So, to directly answer Huckster's question, with an IC AMP, you can usually go up at least one, and often 2, level(s) of impedence with no real ill effects. I would go one for sure, (stepping up another would not be for a unit which plays loud or base-filled music because you could get into trouble).

Watch for Clipping and if so (which I doubt), turn down the gain on the Amp. You generally cannot go down an impedence level with an IC Amp (though you could with a Tube Amp). Hope this helps...

Happy Motoring!... Jim'99
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Old 01-27-2007, 10:31 AM   #8
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Quote:
Originally Posted by MNBoxster
Hi,

The issue about matching impedence is really one of preventing overheating, or local hotspots, in the Amplifier. This was especially true with Tube driven ones. But, modern solid state IC amplifiers do not use matched impedances at all, contrary to myth.

The driver amplifier has a low output impedance such as < 0.1 Ω and the loudspeaker usually has an input impedance of 4, 8, or 16 Ω - many times larger.

Happy Motoring!... Jim'99
The problem with the new amps is not so much impedance matching (as it was in older tube amps), but the specs of the final output devices (or ICs).

It's basic ohms law. Current is voltage divided by resistance (I=E/R). Power is voltage times current (P=ExI). An audio amp will attempt to send the same voltage out to the speaker regardless of the impedance (resistance) of the speaker attached. If you cut the impedance in half, at the same voltage, you double the current flow. Since P=ExI, you've doubled the power. That's why most amps that are rated to drive 2 ohms show they deliver nearly double the power at 2 ohms than at 4 ohms.

The problem is that if the amp is not rated to drive 2 ohms, the final output stages of the amplifier will not be able to deliver that much current. It's internal power supply can also be a current limiting factor.

The results are overloaded electronic devices in the amp, which results in heat and device failure. Also, while the amp is still working, it will be clipping (as it runs into it's own capability ceiling) and that's the worst thing you can do to a speaker. You will damage a speaker much quicker by clipping the amplifier than by overpowering the speaker with clean power. Speaker voice coils *hate* DC current, and that's what a clipped signal is. Clean audio power is essentially AC current that is changing continously to match the vibrations of the sound of the music it represents.

Whew! Enough of that crap. I'm going to drive the Box.
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Old 01-27-2007, 09:39 PM   #9
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Once again, great info - but we're starting to swim at the deep end of the pool. I think I've got the idea why you don't want spkrs with fewer ohms than the radio or amp is putting out. I will wire my spkrs in series and have ohms to spare.

I bought some Kicker KS35s today and have been working all afternoon / evening trying to get them in. They are bigger in all dimensions than the OEMs. Thought I had a soln all worked out but I ran into a clearance problem during a final check. Plan B also had a clearance problem in a different location. Went back to the original plan and modified it. Looks like everything is fitting properly now - on one side. The other side is untouched but now I have a pattern so it shouldn't take long. I was hoping to have the whole project done today as I have a nice 4 hr road trip on the agenda for tomorrow. Guess I'll have to enjoy the new spkrs on my trip in to work on Mon, but it's always better to do a good job versus doing a hack hurried job.

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