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Author Topic: What kind of problem have I created?  (Read 647 times)

Steve Crump

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What kind of problem have I created?
« on: October 11, 2019, 01:52:41 pm »

I have 4 of the Yamaha CM12V monitor cabinets that have spent most of the time in storage for the past couple of years. I have tried twice to sell them with no luck. Anyway just for fun I ordered some coaxial drivers to modify the cabinets, I found some drivers that seem to have the same crossover requirements, which matched the specs given by Yamaha for this particular box. Box size based on documents I found online seems to work for the new drivers and even the port size seemed to work based on some info on the Parts-Express site. When the drivers came the first thing that had to be done is modify the cabinets because the new drivers needed a larger opening, so now the old drivers can't be reinstalled without some major work.

On to my problem. I didn't notice until I was well into this project that the existing Yamaha drivers were rated 8ohm for the 12" driver and 16ohm for the CD. Actually I was done when I noticed, I was cleaning up and picked up the Yamaha drivers trying to decide whether to toss or to keep and that's when I notice the 16ohm rating on the CD. The new coaxial driver is 8ohm for the 12" and 8ohm for the CD. So, what kind of issue can I expect from long term use?, being that the existing passive crossover was designed to work with a 16ohm CD and now there is a 8ohm CD? Do I need to search for a replacement crossover?

I mean it is too late to back up now, my concern is more with reliability. I would hate for the CD to stop working in the middle of a show.

They sound fine, I did a A/B test before I modified all four. I had someone come in the shop and listen without knowing which cabinet was modified and they immediately picked the coaxial, said it was smoother and clearer. My intended use for these would be more acoustic type shows, Bluegrass, Country, Folk, so I am not planning on pounding away on them.     
« Last Edit: October 11, 2019, 02:30:28 pm by Steve Crump »
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Bob Faulkner

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Re: What kind of problem have I created?
« Reply #1 on: October 11, 2019, 02:15:44 pm »

For your intended use of the speakers, I don't seen any issues.  Your amplifier will see a slightly greater load on its output, but this should not present any issues with how the amplifier will function; you will still be able to daisy chain speakers if needed.

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Mike Caldwell

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Re: What kind of problem have I created?
« Reply #2 on: October 11, 2019, 05:33:13 pm »

A passive crossover design in addition to other things is dependent on the impedance of the drivers to operate as designed.

Going to an 8 ohm high frequency more or less doubled the high frequency crossover point, if it was originally 1500hz now it's closer to 3000hz.

Can you get 16 ohm diaphragms for the drivers?

 

Chris Grimshaw

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Re: What kind of problem have I created?
« Reply #3 on: October 12, 2019, 04:46:18 am »


Going to an 8 ohm high frequency more or less doubled the high frequency crossover point, if it was originally 1500hz now it's closer to 3000hz.


Here's what happens when you take a passive crossover (in this case, it's a textbook BW12 at 1kHz, and the speakers are 8ohm resistors) and double the impedance (I just put another one in series):



IMO, any change in driver selection mandates a load of measurements and a crossover modification or re-design.

Chris
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Steve Crump

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Re: What kind of problem have I created?
« Reply #4 on: October 12, 2019, 11:09:44 am »

A passive crossover design in addition to other things is dependent on the impedance of the drivers to operate as designed.

Going to an 8 ohm high frequency more or less doubled the high frequency crossover point, if it was originally 1500hz now it's closer to 3000hz.

Can you get 16 ohm diaphragms for the drivers?

I am going to check on diaphragms just in case...
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Steve Crump

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Re: What kind of problem have I created?
« Reply #5 on: October 12, 2019, 11:28:07 am »

Here's what happens when you take a passive crossover (in this case, it's a textbook BW12 at 1kHz, and the speakers are 8ohm resistors) and double the impedance (I just put another one in series):



IMO, any change in driver selection mandates a load of measurements and a crossover modification or re-design.

Chris

I wonder if the original design intent was to pad the horn? At least (looking at the graph) it looks like the higher impedance attenuates above and below crossover.

When I have time I will setup the cabinets and take what limited measurements I have equipment for and see what I find. Worse case, if something looks off I will buy a crossover from Parts Express and see what happens.
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Riley Casey

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Re: What kind of problem have I created?
« Reply #6 on: October 12, 2019, 05:44:58 pm »

I think you've missed the point of the diagram.  This is an imaginary set of filters showing the effect of terminating a filter in the wrong impedance for that set of filter components.  Not only is the filter now doing unpredictable things it's also ringing.  Bottom line is that the passive components will only do what you expect if you terminate them in the expected impedance.  One very cheap & easy way to test this is to put an 8 ohm resistor in series with the new 8 ohm HF voice coil.  Now the crossover will see a 16 ohm load and will act as designed.  Down side of course is that you've created a voltage divider and will reduce the level to the driver. This might or might not be usable but it will be informative.

I wonder if the original design intent was to pad the horn? At least (looking at the graph) it looks like the higher impedance attenuates above and below crossover...

Steve Crump

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Re: What kind of problem have I created?
« Reply #7 on: October 12, 2019, 11:06:46 pm »

I think you've missed the point of the diagram.  This is an imaginary set of filters showing the effect of terminating a filter in the wrong impedance for that set of filter components.  Not only is the filter now doing unpredictable things it's also ringing.  Bottom line is that the passive components will only do what you expect if you terminate them in the expected impedance.  One very cheap & easy way to test this is to put an 8 ohm resistor in series with the new 8 ohm HF voice coil.  Now the crossover will see a 16 ohm load and will act as designed.  Down side of course is that you've created a voltage divider and will reduce the level to the driver. This might or might not be usable but it will be informative.

You are correct, I did miss the point. I think at this point I am just going to buy passive crossovers based on the info provided by the driver manufacturer, which seems to be the simplest solution.

Thanks for the input..
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Scott Holtzman

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Re: What kind of problem have I created?
« Reply #8 on: October 13, 2019, 03:44:14 am »

You are correct, I did miss the point. I think at this point I am just going to buy passive crossovers based on the info provided by the driver manufacturer, which seems to be the simplest solution.

Thanks for the input..

Have you seen these?  http://www.usspeaker.com/radian%20324-1594-1.htm they have some slick features.  May be a good match for your application.

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Geert Friedhof

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Re: What kind of problem have I created?
« Reply #9 on: October 13, 2019, 05:46:39 am »

You are correct, I did miss the point. I think at this point I am just going to buy passive crossovers based on the info provided by the driver manufacturer, which seems to be the simplest solution.

Thanks for the input..

Or a crossover and biamp.
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Re: What kind of problem have I created?
« Reply #9 on: October 13, 2019, 05:46:39 am »


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