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Old 28th July 2013, 12:46 AM   #1
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Default Distortion in MTM build

I just finished building a pair of MTMs, using Zaph Audio's "Bargain Aluminum MTM" design.

This system uses Dayton DA175-8 woofers and the SEAS 27TBFC/G tweeter.
Crossover is at 1450Hz.

Testing with REW and a calibrated Dayton UMM-6 mic revealed high THD between 100 and 500Hz, tapering off above 500. Both speakers are similar. See plots (I included a shot of a measurement on another speaker I have, a Klipsch RF7-II, for comparison. It shows very low THD).

Notice that above the crossover point THD drops steeply. The tweeters, at least, seem to be on spec. Notice also that most of the distortion is odd-order. 2nd order is actually quite low.

Anyone have any idea what could be causing this distortion? Enclosure probs? Some wiring mistake in the crossover? Or are those woofers just crap? (Zaph's John Krutke thought highly of them).

Suggestions welcome!
Attached Images
File Type: jpg mtmLEQ.jpg (284.5 KB, 221 views)
File Type: jpg mtmREQ.jpg (266.8 KB, 211 views)
File Type: jpg klipschLTHD.jpg (277.2 KB, 208 views)
File Type: jpg mtmbld11.jpg (677.5 KB, 213 views)

Last edited by GaryM44; 28th July 2013 at 12:49 AM.
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Old 28th July 2013, 01:08 AM   #2
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Certainly the distortion level seems high. Probably around 10% on the THD plot up to 400Hz. That should be noticable when listening to music.

How hard were you pushing the drivers when doing the test? At normal listening levels, or much higher? You may be pushing the drivers past their X-Max during the test...

I'm about to start up my own thread as I have a big peak in distortion at 600Hz with my MTM's, and would like to work out what might be causing that.

Just looked up the specs and x-max is 4.25mm which is pretty decent, have you modeled to see what power they should theoretically handle before exceeding x-max?

Tony.
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Old 28th July 2013, 01:47 AM   #3
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How hard were you pushing the drivers when doing the test? At normal listening levels, or much higher? You may be pushing the drivers past their X-Max during the test...
The 85 db shown is accurate. My normal listening level. Not sure how much power that represents. Should measure it, I guess. TS specs say 50W/100W.

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I'm about to start up my own thread as I have a big peak in distortion at 600Hz with my MTM's, and would like to work out what might be causing that.
What drivers are you using?

I originally planned to use the SEAS ER18RNX in these MTMs. But Zaph thought these were a good alternative, and they were 1/3 the price. May have to buy the ER18s anyway. At least the enclosure is the same.
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Old 28th July 2013, 01:53 AM   #4
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Are these drivers in a single enclosure or divided between them?

If not, add a partition. Easy enough to do and will help. They sure look nice
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Old 28th July 2013, 02:34 AM   #5
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Are these drivers in a single enclosure or divided between them?

If not, add a partition. Easy enough to do and will help. They sure look nice
No. Single compartment (xover is in separate compartment at bottom). That's what Zaph recommended.
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Old 28th July 2013, 02:51 AM   #6
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There is evidence that when more than one driver shares an enclosure this causes interaction due to plane waves distorting the shape of the opposing cone. This effect is in both time, affecting phase/time smearing and frequency. It's about 15dB down in level.

It certainly wouldn't hurt to try my suggestion, besides if it costs you a few dollars to reduce that distortion wouldn't it be smarter than simply swapping out for drivers which will set you back hundreds?

I like John a lot, quite a few people here and elsewhere have my sincere respect. Yet also know not one person 'knows it all'.
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Old 28th July 2013, 11:12 AM   #7
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In a symmetrical cabinet, as this appears to be, the partition will make no difference. If we had a cabinet with no stuffing we can imagine a back wave from woofer A running over to woofer B some distance away. At the same time a back wave from B runs over to A. Add a partition and the back wave from A runs to the partition and reflects or reverses course. When it comes back it is absolutely indistinguishable from the previous back wave of B.

Add proper stuffing and it becomes moot as the mid range component is easily fully absorbed.

Anyhow, distortion as shown is not related to any in-cabinet effect. If those measurements are accurate then that would be inherent in the LF drivers. Some drivers have high midrange distortion from magnetic effects. I would also be suspicious of my measurements. Are you sure there isn't filter leakage of some kind giving high distortion readings? Try measureing the amplifier output bypassing the speaker and mic. Does that measure low?

David
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Old 28th July 2013, 01:42 PM   #8
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Quote:
Originally Posted by GaryM44 View Post
The 85 db shown is accurate. My normal listening level. Not sure how much power that represents. Should measure it, I guess. TS specs say 50W/100W.
If it is actually 85db (I'm assuming at 1M) then you definitely shouldn't be exceeding xmax, and you won't be using anywhere near 50W.

Note however that driver specs are how much power the driver can handle without burning out the voice coil, not how much power they can handle without distorting. Moreover the amount of power that they can handle without distorting depends on the implementation.

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What drivers are you using?
Morel MW144's

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I originally planned to use the SEAS ER18RNX in these MTMs. But Zaph thought these were a good alternative, and they were 1/3 the price. May have to buy the ER18s anyway. At least the enclosure is the same.
Something doesn't seem right. I very much doubt that one of zaph's designs would have such high distortion. Do they sound distorted when you listen to them?

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Old 28th July 2013, 04:15 PM   #9
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Are you sure there isn't filter leakage of some kind giving high distortion readings? Try measureing the amplifier output bypassing the speaker and mic. Does that measure low?

David
That's why I included the measurement from the Klipsch. Exact same setup (same amps, etc). It has to be in the speaker.
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Old 28th July 2013, 04:25 PM   #10
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Something doesn't seem right. I very much doubt that one of zaph's designs would have such high distortion. Do they sound distorted when you listen to them?

Tony.
Yes. Its quite noticeable on female vocals. Of course, the distortion has be in the very band where it is most noticeable.

I agree that it is odd. No quality speaker has distortion that high in that band. I'm thinking it's some kind of odd anomaly, like something rubbing the cone, etc. But it's occurring with both speakers. Other oddities are that the FR is quite smooth (better than the Klipsches) and that the distortion is high-order. Usually 2nd order is the biggest component of THD.

Last edited by GaryM44; 28th July 2013 at 04:27 PM.
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