# Jamo D7 Sub THX Ultra 2 Motion Compensation Circuit Failure - Repair or Replace



## DLB (Nov 4, 2008)

I have a Jamo D7 Sub that includes a closed loop motion compensation circuit. This consists of a sensor glued to the 15" subwoofer driver and a circuit introducing compensation back into the signal path. This circuit has failed. I don't know if it is the sensor or something in the amplifier. At first, I thought the voice coil on the driver was going bad, because at very low volume, the sub appeared to work, but then I could observe large excursions and associated sound when turned up slightly. I almost ordered a replacement driver when I decided to disconnect the sensor and the problem went away. Sub working again. Yay! ….except now an awful boomy, muddy low end has replaced the once tight and well controlled sound. 

Does anyone have experience with this issue? Recommendations for repair? or with advancements in subwoofer technology and competitively priced options am I better of just buying a replacement subwoofer? 

I don't want to spend too much money and was looking at SVS and monoprice subs in the $400-$600 range, but want want something tight and fast. Open to suggestions.

Thanks.


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## Da Wiz (May 8, 2019)

There is no such thing as a "fast" subwoofer. That's a bunch of ** repeated so many times since the 1970s, people think is is something real when it is just something dreamt-up by a non-technical reviewer to "explain" something else entirely. If a subwoofer sounds "slow" there is just 1 cause... poor integration with the other speakers it is being used with. The crossover slope or crossover frequency could be "off" of the ideal settings. There could be a phase error that can only be fixed by physically moving the subwoofer in 2-inch increments until the integration through the crossover range is more suitable. If the subwoofer is not the same distance from each of the main speakers, you can end up with integration issues also. 

Getting a subwoofer properly setup is MUCH HARDER than various instruction sheets would lead you to believe. In fact, it is so hard, I can't recall ever seeing the entire process ever written up comprehensively. Somebody might have written that, but I've never seen it. Some fine points you RARELY see mentioned: If the sub is 4 feet from one main speaker, it should also be 4 feet from the other main speaker. Setting the sub precisely in the center of the width of a room is never the correct location--the subwoofer (and other speakers in the system) should be off-center by around 6 inches in order to minimize reinforcement of room modes. You can NEVER correct a "suckout" in bass response because those come from cancellation of a specific narrow band of frequencies. If you try to increase output in the suckout range by making that frequency range louder, the cancellation just gets stronger and you STILL have the suckout. Your only chance is moving the subwoofer 2 inches at a time until you find a spot without a suckout. That's nearly impossible to do without an SPL meter as a guide--and a fairly extensive collection of test tones in the bass region. 

Most discs with test tones will have no more than 10 frequencies between 20 Hz and 100 Hz. What you really need are 20 to 40 test tones from 16 Hz to 100 Hz and a means of viewing the results without having to manually create graphs... free software, like REW (RoomEQWizard) used with a suitable compatible USB microphone helps tremendously with the data gathering/visualization process. The more time you spend on setup, the better the end results will be in every way. If you think you found just the right combination of position and settings after 2 weeks, you are fooling yourself. 2 months is probably not enough time for an average enthusiast to get subwoofer setup optimized. I've been setting up subwoofers since the 1970s and it STILL takes 3 months or so to "learn" a new room I haven't used before.

You should consider Hsu Research subwoofers also. I find they perform a bit better than the others you mentioned in the $500-$600 price range (deeper bass, below 20 Hz). The other thing about Hsu subs is that most of them have 2 ports and come with 2 plugs for the ports. You can run them with 2 open ports for maximum low bass extension, with 1 port open and 1 port plugged for "tighter" sounding bass, but still fairly deep bass, below 20 Hz. Or you can run them with both ports closed to simulate a sealed-box. That configuration typically sounds best when you are listening to music.

One subwoofer location overlooked by a lot of people is placing the subwoofer beside or behind your main seat so it is literally just a foot or two away from you. This location is likely to produce the most linear bass response of any location in your room because you will hear the bass from the subwoofer, before it has a chance to interact with the room dimensions to produce response peaks and suckouts. 

You should contact Jamo about the cost of repairing the existing subwoofer... assuming they are still in business, of course. It's possible they have a swap program where you send in the old driver and electronics module with some cash and they ship you a replacement driver and electronics to install in your existing cabinet.

This info just scratches the surface of WORTHWHILE details involved in finding the best location for a subwoofer.


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