JXR6 design analysis for near field setup - diyAudio
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Old 26th January 2006, 12:04 AM   #1
deandob is offline deandob  Australia
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Default JXR6 design analysis for near field setup

OK, my local distributor (Decibel HiFi) just down the road from me has the JXR6 in stock now, and a significant discount to the JX53 (about USD $220 for a pair) which is low enough for me to experiment with a pair.

I'd like to start a new thread that discusses the spreaker design using these drivers. There is some information in the other threads and the EJ Jordan site but not enough for me to feel comfortable with spending time/money on developing a quality speaker with this driver. I have an existing setup using Tang Band 871 speakers (6 litres, drivers mounted high on a thin front baffle) and subwoofer which I'll refer to in this thread as a reference. I'd appreciate input from those more qualified in speaker design & more familiar with the JXR6 to help me build a speaker as optimised as possible. Hopefully this thread will be useful for others also looking to develop a single driver monitor with the JXR6.

Firstly to describe how the new speaker will be used, to give you some context for the design.

Setup - Near field monitors where I'm sitting about 2 -3 feet from the drivers. The speakers will be separated by 2.5 feet and a LCD monitor inbetween (flat surface) slightly recessed from the front of the monitors. The current setup with the Tang Band has excellent imaging, like listening to headphones - imaging is better than any far field setup I have heard. A wall is about 1/2 foot behind the drivers, and the back of the speaker is about 1 inch away from the wall.

Source - High quality computer audio. Please no comments about computers being no good as a source, this was debated in the other JXR6 thread, assume that the source quality justifies investment in the JXR6. I listen to a variety of music, mostly rock/pop, alternative and chillout. Pink Floyd, Peter Gabriel, Tori Amos, Dire Straights, Dave Matthews Band etc.

Amp - Tripath TA2022 DIY AMP3 (similar to Sonic "t-amp" but better). 20W @ 0.02THD Class D. A well executed Class D amp will express lots of detail in the source (eg. able to articulate each instrument in a complex passage), has superb soundstage and precise bass. This particular amp has a very smooth & clear top end in comparison to other good Class D amps. The JXR6 will be an interesting match for this amp, as the JXR6 detail and low distortion should shine in this setup with decent quality recordings although I suspect it will also show up poor recordings as poor.

Subwoofer - I'll be crossing these drivers at 100Hz and I have a single Peerless 8" XLS downfiring woofer in a sealed 20l box. It is driven by a linkwitz transform to achieve 30Hz and has a second order active low pass filter at 100Hz designed to match the active high pass filter and a system Q of 0.5 (Critically damped for better matching to the monitors). This subwoofer has good transient response and a warm, firm bass. It is located close to the monitors and currently integrates well with the TB 871's I'm using, adding a lot more body & authority to music.


Questions about the driver to help clarify the design.

There is some discrepancy with the data posted in this thread
http://www.diyhifisupply.com/docs/JX...0Para%2006.pdf and what is on the EJ JOrdan site. Does anyone know what T&S data is correct? Is Brian Cherry's data a measurement from a production unit? That is also a pretty impressive waterfall chart!

Frequency response curve.
The graph at the EJ Jordan site is pretty impressive, this is one of the flattest curves I've seen for a full range driver, especially considering its a 2" driver. However its not perfect and has 3 areas for concern:
1) Forget anything below 100Hz. OK for me as I'll be rolling off the speaker with a second order active HP filter at 100Hz. I do this on my TB monitors and it does make a difference to its power handling ability and improves higher frequency distortion as the cone is not modulated by strong low frequency energy while trying to reproduce high frequencies. As the JXR6 is a light metal cone I'd assume this HP filtering is even more important than a "sluggish" paper cone like my current 871's.
2) Frequency response above 12K starts rising by a few db. I'd say this is a good thing as it will add more 'air' to music. Most music has little energy past 12K as well as my hearing starts to deteriorate around 15/16K a small amount of boost wont matter here. So nothing needs to be done.
3) THere is a 3db boost from about 2.5Khz to about 5K. Now this is an important area of the frequency spectrum especially for voice. Looking at Brian's plots, this hump is less pronounced, especially if slightly off axis.


4 ohms. Should be no problems as my Tripath TA2021 amp only delivers 10w @ 0.01 THD 8 ohms and 20W @ 0.02 THD with 4 ohms. The extra 4 ohms headroom is nice although not really needed, and the increase in THD wont matter as distortion is already very low and the driver distortion is much higher, even a high quality one like the JXR6.

Sensitivity at 89db/m is pretty good, 2 db higher than my 871, so coupled with the higher output of the amp with 4 ohms I wont have to drive amp as hard with the JXR6.

Rated power. 100W "music" power. Huh? Music power means nothing? I think I'd be OK with my 20w amp, with the 100Hz high pass filter, near field listening and reasonably high sensitivity. Xmax at 6mm p-p is pretty amazing for a small driver which should give it some additional resilience (the 871 as a point of comparison is 0.5mm Xmax).

Baffle step compensation & location of the driver. I decided not to do baffle step on the current Tang Band speakers for the following reasons:
- Speakers are against a wall (adding a bass boost)
- The front baffle has rounded edges, with the driver almost on the edge in the horizontal plane, and very close to the top edge.
- high pass at 100Hz and baffle step really only a problem from 400Hz & below
THe same theory applies for the JXR6, in fact looking at Brian's frequency plots there is a pronounced mid bass hump from 150Hz to 700Hz, so not having baffle step compensation will remove this hump (assuming Brian's plots were not measured with a driver in a box?). So sounds like baffle step compensation is not needed. Although I see most Jordan designs with a really wide front baffle, I'll stick with a thin front baffle (as I use now with the 871's) with the driver placed near the top, it looks good, reduces baffle step and has the drivers at ear height.


Toe in & off axis response. The driver will be located at ear height, about 2 feet from my ears. Jordan drivers are suppost to be driven with lots of toe in so that you get the on axis response for the drivers, crossing just in front of you. With my current tang bands I do get excellent imaging with the drivers pointing stright ahead, but I do notice better high frequency response if the drivers are pointing at my ears. For the new JXR6 setup, I'll slope the front baffle so that the drivers cross just behind my head for a couple of reasons:
- Brian Cherry's frequency response shows rising high frequency on axis and falling high frequency @ 30 deg. So slightly off axis should make it about right.
- Initial comments about this driver is that they are bright at the top end
- Listening near field should not make the toe in and axis response as critical as far field.
- beaming the driver near field from 2 feet away directly into my ears will accentuate the top end.
- Looks better.

Compensation circuit. I'd like to get away from any passive components in the chain as possible. As I'll be driving the speakers slightly off axis the mid range hump & rising top end should equalise, avoiding the need for a compensation circuit. However I appreciate that these things are best measured in the real world and not from theory, but no circuit sounds like a good place to start and simple.

Box size. Not sure on this, Win ISD shows optimal about 3l with -3db at 100Hz, but I dont know about this driver without a rear suspension, and how to keep Q down below 0.6.

I'm interested in your feedback, tips & critique of my analysis above.

Anything else I need to think about?

Regards,
Dean
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Old 26th January 2006, 09:47 PM   #2
Colin is offline Colin  United Kingdom
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Phew, that's a pretty exhaustive analysis of your requirements. And now I see what you mean about high quality computer audio ...

Spec sheets for the JXR6 seem to be variable at the moment - I'd stick with the ones on Ted's site. He developed the drivers and he has his own ways of measuring things (he said once that he derived some of the measurements that others later put their names to). His measurements will be the most rigorous as he's not a fan of rose-tinted specs. It's also possible the manufacture is still settling in.

On that basis, to reach 100Hz at 0.6 Qts, go for the largest recommended enclosure of around 4 litres.

On axis sound - although I haven't experimented with my JXR6s yet, my experience with the JX53s is it has much less variation on and off-axis than the JX92. The 92 is very sharp on axis. The 53 does require slight compensation to balance out the sound, it's part of the 500Hz crossover circuit and may be there to deal with that 2.5-5k rise. Until I have my own units running, I don't know if the JXR6 needs it.

Power handling - the 100 watts is probably in-system use, crossed over to a suitable bass. FWIW, Ted told me he once fed a JX53 with a sine wave at deafening levels and left it like that for an hour or two to see if he could destroy the driver. When he came back, the cone and chassis were too hot to touch but it was still happily churning out the sine wave.

Baffle - the more I play with Ted's drivers, the more I come round to his idea of mounting them on a wide baffle. It does seem to do something magical to the lower mid-range. But if you are using yours close to a wall, it won't be so much of an issue.

Okay, I'm not sure how much this helps you. Not many of us have experience with the new drivers. Others will be more helpful on the technical aspects of boxes and crossovers, my own experience relates to using listening to Jordan drivers for the last 8 years and chatting to him when I've seen him.

I see the glue has finally set on my new boxes so I'll have a pair of JXR6s up and running this weekend and will report on how they sound.
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Old 26th January 2006, 09:58 PM   #3
VT67 is offline VT67  Belgium
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Hi guys

I will follow this thread with big intrest because my pair arrived this week.

Regards
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Old 26th January 2006, 10:46 PM   #4
Dumbass is offline Dumbass  British Antarctic Territory
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My initial reaction is, if the Tang Bands work, the Jordans will work better.

Here are my votes:

~ 3 liter enclosure, i.e. system Q of 0.7, if you can spare the room.
Wide-and-shallow approach, like shown on Jordan web site.
Consider the front baffles of speakers as extensions ("wings") of your computer monitor.
One comment I've heard about Jordans is that they improve with break-in. You might consider breaking them in at higher-than-normal levels.
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Old 26th January 2006, 10:46 PM   #5
deandob is offline deandob  Australia
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Thanks for the feedback Colin. Do you think the later JXR6 manufacturing batches may be more optimal as the production process is tweaked based on the testing of the initial batches?

It looks like there is nothing fundamentally wrong with my analysis.

The compensation issue (if or what is required) is best sorted with real world measurements in a box, although the paper analysis looks promising. It is also easy to add later.

I have a large amount of blackwood veneer MDF waiting to be cut but as I'm not as familiar with these drivers (or speaker design) I'll hold out until the first DIY reports come in.

Regards,
Dean
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Old 27th January 2006, 04:30 AM   #6
deandob is offline deandob  Australia
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Hi Dumbass.

Hmm. Good idea on the wide and shallow front baffle, that would look really good on either side of the monitor.

What about baffle step with such a wide baffle? My understanding of speaker design is that the less baffle on the front the better. For example, Dunlavy speakers use felt around the driver edges to absorb edge diffraction.

Regards,
Dean
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Old 27th January 2006, 05:01 AM   #7
Dumbass is offline Dumbass  British Antarctic Territory
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Quote:
Originally posted by deandob
My understanding of speaker design is that the less baffle on the front the better.
Untrue. Read the Jordan FAQ.

A lot of folk are coming around to wide baffle idea.

Larger baffle means baffle-step cutoff freq is lower, i.e. a good thing. Freqs above BSC freq radiate directly into half-space. Freqs below BSC need some sort of boost because they radiate into 360deg, not 180deg.

Jordan himself recommends wall-mounting. What I'm saying is that your video monitor plus two speaker baffles can act as pseudowall.
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Old 27th January 2006, 06:54 AM   #8
deandob is offline deandob  Australia
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Hi Dumbass (love the name BTW!)

The computer monitor between the speakers and a wide baffle would make it equivalent to having the drivers flush mounted in the wall. Is this a good thing for Jordan speakers (as opposed to near a wall)?

Quote:
Ref the imaging, Ted's drivers are toed in about 60 degrees and cross in front of you. I've found anything large and solid in the way of my set up in the lounge causes the imaging to deteriorate, so I would think the same would happen in the nearfield. Having the speakers slightly forward of the monitor may work or perpahs a LCD monitor might not create such a strong effect. Otherwise, it's down to finding a way to mask the monitor in some way, soften the edges and make them sound absorbing.
Colin posted this in one of the other threads, I take this to mean that if the monitor was midway between the speaker and the listener in the soundfield it would be a problem? This contradicts what you are saying (and written up on the jordan web site) as the monitor in this situation would be flush or behind the line of the speakers.

But I'm keen to give it a try with a wide baffle - Ted Jordan certainly recommends this and he should know.

Regards,
Dean
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Old 27th January 2006, 08:16 PM   #9
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deanbob

I have to agree with you on your observation of the waterfall
chart produced by Brian DiyHifi.Those graph are truly a thing of
a beauty IF it holds true.
Although i've heard others brag about the high distortion at
200Hz & below (1.5%), this driver is still remarkable.

I hope those with JXR6 able to come up with some of their own measurement especially the waterfall plot in this thread for us..
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Old 27th January 2006, 10:15 PM   #10
Dumbass is offline Dumbass  British Antarctic Territory
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Quote:
Originally posted by deandob
Hi Dumbass (love the name BTW!)
It's an homage to Red Forman.
Quote:
Originally posted by deandob
The computer monitor between the speakers and a wide baffle would make it equivalent to having the drivers flush mounted in the wall. Is this a good thing for Jordan speakers (as opposed to near a wall)?

Colin posted this in one of the other threads, I take this to mean that if the monitor was midway between the speaker and the listener in the soundfield it would be a problem? This contradicts what you are saying (and written up on the jordan web site) as the monitor in this situation would be flush or behind the line of the speakers.

But I'm keen to give it a try with a wide baffle - Ted Jordan certainly recommends this and he should know.

Regards,
Dean
IIRC Colin has his speakers placed directly against the rear wall, so I assume he means something like a couch, armoire, TV, etc, between the speakers.

Here's what Ted Jordan says about wall-mounting speakers:

[D]espite convention, there is strong argument in favour of placing the loudspeakers as close to the wall as possible. This reduces the time lag between the direct sound and the reflection from the 'virtual images' and substantially improves coherence and spatiality. (The ideal, but usually impractical, solution, is to mount the drivers in the wall where there would be no virtual image).
http://www.ejjordan.co.uk/basics.html
http://www.ejjordan.co.uk/faq.html

A lot of food for thought on those two pages.

It just occurred to me, if you do choose to go wide baffle, you might want to place the drivers off-center (mirrored pair), because otherwise you might be forced to have the drivers too far from each other, given the nearfield setup and monitor in the middle. You also might want to have the front baffle leaning back at the same angle as your video monitor, just for kicks.
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