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PostPosted: 12 Jul 2010, 16:10 
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Location: Christchurch, NZ
Hi All,

I am looking for a suitable 8 inch midwoofer for use in a MTM (Midbass-tweeter-Midbass) 2 way floorstander project. Tweeter used will either be a Peerless HDS (93dB/w) or Wavecor silkdome tweeter TWA05 with ferrofluid(91dB). Crossover point is at 1500Hz, 4th order acoustic Linkwitz-Riley. One woofer that I feel is suitable for the task is a Scanspeak 21W8554. It is an old driver but Linkwitz has used it as a MTM for his Pheonix dipole project and seems to regard it as a great midrange. The driver has an efficiency of 90dB, so if i parallel it with the second woofer, electrically I would get 96dB/watt, but in reality I will achieve around 93dB/watt right?

Greatly appreciate any other suggestions and experiences of anyone who has built a 2 way MTM with 8'' woofers. Assuming that I will be designing my own crossovers :

Goals for the project :

1. Sensitivity of at least 93dB/watt/m in the 50-2000Hz range
2. Simplicity of a 2 way crossover
3. Powerful and deep bass ; F3 of 30Hz in room.
4. Highly dynamic sound and can easily be powered by (5-10) low watt SS/valve amps at 2meter
listening distance.

You may be wondering why use an 8'' for the midrange? Most of the 2 ways I have built using 6.5inch midwoofers don't sound as dynamic as a single 8''. Also they cannot achieve the required efficiency in a MTM format. I could go with multiple 6.5inch drivers to get a higher sensitivity but they still cannot go as deep as a single 8''. A larger driver just has that sense of ease when played loudly. Totally free of compression when compared with the smaller 6.5inch. (assuming drivers of similar quality are used.) Also, I would like to avoid a 3 way due to complexities with crossover and since I already have a sub to complement the 15-35Hz region. Thoughts anyone?

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PostPosted: 14 Jul 2010, 02:52 
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Joined: 24 Jun 2010, 00:48
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Location: Christchurch, NZ
A short compilation of some 8'' midwoofers...

1. Jantzen JA-8008 paper cone
2. Visaton AL200 alu cone
3. Scanspeak 21w8554 Kevlar
4. Seas CA22RNX H1288 paper cone
5. Fostex FW208N paper cone
6. Scan-Speak Discovery 22W/8534G00 Fibreglass cone

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PostPosted: 17 Jul 2010, 11:33 
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Joined: 24 Jun 2010, 00:48
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Location: Christchurch, NZ
After some recommendations by TonyGee from humblehomemadehifi, I have ordered the following drivers :

1. 4X Scanspeak 21W8554 8'' Kevlar Midwofer
2. 2X Scanspeak D2904 7100 Softdome tweeter

Enclosure = MTM 2 way with sealed enclosure for tweeter. This chamber will house the crossover.
Each woofer chamber will have 35L net internal volume, port tuned to 33Hz. Thw woofers do not share the same enclosure. Expected sensitivity of 93dB/watt, 30Hz F3 in a 17X24 ft room.

Some simulated pics using Google Sketch Up...
Simulated crossover design to come soon..


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PostPosted: 18 Jul 2010, 10:37 
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Joined: 28 May 2008, 21:53
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Location: Winnipeg, CANADA
Oh I like. Can't wait to see how you make out.
Cheers

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PostPosted: 18 Jul 2010, 12:54 
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Location: Christchurch, NZ
2 crossover versions : perfectionist (V1) vs minimalist versions (V2).
Both have a notch filter on the tweeter to reduce the harsh impedance peak @ Fs (520Hz)
Kevlar midwoofer has a nasty but gradual breakup at 900Hz and ends at 2900Hz (7dB peak).

The series notch filter on the woofer in V1 is centred at 2900Hz which will smooth the response such that at 2800Hz, it will be 6dB down. This then allows the rest of the woofer filter to achieve a smooth 18dB/oct rolloff after 1700Hz.
Attachment:
V1 Crossover.jpg

V2 has a single cap on the tweeter to start an early but shallow rolloff which begins to get much steeper at 1500Hz. Larger inductor on the woofer to act as a baffle step compensator as well as inducing a 12dB acoustic rolloff. No zobel network needed for the woofer as it has a remarkably flat impedance to 20kHz !! (14Ohm at 20KHz). With the use of a single small value inductor in the woofer's signal path, much less power losses would occur. This would probably result in a superb bass response. I have noticed that the larger inductor used in the lower woofer of an TMM floorstander tends to make the bass sloppier and lose its "punch". This is what I am facing for my current speaker. Might have to resort to a 5.6mH C-Coil from Jantzen @ $50 a piece !!
Attachment:
V2 crossover.jpg

V2 will be tried first. If it is too harsh then V1 will be used. Harshness could be due to the insufficient damping of the Kevlar cone breakup. Zaph from zaphaudio.com tested the D2904 and it has ultra low distortion all the way to 1000Hz. Totally confident about the tweeter; not so with the midwoofer.

Pls comment about the crossover.


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PostPosted: 18 Jul 2010, 20:32 
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Joined: 28 May 2008, 21:53
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Location: Winnipeg, CANADA
Look good. Have you run an analysis?
Cheers

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PostPosted: 18 Jul 2010, 22:25 
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No analysis done yet. I don't have a measurement/modelling design software like LEAP 5 yet.
I am using Crossover Pro by Harris Tech which helps me design the electrical filter slopes. Thus, if I specify 4th order L-R, it will provide a 24db electrical filter. Actually not all the components are needed for a 4th order acoustic rolloff. For example I could use a 12dB electrical + notch filter on tweeter to produce an acoustic LR4 order rolloff after the crossover point.

Usually once I'm satisfied with a crossover, I use an SPL meter to record various test tones at 1m distance to identify any peaks in each speaker. That is just a rough method of ensuring that the crossover works. For example my current set up has the tweeter crossed at 2500Hz @ LR2 acoustic.
I then played a 1250Hz(one octave lower) test tone and found it to be 12dB lower than that of a 3000Hz tone. So fairly close.

In the future I would definitely get some measuring/modelling software like the Clio or LEAP5. Imagine how great it would feel to build a crossover that measures well and sounds good ! :D

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PostPosted: 19 Jul 2010, 12:00 
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Location: Winnipeg, CANADA
The Dayton Audio Woofer Tester 3 (WT3) is very handy and easy to use for quick impedance and phase plots of speakers. It can also be used to measure the inductance of coils.
Cheers

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PostPosted: 20 Jul 2010, 08:08 
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Joined: 24 Jun 2010, 00:48
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Location: Christchurch, NZ
Freshly sliced MDF panels straight from the MDF factory today. They allowed me to operate the table saw and since they had no workers at the shed, it was totally free for use for me and dad. Lots of prior experience paid off. All panels hand cut to +- 0.5mm. A pair of speakers cut including bracing in an hour.

An old CPU enclosure is being abused. Not bad for flimsy alu; able to hold 40kg of MDF very well.
Panel dimensions :

Front panel : 700 X 350 X 25mm MDF
Side panel : 650 X 450 X 25mm
Top panel : 350 X 450 X 25mm
Horozontal braces : 300 X 450 X 25mm
Vertical braces: 265 X 300 X 25mm


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PostPosted: 29 Jul 2010, 03:34 
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Joined: 24 Jun 2010, 00:48
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Location: Christchurch, NZ
The cut panels are test fitted and show the side view of the enclosure.
Thankfully no mistakes in the dimensions - everything fits like a glove !
Front baffle is 2'' thick. More marking out to be completed.


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