DIY Audio Projects Forum
http://diyaudioprojects.com/Forum/

2-Way MTM Loudspeaker with Scanspeak 21W8554 and D2904-7100
http://diyaudioprojects.com/Forum/viewtopic.php?f=11&t=2235
Page 3 of 5

Author:  kkkittehz [ 04 Aug 2010, 11:32 ]
Post subject:  Re: Suitable 8'' Mid/woofer for use in an MTM 2 way speaker.

At least you can go to bed at night without locking any doors and leaving all the windows wide open, I'd like to see someone steal them, just check the hospital the next day for people with chronic back pain and a crushed foot from when they dropped them :D

Do you have an idea for a colour scheme? Please dont just paint them black, or if you do, make it glossy black not just some matte c**p from a tin :P

Author:  dtsup1 [ 07 Aug 2010, 07:07 ]
Post subject:  Re: Suitable 8'' Mid/woofer for use in an MTM 2 way speaker.

@ kkkittehz - I'll do a gloss black with an acrylic top coat.

Some of the problems of the speaker are becoming more clear. Vocals sound a little nasal and an investigation reveals why ...

These measurements are taken at discrete frequency points using an SPL meter. Values are then plotted in Excel. First graph shows the SPL vs Frequency of the right speaker taken at 0.5m distance.
Attachment:
Freq vs SPL of right speaker at 0.5m.jpg

The second graph shows the response of the tweeter via the 24dB LR4 crossover at 1500Hz.
Perfect 24dB/oct acoustic rolloff !
Attachment:
Tweeter freq response with 24dB-oct filter in place.jpg

Third image is the crossover schematic. Both drivers are wired in phase ( as they should be in an LR-4 crossover with 360deg of phase shift).
Attachment:
4th Order L-R @ 1500Hz.jpg

Notes :
I used a 0.68mH inductor for the woofer's network and maybe that could be the reason for the dip.
A 0.55mH is required for the correct response. Need to go get a 0.55mH to do more testing...
Looks like components need to be of tight tolerances in a 4th order design. No measurements have been made yet for the tweeter with out of phase wiring nor have I measured the response of the woofers alone. The tweeter's response starts rolling off gently at 7Khz due to limitations in the soundcard. Front baffle is 12'' wide and it shows in the measurements that the baffle step transition is at 450Hz.

Any suggestions as to how to get rid of that horrible dip at 1750 Hz ? This is worst BBC dip ever seen LOL.

Author:  Gio [ 07 Aug 2010, 12:14 ]
Post subject:  Re: Suitable 8'' Mid/woofer for use in an MTM 2 way speaker.

dtsup1 wrote:
Any suggestions as to how to get rid of that horrible dip at 1750 Hz ? This is worst BBC dip ever seen LOL.

Hi Dinesh, since you are using a 4th order cross-over you should not need to invert the phase of the tweeter relative to the drivers. Try reversing the polarity of the tweeter and lets see what we get.
Cheers

Author:  dtsup1 [ 07 Aug 2010, 13:30 ]
Post subject:  Re: Suitable 8'' Mid/woofer for use in an MTM 2 way speaker.

Oops I meant flipping the polarity of the tweeter, not the phase. Yep, i'll try that and see how the measurements are in the morning. I am expecting even more of a dip but let's see hahaha...

If it ends up worse, that should indicate that the woofer's inductor value is too high. 0.55mH to be used then...

Author:  dtsup1 [ 08 Aug 2010, 05:50 ]
Post subject:  Re: Suitable 8'' Mid/woofer for use in an MTM 2 way speaker.

System tested with tweeter polarity reversed. A little bit better but the dip is still present. This translates to a relaxed midrange, slight nasalness still present.

Conclusion : Crossover points are not accurate; time for inductor changes.

Author:  Gio [ 08 Aug 2010, 15:24 ]
Post subject:  Re: Suitable 8'' Mid/woofer for use in an MTM 2 way speaker.

I would quickly try a zobel circuit before changing costly inductors. You can use this Speaker Zobel Network / Impedance Equalization Circuit Calculator to size one up.

Did you use the actual impedance of the drivers at the crossover frequency when sizing up the crossover. A common mistake is to use Re from the datasheet which could be quite different that the actual impedance at the crossover frequency.
Cheers

Author:  dtsup1 [ 08 Aug 2010, 15:34 ]
Post subject:  Re: Suitable 8'' Mid/woofer for use in an MTM 2 way speaker.

Hi Gio,

Ya I noted the exact impedance at crossover freq. The 8'' nominal 8Ohm driver has 0.2mH inductance and is barely reaching 9Ohms at 6000Hz. Scanspeak's amazing SD motor !

At 1500Hz , which is my crossover point, it is exactly 8Ohms. I'm quite happy with the tweeter's response and I am a little hesitant about lowering it too much. Already at 1500Hz 4th order, the tweeter is being pushed a little hard - though not audible.

Will try some spare inductors later on.

Author:  dtsup1 [ 13 Aug 2010, 05:42 ]
Post subject:  Re: Suitable 8'' Mid/woofer for use in an MTM 2 way speaker.

New update : The old 4th order crossover that totally sucked the life outta the music was dumped into the garbage (figuratively speaking). A new 2nd Order is in place producing a more open and lively sound while really increasing sensitivity of the speakers by 3dB. Imagine that for the 94dB tweeter, no L-Pad is necessary. System sensitivity is estimated at 93dB/m. All capacitors Mundorf M-Caps. Inductors 14 AWG.

The high end version is shown below :

Author:  Gio [ 13 Aug 2010, 19:04 ]
Post subject:  Re: Suitable 8'' Mid/woofer for use in an MTM 2 way speaker.

Well, that is good news. Less parts, lower XO costs and improved results. ;)
Cheers

Author:  dtsup1 [ 07 Sep 2010, 07:42 ]
Post subject:  Re: Suitable 8'' Mid/woofer for use in an MTM 2 way speaker.

I recently got a pair of Mundorf 4.7uF supreme silver/oil ($100/pair) on loan from a local dealer. These replaced a 4.7uF Mundorf M-Cap ($14/pair).
The results are out of this world. :eek: :shock: :up:

Am I listening to a different speaker !?!
Soundstage width + depth increased by 1m.
Much better layering effect with instruments in their appropriate places.
This allows me to focus on a particular instrument in the recording, and yet it sounds amazingly clear. The particular instrument does not get lost as mush/haze in a complex passage.
Fatigue has reduced to nothingness. No more "edgy" highs that give false impressions of detail.
Smooth, lush, earthy and organic highs.
Amazing difference. I never was a strong believer of boutique caps.

I will be keeping these. Ouch @ $140 a pair !! :( :$: :$:

Page 3 of 5 All times are UTC - 6 hours [ DST ]
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/