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My MTM stand mount loudspeakers

Marantz offers at HiFiMART.com

t.antony

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This project came about due to the need for a high quality small speaker for music and home theater

Design Goals
  1. Less demanding on the room size and better integration in smaller rooms.
  2. Should provide well balanced sound output suitable for music and good power handling and dynamics for home theater use.
  3. Easy integration with sub woofers should cross at a frequency suitable for a good 12 sub to take over
  4. Small footprint and WAF
  5. Match the quality and finish of speakers that cost much more.

Driver alignment
After doing some research on the web for about a year finally convinced that an MTM(mid-tweeter-mid) configuration will satisfy my requirements. There are some advantages that an MTM has over the usual TM and TMM alignments.

  1. Compared to a TM, MTM with the same drivers will have better power handling and dynamics and can go louder as well all good traits for a HT speaker
  2. MTM design will have roughly the same horizontal dispersion as a TMM or TM design
  3. The MTM will have a narrower vertical off axis response, compared to TM and TMM. This can work out well for small rooms and cramped spaces as it minimizes the effect of floor and ceiling reflections.
  4. MTM has a symmetric dispersion pattern and coherent phase and amplitude characteristics when the listening position is at the tweeter level this is something difficult to achieve with TMM or TM designs.

Choice of Enclosure
The low frequency cut off of the enclosure is planned to be around 45Hz as this will allow a smooth transition to the subwoofer in case of a HT use. There is another advantage of crossing to a sub at this frequency, due to the non-directional nature of sound at these low frequencies, there is more freedom to position the sub to blend -in and become invisible sonically.

I was confounded between floor standing(tower) and stand mount(mini-monitor) options. Few months of research helped in deciding in the favor of a stand mount . Here are the reasons for this conclusion.

  1. Cabinet resonance problems are comparatively less in a short stand mount enclosure. They are cheaper to make, brace and finish.
  2. With a proper stand, stand mount can be positioned in axis with the ears, this has some good sonic benefits as indicated before. Even though they increase the overall cost, I think this cost escalation does not outweigh the flexibility.
  3. Simulations did not yield significant low frequency extension to a floor stander compared to stand mount.

For ease of construction a ported enclosure is chosen, decided to go with a forward firing port since it allows the speaker to be kept closer to the back wall useful for small rooms and HT use

Drivers
Of all the drivers I had collected these years, I am planning to use two of my favorites - 5.25 paper composite midbass drivers and low Fs silk dome tweeters manufactured by Peerless India. That would make this project cost effective and 100% Indian as well :). These midbass drivers are very similar to the ones used by GR-Research M-130 which is also manufactured by Peerless India. Here is a quick comparison.

  • The M-130s have been lauded for their midrange clarity that accounts to the curvilinear cone structure that allows smooth roll off into the higher frequencies and a paper composite cone which provides smoother breakup modes. These features make the crossover design easier. The Peerless India Standard line (SL) drives I am using for this project also has the same cone.
  • Both have polycarbonate frames. This does not transmit vibrations into the front baffle like a metal frame. The difference of GR- Research drivers is that the frame shape is suitable for surface mounting.
  • The GR research drivers seems to be having smoother high frequency response . But till 2.5Khz they are almost identical with each other.
  • The SL driver seems to have lower Fs than the GR research drivers.
  • The SL drivers have an impedance of 4ohms against 8ohms and 16 ohms versions of M-130. This means that these drivers need to be in series for an effective impedance of 8ohms. This results in an overall sensitivity of 88dB(ie: the sensitivity of a single driver) which should be sufficient for driving them with a medium power amplifier.

The Silk dome tweeters have an Fs of 880Hz and can be easily crossed around the intended crossover frequency around 2500Hz. So my expectation is that these two drivers are good candidates for building a cost effective system of high quality.

Driver pics





First I am planning to test these drivers in a set of experimental enclosures with 3 variations of front baffle with different driver alignments. This is basically to check the vertical dispersion effects and tweeter offset (Dappolito configuration). A pc based active xover is going to be used to determine the final crossover frequencies and slopes.

The test mules (enclosures) have already arrived from the carpenter. So this weekend is reserved for Diy :)
 

kaushik

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very well planned approach ! waiting for measurements and progress report.

which pc based crossover u r using?
 

t.antony

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Initial listening impressions...

Due to PC issues I had to use the passive crossover from my earlier project. So no response plots yet.

Here is a pic of one of the test boxes.


Each box was stuffed loosely with Dacron(about 100g per box). Used my ipod shuffle as source. Here are the listening impressions after about 10 hours of playing.

- Bass seemed tight and articulate. The nuances of different percussion instruments and bass guitar were reproduced well with good instrument separation.

- I used a DIY 60w RMS amp to drive these and these speakers could play clean and loud. The bass output also seemed a bit higher than my earlier bipoles despite half the size :).

- It's too early to comment on the mids and highs since the crossover in use is not originally intended for these, also the drivers are not fully broken in. However they were sounding nice. I was listening to Hendrix and his guitar seemed to blend well with the bass and vocals, it was not very forward sounding or bright. The highs are also becoming more crisp as the tweeter is breaking in.
 

neeraj

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Great Job Antony.:clapping::clapping:
Keep Updating.
I am Also In Q.
Could you please let us know the Driver Prices.
 

captrajesh

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What are the dimensions? What is the thickness of the plywood (It appears to be plywood and not MDF)
 

t.antony

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What are the dimensions? What is the thickness of the plywood (It appears to be plywood and not MDF)
It's 19 mm MDF with a veneer design on one side :). The external dimensions are H21" x W9" x D10.5". The front baffle is temporary and will be replaced with a CNC milled panel as I plan to flush mount the drivers.
 

t.antony

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Here is an overlay of free-air responses of the two drivers - Peerless Standard Line and GR-Research M-130



The SL driver has slightly better low end and M-130 has extended high frequency response. The good part is that till about 2.5Khz they are almost identical and looks very favorable for crossing around that frequency. Few GR-Research kits are crossed around this frequency as well. Since the SL driver's high frequency extension is not as smooth as the M-130. I am planning for a 3rd or a 4th order slope. This higher order is also beneficial in the MTM alignment as it minimizes the lobing effect between the midwoofer and tweeter.
 

goldyrathore

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I am planning for a 3rd or a 4th order slope. This higher order is also beneficial in the MTM alignment as it minimizes the lobing effect between the midwoofer and tweeter.
That being a waveguide loaded tweeter, IMO, will allow you to cross much lower at about 1500hz-1800hz.
3rd order electrical may give 4th order acoustic slope near this freq. Eager to know what works out best from your listening tests.:)

Goldy
 

t.antony

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@ goldy & kaushik,
I am traveling, will reply in detail once back
 
Last edited:

t.antony

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That being a waveguide loaded tweeter, IMO, will allow you to cross much lower at about 1500hz-1800hz.
3rd order electrical may give 4th order acoustic slope near this freq. Eager to know what works out best from your listening tests.:)

Goldy
Yes it's possible to cross these tweeters lower, It may be even advantageous in reducing the lobbing effects as well since the shared bandwidth between the drivers are reduced. The inbuilt waveguide is very small and the diameter of the mouth will be around 2.5 inchs. I don't know yet if it give any perceivable advantage to the overall directivitiy. Also it's possible that directivity will change when the tweeter is flush mounted.

Currently the center to center distance between the midwoofer and tweeter is about 13cm which corresponds to a frequency around 2600Hz. The reason behind choosing a crossover point around this frequency is that the midwoofer will cover almost the entire vocal spectrum. I am not yet sure if there will be any added advantages of this CTC distance w.r.t time alignment when crossing at 3rd order(electrical) or higher. However the degradation due to lobbing could be easy to recognize and my approach will be to find the best frequency/slope by trial and error :)
 
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