# Repurposed MTX



## Sonny_ss (Apr 7, 2012)

Hi everyone,
I'm a noob to this forum, but have been making speakers as a hobby for a while now. I see the value of DIY, and prefer it to store bought - maybe I'm just cheap??
Anyhow….I have big plans for a 7.1 or 7.2 HTPC in my house, and I want to start off by improving the bass of my current system. I've owned 2 self powered subs before and have never been all that happy with them. One was down firing and the other was forward firing. The forward firing model was stolen along with my Ensemble II speakers.
I have from my younger years 2 subs from MTX that I was thinking about re-purposing for HT. 
They are TP840 series. 
Here are the parameters:

Fs- 35 Hz
Qms - 7.23
Vas - 0.96
Cms - 0.0706 in/lb
Mms - 1.81 oz
Rms - 3.439 lb/s
Xmax - 0.27 in
P-Dia - 6.57 in
Sd - 33.9 sq in
Qes - 0.35
Re - 3.72
Le - 1.45 mH
Z - 4 Ohms
BL - 2.459 lb/A
Pe - 200 Watts
Qts - 0.33
no - 0.32%
1-W SPL = 87 dB

Here is where I need some help. I can plug the T/S values into different online calculators, but unfortunately I tend to get different answers depending on the site.

First question - is WinISD the site to use for online calculations?
Second question - what would the consensus be for the type of box to use? If vented, isobaric, BP-4th, BP-6th?
I really don't have any limitation to the size of the box, within reason of course.

What I'm looking for, if possible with these subs, is good response down to 20 Hz using AR S12HO plate amp (150 W & 4 Ohm) from a previously mentioned sub. The room is about 2500 cu ft.

BTW my current system is a set of JBL 5.1 HT speakers (to be replaced), Pioneer VSX-D5065 receiver (to be replaced), Pioneer PD-F508 CD player (to be replaced), Bell 9242 PVR, Sony BDP-S280 Blue Ray, Samsung 56" LED DLP. I used to have a set of Ensemble II speakers from Cambridge Soundworks and I really miss them (they were stolen during a house break-in)- the JBL are NO comparison.

Thanks to everyone for their input and help.

Sonny_ss


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## GranteedEV (Aug 8, 2010)

Sonny_ss said:


> I'm a noob to this forum, but have been making speakers as a hobby for a while now.


Welcome to HTS, fellow Albertan 



> First question - is WinISD the site to use for online calculations?


Calculations wouldn't be the right word. It's a modelling program. It will show what a given box parameter will do to the response. It's mostly fine for a subwoofer, although I would not model a main speaker with it.



> Second question - what would the consensus be for the type of box to use? If vented, isobaric, BP-4th, BP-6th?


Well, things like Isobaric and 6th order bandpass are probably not worth it for HT purposes.

- For sealed, you need a lot of power handling, low fs, and displacement. I already feel that would not be worthwhile with these MTX drivers. They might work fine sealed for car audio, but for HT you're probably wasting your time.

- I will model it vented in a moment. Hold on.

- If box size requirements are not a limiting factor, consider a tapped or front loaded horn. They are bandpass if you want to get technical but they will have low distortion and high output. Your operating bandwidth will be limited so you will need very capable mains to mate to a horn though.



> really don't have any limitation to the size of the box, within reason of course.


Well the definition of "within reason" will need to be defined :whistling:



> What I'm looking for, if possible with these subs, is good response down to 20 Hz


Again I haven't modeled or measured it, but i'm pretty skeptical of getting such results from an fs = 35hz woofer. 



> using AR S12HO plate amp (150 W & 4 Ohm) from a previously mentioned sub


You may need to reconsider this. Plate amps will likely have high pass filters, which will throw any computer assisted design right out the window.


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## GranteedEV (Aug 8, 2010)

Well, I'm running this thing through sims... I don't know if this driver is worth your time for an HT, especially if you want _good output_ down to 20hz. This driver may have good enough output to do pipe organs in music, but probably _can't_ handle movie LFE. If you do want to squeeze what you can out of these, you will want to put both of them in the same box and wire them _in series_ for an 8 ohm load. Let's say on paper the amp can do 100W @ 8 ohms. Your F3 will be around 48.5hz, however with F6 around 23hz you'll get pretty decent extension.

In that case, *5 cu. ft* plus 4" diameter x 38.5" length port will do well giving you a tuning frequency of 18hz. A 6" x 2" x 37" slot vent will do the same and could be folded into an L shape.

You will need a high pass filter cented at 13hz/LR4 or 16hz/BW3 to protect the drivers - I have NO clue what kind of high pass filter is in the plate amp you plan on using which makes any modelling a guess. Attached is the model (Fuschia graph). It's basically best case scenario, assuming the driver actually sounds good to its rated xmax of 7mm and the amp doesn't roll off everything below 30hz. I'm not sure about the back-EMF related consequences of wiring subs in series, but that's the only solution I can really think of besides a very large horn (i'd imagine 12 - 15 cu ft)

Else, consider starting from scratch with a CSS Trio 12, and a SPA500 amp. It is also shown modelled (Green graph). This is the kind of performance I consider more acceptable for movie LFE. That is a 5 cu ft box tuned to 17.5hz with a BW3 filter at 17hz


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## Sonny_ss (Apr 7, 2012)

Thanks for the education GranteedEV. You are right, I need to have a different solution. If I'm going to buy components for a DIY sub, in your opinion, what do you think about Rhymik Audio servo subs? About twice the money delivered to Alberta, do you see value in them?

It is overwhelming to start searching for the right product and have so many choices - brands, sizes, new materials and technologies. 

I am a little partial to the products from PartsExpress. I've used them in the past and have had a great experience. They have some talented staff that are avid DIY guys too.

If you are willing to point me in the right direction that would be great. Do you know of any Calgary based retailer of quality products at competitive prices?

Thanks


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## GranteedEV (Aug 8, 2010)

> If I'm going to buy components for a DIY sub, in your opinion, what do you think about Rhymik Audio servo subs?


Rythmik's amps and 15" drivers are pretty good, but i'm not really convinced the servo provides any SQ advantages. 



> About twice the money delivered to Alberta, do you see value in them?


Yes, but not the best value either.



> It is overwhelming to start searching for the right product and have so many choices - brands, sizes, new materials and technologies.


Just slow it down and take it step by step. You can get great performance if you look at the big picture and figure out what you're willing to live with.



> I am a little partial to the products from PartsExpress. I've used them in the past and have had a great experience. They have some talented staff that are avid DIY guys too.


Parts express is nice. Their RSS drivers don't really have the most xmax in the world but they're well designed.



> If you are willing to point me in the right direction that would be great.


Well there's a few basic aspects to great bass

1) Flat Frequency response. Tight, accurate bass requires fidelity to the source. The subwoofer should be flat in-room, and also it should not stop being flat at higher SPLs.

2) Uniformity. If you have plenty of bass at the right side of your head, but less bass at the left side of your legs, you don't feel it all that well. Outdoors bass is uniform, but in rooms it is at the mercy of how it interacts with the room, and big "suckouts" caused by a room are very real, as are big "peaks". Now the peaks can be EQ'd down for a specific point in space, but not all, and the valleys should not be equalized. This also means that the likelyhood of flat frequency response is very LOW!

3) Headroom. You don't want distortion, or inability of your system to reproduce what's there. So having 110+ db capability down to 15hz or lower is by NO MEANS an irrational goal. 

Given the above, you need two things

1) Displacement (and amplification capable of utilizating it)
2) To average out the frequency response in the room across multiple physical locations

Both of these things are acheived with MULTIPLE subwoofers placed separately from each other.

Now such response down to 15hz can be handled by a vented box, but it will be very large.
Sealed subwoofers require significantly more displacement from the driver, but will be smaller. And because the rolloff is a gradual 12db/octave, you can extend even down into single digit frequencies. That I suppose is the "ultimate goal". You should only expect usable response down to the tuning frequency of a vented sub on the other hand.

What is my advice dollar for dollar? Well, if you're not concerned about size, four of these, one in each corner of the room:

http://www.hometheatershack.com/forums/creative-sound-solutions/51662-css-trio-12-loaded-horn.html

And some equalization to get things flat...I would not waste time with plate amps. Go straight to some pro amps.. a single crown XLS1000 for example can drive two of those subs simultaneously to earbleed levels. That could be pretty deadly. You could start with just two and add the other two later when funds open themselves up.



> Do you know of any Calgary based retailer of quality products at competitive prices?


Not really. I order most stuff online. Creative Sound Solutions however is B.C. based and their products are very well engineered. Mach V audio is based in Manitoba and their UXL-18 was recently evaluated by Josh Ricci - http://www.data-bass.com/


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