Speaker Related Projects

   Vector2
(A T-line and ribbon tweeter 2-way. August-2012)

   Polaris
(A tall, thin, upwards firing omnidirectional speaker. May-2010)

   Shiva_PR15
(A powered subwoofer using a 12" driver and 15" passive radiator. Jan-2010)

   Can-Less
(A computer speaker; redux. December-2005)

   Can-Can
(A computer speaker in a light canister. Jan-2005)

   Sonosub
(10" vented subwoofer in a cardboard tube, powered by a Parapix amp. May-1999)

   MTM Center Channel Speaker
(A Madisound design. Nov-1997)

   2-way Surround Speakers
(5" woofer and 1" tweeter. July 1997)

   3-piece mini system
(6" DVC bass module mated to 4" car speaker. June 1997)

   3-way Vented Floorstanding Speaker
(vented 10" woofer, 5" mid and 1" tweeter in a 4 ft tower. Summer 1995)

   NHT1259 Subwoofer
(A 12" woofer in a sealed architectural pedestal. Winter 1994-95)

   Inexpensive Speaker Stands
(Particle board, sand and spray paint. Fall 1994)

   2-way satellite
(6.5" woofer and 1" tweeter. Summer/Fall 1994)

Electronics Related Projects

  900 MHz Audio Receiver
(Better use for bad headphones. Jan-2008)

  Buster - A Simple Guitar Amp
(Perfect for the beginner. Jan-2010)

  A PC-based Audio Console
(Use a PC to play tunes. Jan-2010)

  LM-12 Amp
(Bridged LM-12 opamps. Aug-2003)

   CeeDeePee
(A CD player and FM tuner from spare computer parts. Oct-2002)

   Quad 2000 4-Channel Amp
(Premade modules by Marantz. May-1998)

   Zen Amp and Bride of Zen Preamp
(by Nelson Pass. Apr-1997)

Articles

  Backing-up LPs to CD-R
(Whiningdog.net 10-Dec-2002)

  Using Wood in Speakers FAQ
(Work in progress)

   MDF FAQ for speaker builders

   Woodworking Tools for the DYIer
(HomeTheaterHiFi.com Oct-1998)

  Some Thoughts on Cabinet Finished for DIY Speakers

   Large Grills Made Easy

   Some Parts Suppliers
(Outdated)

Other Useful Stuff

   DIY Audio Related URLs

  Veneering Primer
(by Keith Lahteine)

   How to get a Black Piano Finish
(by DYI Loudspeaker List members)

   Sonotube FAQ
(by Gordon McGill)

   Excerpts from the Bass List
(Oldies but Goodies)

DIY Loudspeaker List

   Current DIY Loudspeaker Forum Home

   Former DIY Loudspeaker List Subscription Page

  DIY Loudspeaker List Archives

MTM Center Channel Speaker

Introduction

In building this center channel speaker, I combined a number of goals and interests into one project.  The points of interest for me are (in no particular order) : to build and listen to a mid-tweeter-mid (MTM) speaker, to match the timber of my main (front left and right) speakers as closely as possible, to audition and compare an existing kit design, and to build it all cheaply by using whatever materials I already have.

My main speakers are a 2 way sealed design using an Audio Concepts (ACI) AC-7 woofer and a  Vifa D25AG-05-06 tweeter.  Neither of these drivers are shielded for A/V use thus making them unusable for a center channel.  Luckily, Vifa makes a shielded version of the tweeter - the D25ASG-05. As for the woofer, the AC-7 is very similar to Vifa's C17WG drivers.  The C17WG is not shielded but others in the product line are; notably the M13SG-09-08 and M17SG-09-08.  Both of these mid bass drivers have a coated paper cone like the AC-7.  The shielded drivers differ from the AC-7 in their use of cast frame, and a rubber surround.  My original plan called for the use of two M17SG drivers.  However the resulting box size just didn't seem right sitting atop my television.  Using the M13SG driver yielded a more reasonable box size, though still rather large.

The Kit

As luck would have it, Madisound has a Vifa kit utilizing two M13SG-09-16 and one D25ASG-05. The kit can be purchased complete with all parts and cabinet or as individual pieces.  Since I wanted to build the box to suit my taste and decor, I bought the woofers and crossover (I already had the tweeter).  Incidentally, the M13SG-09 comes in both an 8 ohm version (-08 suffix) and 16 ohm version (-16 suffix).  The 16 ohm version allows two such drivers in parallel to yield an 8 ohm nominal impedance.

The claimed response by Madisound is 40 Hz to 35 KHz vented and 80 Hz to 35 KHz sealed.  My own calculations yielded an F3 of above 100 Hz in a sealed cabinet.  While a DVD center channel (Dolby Digital) is full range, most decoders allow the bass to be transferred to other full speakers. Since I'm not as fond of the sound of vented cabinets speakers in general, I sacrificed the bass response and built a sealed speaker.

For the cabinet volume, I took the stock Madisound cabinet (18.5 H x 8 W x 10.5 D when placed vertically), subtracted 3/4 inches (19 mm) on each face for the cabinet walls and arrived at an internal volume of 0.58 cu. ft.  For a center channel, I was willing to stretch the speaker out sideways (height) in exchange for a reduced depth while keeping the width the same.  Reducing the depth makes it an easier fit for the television.  Another limiting factor was my supply of construction material.  As was the case with my surround speakers, I wanted to consume leftover MDF and particle board and avoid having to buy new stock. After juggling with the materials on hand, I came up with a box measuring 22-1/2 x 7-3/4 x 8-7/8 (external). The internal dimensions were 21 x 6-1/2 x 7-3/8 or 0.58 cu. ft.  More on the box later.

The crossover

Madisound's crossover comes pre-assembled on a single sided glass epoxy printed circuit board.  The PCB looks like it was designed for a generic layout. All parts were soldered on the foil side of the board and held down with some silicone-like material.  Tabs allow for signals to enter and leave the PCB.  Four holes at the corners are used to secure the PCB to the cabinet.

The quality of parts varied with function.  Critical capacitors in the tweeter signal path are polypropylene, while other less critical caps are mylar and NP-electrolytic.  All inductors are air-core of varying gauges. Resistors are sand-cast.  Overall, the quality is very good, making reasonable tradeoffs to keep cost down. This is not a cost-no-object crossover, but rather a smart and economical implementation of a design.

The circuit consists of a second order low pass filter on the woofer, a third order high pass filter on the tweeter with impedance compensation and an LCR circuit whose purpose I am not totally sure of.  It never occurred to me to measure the drivers individually with the crossover prior to final assembly !  I also do not have any manuals or official kit information (since I only bought the crossover).   My guess based on the crossover parts is that the 2nd order low pass filter is set at about 2500 Hz while the 3rd order high pass is set to about 4000 Hz.

Measured Results

A Radio Shack SPL meter was used to measure the resulting in-room response. This meter is limited to about 10 KHz on the high end, and the speaker is limited on the low end. I therefore measured the speaker in the range of 100 Hz to 10 KHz. Three sets of measurements were taken - on-axis, 30 degrees off-axis horizontally and 30 degrees off-axis vertically.  The speaker was laid on its side on top of a television with the tweeter's center about 52 inches above the floor.  This probably seems high but the current setup of my room left me with only this options.  The off-axis degrees are approximate. Input voltage is 2.81 volts and the test source was Stereophile's Test CD2. All references made here relating to orientation (up, down, left, right, width, height, etc.) are made relative to the speaker lying on its side.

As expected the on-axis response at 100 Hz is down considerably (78 dB) - over 10 dB below the level at 125 Hz (88.5 dB). The response from 125 Hz to 3150 Hz is very flat, measuring 90 dB +1, -1.5 ! Between 4 KHz and 8 KHz, the measured response suddenly rises as much as 6 dB before returning back to 91 dB at 10 KHz.  In listening tests, there is a slight brightness when listening to some classical pieces but it is not harsh nor "tinny". Vocals are very smooth, probably in part due to the nice mid bass response curve.  Bass is practically non-existent as expected.

Off-axis, the 30 degrees vertical (down) measurements were within 2dB of the on-axis measurements showing the excellent dispersion of the MTM topology. This is consistent with the Madisound claim that this speaker is designed for wide horizontal dispersion when standing up.  In the horizontal plane, the 30 degrees off-axis measurements is down about 6 dB between 800 Hz and 3150 Hz. Above this, the tweeter's output dominates and is about 2 dB down from on-axis measurements to 10 KHz.

The box

As mentioned earlier, I made this box from what is essentially leftover scrap. Again with the speaker lying on its side, the front and rear panels are 3/4 inch birch veneered MDF.  The short side panels are 3/4 inch thick high quality particle board I salvaged from a box used in  shipping heavy equipment.  The top and bottom panels are 5/8 inch particle board typically found in your local home center.

I began construction with a router; cutting out four circles - a terminal cup on the rear panel, two woofers and a tweeter on the front baffle.  All drivers are recessed so as to mount flush with the baffle. The woofer and tweeter spacing is minimized as much as possible.  Five panels (excluding the front baffle) are then glued together with yellow wood glue using simple butt joints. Before all the glue sets, I scrap off leaking glue on the inside joints. This is done to provide a nice clean right angled surface to glue corner reinforcements on all inside edges.  Inside reinforcements are also added for the not-yet-mounted front baffle. These later double as baffle supports.  The rear, top and bottom panels gets an added rib to roughly divide the middle of the panel.  The front baffle gets 2 ribs, one between each woofer and tweeter. The rear, sides, and bottom then received a single layer of 50 pound roofing felt in the remaining space. For the top I tried an experiment by gluing a layer of thick egg crate foam. Why you ask ?  why not !  All seams are caulked and the crossover board (with wires pre-soldered) was mounted before the front baffle was glued in place with a very generous amount of glue.  Additional caulking was added on the baffle's inside seams.

With the box done, I ran it through the table saw to remove the four front-to-back edges.  These rectangular spaces are eventually filled with solid cherry wood.  The back and front baffle were actually spray painted black and top-coated prior to assembly.  The remaining sides were veneered with rosewood to match the other speakers in the setup.  The finish is Danish oil and wax.

Just before mounting the drivers, I added some R19 fiberglass insulation to the chassis cavity.  The crossover wires are then soldered to each driver, being careful to keep the polarity correct. A thin layer of weather-stripping rope caulk is applied to each driver opening before the driver is finally screwed down with particle board square drive screws.

For the baffle, I decided against a grill that would cover the entire baffle, as I have done with the other speakers in this setup.  Instead I wanted a grill that appeared somewhat inset.  To do this, I added a solid cherry edge around the front baffle. This edge has a 1/4 inch round over and adds about 1/2 inch to the overall depth of the speaker. The grill then fits inside this edge, protruding about 1/4 inch. The grill itself is a 3/4 inch particle board frame with black acoustical fabric stretched and hot-glued on the inside edges.  Ball and socket connectors are used to mount the grill to the baffle.

The resulting box is quite heavy, dense and rigid. It does not exhibit noticeable vibrations when in use. 

Here is what the completed speaker looks like with the grill on and off.

The result

Looking at what I set out to do, I have to say that I am very pleased with the results. The sound from this speaker is extremely smooth and the wide dispersion will be handy since I doubt I'll be able to have the center channel speaker perfectly pointed at my sitting position.  The cost is reasonable - about $30 for the crossover, $35 for each woofer and tweeter.  Construction materials were leftovers and thus effectively free.

Since I only made one unit, I cannot comment on its imaging in a stereo pair.  Nonetheless, I definitely recommend this Madisound design for others willing to try a kit.

29-September-2000


Note: The contents in these pages are provided without any guarantee, written or implied. Readers are free to use them at their own risk, for personal use only. No commercial use is allowed without prior written consent from the author.