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DIY Vifa MTM speaker - XT25 + MG18




This project's design goal is to make a pair of speakers which has a clear, detailed sound with good imaging and soundstage. The bass should extent low to 40Hz and high to above 35kHz for SACD reproduction.

 

Vifa speaker units are choosen because they received good reviews, and they are sold at a reasonable price.

 

The woofer model is Vifa MG18WK09-08, it is a 8ohms driver, I connected 2 of these in parallel. Tweeter is XT25TG30-04, it is a 4ohms tweeter.

 

I chose MG18 woofer because the specification said it has a Fs of 34Hz, which is low enough for the bass I want.

I was looking for a tweeter that has flat response and able to go higher frequency. I found the XT25 has a flat response from 1kHz to 40kHz, so I decided to give it a try.

The enclosure is made of 1" thick MDF. I have the enclosure made by a furniture company. In order to have a clean sounding bass a big enclosure is needed. Unibox is used to determine the optimal enclosure volume, which is 63 liters. Dimension is 46"H x 10"W x 13"D.

The speaker has a 3" port tuned to 35Hz, located 21 inch from the bottom.

The crossover is a simple 2nd order crossover. Crossover frequency is at about 2500Hz. I started the crossover design by simulating the standard text book 2nd order crossover in OrCad, then fine-tune the value until I got a flat frequency response. Normal tweeter polarity is used here. One of my design objective is to keep things simple and to minimize the number of components in the signal path. Only 2 inductors, 2 capacitors and 1 resistor are used in the crossover. Components used are Jantzen Cross-Coil, Cross-Cap and Mills resistor.

 

Basic crossover schematic

 

OrCad simulation result :

I used the free version of OrCad to do a simulation of my crossover design. You can compare the simulation result with the measured result. I find both are very similar.

 

Measured far field frequency response:

 

The speaker measures +/- 3dB flat until 20khz. XT25 should go up to 40kHz, but my mic only flat to 20kHz. The mic I used is Behringer ECM8000. Mic preamp used is the Behringer UB802 mixer.

 

The software used for this far field measurement is RMAA, I also tried speaker workshop and obtained similar results.

 

 

[Updated Aug, 2007] Baffle Step Compensation

 

Initially my speaker design did not included the baffle step compensation. After using the speaker for some time I feel I need a bit more bass. So a +4dB baffle step compensation is added. The BSC is calculated from a excel BSC calculator from http://www.quarter-wave.com/General/General_Articles.html.

 

The added BSC compensation circuit requires additional crossover components. After adding the BSC, the final crossover schematics is as follow :

Below is the measured woofer near field response (blue) and port response (red) after the BSC is added. The measurement is done by playing back a white noise signal from a CD player, and then use Adobe Audition to record and do a frequency analysis. The combined port and woofer frequency response should be extended down to 35Hz.

 

Woofer near field response:

 

The finished speaker sounds detailed, natural and smooth, with clean and deep bass extension. Soundstage and imaging is also good. XT25 is a very smooth and natural sounding tweeter. Yet it provides all the details.

 

 

 

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