5 inch 2-way
13 Liters Bass Reflex Bookshelf

by Michael Chua

Measurements by LMS version 3.0
LMS is manufactured by LinearX

This design is free for DIY. Not for Commercial Use.
This article may not be published in part or full without the express permission of AmpsLab.

Magnetic Field vs Performance

When I came across HiVi's "Symmetric Motor Drive (SMD)", my curiosity was naturally piqued. I was instantly transported back to the 80's, in an era when JBL Studio Monitors reigned supreme. It was during that glorious period in time, JBL's top of the line woofers sported their state of the art technology, "Symmetrical Field Geometry (SFG)".

Could it be that 30 years later, this technology finally trickled down to mass consumer level? Their claims are remarkably similar. Substantial reduction in distortion by way of symmetrical flux distribution along the voice coil path. In simple terms, the cone is better controlled by the magnetic field. These SFG woofers were reputed for their precise bass reproduction.

Loading the M5a

Upon extracting the Thiele & Small Parameters with LMS, I was pleasantly surprised to find that the M5a could be loaded into a compact 13 Liters Bass Reflex box with a fairly low F3 of 53Hz. At this frequency, the M5a would be able to stand alone without a subwoofer.

In order to determine the real acoustic roll off, near field measurements were made of the M5a and port output. Plots were then reference back to 1 meter before mathematically combined by LMS.

The Violet plot on the right is the Summed Response of the M5a with the port. F3 registers at an admirable 53Hz. This deviation from box simulated response is due to an increase in box volume from stuffing the inside with R11 fiberglass wool.

Click Pictures to Enlarge

Predicted Low Frequency Response
Bass Reflex (Vb=13L Fb=60Hz)

Far Field Summed Response (1W1M)

The Venerable D25AG35

I decided to mate the M5a with Vifa's elderly 1" aluminum dome tweeter, the D25AG35-06. Though outdated, it still commands a large following due to its bewitching highs. The D25AG35 is demanding. An LCR (Series Notch Filter) must be used to further damped the tweeter's resonance. Neglect that and it will irritate, especially when crossed low.

For a more forward vocal projection, I crossed the D25AG35 at 2,500Hz with a slope of 18dB/octave. The Violet Plot on the right is the unsmoothed, Gated Summed Response of the tweeter with the M5a. I'm not overly concerned with the peak at 4,000Hz. Even with a gated measurement, I believe the cause of uneveness in the tweeter's response is from room reflections.

The final plots are the Frequency Response and Impedance of the Robin. The all important critical midrange region is smooth. No peaks. No valleys. Overall impedance is friendly. Generally 8 ohms from bass to the mids, dropping down to 4 ohms at 3,000Hz. Nothing that any competent amplifier cannot handle.


The Robin

After all that's said and done, how does it sound? First off the blocks was Strauss. Violins are not strained. Cymbals don't sound like glass shattering. Just the right amount of Oomph in the lows. Promising, so far.

Jewel was next. "Loved by You" in her latest album "Perfectly Clear", is particularly demanding on the crossover. This track is far better than a warble tone generator. Jewel's heart wrenching rendition came through without any edginess.

On a brighter note, Mindi Abair followed. This girl is outstanding. She can hold her own with the best in the business. Her sax is precise and commanding. Lastly, Armik's guitar came on for transients. The attack in the strings was fast and crisp. The mellowness one expects from an acoustic guitar was faithfully reproduced. Most important of all was the upper bass ability to keep pace with the speed of his guitar. This is where the "Symmetric Motor Drive" technology shines. Its in the articulation in the upper bass.

In summary, I'm exceeding pleased with the Robin. For this level of quality, a pair of bookshelf would easily cost more than $1,000 retail.


next > Robin | Robin-II | Robin-IIa



Speaker Type  2-way Bass Reflex Bookshelf
High Frequency Transducer  Vifa D25AG35-06 1" Aluminum Dome Tweeter
Low Frequency Transducer  HiVi M5a 5" Metal Cone Midwoofer
Crossover Frequency  2,500Hz
Crossover Network  Butterworth Low Pass (12dB/oct)
   Butterworth High Pass (18dB/oct)
 Frequency Response (+-3dB)  53Hz-20kHz
 Sensitivity (1 watt / 1 meter)  86dB
 Nominal Impedance  6 ohms
 Enclosure Internal Volume  13 Liters
 Enclosure Internal Dimensions (WHD)  7" x 12-1/2" x 9"

Vifa D25AG35-06 (pdf)
HiVi M5a (pdf)
M5a (LMS) T/S Parameters

Gated Crossover Response
No Smoothing Applied
System Frequency Response

System Impedance
High Order Crossover Network
Enclosure Plans

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