Craig J Coley
$100 – $500
This project is called the Coley OB-4 Desktop Open Baffle speaker. It is a miniature full range open baffle speaker with a frequency response of 50 Hz to 20 KHz. The circuitry presents a high impedance to the amplifier at high frequency, preventing high frequency instability with simple amplifier designs.
This project began with two goals, (1) determine the minimum possible size for a full range open baffle, and (2) design a set of speakers suitable for desktop use at the office. Considering that small, low distortion Class D bluetooth amplifiers were readily available, it was decided to accept 20-22dB of efficiency loss to make the speaker full range. This efficiency sacrifice may sound unacceptably large but with a 30W low distortion amplifier costing only $30.00 or so, there is still plenty of volume available to achieve a high fidelity sound.
A Mark Audio CHN-70, Tang Band W4-1320, and Tang Band W4-1720 were evaluated for this project. I was surprised to learn that my own measurements of the Tang Band W4-1720 woofer showed it to be a better full range speaker than either the Mark CHN-70 or Tang Band W4-1320. The W4-1720, even though advertised as a woofer, had a flatter high frequency response than the manufacturers data sheet implied, as well as better Xmax and a lower free air resonance.
The baffle for the OB-4 is a simple flat board made from 1 x 6 cedar with a 3 inch foot made from 1 x 4 cedar. Cedar was chosen for its low cost, pleasant grain, and lack of formaldehyde that can trigger allergic reactions.
Because the OB-4 is a simple open baffle, construction is quick and easy. The baffle and foot were first rip cut and then sanded smooth using an orbital sander. All edges were treated with a router and round-over bit. One edge of the 3” foot was cut at 10 degrees with a band saw and two pocket holes were drilled for attachment to the main baffle. The driver hole was cut with a 4” hole saw and the hole edges rounded with the router. Because this was solid wood, all holes were predrilled to avoid splitting.
Once assembled, the finish used was a Minwax water based walnut stain that is fast drying, low odor and low allergy potential.
The crossover is a simple LR shelving circuit used to provide a conjugate match to the natural bass rolloff of the driver. Once initial measurements were taken of the driver itself, the driver characteristics were entered in SPICE and the circuit modeled to achieve the desired conjugate characteristics of the circuitry for compensation. Approximately 22dB of efficiency loss was required to flatten the driver response from 50 Hz to 20 KHz.
Tips & Tricks:
The baffle isn’t critical in either size or material and can easily be made from any convenient material such as MDF. The 30mH inductor also isn’t critical and can be made from a wide variety of solid and air core inductors as long as the rated saturation current is at least 1.0 amp.
The Coley OB-4 performs extraordinarily well as a desktop speaker system, bringing high fidelity sound to the office at low cost.
It may be difficult for some builders to get past the low efficiency of the OB-4 but this shouldn’t be a deciding factor. The overall sound is excellent and flat in frequency response, betraying both the speaker size and the low efficiency. I have tried these speakers with a wide range of amplifiers from my $1100.00+ Coley 215SE high end SET amplifier to low cost Class D amplifiers cost only $30.00, all with excellent results.
About the Designer:
This project was designed and built by Craig J. Coley of Burleson, TX. I work as an electrical designer and am listed as inventor on 8 US patents in the field of avionics. Contact me at [email protected].
Project Parts List:
|Tang Band W4-1720 4″ Underhung Midbass Driver|
|100 Ohm 5W Resistor Wire Wound 5% Tolerance|