Designer: Triode

Project Category
Furniture

Project Level
Intermediate

Project Time
8-20 Hours

Project Cost
$100 – $500

Project Description
I won an Eminence Lab 15 as a door prize at the DIY KY 2015 speaker event this year and thought I’d retrofit it into a RCA 6-HF-4 phono console that I have in my diningroom. There is a mandate in the household for no visible speakers in the dining room so this was a good way to hide a new speaker in a piece of furniture already in the room.

Design Goals
A mono full range 3-way speaker with sealed enclosures using the Lab 15 for a low end that would fit in a RCA 6-HF-4 chassis

Driver Selection
I started with the Lab 15, I knew I wanted a sealed enclosure for the mids, and based on the available area in the box I thought a MTM configuration in the top section of the existing grill would work nicely. I wanted to maximize bang for the buck, given these parameters the Dayton Audio ND140-4 5-1/4″ Aluminum Cone Midbass 4 Ohm Drivers and a Dayton Audio DC28F-8 1-1/8″ Silk Dome Tweeter seemed to fit the bill.

Enclosure Design
The external dimension of the box were fixed based on what would fit inside the RCA box. A divider was used to separate it into 2 sealed cavities, with the lower cavity for the sub about twice the size of the upper cavity. A 3d model is available here:
https://cad.onshape.com/documents/2f07eeb101144286bc4ca1f0/w/fb867ff8ca32462c804dba2d/e/e837701ad9924774ba9d00c1

Enclosure Assembly
The box made of 3/4″ MDF fit snug in the existing chassis of the 6-HF-4. Of course I had to remove the existing speakers and record player, and cut out the shelf that separated the speaker and phono sections. To cut a clearance hole for the Lab 15 in the RCA chassis I laser cut a template out of 1/8″ plywood and used a router bit with a bearing to cut it out. The new box slid right in the top and was screwed in place to avoid any rattling. I removed the original baffle, peeled back the top section of the grill cloth and laser cut a new opening for the MTM top section.

Crossover Design
For a crossover I decided to go active and use minidsp 2×4. I measured the impedence curve for the Lab 15 once it was mounted in the box and used a linkwitz transform based on the measured Fc and Qtc in order to achieve an F3 of 20Hz with a Q of .7
For the mids I used 48dB/oct Linkwitz-Riley filers at 125 Hz and 2250 Hz .
For the highs I used a matching 48dB/oct Linkwitz-Riley filer at 2250 Hz.
I set the gain of each crossover section based on the published sensitivity numbers, then tweaked a bit by ear.
The power amp I have on the sub can do about 450 W into the 6 ohm Lab 15.

Conclusion
I have to say that the bass response on the finished product is quite satisfying.

About the Designer
I’ve been an electronics design engineer for 15 years and have been a speaker building hobbyist on and off for about 20.

Project Parts List