Programmers' Pain
8Nov/110

Things you shouldn’t do #3: Cut 19-inch amplifiers and subwoofer crossovers in half

The coffee tables sound system in the original and installed state

The coffee tables sound system in the original and installed state

Quite some time has passed by since my last blog post about my 512 RGB LED Coffee Table project so that it’s about time to give you a brief update what has happened the last ~8 weeks. Mid of September my carpenter was done with constructing the coffee table so that I could finally start to integrate all the electronic components and the needed wiring into it. Thanks again to especially Gunnar Schwinn from http://www.massivholz-moebel-schreinerei.de/ which did a great job in bringing my ideas to life and giving valuable feedback. As soon as the coffee table arrived at my place I focused on the sound system since the installation of it is rather a standalone task beside the installation of the RGB LEDs.

Thomann THE T.AMP S-100 2x100W Stereo 19″ PA amplifier

Thomann THE T.AMP S-100 2x100W Stereo 19″ PA amplifier

The sound system consists out of two Thomann THE T.AMP S-100 2x100W Stereo 19″ PA amplifiers that have been ‘optimized’ to fit into the table. ‘Optimized’ in that context means that I’ve chopped both amplifiers in half, cut the housing into several pieces that do allow me to fit the circuit boards into one of the tables 20x23cm technical compartments. The power transformers were mounted against the compartments sidewalls and the cables to the front panels providing the volume regulators have been extended to fit my installation. It’s just the average modifications that makes every DIN-Standard-evaluator start to cry – instantly.

Each stereo amplifier will drive a QX-62 two-way– and a QX-6.2W kickbass speaker pair from ESX that are meant to be installed into car hifi setups. Since I only have a stereo sound source I had to find a way how to connect both stereo amplifiers together. This is aggravated by the fact that all four speakers will be installed into relatively small compartments which will make it even harder to calibrate the speakers to deliver an acceptable sound. To do so I’ve installed a Reckhorn S-1 active subwoofer crossover that allows me to finetune the exact cutoff- and crossover frequencies between the two stereo amplifiers so that I can operate the kickbass speakers only in the desired frequency range and that the two-way speakers don’t have to handle those the lower frequencies.

The four speaker compartments after they've been damped with bitumen plates

The four speaker compartments after they've been damped with bitumen plates

The installation of the speakers was quite work-intensive since I first damped the speaker compartments with some bitumen plates to prevent the wood from picking up the vibrations. Afterwards I’ve glued them into the table so that I was able to mount the speaker chassis itself. To tighten up the sound I’ve also muffled each speaker compartment with a bigger piece of insulating material since especially the kickbass speakers would usually need a larger compartment volume than my ~2.5l compartments can offer. As a final step I’ve installed two bass reflex tubes into each compartment to give the speakers some air to do their thing. Although those two bass reflex tubes are a bit oversized for the compartments a prototype compartment with only one or none bass reflex tube wasn’t able to deliver a better sound in my opinion.

One of the two speaker stacks with a two-way speaker on the bottom

One of the two speaker stacks with a two-way speaker on the bottom

At the end a two-way- and a kickbass speaker have been stacked on top of each other as shown on the picture. Both speaker stacks have been installed between the bottom plate and the base plate of the tables technical compartment that does contain all the wiring needed to drive the RGB leds. To allow the sound to get out of this confined place a 45° cone between both speaker compartments does reflect the sound of the two-way speaker to the outside. Since I wanted the speaker stacks to appear as a single unit the space between the compartments will be covered with acoustic cloth to get a cleaner look.

The most work-intense part was for sure the modification of those PA amplifiers so that they do fit into the table at the end. Beside constructing a housing out of the original 19″ enclosure I also had to extend some wiring to be able to connect the front panel controls and the power transformers to it. Also I’ve disassembled the subwoofer crossover and cut its circuit board in half so that I can mount the potentiometers to the front panel. The other half of the circuit board containing the power supply for the active subwoofer crossover is mounted inside the technical compartment of the coffee table.

Mounted amplifiers with extended front panel wiring and installed power supply

Mounted amplifiers with extended front panel wiring and installed power supply

After the first tests I’m pretty satisfied with the sound of the coffee table especially since this was my first attempt to build anything like this. Of course there isn’t a massive bass foundation coming out of the table but this was expected since I only used 16.5cm car hifi speakers in a very limited environment. Usually you would need a lot more space to construct an usual subwoofer compartment. Nevertheless due to the combination of a full-range two-way- and a kickbass speaker pair the table is able to deliver an more than acceptable sound from my point of view.

Be advised that applying those kind of modification to consumer hifi / PA devices do require a broad knowledge of electronic components in general. Please be aware that you can easily end up in a life-threatening situation since you have to deal with line voltage directly at several places while doing those kind of modifications. Therefore I have to close the post with:

So, things you shouldn’t do #3: Cut 19-inch amplifiers and subwoofer crossovers in half
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