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October 5, 2011

The Planet10 Frugel-Horn mk3 Backloaded Horn

While high-end audio may be a niche following, there are further sub-niches for products that have a specific appeal within the marketplace. Most speakers on the market are a 2-way design, meaning they make use of both a woofer for the low-frequencies, a tweeter for the high frequencies and a passive internal crossover consisting of a network of inductors, resistors and capacitors that splits up the frequency range between the two. You may have seen 3-way speakers that add a midrange to the mix, or other more complex designs that split up the range across many drivers such as a line-array. Then we have the full-range speaker, which makes use of a single driver to reproduce as much of the audible frequency range as possible. This design does not make use of an internal crossover, leading to what listeners feel is a more natural sounding response, but often times this particular design sacrifices the highest and lowest frequency response.

There's a solution to this issue by making use of technology that stretches all the way back to the mechanical gramophone in 1857; the horn. Thanks to the advent of computers and more advanced sound modelling technology, horn development has really come a long way in recent years. For full-range drivers, the horn attaches at the rear of the driver, allowing the front of the cone to radiate the high-frequencies directly. The level of the lower frequencies are increased to the same level as midrange and treble by making use of the horn. Because bass frequencies have a long wavelength, the length of the horn needs to be fairly long  to cover the low frequencies. A completely straight horn would be obtrusively long, so they are usually folded to take up less real-estate. The Planet 10 team, which includes David, Chris and Scott, has been working on a number of back-loaded, folded horn designs, the latest iteration being the Frugel-Horn mk3, with special help from Ron Clarke who designed the signature curved mouth opening.

One of Planet10's original Frugel Horn Designs

The Frugel-Horn mk3 can house a variety of full range drivers. Candidates include the Mark Audio Alpair 7, CHP70 & EL70, CHR70, Fostex FE126e, and 126En among others. The carefully calculated design is simple but elegant. It's apparent that Planet10 was concentrating on making the design as simple to assemble as possible while still maintaining the performance aspect of the speaker. All the wood panels have been CNC fabricated, so the dimensions are much more exact than if you were cutting by hand.

I began my journey into backloaded horns by purchasing a Frugel Horn "flat-pack" from Planet10, along with a set of his matched and modified Fostex full-range drivers. A flat pack is a stack of CNC cut wood that's ready to be glued together. It's pretty much essential to have this wood pre-cut because the design would be quite difficult to fabricate by hand.

The assembly process began by checking the fitment of the wood boards and lightly sanding as necessary. Titebond wood glue was then placed in the recess and wood boards fit in place. A small spacer piece was glued and added, the boards were checked to be level and clamps were put in place.

First step assembling the Frugel Horn
Next came the placement of the front baffle.

Gluing the baffle in place

Followed by the top, which required pressure in a number of dimensions.

Fitment of the top of the speaker

And finally the bottom piece.

Gluing the bottom in place

Another perspective

Once everything had ample room to dry, I applied silicone sealant to the interior edges to prevent air from escaping. Then came the process of adding the damping to prevent reflections around the driver.

Internal damping within the Frugel Horn

I used approximately 1/5 of a pound of Acousta-Stuf from PartsExpress for each speaker to line the interior horn area. The batting had to be appropriately "teased" to fill the opening adequately.

Acousta-Stuf filling

Before the final wood panel would be glued in place, a set of holes were drilled to allow for "hurricane nuts to be inserted on the inside. These would connect to either carpet spikes or out-triggers. Wood glue was then placed generously as the excess could be wiped away with a damp towel and sanded later. All the clamps I had available were used to compress the speaker together during the initial placement. 

DAP stainable Plastic Wood was used to seal up any small gaps in the final fitment. Once the Plastic Wood has dried, it is sanded down smooth.

Once all the surfaces were sanded down smoothly with 180 and 220 grit sandpaper, the staining process could begin. I decided early-on that these would be a nice deep red finish, something hopefully similar to the sweet looking Zu "Sangria" finish. Since there aren't any off-the-shelf solutions for oil-based red stain at the local hardware store, I procured some water-based clear tint base that I would be tinting red. Bright Red Trans Tint was used for this purpose. Water-based stain has it's own set of products for pre-stain prep and finish, which you can see in the below photo.

The initial prep includes a pre-stain layer, which is followed by a light sanding. This prevents the grain for raising inopportunely during the staining process. This is followed by the wood stain. The TransTint is to be mixed at a half bottle per 32 ounces; in this case the whole bottle was used for a deeper, more saturated tone. Four coats of this stain were applied to get the wood evenly dark. Finally, three layers of water-based satin Polycrylic are brushed on to seal up the finish. It's a relatively long process going through all these coats, but if one takes their time, it's worth it in the end.

There were a few nice adornments that were planned for this speaker to give it a more professional look. I created some custom art files and had them engraved on plates for the front and back. The back plate would cover the circular hole on the back that was designated for a basic set of terminal posts and house a pair of bare copper Cardas posts. I find these to be the most attractive posts on the market, albeit a bit expensive.

Custom engraved speaker back plate

On the backside of the engraved plate is a circular piece of Dynamat Extreme to help prevent unwanted sound reflections in this small concave area. Cardas 15.5 gauge litz wire was used for the speakers internal wiring. A foot of red and a foot of black for each speaker was more than sufficient. Heatshrink was used around the soldered areas for a clean look.

Rear of the back plate
A healthy amount of speaker caulking was used to seal the area between the plate and the speaker opening to ensure a proper seal. Once the plate was set, a small brass wood screw was placed in each corner. The speaker caulking was then compressed and smoothed by hand around the back opening from the inside. 

Installation of the back plate
Fully installed

The front also had a smaller decorative plate made with a cute little horn icon.

Detail of the Planet10 FE126eN EnABLed driver

A beautiful set of Frugel Horns

In the interest of providing the Frugel Horns with a wider and more sure-footed stance, some outriggers needed to be fabricated. I chose to order a set of ten-inch wide aluminum bars that would attach to the speakers at the bottom and hold a carpet spike at each end. 

Bare aluminum bars to be used as outriggers

Each aluminum bar had four holes drilled, two for the outer carpet spikes, and two to run screws through to attach to the hurricane nuts in the speaker. All edges were sanded down to avoid a "barefoot surprise". Once sanded, they were given to the powdercoater for a matte black finish. A set of low-cost carpet spikes were selected from Partsexpress to mount to the newly painted outriggers. 

Black carpet spike from Partsexpress

Once these were mounted, the outtriggers could be mounted to the bottom of the speakers using 1/4-20 brass bolts which screw into the hurricane nuts that were mounted previously.

Outriggers mounted to bottom of speaker

The Frugel Horn is now finally complete.

The completed Frugel Horn

Looking to take your audio rig to the next level with a set of custom audio cables? Contact Zynsonix Audio for some great solutions.
The Fine Print:
The above steps detailing the building of a speaker are for entertainment purposes only, and are not intended to be used as instructions. Zynsonix Audio LLC and DIYAudioBlog do not have any affiliation with Planet 10 Audio or Fostex. The owner of this blog and all associated parties can not / will not be held responsible if you attempt the process posted and cause physical harm to yourself, your surroundings or your property. Please keep this in mind.


  1. Nice looking project you have here. Are you using your Bottlehead SEX to drive the speakers?

  2. I'm actually working on a TubeLab SE to go with these, I'm hoping the two will compliment each other nicely.

  3. Sweet! Love the font on the plates.