Sunday, July 21, 2013

Plywood Horns and Poor Man's Horn Driver

For quite a long time now, I have been trying to achieve the best sound reproduction possible while keeping the cost to a minimum.

Forty years ago, I had built from scratch a version of the Klipschorn folded corner bass horn.  I modified an Allied or Electro Voice 15TRXB tri-axial speaker by removing the horn tweeter.  This, I installed as a bass driver in the big plywood horn.  This seemed to work quite well.  I had the high frequency horn tweeter from the 15TRXB.  What I needed was a midrange horn to go with it.  I tried layering up a polyester resin and fiberglass horn over a wood mold but ran into trouble when the mold would not fully "release" from the horn.  I had to destroy the fiberglass horn in order to remove it from the mold.   I still have the original mold with resin stuck to portions of it.


Midrange Horn Mold #1
Midrange Horn Mold #1
 
After this, I made a second mold and tried bending gluing, and clamping 1/4 inch plywood onto the mold.  I think I used waxed paper as a mold release.  I used resorcinol glue and very small nails along the joints.  After the glue set, I was able to pull the horn off the mold.  Even though I did not yet have a second big folded bass horn to go with it, I went ahead and made a second midrange horn.


Midrange Horn Mold #2
Midrange Horn Mold #2


The big folded horn appears in this old photograph center right in the corner between the barn and the house.  You can see how big the horn is by comparing the size with the two red cars, the 544 Volvo and the 142 Volvo, at about the same distance away in the photo.


Listening to music through the big folded corner horn
Listening to music through the big folded corner horn

Well, I had to let the big folded horn go during a move.  I dropped it off at the Boothbay Transfer Station on Country Club Road at some point during the 1980s.  I hope that someone recognized it for what it was and rescued it from the construction demolition grinder.  I held onto the two plywood midrange horns.

A few years ago, I removed a couple of 2 1/2 inch speakers from small plastic computer speakers that used to come with desktop pc computers.  Recently, I was cleaning up my shop and found the pair of small speakers and thought it might be interesting to see how they would work as drivers for the old plywood horns.  The speakers have rubber surrounds and 1/2 inch voice coils and seem to be pretty well made. 

First, I made a couple of new plywood flanges for the horn throats.  To prevent the rubber surrounds from hitting the flange, I carved a circular groove out of the flange.   Using the special mounting hardware that I had saved, I installed the speakers on the horn flanges.


Plywood horn with new flange and wired for testing
Plywood horn with new flange and wired for testing
 
I have an old Benjamin Miracord turntable and pioneer receiver on the bench for a signal source.


Benjamin Miracord Turntable
Benjamin Miracord Turntable
Pioneer Receiver
Pioneer Receiver
I connected one channel directly to one of the horns and tried listening to an old 78 recording.  I did not use any crossover or series capacitor.  The sound of this horn was really impressive.  It did not sound like a midrange horn at all.  There was none of the "barking" or "edge" to the sound.  Instead, the sound was effortless and smooth.  The frequency response seemed to extend way beyond what the horn should be capable of reproducing at both the low and high ends of the spectrum.  I would describe the sound as "lively".  I immediately connected the other horn to the other channel and tried listening to an iPod track in stereo.  The stereo effect of the horns was stunning.

At about this time, I became aware of the Class D 12 Volt amplifiers made under the Lepai brand.  The sound of these horns through this amplifier is about as clean as I can recall ever hearing.


Lepai amplifier with "Poor Man's Horn Driver"
Lepai amplifier with "Poor Man's Horn Driver"
I am fully aware that midrange horns typically have "compression drivers" and that the drivers typically have large voice coils, heavy magnet assemblies, and specially designed acoustic loading built into the horn throat.

Compression drivers can be quite expensive.  The "poor man's horn driver" I have stumbled upon is definitely "under-engineered" and perhaps even "heretical" in that it appears to "get around" an apparent cost barrier to the enjoyment of high end sound reproduction.

Now, I really need a couple of bass horns to go with these plywood horns.  The next project I will undertake is an improvement on the Klipschorn folded corner bass horn.