New Meets Old — A Horn Loudspeaker
I’m here again this time with an original design for a crystal radio loudspeaker. I was struggling to find a name for this project, thanks Dave for the Idea !!
As the name implies, this is a project where new technology (solid state) meets the old period appearance; this was intentional. I wanted to make a stand-alone loudspeaker that was in keeping with my (rapidly) growing collection of home brew crystal radios and I think this fits the bill.
For ease of construction and reproduction, where possible, I have used standard “off the shelf” parts The construction can be split into three main stages
1) The speaker horn
2) The amplifier and sounder
3) Finally, the housing
This write up is not the only way of achieving the desired result but is a general over-view. It is up to you to apply your own final touches so you will be making a totally original addition to your own collection. Down to business.
1) The Speaker Horn
The speaker horn is made in segments using the pattern included as figure 1. A total of 16 or 14 segments are needed to make the horn, half with glue tabs and half without. The 16 segment version gives a “flatter” horn mouth and the 14 segment version gives more of a conical horn shape. If in doubt which you want I suggest you aim for the 16 segment version, you can always leave one out in the final stage.
The horn itself can be made from either cardboard (the ideal cardboard source could be old cereal boxes) or brass. If brass is used only 7 or 8 segments are needed with no tabs the joints would be butt soldered.
In both cases the lengths, shown are relatively critical for the best performance as a great deal of background work went into the design calculations and you should try to keep as near as possible to my design.
The horn is built up as shown in figure 2. The first layer of 8 (7) which shows the first 3 segments glued together. Make sure the tabs are on the outside of this layer. You will find the shape forms automatically as the pieces are glued together. I found super glue (Cyano acrilate) was the best to use as this gave an instant joint, the trick being to position the tab and then apply a small drop of glue, making sure you don’t glue yourself to the horn.
When the completed horn inner is made put it to one side let the glue fully set before continuing. This gives you a chance to get the excess glue off your fingers !!!!
At this stage the horn will be reasonably rigid for you to gently remove any high spots from the tabs by gentle sanding.
The next step is to reinforce the joints with small strips of thin paper soaked in PVA adhesive, put to one side and allow to dry thoroughly.
Gently sand the outside to remove any high spots and smoothe the surface with fine sandpaper.
Nearly there !!
The next step is to fit the plastic tube into the horn and gluing it in place, some trimming will be needed for a snug fit, I again used superglue. Make sure the tube is in line with the horn’s centre.
The final stage of construction is to glue another set of the segments to the outside using PVA adhesive thus making a two ply card horn.
The finishing stage just involves filling any major holes with filler and gently sanding to provide a reasonable surface and glueing a length of split plastic tube around the edge of the horn mouth to give a bit more strength to the horn edge.
Complete the construction by painting with paints of your choice.
The horn is made. (See figure 3)
2) The Amplifier and Sounder
The amplifier is a standard little amplifier made using an LM386 chip and the clever bit an 8ohm transistor output transformer used in reverse, giving approximately 2Kohm impedance to drive a standard Piezo tweeter driver for the speaker.
Note I had to change the driving element to one of a lower frequency response for the best results.
I chose this approach to save the hassle of making, begging, borrowing or stealing a suitable enclosure for the sounder. The piezo drivers are already in a housing which looks right and is cheap if you look around. It just needs slight modification to fit securely on the end of the horn.
3) The housing
I made my housing from a wooden cigarette box bought cheaply off E-bay and a cheap Microphone clip, again from E-bay. The fine details, I’ll leave to you to apply your own imagination.
All the best and happy building
PS I am at present working on a carbon beam amplified version which will be shown when completed and debugged, keep an eye out for “The Cat’s Whisker”.