Wednesday, October 22, 2014

The Sound Transformer - An Early Prototype

This is a continuation of Bass Horn Project - Part Three.

First a little review of how we got here.  A very long time ago, I developed an interest in high fidelity sound.  I plunged into the field at a very early age by removing all of the "recus" from my parents, record cabinet and spreading them around on the floor.  By the time my parents discovered this, I was sitting on top of a haphazard pile of 78 discs.  Later, when I could walk, I conducted an investigation of the loudspeaker in my father's Zenith Cobra Radio Phonograph.  This involved climbing up on a chair and poking a pencil through the grille cloth into the speaker cone.

A few years later, a very kind uncle kindled my interest even further by dropping off at our house several vintage audio devices for me to explore.  Those included a Webster-Chicago wire recorder, the precursor to tape recorders, an Ampex ten-inch reel-to-reel tape recorder, a Thordarsen twelve watt amplifier, and a Garrard record changer.  My first "hi-fi" was the Garrard plugged into the Thordarsen with a 4" x 6" speaker connected to the output.

I continued my formal education later on pursuing physics, mathematics, and electronics.

Finally, after all of this preparation, I am able to make a contribution to the field of high fidelity sound.  The "Sound Transformer" described below probably offers the most bang for the buck of any loudspeaker available today.  It is made of components removed from an "acoustic suspension" speaker, specifically a KLH model CL-3A.  The parts used include a twelve-inch woofer in the throat of the bass horn and the three-way crossover.  The midrange driver attached to the plywood horn is a two inch speaker removed from a small computer speaker box.  The tweeter is a Sansui unit removed from another speaker.

The Sansui tweeter is so efficient that it was necessary to use a dividing network to attenuate the treble response to achieve a balance across the frequency spectrum.


Sound Transformer - Midrange and Tweeter Horns
Sound Transformer - Midrange and Tweeter Horns


Sound Transformer - Showing Lepai 2020A and Crossover
Sound Transformer - Showing Lepai 2020A and Crossover
The plywood and Masonite panels used in the bass horn were recovered from scrap at local recycling and transfer stations.  Although large, the bass horn is light in weight and separates into two sections for transport.  The inexpensive Lepai 2020A amplifier provides the cleanest sound through this.

This assembly is called the "Sound Transformer" because the horns provide an acoustical impedance match between the relatively small speaker drivers and the air in the listening area.  The bass, midrange, and treble drivers are located in the same plane so the sound can travel from the drivers to the listener in the same length of time.

Tuesday, October 7, 2014

Even More on the Plywood Horns

I needed to make more experiments with the "poor man's" midrange horn driver concept.  I regard the matching plywood horns as a prototype not to be further altered.  They sound quite fine the way they are.  I needed a quick and simple way to fabricate a spare midrange horn to work on and try some new inexpensive drivers with.  Searching online, I came across a technique for building a horn using a combination of plywood or MDF panels and foam core panels available in almost any art supply store.  Here is a photo showing the horn glue setting:


Foam Core Midrange Horn I
Foam Core Midrange Horn I
Here is another photo showing the pine mold inside the foam core construction.  It requires many elastic bands to hold the glue joints.  Particularly on the narrow sides with the sharpest curve, it really helps to carefully score the foam core parts so they will bend easily into the shape you are trying to make.
 
Foam Core Midrange Horn II
Foam Core Midrange Horn II
I already had some 3/16 inch black foam core.  I will post more pictures of this later after it comes off the mold...

Bass Horn Project - Part Three

A Fully Horn-Loaded Loudspeaker Made Entirely From Parts And Materials Recovered From The Dump.

This is a continuation of Bass Horn Project - Part Two.

Part Three was supposed to be about constructing the third "throat" section of the Bass Horn.  Instead, the present two-thirds of the Bass Horn are now set up as the bass portion of a fully horn-loaded speaker in the barn attic.  The bass driver currently connected to the Bass Horn is a twelve-inch woofer taken from a KLH model CL-3A.  The woofer is connected to the crossover network also removed from the CL-3A.  The plywood midrange horn is connected to the midrange wiring in the same crossover and the Sansui tweeter is connected to the tweeter wiring.  I set the midrange and tweeter horns on a box so they are about the same distance from the listening area as the Bass Horn driver.

Horn-Loaded Speaker System - View 1
Horn-Loaded Speaker System - View 1
Horn-Loaded Speaker System - View 2
Horn-Loaded Speaker System - View 2
Horn-Loaded Speaker System - Plywood Midrange and Tweeter Horns
Horn-Loaded Speaker System - Plywood Midrange and Tweeter Horns
Horn-Loaded Speaker System - KLH CL-3A Crossover Network
Horn-Loaded Speaker System - KLH CL-3A Crossover Network
Horn-Loaded Speaker System - Lepai 2020A Amplifier
Horn-Loaded Speaker System - Lepai 2020A Amplifier
I am listening to the speaker powered by one channel of the Lepai 2020A Class "T" amplifier.  I have only one speaker like this right now so I have been listening mostly to monaural LPs recorded in the mid 1950s.  The sound is surprisingly good.

Monday, September 1, 2014

Bass Horn Project - Part Two

There is a lot to report today.  I'll get to that later.  First, before I could complete Bass Horn Project - Part One, which is the largest portion of this project, being the "mouth flare" of the horn, I needed to build the middle part of the horn.  This was necessary in order to fit the mouth flare to the middle part properly before installing remaining cleats and fasteners.  I do not have a photo of the middle section by itself because I went ahead and clamped the two sections together to begin the fitting process.  Here is a photo of the clamped mouth flare and middle section:


Bass Horn - Part-2-01
Bass Horn - Part-2-01
Of course, I couldn't resist preliminary testing.  The first thing I tried was listening to the world through the horn throat.  This worked pretty well with the garage door open and the horn propped up at an angle.  I could hear sounds through the horn that were inaudible to me without it.

I wanted to try a driver mounted at the throat to see how this might sound.  I tried a wide range GE S-1201D featured in an ad in Audio Engineering February 1952.  You will find an image of this advertisement on the page at The EH Dirt Road Special is Back for repairs posting.  The S-1201D I tried is in an old open back baffle originally part of a Califone record player.  Measured DC resistance of the voice coil is 8 Ohms.

Just about the time I got this lash-up ready, I realized that the power had gone off.  So instead of plugging anything into the wall outlet, I set up my 12 DC marine battery with a small inverter and plugged into that.  Here is a photo of the setup for testing:


Bass Horn - Part-2-02
Bass Horn - Part-2-02
This worked quite well playing music from one channel of my iPod through a Lepai LP-2020A.  I could have powered the amp directly with 12 VDC but decided to try out the small inverter instead.  Here is a picture looking directly into the horn with the Califone baffle and screen visible.

Bass Horn - Part-2-03
Bass Horn - Part-2-03
Here is how the driver and Califone baffle look from the throat end:


Bass Horn - Part-2-04
Bass Horn - Part-2-04
At this point, pleased with the preliminary testing so far, I removed the driver and baffle and continued to fit the two horn sections.  This work consisted of ripping and fitting four cleats to fasten to the flange at the smaller end of the mouth flare section and fastening the edge of the Masonite panels to the cleats.  In addition, I fastened the panels at the big end of the mouth flare section to the four longer cleats at the horn mouth flange.  Two of the beveled 3/4" x 1" cleats are visible in this photo:

Bass Horn - Part-2-05
Bass Horn - Part-2-05

Here are a couple of photos showing the interior of the horn after the eight cleats have been fastened:


Bass Horn - Part-2-06
Bass Horn - Part-2-06
Bass Horn - Part-2-07
Bass Horn - Part-2-07
This second close-up photo has the Califone and GE driver back in place.  The Masonite panels are much stiffer with the additional fastening.  By this time, the AC power had come back on, so I disconnected the inverter, put a little charge in the 12 Volt battery, and plugged the power supply for the Lepai LP-2020A into an AC outlet for further testing.

This project is about a bass horn.  However, preliminary testing with the vintage GE S-1201D with a variety of music indicates that, if you can live with a little directivity at the higher frequencies, this seems to work quite well as a full-range horn.

Sensitivity is extremely high.  While working in the garage today about eight feet away from this horn while it was playing, I had the volume set for a comfortable listening level.  Without changing anything, I connected an AC voltmeter directly to the amplifier speaker output terminals and measured peaks of about 200 millivolts going into the speaker.  At 8 Ohms, this is the equivalent of 5 milliwatts of peak electrical power.  While fastening the Masonite panels, I was essentially inside the horn listening to the music and I had to turn the volume down even further.

Before I set up the iPod music player, I tried a sine wave generator and got very strong output down to about 40Hz.

So far, this speaker is relatively lightweight.  I will try to weigh it tomorrow.  Using an SPL meter, it would be interesting to measure the efficiency of this speaker at several different frequencies.  Another test I would like to try is applying a square wave and watching the waveform in the air in front of the speaker with an oscilloscope.  Time coherence should be quite good because all of the sound is coming from a single driver without the need for any crossover.  I will probably take it outside for square wave testing to avoid room boundary reflections.

Even without any further testing, the musical sound from this speaker is really stunningly good, especially from well-recorded percussion instruments.

This bass horn project continues with Bass Horn Project - Part Three.










Friday, August 29, 2014

Bass Horn Project - Part One

Before attempting a New Folded Corner Bass Horn design that is likely to be very complicated to build and only worthwhile if it improves upon the performance of the highly regarded Klipschorn, I decided to build a couple of straight bass horns instead.

Bass Horn Project - Part One is the construction of the largest of three separate horn sections that will eventually be joined together to make one straight Bass Horn.  The point of having three separate sections is mainly for transport and storage in the smallest volume.  Theoretically, the smallest section will fit inside the middle section and the middle section will fit inside the largest section.  Separately carried, all three sections will fit through a standard 3' 0" x 6' 8" exterior door clear opening.  Nested together, they should fit in most SUV vehicles.  The fully assembled horn will be eight feet long without driver and back chamber so it could be tied down to a small boat trailer fully assembled.  Here are pictures of the progress so far:


Bass Horn - Part-1-01
Bass Horn - Part-1-01
This shows the upper and lower flanges connected by two pairs of ribs on opposite sides.  The larger flange has a cleat tacked to each side of the flange.


Bass Horn - Part-1-02
Bass Horn - Part-1-02
A five gallon bucket on the left illustrates the scale of this thing.  Here, the first of four Masonite interior walls is being fitted.


Bass Horn - Part-1-03
Bass Horn - Part-1-03
The first Masonite side is being fastened to the pair of ribs provided.  This material bends easily and can be stiffened with sprayed-in-place foam insulation later on if necessary.



Bass Horn - Part-1-04
Bass Horn - Part-1-04
Both sides are fastened to respective ribs.  These first two sides are trapezoids.  The next two sides will fit inside these and will be scribed, fitted, and fastened to complete the basic assembly of Part One.  I will be posting more photos as the project progresses.

Here is the completed basic assembly of Part One:


Bass-Horn - Part-1-05
Bass-Horn - Part-1-05
This bass horn project continues with Bass Horn Project - PartTwo.









Monday, July 21, 2014

Outdoor Party Speaker Pair Upgrades

In June, 2014, I found the Party Speakers next door at a yard sale.  Considering that they appear to have been built as long ago as 1958, they were in remarkable condition "as found".

However, I have been able to make several upgrades to improve the overall condition and sound.  First, I was listening to a pure sine wave generator played through the speakers outside from a distance of about 40 feet and noticed that the output between 400Hz and about 1200Hz was somewhat diminished.  Later, I disassembled the Calrad crossovers and discovered that the original crossover capacitors had developed leakage and no longer worked as well as they probably did 56 years ago.  I removed a couple of new film capacitors from a pair of crossovers that I had on hand and replaced the old ones.  This helped to improve the uniformity of sine wave response vs frequency.

The finish laminate had begun to separate around the bottom edges on both speakers so I glued it back down.

To make it easier to connect the speaker wire, I removed the old terminal blocks and replaced them with banana jacks spaced about 2 1/2 inches apart.  I filled the old screw holes with Durham's water putty and applied mahogany stain over the putty and on scratches in the original finish.  Finally, I applied a coat of varnish to the whole exterior.

A couple of days ago, I removed what appeared to be burlap used as grille cloth.


Upgraded Party Speaker Pair
Upgraded Party Speaker Pair

Meanwhile, I have installed the Jensen horn tweeters on top of the corner cabinets.
 


Corner Speaker Pair and Jensen Tweeters

I set up the Jensen tweeters atop the Corner Speaker Pair after winding new air-core inductors and connecting film capacitors in a "series connected" crossover similar to the Calrad CN-2.


Jensen Tweeter with EV Corner Speaker
Jensen Tweeter with EV Corner Speaker

Jensen Tweeter with EV Corner Speaker and Series Crossover
Jensen Tweeter with EV Corner Speaker and Series Crossover
This combination sounded great so I made a more permanent installation.


Jensen Tweeter with EV Corner Speaker Nearly Complete
Jensen Tweeter with EV Corner Speaker Nearly Complete
These work best when placed in a corner.  I would like to try them in a much larger room.

Thursday, June 5, 2014

The Latest Find: Outdoor Party Speaker Pair

Well, I have been on the look-out for highly efficient speakers that will play at reasonably high levels outside.  While I was mowing my lawn a few days ago, I noticed a yard sale in progress just up the street at a neighbor's house that had recently sold.  Somewhat impatiently, I finished mowing the lawn and then went inside to get my camera just in case anything of interest turned up at the sale.

Instead of taking pictures, I wound up acquiring, through an exchange of equal value, a pair of owner-designed and built bass-reflex cabinets.  Inside each cabinet, I found an Electro Voice SP12B, a Calrad CN-2 crossover, and an Atlas Sound HR-3 horn tweeter.



 Atlas HR-3 - Calrad CN-2 - Electro Voice SP12B
 Atlas HR-3 - Calrad CN-2 - Electro Voice SP12B

The Atlas HR-3 tweeter appears to date the bass-reflex project at sometime after 1958.  Here is some advertising from the magazine Audio, February 1958:


Atlas Sound HR-3 Horn Tweeter (Audio February 1958)
Atlas Sound HR-3 Horn Tweeter (Audio February 1958)
 
The Atlas HR-3 tweeters in these cabinets look like this:


Atlas Sound HR-3 Horn Tweeter 2
Atlas Sound HR-3 Horn Tweeter 1


Atlas Sound HR-3 Horn Tweeter 2
Atlas Sound HR-3 Horn Tweeter 2
Here are images of the Calrad CN-2 crossover:


Calrad CN-2 crossover label
Calrad CN-2 crossover label


Calrad CN-2 crossover schematic
Calrad CN-2 crossover schematic
The Calrad CN-2 pictured is a series dividing network.  As you may already know, this means that the woofer and tweeter are wired in series.  I verified this by drilling out the four rivets holding one of them together and peering inside at the innards.  The ferrite ring core inductor and leads are potted into the case so you cannot see them.  The inductor is tapped and there are two capacitors so that you can change from 2500Hz to 3500Hz by shorting or un-shorting a pair of screw terminals.  They are found configured at 2500Hz in this instance.

The SP12B woofers are the same as you will find in a previous post, Corner Speaker Pair with ElectroVoice SP12B Speakers.   Here is some advertising from Hi-Fi Stereo Review, February, 1962, courtesy of http://vintagevacuumaudio.com/:


SP12B Ad in Hi-Fi Stereo Review February 1962 - p1
SP12B Ad in Hi-Fi Stereo Review February 1962 - p1


SP12B Ad in Hi-Fi Stereo Review February 1962 - p2
SP12B Ad in Hi-Fi Stereo Review February 1962 - p2
Here are images of the interior of each cabinet:


Bass-Reflex Cabinet Interior 1
Bass-Reflex Cabinet Interior 1


Bass-Reflex Cabinet Interior 2
Bass-Reflex Cabinet Interior 2
And here they are set up outside for listening tests:


Bass-Reflex Pair Outside For Testing
Bass-Reflex Pair Outside For Testing


These have a very lively sound and will definitely make great Party Speakers.  I moved these with the wheelbarrow pictured.  I am thinking of adding the Jensen tweeters found in the Magnavox speakers featured in the post,  More on Plywood Horns and Poor Man's Horn Driver, to the Corner Speaker Pair with ElectroVoice SP12B Speakers.  

Saturday, April 26, 2014

Mike Matthews Dirt Road Special - Speaker Upgrade

Several months ago, I posted about a repair on the Mike Matthews Dirt Road Special.  A few days ago, I had the good fortune to run across an exact replacement, electronically and acoustically, for the Celestion 12 Inch original speaker for this amplifier.  For example, the new Fender 12 Inch replacement speaker has almost exactly the same free-air resonance frequency.  The two sound nearly identical under a "tap" test.  By removing the GE (antique) temporary replacement and installing the new Fender, it is now possible to re-install the original power supply transformer to get the original 70 Volt DC power supply level.  The small stone phase shift effect is almost infinitely adjustable with the full 70 Volt supply.  Here is the Mike Matthews Dirt Road Special with new Fender replacement speaker:


Mike Matthews Dirt Road Special with Fender Speaker
Mike Matthews Dirt Road Special with Fender Speaker



Monday, April 7, 2014

Foam Core Midrange Horn

I made a foam core horn using the same mold from the two plywood horns.


Foam Core Midrange Horn
Foam Core Midrange Horn
This is not as rigid as the plywood versions.  The sides resonate a little.  I believe this could be corrected by spraying foam on the horn exterior to stiffen the construction.  This would not add significantly to the weight.  As pictured above, this horn including driver weighs less than one pound.

This type of construction might be applied to the bass horn to reduce the weight.  The basic horn interior horn shape could be built up from foam core and spray foam used to fill in behind the sides to stiffen them.  While still quite large, the bass horn would be light enough to be carried by one or two people.  Maximum dimensions should be less than the standard exterior door opening, about three by seven feet.