Friday, November 1, 2013

More on Plywood Horns and Poor Man's Horn Driver

A while back, I posted about the Plywood Horns and Poor Man's Horn Driver.
Since then, I have had the good fortune to discover a couple of  aluminum horn tweeters inside Magnavox 1S8730 speaker cabinets.

Inside The Magnavox 1S8730 Speaker Cabinet
Inside The Magnavox 1S8730 Speaker Cabinet
 I removed both horn tweeters and connected each through a 10 uF capacitor in parallel with a plywood horn driver.

580088-1 Horn Tweeter with Plywood Horn
580088-1 Horn Tweeter with Plywood Horn

The plywood horns are even more efficient since I enclosed the back of the drivers.

580088-1 Horn Tweeter with Plywood Horn and Back Enclosure
580088-1 Horn Tweeter with Plywood Horn and Back Enclosure
When I first tried listening to the midrange plywood horn and the tweeter horn together, the level for the tweeter was higher than the level for the midrange and I had to pad the tweeters with a series 5 Ohm resistor.  After I enclosed the back of the midrange driver, the padding resistor was not necessary.

I have not yet built bass horns to go with this setup.  For the time being, I am using the bass drivers in a pair of KLH acoustic suspension speakers.  I disconnected the midrange and tweeter drivers in the KLHs and the twelve inch woofer and crossover handle the bass quite well.  This setup is "bi-amped" with a Lepai amplifier for the horns and a second Lepai amplifier for the bass.

The volume control for the bass is set quite a bit higher than the volume control for the horns because the horns are more efficient than the KLH bass drivers.

Just today, I listened to the first part of a CD of "Romeo and Juliet" by Berlioz.  This sounds fantastic.

Wednesday, October 16, 2013

Corner Speaker Pair with ElectroVoice SP12B Speakers

This pair of corner speakers arrived with a little dust but otherwise in almost new condition.  Here is how they looked when I first got them:

Corner Speaker Pair with ElectroVoice SP12B Drivers
Corner Speaker Pair with ElectroVoice SP12B Drivers
Of course, I took them apart to see what was inside and discovered SP12B Drivers in pristine new condition.  EV manufactured several versions of these over the years.  Here are some photos of the ones in these boxes:

ElectroVoice SP12B Speaker
ElectroVoice SP12B Speaker

ElectroVoice SP12B Speaker Label
ElectroVoice SP12B Speaker Label

ElectroVoice SP12B Speaker Cone and Whizzer
ElectroVoice SP12B Speaker Cone and Whizzer

ElectroVoice SP12B Speaker Voice Coil Dust Cover
ElectroVoice SP12B Speaker Voice Coil Dust Cover

ElectroVoice SP12B Speaker Terminals
ElectroVoice SP12B Speaker Terminals
I weighed one of them on a spring scale:

ElectroVoice SP12B Speaker Weighs 12 1/2 Pounds
ElectroVoice SP12B Speaker Weighs 12 1/2 Pounds

Here is what the back of the cabinets looks like:

Corner Speaker (Back)
Corner Speaker (Back)

The terminals on the back look like this:

Corner Speaker Terminals
Corner Speaker Terminals

To sound best, they should be positioned in a corner:

Corner Placement For Best Sound
Corner Placement For Best Sound

These are among the best sounding speakers I have heard.

The EH Dirt Road Special is Back for repairs

Well, quite a lot has happened since my last post on the great little amplifier, the Electro Harmonix Mike Matthews Dirt Road Special.  I replaced the original 70 VAC secondary transformer with a 12 VAC transformer.  My laborious temporary speaker repair did not hold.  I have replaced the original Celestion speaker with a vintage replacement speaker that I have had for quite a long time. 

Here is a photo of the 12 VAC transformer installed in the amplifier:

12 VAC Transformer in Dirt Road Special
12 VAC Transformer in Dirt Road Special

I got the transformer at Radio Shack.  It has a three Ampere rating.   You can see that I have used heat-shrink tubing over the splices I made in the primary leads.  Here is a photo of the amp with the replacement speaker installed:

Electro Harmonix Mike Matthews Dirt Road Special Amp with GE speaker
Electro Harmonix Mike Matthews Dirt Road Special Amp with GE speaker

This is an early 1950s speaker I inherited from a guy who used to hold "record hops" in his barn a few miles from here.  He had several of these speakers, each in a plywood open back box hanging from a barn beam.  He ran speaker wires to them from a record player on which he played mostly 45 rpm records of tunes that were popular at the time.  At any rate, I have been trying to find out more about the speaker for quite a long time.  It has an Alnico magnet and is similar in appearance to a GE model S-1201D speaker illustrated in the following scan of page 7 of the February 1952 issue of Audio Engineering magazine.

GE S-1201D Ad in Audio Engineering February 1952
GE S-1201D Ad in Audio Engineering February 1952
The speaker still has its original felt dust cap.  The speaker is very efficient and sounds great in this amplifier.  It ought to.  If you apply the inflation calculator at this speaker would have cost $1279.50 in 2012!

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.

Wednesday, May 15, 2013

Take 7 by Natural String Effect™

This is a recording of the Natural String Effect™ responding to a midi file.  The video is a result of playing the recording in Winamp with a Visualization plugin activated.

Take 7 by Natural String Effectâ„¢
Take 7 by Natural String Effect™
Here is the actual link:

Wednesday, March 13, 2013

Natural String Effect™ - Vol II

Here are five tracks of the most recent setup.  This one has two strings, tuned to E-flat and A-flat, each string with a String Driver™ under it.  The String Drivers™ are wired in parallel to get the most power from one channel of a 2 Watt stereo amplifier.  With this setup, the coils get slightly warm during operation due to resistive loss.  This is quite an improvement over driving the coils in series with a powerful amplifier.  With the old setup, you had plenty of power, but the coils could get so hot that they came loose from the fiberglass board substrate!  Now, there is no chance they will overheat.  Plus, I have the other 2 Watt stereo channel available.

Here is a photo of the old shop radio I listened to in 1964 while building the pine bass guitar I am currently using in the Natural String Effect™ experiments.  This 1939 Philco console radio still works!

1939 Philco Radio
Here are links to the five tracks on Natural String Effect™ - Volume II:
Long Song Square Reverse is Long Song Square played backwards.
Square is ascending and Square Reverse, because it is played backwards, becomes a descending composition.  However, a look at the waveforms indicates it is not the same as recording a descending track in the first place.

Wednesday, February 27, 2013

Natural String Effect™ - Vol I

Here are the first five tracks ever recorded using the Natural String Effect™.  This short compilation, Natural String Effect™ - Vol  I., provides a brief introduction to the instrument.

If you wish, scroll down and click on the links at the bottom of this page.

The musical instrument I call the Natural String Effect™ is extremely easy to play.
It is so easy to play that I believe anyone would be able to play it, particularly those individuals who like listening to music but believe they have no musical ability.

The Natural String Effect™ project has emerged from my ongoing effort to apply engineering principles to musical composition and performance.  As an engineer, I am always seeking to find the simplest and most efficient process.  In the case of the Natural String Effect™, we allow a simple electro-mechanical system to respond to input with limited intervention.

What the Natural String Effect™ does, among other things, is to make musical composition and performance accessible to everyone.  No training, classical or otherwise, is necessary.

If you wish to hear the first five tracks ever recorded using the Natural String Effect™, please turn your speaker volume or headset volume control down because these are high level signals.

It is likely that you have never heard sounds like the five tracks linked below.  I know I haven't.  Still, you might be reminded, as I was the first time I heard them, of sounds and music familiar to you.

Track 01 - Take 1  This steady state composition reminds me of a power plant or propulsion system.

Track 02 - Take 2  This is another power plant or propulsion system.  This one is in high gear.

Track 03 - Take 3  In this one, you are shifting between high and low gear.

Track 04 - Take 4  This is a short musical composition with a three-chord sequence repeated.

Track 05 - Take 5  This is a three-minute musical composition with a succession of chord changes I find very relaxing.  I hope you do too :)  Believe it or not, Track 05 was the easiest one to play!

Friday, February 22, 2013

Take 5 by Natural String Effect™

Introducing Take 5, Natural String Effect's first composition:

This is about three minutes long, a very relaxing instrumental.  I will upload the audio track in a few days.

Sunday, February 17, 2013

Natural String Effect™

The Natural String Effect™ refers to the output set of signature sounds and vibrations induced in a tensioned string, of variable length, by any time-varying magnetic field or mechanical vibration inputs applied at points or positions at fixed or variable distances along the string length.

A portion of the output set, of variable relative amplitude and phase, is summed with the input set.

The range of  induction points or positions includes any point or position along the string length including either or both string ends.  The Natural String Effect™ refers to the assembly of devices described above, its deployment, the process of output set collection, and the actual output sets themselves.  String Driver™ and String Speaker™ refer to the magnetic and mechanical devices comprising the input string-driving function.

The inputs induce, mechanically and/or magnetically, transverse, tortional, and longitudinal string vibration components.

The output sets include transverse, tortional, and longitudinal components of string vibration.

Here is a demonstrative video of one embodiment with three separate strings.

Applications for this device include music experimentation, composition, and performance, sound effects, movie soundtracks, musical instrument simulation, and musical string testing.

Saturday, February 16, 2013

Beware Of Old Gear!

To all engineers and technicians out there:  Beware of old gear that appears to be in working condition!  Last year, a friend dropped off an Electro-Harmonix Mike Matthews Dirt Road Special here.  It was actually working but there was extreme crackling noise in the volume pot.  I was thoroughly impressed by the sound of the built-in "Small Stone Phaser".  The label on the highly regarded British made Celestion Speaker was visible through a bass port on the back panel.  Here are some pictures of the amplifier.
Electro-Harmonix Mike Matthews Dirt Road Special

Top view

To clean the pots, all of which were quite noisy, I decided to remove the back panel and take a look inside.  This is when things began to get interesting.  I had noticed that the Philips screw fastener heads visible on the exterior were considerably rusty.  However, I was surprised to find that the three wood screws along the bottom edge of the back panel were securely rusted into the wood.  One of the three would not even begin to turn.  I gently tapped on each of them with a Philips driver bit and eventually removed two of them but the third did not respond.  Finally, after stripping the screw head, I had to drill the head of the screw away so that only the shaft remained.  Then I could remove the back panel.  Here are all of the wood screws and a couple of machine screws removed later.

Rusted screw fasteners
What I noticed right away was that the amp must have been in a flood because, not only were the screws rusted but also the back of the speaker was rusted with much of the paint flaking away.

Anyway, the pot interiors were accessible for spraying with contact cleaner without any further disassembly.  With the pots cleaned, I applied power and there was no sound at all from the speaker!  I unsoldered one of the speaker terminals and found that the speaker's impedance was an open circuit!  Next, I removed the speaker and installed another twelve inch speaker.  There was a steady hum from the amplifier but otherwise no sound!  After finding a schematic of the output circuit online, the next thing I did was to remove the amplifier chassis, unsolder the two power output transistors and find that the PNP transistor (TO3 case) had a collector-to-emitter short.  The NPN transistor pn juncitons tested fine with no shorts.  Here is a photo with the PNP transistor removed:

PNP Transistor removed
Here is another close-up of the board for reference.

Printed on the board is the number EH1313 B

I tried checking the connections to the speaker's voice coil and could find nothing amiss.  I finally decided to remove the cone/surround/spider/voice coil assembly to try to get a look at the voice coil because it appeared that the coil wire must have parted somewhere in the coil.  In addition, the coil was rubbing against something in the gap and not moving freely.  With the cone assembly removed from the basket, I noticed that there was really heavy rusting around the spider edge and the spider glue had disintegrated into small fragments, some of which had fallen into the voice coil gap.

Rusting on speaker basket

Rust and glue particles in voice coil gap

At some point, with the cone assembly removed, I measured a voice coil DC impedance of 7.2 Ohm.  This was surprising.  I decided to reinstall the cone assembly after cleaning the rust and clearing the debris from the voice coil gap.

Close examination of the voice coil showed a little scraping but did not reveal any obvious broken wire in the coil.
Scraping on voice coil

Scraping on voice coil

After thorough cleaning with vacuum and fine tweezers, I temporarily covered the gap with black electrical tape to keep filings from entering the gap while cleaning loose rust away.

Loose rust cleaned away

With the rust cleaned out, it was possible to reinstall the cone assembly and align the coil in the gap.

Speaker cone assembly glued and aligned with paper strips


However, after the glue set, the voice coil had opened up again and the carefully restored speaker did not work! Here is a picture of the setup showing a voice coil open circuit on the meter.

Speaker still does not work!

The only way I could get the open voice coil to close was to insert a short wood brace across the diameter of the coil former.  As soon as I discovered this, I positioned the brace for maximum effect and glued the ends to the former.  Here is a photo showing the speaker working with an AC voltage applied to the terminals.

Celestion speaker temporary repair with wood brace

With the speaker working, if only temporarily, I decided to order a matched PNP-NPN transistor pair to replace both output transistors.  I selected transistors with robust specifications, including maximum ratings not likely to be exceeded in this amplifier design.

Here is a picture with the new transistors installed.

Amp with new output transistors installed

I powered this up using a Variac autotransformer.  Everything is working fine with the Variac high enough to make 14 Volts DC from the power supply.  With a full 120 VAC applied, the power supply voltage would be much higher at about 70 Volts.  My friend is suggesting that his potential customer might be interested in doing some studio work with this amp.  If that is the case, I may decide to change the power transformer to one with 12 VAC or even 9 VAC secondary to reduce the power and protect the original speaker, as repaired, from too much power from the amp.  This amp is really loud with only 14 Volts.  In the event someone installs a new speaker, the original transformer should be used.  On the other hand, the amp should work great powered with a 12Volt car battery instead of the AC power supply!  I will post further photos soon...

For more on this, check out the latest post at EH is back for repairs.