A little bit of catching up…

I regret not having enough time for writing these days. There are a number of reasons for this. Firstly, much of what I do running my business is mundane and not worth noting. For example; today I am going over work reports and reconciling the bank account. Necessary, but about as exciting as watching the grass grow or reading about drying paint.

However, the rest of the time I have been working on various projects around the northeast, to wit:

In Boston, I took part in converting an LPTV station to ATSC 3.0. That was interesting and I am enjoying the TV work.

WCRN-LD exciter GUI
Boris Johnson resigns, also one of the first ATSC 3.0 images transmitted OTA in Boston, MA

In Syracuse, we had to lower a TV transmitter from the 23rd floor to the 22nd floor on the outside of the building. The transmitter itself became marooned because an electrical conduit for an alarm system was installed restricting the size of the stairwell.

Carefully lowering a 2KW UHF TV transmitter, State Tower Building, Syracuse, NY
Transmitter re-assembled and on the air

Fortunately, we hired a moving company to do this. I am pretty sure that our insurance does not cover damages from transmitters falling 22 stories.

In NYC, I installed two FLX-40 transmitters for GatesAir.

WXBK FLX-40 transmitters, Rutherford, NJ
FLX-40 Heat Exchangers
“Do you see over yonder, friend Sancho, thirty or forty hulking giants? I intend to do battle with them and slay them.” “Take care, sir, those over there are not giants but buildings on the island of Manhattan.”

In Kingston, NY a used BE AM1A (along with a coax switch and dummy load) was installed at WKNY.

Slightly used BE AM1A installed as backup at WKNY, Kingston, NY
7/8 inch coax switch and 2.5 KW oil cooled test load suspended from the ceiling

It is nice that this station has a decent backup transmitter to buttress the aging, yet very reliable Nautel ND-1.

Even though it is a short drive away, I had never visited the 1969 Woodstock Music and Art Fair site in Bethel, NY. It was interesting and being sort of an audiophile, I enjoyed this exhibit in particular:

Your author, standing in front of a “Woodstock Bin.”
Back of high-frequency horns

From the display:

This speaker stack sat on scaffolding high in the air… festival sound engineer Bill Hanley custom-built eight speaker cabinets for Woodstock, amplifying music and stage announcements across the large festival site… Afterward, the design would be known in the industry as the “Woodstock Bin.”

Bethal Woods Performing Arts center Museum, August 10, 2022

The high-frequency horns used Electrovoice diaphragm S/A compression drivers. I don’t know which driver was used for the bins.

Site of Woodstock ’69 stage, looking up into the field
Looking down from the field to the stage area. Person(s) for scale.

I am also writing articles for Radio Guide, I hope that you are enjoying them!

Distributed Mode Loudspeakers

My journey into home stereo speakers and high quality audio continues…

This is not a broadcast related post, but I find this type of thing interesting nonetheless. A few years ago, I began fiddling around with flat panel speakers or Distributed Mode Loudspeakers (DML). My first attempt was lackluster, although adequate and not very expensive.

I enjoy listening to all types of music. I notice that some music reproduces better than others. Piano is a particularly difficult instrument to record and faithfully reproduce because of the complexity of the instrument itself. It is a percussive string instrument. When one of the hammers strikes a sting, the string itself does not make a very loud noise. The string is connected to a sound board through a bridge. The much larger soundboard vibrates and amplifies the initial sound. Thus, the wood itself imparts a noticeable quality to the sound. Several other instruments such as acoustic guitar, violin, etc also use tonewood to amplify sound. Thus, when listening to a piano being played back on a set of speakers, it lacks much of the original texture of the actual instrument. It sounds like a recording of a piano, not an actual piano.

This made me think of ways to improve that reproduction.

A DML uses a large flat surface to reproduce sound. The large surface area and the bi-directional nature of the panel means the sound will be defuse and not directional as is the case with conventional speakers at higher frequencies.

The principle characteristics of material used in a DML is stiffness and density. The less dense (light weight) and the stiffer the better. All materials have internal resonances, some decay quickly and others, such as steel will ring for a long time. A ringing reverb sound would be undesirable in most cases. Tonewood has suitable stiffness, a relatively high sound radiation coefficient, varies in density according to species and generally dampens high frequency harmonics. Pianos often use Sitka Spruce for sound boards.

I made two terminated transmission line speakers last summer. Those sound great, but using one driver to reproduce all sound from 20-20,000 Hz has a drawback; the high end lacks detail and is very directional.

A bit of research shows that Poplar has good to very good tonewood qualities while being inexpensive and readily available at the local big box hardware store. Thus, I purchased several 1/4 x 6 x 48 inch (6 x 152 x 1220 mm) planks and a few pieces for bracing to make two 33 x 22 inch (838 x 559 mm) flat panels. The idea here is to gently cross over the transmission line speakers at 4,500 Hz with these flat panels.

The assembly process was pretty easy. It took perhaps two hours to cut all the pieces to the right size. I used a router to chamfer the edges of the back bracing, mostly to reduce the weight of the brace. Everything was glued up and allowed to dry for 24 hours.

Gluing the parts together
Back bracing

The bracing is designed to keep the panel stiff and helps with structural integrity. I sanded the fronts and backs of the panels, then finished them with Tung Oil.

Dayton tactile exciters

Each panel uses two Dayton DAEX32QMB-4 tactile exciter wired in series. There is also a Zobel network which is important for a tube amp. Most driver’s impedance will rise as the frequency increases. This rising impedance is mostly reactance from the driver coil. For solid state amps, this is not much of a problem. However, tube amps, because of the output transformer, need a fairly constant impedance across there output range. This is because the output transformer is a ratio device. The primary impedance is derived from the impedance on the secondary (e.g. what the secondary is terminated with). For example, a certain tube has a 5,000 ohm plate impedance. The output transformer is chosen to match this design specification. The output transformer is designed to drive an 8 ohm loudspeaker. However, the driver impedance rises to 16 ohms at 15 KHz and 24 Ohms at 20 KHz. Under those conditions, the tube would be seeing a 10,000 ohm impedance at 15 KHz and a 15,000 Ohm impedance at 20 KHz. This will lead to added distortion. The Zobel network keeps the impedance fairly uniform across the entire audio range.

Rather than build a cross over into the transmission line speakers, I made an external unit. This allows the speaker system components to operate independently if needed. I can easily change the cross over components for a different crossover point. I can also mix and match things in the future. In other words, it gives me a great deal of flexibility for future projects and experimentation.

Crossover and HF attenuator
Crossover and attenuator components

I made these small boxes to match the speakers. These contain the crossover and HF attenuator. The attenuator is necessary because the difference in efficiency between the transmission line driver and the tactile drivers on the DML.

The schematic looks like this:

Cross over diagram

I kept things as simple as possible. Yes, I could have made a 12 dB per octave crossover with a few additional components, however, I like the gentle blend of both drivers in the mid ranges.

A basic sweep with Room EQ Wizard shows a fairly flat response. There is a room resonance around 90 Hz which I intend to correct with some sound deadening materials in the corners and possibly a bass trap.

Room EQ Wizard waterfall display

This shows that the speaker system is relatively flat from about 200 Hz to 10 KHz. Those bumps and dips from 88 to 250 Hz are all room due to room resonance.

Room EQ Wizard SPL graph

How does it sound? Pretty good! Here is some copyright free music, recorded from youtube, then re-uploaded to youtube! It is not the best video, there is some background noise, etc. However, it give a pretty good idea of how these sound.

A few pictures

Some things I have been working on lately:

A nice row of transmitters
A nice row of transmitters

Finishing up a transmitter site rehab.  The BE FM20T is nearly 20 years old.  The BE FM2C transmitters are new.  There is also a rack of new fiber equipment and CODECs.  This site has good utilization; there are three stations on one tower with a shared STL antenna and generator.

Energy Onix ECO-6
Energy Onix ECO-6

Energy Onix ECO-6 tube type transmitter.  One of Bernie’s better designs, a grounded grid tube with solid state driver section.  This one needed some fans replaced and a new tube.

AM transmitter site.  Looks like these vines have not been cut in a couple of years.
AM transmitter site. Looks like these vines have not been cut in a couple of years.

I wonder how much the guy tensions have changed…

Noticed this after some particularly strong thunderstorms
Noticed this after some particularly strong thunderstorms

The reason why you do not use a POTS line phone during a thunderstorm.

USS Slater radio room
USS Slater radio room

I took a tour of the USS Slater, a museum ship in Albany, NY.  The museum has painstakingly restored the ship to its WWII configuration.  The main transmitter is the RCA TBL-8 seen in the left/center of this picture.  This unit put out 200 to 400 watts CW or 150 watts AM phone.  During the hostilities it was turned off as allied ships observed radio silence unless they were sinking (and sometimes even then).

A little ChiFi tube type RIAA phone preamp.
A little ChiFi tube type RIAA phone preamp.

I have been fooling around with this little 6AK5 preamp.  I find it works very well and sounds better than the built in phone preamp on my Kenwood VR-309.  The FU-29 tube amp did not come with a phone preamp.

This is a short video clip of an audio processor at one of our transmitter sites. The fancy lights around the control knob are designed for the program director. They are saying “Buy me… Buy me…”

The Sonotube subwoofer

Continuing with my speaker projects; I have finished building a Sonotube subwoofer for my Box One speakers. As I discovered, those speakers miss a significant part of the bottom end. I made up for this by using a Polk Audio PSW-10 subwoofer that was part of my home theater system.

Sonotube subwoofer, downward firing configuration
Sonotube subwoofer, downward firing configuration

I found the PSW-10 was certainly loud enough, but lacked definition which became noticeable when the new speakers where installed. Thus, I began thinking about a sealed box subwoofer. When designing the enclosure, there is a good bit of math involved in calculating the box volume.

Sonotube subwoofer, upward firing configuration
Sonotube subwoofer, upward firing configuration

I now have a bit of a dilemma; should I set the subwoofer up downward firing into the floor or should I set it up upward firing into a dispersion cone?  I have tried it both ways and in the downward firing position, it shakes the floor.  That is a great effect for watching movies like “The Hunt for Red October,” where there is a lot of bass rumbling.  It is also great for blasting some Led Zeppelin or Pink Floyd Dark Side of the Moon.   However, the idea of a sealed subwoofer was to add detail to the bass and that it does do.  The Polk Audio PSW-10 certainly had bass, but it was ill defined.  The sealed sono-sub has less low, low bass but the bass instruments now sound as good as the rest of the orchestra.  In the upward position, I get clean omni directional bass which sounds fantastic when listening to P.I. Tchaikovsky Concert #1 in B flat minor, opus 23.  That was the idea when I started making this.  Still, blasting Led Zeppelin is fun and I recommend it for everyone.  Decisions, decisions…

After a couple months of evaluation, I finally decided on the downward firing configuration.  It does add another dimension to watching movies.  Now, I kind of want to get a bass shaker for the floor underneath the sofa.

Piece of left over, 12 inch diameter sonotube
Piece of left over, 12 inch diameter sonotube

In any case, the technical details for this subwoofer are as follows:

  • Driver: Dayton Audio RSS265HF-4, 10″ subwoofer, 4 ohm
  • Plate Amp: Dayton Audio SA70 70 watt
  • 12″ (30.48 cm) diameter sonotube, 19 3/4″ (50.165 cm) long
  • Miscellaneous pieces of wood, paint, screws and foam that were laying around

Sonosub driver, Dayton Audio RSS265HF-4 4 ohm aluminum cone driver
Sonosub driver, Dayton Audio RSS265HF-4 4 ohm aluminum cone driver

Sonotube subwoofer foam lining inside of sonotube
Sonotube subwoofer foam lining inside of sonotube

I found a driver that will work well with a sealed enclosure.  This is important because some drivers work better with ported enclosures and some with sealed enclosure.  Another detail is whether or not the driver can be mounted horizontally.  Some driver cones will deform if placed in the upward or downward firing position.  The Dayton subwoofer selected will not suffer from that.

I used an online calculator called speakerbox lite to calculate the volume of my sealed subwoofer enclosure.  There are several choices for the type response the designer is looking for.  Initially, I thought about critically damped, but the box volume was 125 liters, which is more than a 12 x 48″ (30.48 x 122 cm) sonotube.  I settled on Bessel-2, which has a box volume of 56.68 liters.  That was easily obtainable with the sonotube I had on hand.

The plate amp is a run of the mill 70 watt Dayton unit.  Truth be told, it runs a little bit hot and I’d consider something else if I were making this design again.

For construction, I carefully cut the sonotube to the right length with a jig saw.  The outside of the sonotube was roughed up with some 220 grit sand paper before painting it flat black.

The individual pieces were glued together with gorilla glue.  The wood braces on the outside of the sonotube connect with wood braces in the inside of the sonotube with brass wood screws.  The plywood rounds were cut with a router to a close fit.  Being that this is a sealed design, I took some extra time with a non-silicone based sealant to make sure that the entire enclosure was tight.  All in all, the enclosure certainly feels tight.

It sounds great and I feel like my subwoofer now matches the quality of the other speakers I am using.