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Author Topic: Victorian Boombox Mk III. A brand new start.  (Read 129 times)
J. Wilhelm
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« on: February 14, 2020, 11:36:44 am »

Dear ladies and gents, I have decided to give the pink slip to the Mk. II Boombox because I was not satisfied with the quality of the electronics, and while trying to compromise and address it's shortcomings, I basically engineered myself into a cost overrun of what was supposed to be a cheap, easy to build system. So for the moment I will abandoning further development of the Mk. II, and instead I will revisit the Mk. I concept.  


PROJECT OUTLINE (sorry for the wall of text - I'm just laying down the rules of the project).

The original Mk. I was a "big one-off" project built around a salvaged Altec Lansing 2.1 computer subwoofer speaker system from circa 2000, back when Altec Lansing was a more reputable and expensive sound house. The sub-woofer box provided a body onto which you could screw or bolt all sorts of steamy paraphernalia, including two cast aluminum lamp shades that served as housing for the relatively small satellite speakers and doubled as decorative Victrola horns. Enormously popular and successful, the Mk. I was paraded for years at Steampunk meetings, like Jeff Vandermeer's Steampunk Bible release party in Austin in 2011, and the streets of downtown Austin every single South by Southwest Festival since about the same time, but it was a monster. Too heavy to haul around and too delicate and expensive to sell. To make matters worse the heavy triple-transformer power supply was external and you needed to haul that too if you wanted to demo the unit (it's not battery powered).

Over the years the old box has developed issues related to multiple loose connections and a maze of wires that have not aged gracefully, and are rather difficult to replace. Nothing worse than an amplifier built around separate modules connected by a maze of thin wires that can rust and break. Unfortunately most large sub-woofer systems are built in a similar way. They were simply not designed to me moved around a lot or to have a long life. Not that I can't fix or replace the connections, but it will require open heart surgery and I'm, still left with a monster that is increasingly becoming more difficult for me to carry.

What I'm proposing now is a new architecture, a second "one-off" built around relatively expensive systems, but using different technologies that will let me change any boombox I build from now on. The Mk. III, as I call it will revolve around 3 separate technologies:

1. A central Hi-Fi receiver/amp unit, with a built-in power supply. I will try a relatively high quality (10% THD at 30 W) digital amp to save on weight and size.

Sony CMT-MX 500i mini shelf system

2. A set of two full range speakers instead of the central sub-woofer with smaller satellite speakers. The driver units will be mounted at the extreme ends of a structural beam that will double as a Transmission Line speaker enclosure (Inverted Horn Wave Guide Speaker).

Sony 1-826-587-12 100W full range speaker (plus 1-826-589-11 horn tweeter not shown)

3. A computer sound card that is able to partially operate (mixing analog and digital sources, microphone with sensitivity control, D/A conversion, Dolby AC-3 surround sound decoding, Creative CMSS surround synthesis) without being connected to a computer, and when it is connected to a computer by way of USB, is able to provide much higher functionality as you all know from using computer sound software. The sound card serves as both a switch/mix hub to allow multiple input and output connections as well as a quality headphone monitor output, also with a separate volume control.

Creative Extigy SB0130

I don't intend to use the 5.1 surround output for the boombox itself for obvious reasons. There are no sub-woofer, center and rear amplified speakers. Instead the front R/L channels decoded from 5.1 AC-3 could be routed to the boombox speakers and a separate set of rear channels and sub-woofer/center channel would use an external amplifier. This is a limitation of the digital amplifier. Each R/L channel is a closed loop with no common electrical ground. I can't "borrow the signal" to power a center channel, say, without needing to build a pre-amp circuit and a separate amplifier.

For other 2-channel surround sound synthesis methods (known as "Virtual Surround," I will probably post on that later), the boombox' built-in speakers alone are enough - I will use the sound card connected to a computer through the desired surround software -or- if the surround system is simple enough, like SRS surround -which is analog- then any stereo input from a decoding source, wired analog, digital wired/optical or Bluetooth, will be able to pass along the surround with no problem to be reproduced by the speakers - in theory. It all depends on the quality of the amplification - but should be no problem for the Sony amp.

In the Bluetooth case, the surround sound decoder is in the form of an Android or iOS based app - if it exists. I don't think that a wired connection directly from a tablet or phone would be able to transmit a good surround signal on account that most Android phones use really crappy internal power amplifiers to save the battery - but you can try. I don't know about present day Apple tablets/phones (I'm too poor to own one). But I'm willing to bet my lunch money that my ancient 2nd Generation iPod Touch beats the crap out of most present day Android phone amps - quality wise... Android needs Bluetooth.

The advantages of the new system architecture are multiple. The use of two large speaker drivers is meant to eliminate the need for the sub-woofer.  It will be a less delicate system, it won't require a separate power supply, it will be ready to accept next-generation sound sources, such as a Bluetooth connected smartphone or tablet, as well as more traditional Line Level RCA-based  connections, MIDI DIN based connection, RCA digital and laser optical connections (common things in computer sound cards), etc.

Instead of a sub-woofer I will place two high-power speakers into what is known as a Transmission Line enclosure. The "Transmission Line, really is a wave-guide, shaped as an inverted horn (the sound source is placed at the mouth of the horn instead of the neck of the horn). The theory is that the speaker acoustic loading (impedance) is provided by the mass of the air trapped in the wave-guide which dissipates the undesired sound next to the driver and channels the remainder of that sound far enough to a small open port (the neck) where the sound emerges at the same phase as the sound emanating from the front of the driver in order to avoid destructive sound interference.

An inverted horn transmission line speaker

The end result is similar to a bass reflex system, and not entirely unlike a regular "non-inverted" horn, like you see in Victrola horns. The energy in the undesired sound waves is re-routed to favor certain low frequencies that make up for a poor performance of speakers at the low range. There's a number of terms used to express this acoustic phenomenon, from "low pass filter," to "impedance matching," etc.  It's all the same, just seen from different spring-mass-damper perspectives. It's an acoustic transformer, basically. You want to discard undesired sound and pump what you can as usable low frequencies in the right phase if possible. That is how I can get rid of the sub-woofer altogether.

In fact I'm certain you've heard this system before, albeit in a much smaller scale. Does the name Bose ring a bell? Well. I'm going to use bigger speakers and use the "folded horn" as a structural beam for the Mk. III system. I will have to compromise and use low weight materials. I had considered using PVC pipes, but they're very heavy and can be expensive (5 to 7-inch diameter pipes). I had considered using tin chimney flue and A/C climate contrul ducts as well. They're too thin and will not dampen undesired sounds, plus they will resonate when your supposed to dampen any sound instead (this is NOT like a violin or guitar where you want the enclosure to resonate).

I need high density and rigidity. Particle board or MDF would be ideal. But I may consider strong light wood like poplar if its well braced structurally (more on that subject later). I can make due with rigidity and dampen the rest with batting if need be...

I'll see what I can come up with in the next few days... Until then,

Cheers!

J. Wilhelm
« Last Edit: February 14, 2020, 10:05:43 pm by J. Wilhelm » Logged

Synistor 303
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« Reply #1 on: February 15, 2020, 05:55:12 am »

Strewth!
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Banfili
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« Reply #2 on: February 15, 2020, 12:23:18 pm »

Strewth!

Indeed!
Looking on with interest to see how this develops!
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J. Wilhelm
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« Reply #3 on: February 15, 2020, 04:25:40 pm »

The build started yesterday with me picking up the Extigy I bought at eBay from the post office. I got off to a rocky start. The sound card works - mostly - and it's in very good shape externally, except that during its life someone dropped the unit to the floor, breaking the optical in /out jacks by way of the tiny plastic plugs that cover the led and photosensor. When I opened the enclosure, the jack modules were sheared off clean at their solder points.


I knew I had to either remove the units or try to solder them back before applying power to the unit (avoiding an electrical short circuit). I did the latter, but clearly the LED transmitter (which is visible red and on at all times) is not working. So I know I don't have Optical pulse modulation out. The photosensor also was sheared off and I re soldered it. It remains to be seen whether that is still working. I need to dig up an optical cable and try to hook two sound cards together.


In any case this will not stop me. I have to confess that I have another Extigy of mine that I have tucked in a box inside a storage unit somewhere in the city. Alas, the storage unit is far from where I live and I don't have a car, so it's a major expedition just to go get it (or an expensive über ride to go get it).That's why I bought one from eBay, the Post office is closer!

Anyhow, if anybody has a transmitter and receiver pair of Toshiba modules ( TX179 /RX 179) let me know! Otherwise I'll try to get them or it may be even easier to get another sound card. I will continue work with this one as is until further notice.

The card otherwise works well. I discovered that Debian Linux offshoots like Ubuntu have next to no support for the Extigy. The sound is too low when connected to the computer, and the card functions much better in stand-alone mode. All analog input /output types seem to be working. I haven't tried digital coaxial yet or MIDI in/out.

In the days when I was more savvy and compiled my own software in Slackware Linux, I remember extracting more functionality out of the same sound card. I probably need to compile another sound server and get some Source Forge drivers from some geek out there who has managed to run the Extigy in Linux.
« Last Edit: February 15, 2020, 05:24:03 pm by J. Wilhelm » Logged
J. Wilhelm
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« Reply #4 on: February 17, 2020, 06:13:03 am »

So à lot has happened in the last 24 hours, and I had an inspiration after visiting the local hardware store on my way home last night.

I had already drawn on paper possible arrangements for the sound wave horns in the SoundBeam™. The maze of folds in the transmission line enclosure could be made from the smoothest, straightest boards of 1cm pine craft boards that you've ever seen. Now, pine is not known for its acoustic qualities, but I'm not making a violin. The wood is not supposed to resonate but rather form a horn, or rather two folded horns. The wood just needs to be dense and rigid enough, and the maze of passages in the SoundBeam™ ensure that the enclosure will be very rigid. I can't put something heavy and gummy like particle board on account o need to be able to lift the boombox with my hands!

The arrangement of the horns is extremely compact. The theory is that the wavelength of the fundamental frequency of the speaker alone will determine the length of the horn. The speaker will go "on the wrong end" of the horn, acting as a vibrating closed end of a quarter wave resonator. I measured the speaker's fundamental frequency to be 84 Hz or so, which corresponds to à wavelength of about 4 m. Hence the wave guide should be about 50 cm folded in half...


From the outside the transmission line will look very unassuming just a rectangular beam


The idea is to finish the SoundBeam™ first and test it with different sources and perhaps stuff fiberglass batting in it as needed. A large area on each end of the speaker enclosure will connect to the speaker's back in an unspecified manner.


I have actually made more progress than I've shown, and I apologize for not disclosing that now, but I hardly have any time tonight to post anything, and thus I'll leave you in a cliffhanger... What I can tell you now is, "wow" this project is showing strong signs that it wants to live! You know how sometimes too many things just align together to be some sort of coincidence? This is one of those times.

Now, I must go back to my SundayChores™, because I only have one hour before midnight and I still have to eat and do the laundry!
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J. Wilhelm
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« Reply #5 on: February 17, 2020, 10:16:52 pm »

Do you know how sometimes things just fit together for no reason? I couldn't have found a perfect match for the speaker drivers if I was looking for it. One of the things I hated the most about the boombox was looking for suitable lamp shades or bells to use as Victrola horns. This time, I found this pair of crown molding quoins or corners, which are relatively expensive at $15 per piece, but they were far too convenient to overlook. They simply save me a lot of time and work.




Even though the pictures suggest the method of attachment is trivial, the edge of the speaker magnet and the rim of the driver cone actually interfere with an easy installation to the waveguide box. One solution is to add a "neck" to the crown molding quoin, and I will probably use a smaller wooden box serving as a tube connection.





In my next post I'll explore methods of attachment for the units above...
« Last Edit: February 17, 2020, 10:28:23 pm by J. Wilhelm » Logged
J. Wilhelm
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« Reply #6 on: February 18, 2020, 10:51:09 pm »

When I was at the hardware store, I was inspired by the way the molding crowns fit over the boards I was using to build the waveguide. It just seemed like a perfect match and the geometry of the boombox was emerging naturally from the materials.




The only problem is that installation is not as easy as shown in the pictures. The magnet and rim of the Sony driver actually interfere with the install as shown. My first thought on that is that a small rectangular box could serve as a connection between the crown molding and the waveguide beam. This box is critical, because it adds length to the waveguide, so I will have to adjust the waveguide, probably by blocking the neck and using a perforation on the side of the box - or not, we shall see. But what I know is that I have to match the 1 meter quarter wavelength. I think that blocking the exit of the horn and making a hole of the right diameter on the right location (like a flute) might be enough.




The geometry of the crown molding box doesn't bother me much. It's sufficiently "spherical" inside, so it makes a good bend in the waveguide, like the bend in a saxophone. It's mostly keeping the length of the waveguide that matters, so there will be some tuning of the system while it's being built
« Last Edit: February 18, 2020, 10:53:08 pm by J. Wilhelm » Logged
Banfili
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« Reply #7 on: Today at 02:22:13 am »

Looking very good so far, J.Wilhelm - no doubt the technical problems will sort themselves out as you go - all part of the fun!
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