Saturday, June 15, 2019

Portable Stereo Stage

This channel has been silent for some time by now, sorry for that! Will be getting more active in the near future.
While this blog started as a RF only outlet, slowly I drifted to all sort of electronics, including speaker enclosure designs using PVC piping material. Yes, those pipes are still in my living room, connected to a class A tube amplifier and fed by some CD players. While sounding pretty good, this is not really portable per se, although, seen against cement speaker enclosure, this is probably relatively portable.
Travelling back and forth nowadays, I was searching for an alternative.

Lately, I found said alternative. Got a couple of JBL Flip 4 BT speakers. Those are connected to my phone using BT.

Those JBL Flip 4 BT speakers can be used as a pair of stereo speakers. Not having read any instructions, I took me some to to figure out how to actually create that stereo stage. Having tested the setup with different kind of listening, from Vivaldi over Herbie Hancock to Dream Theater, I am deeply impressed!
Seen that the setup is that small and inexpensive that fact that I could not find any flaw in the sound experience tells me that I found my next stereo system.
Although in comparison to my tube class A amplifier, some additional noise is evident.

For aesthetic reasons, I chose both Flip 4s to be black. Of course this makes them a little bit more difficult to setup in the right way. A small sticker indicating which is left and which is right should not be a bit deal however.

If you can live with a little bit of pink noise, a setup of dual JBL Flip 4 speakers is highly recommended.

Sunday, April 8, 2018

PVC Tubes Driven by Tubes

Dear reader,

please accept my apologies for keeping you in suspense over my latest projects for so long. 2017 was a very intense year for me. During said year, I changed my day-jobs twice, so, the radio hobby and the audio hobby had to take a break. Concerning jobs, I believe to have finally found the position of my dreams with a lot a really nice people in the company... I believe I have never been happier before!
Anyway, this is not about life, this is about driving tubes, PVC that is, with tubes, such as in vacuum.

My pipe dreams, as you know, are employing 3W (8Ohms) broadband drivers, which came with a super-cheap set. By now, I am convinced that those drivers were the most valuable part in the entire kit.

In search for a 2 x 3W class A amplifier, I came across a Chinese kit supplier (Douk Audio, cf. ebay), who sells single-ended class A amp kits based on 6N1 and 6P1 tubes. Said kits are comparably inexpensive and, while provide good quality parts, come without a mains-transformer.

My choice of mains transformer fell on a 100W transformer offered by a supplier in the UK, search for "big.daddy!" on ebay.

The combo works as a charm! The amplifier kit seems to be made for my pipe dreams.

As an audio source, I am presently using a professional grade table top DJ CD player by Numark.
In the future I consider to add a passive equalizer / tone control circuit to the setup in order to allow for compensating deficits from compressed digital audio formats. For now, I am pretty pleased with the raw performance from CDs.

Saturday, February 11, 2017

More Pipe Speaker Improvements

The last issue I had with all of my PVC-Voigt-Pipes: resonance of the horn. I read about it, I knew about it... and now, I was bugged by it too.

The solution is very simple: a tripe of cleaning cloth, just below the joint of the driver unit to the horn. The effectiveness of which can be tested by slapping a flat hand on either end of the horn. Without the damping cleaning cloth sound would resonate creating some BOONNNNNG BOONNNNG. With the cloth inserted, this changes to BooB BooB.

BTW, this was effective with the designs I published on this blog.

Saturday, January 21, 2017

Pipe Dreams revamped

My prototype pipes always lacked a certain tonal depth. In the past, I laid that down by the fact that I was using 2" pipes (have a look).
The drivers I was using to that time came from a TV set. While this drivers taught me a lot and enabled some experimenting, which lead to the latter 3" design, which sits in my living room, now in a folded manner.
Strangely, the 3" drivers seem not to be of that much better quality... however, the 3" setup a much butter punch in all aspects.

Willing to further experiment with the PVC prototypes, I ordered a pair of 2" 3W drivers, similar to those.
Yep, those cost next to nothing. They sound surprisingly good when driving my folded "XXL PVC" prototypes.

The question remains if it would be worth to further fold the pipes for improved portability.

Sunday, July 24, 2016

PVC Pipe Dreams updated design (audio)

My PVC Voigt pipe speakers were in heavy use during the last 2 years. My study allows for housing those slim monsters.

Anyway, I found 2 problems with them:

  1. too tall
  2. very directional
So, very easily, since those things are all pipes, I slightly changed the design. Have a look!

modified PVC Voigt pipe design
To reduce the length, I changed the position of the speaker in the T-piece and added an elbow to bend the resonator parallel to the the horn. The above image shows the 2 different designs I posted earlier.

Also, I inverted the orientation of the full range drivers. There are some advantages:
  • the drivers breathe directly into the resonator (similar to the folded TQWT)
  • the direct audio of the driver is less directional
  • the drivers are better protected
As a first observation I would like to mention that treble and bass seem prominently present in the horn, while the mids are emitted by the driver directly.
As a consequence, that could mean that the horn should be above the driver, since the stereo field is determined by high frequencies.
The next step would be to put a wooden box around the T-piece driver part of the design. This way, the contraption can be held upright and all the ugly part would be hidden away nicely.

If it all seems rubbish what I write about PVC plumbing, give it a try yourself. All parts are readily available in your local hardware store. 

Concerning the inverted speakers, I am not yet sure myself. However, for the time being they appear to create a pleasant listening experience.

Sunday, July 12, 2015

The Mobile Shack

Followers of my IT blog already are aware of the fact that I am fitting out a worker's van for camping and other spare time activities.
Over the last years (maybe even decade) I was neglecting my radio hobby. The van is meant to be one of the remedies. It is presently arranged to survive a few days in it, however, the van still lacks radio equipment.

Readily available options are the following:
  • 20m PSK as base gear
  • the 20m Pex-Al-Pex loop
  • fishing-pole elevated dipole
  • 30m PSK for mail
  • TenTec Scout #555 (low power consumption and punchy audio)
  • Icom IC-M710 (even punchier audio) w/ respective auto-tuner
  • Vertex Standard VX-1700
  • PCT-IIe

Further thoughts add digital radio such as D-STAR or DMR. Since I got zero experience in that field, I just started my research. As appealing the digital world is, as confusing it is!

Presently, I am somewhat sure about the computing device I will use:
  • HP Stylistic ST4120
  • respective docking station
  • respective 12V power supply

This thing presently runs WinXP.  Years ago, I bought this thing (used) to be the board computer of my sailing boat. For unknown reasons, the PC never made it aboard.

Things I need to do before getting serious with radio in the van:
  • install one or more service batteries
  • install a charger for the service batteries
  • install a VHF/UHF antenna
  • solar panels?
  • create an RF-ground device 

The van also needs a decent WiFi solution. I presently research the possibilities.

BTW, the van is a Mercedes-Benz Vito (2008). Some told me that Vito's are supposed to be like regular cars. Well, driving one, I can tell you that the allover feel of a Vito is more like driving a semi than a people carrier.

73 for now, stay tuned....

Wednesday, April 2, 2014

Finally: The PEx/Al/PEx Loop!

Yeah, that was a good one today. I took a couple of hours for tinkering and finally got to build my long planned magnetic loop aerial made from the recently discovered light weight copper substitute PEx/Al/PEx.

As previously mentioned, RG213 snug fits into the tubing material. This gave me the idea to actually slide in the coax cable in order to form a Galvanically isolated capacitor. Two reasons not wanting connect anything electrically to the aluminum: 1) it is nearly impossible to solder and 2) it will corrode in rapid rate.

As a result, the coax needs to be inserted in both open end of the loop, thereby closing the same capacitively. In principle this is like any other magnetic loop using a butterfly capacitor.
Just to remind you, this means that 2 capacitors are in series, i.e. they don't add up their capacities, they do this instead:
with Cr being the right capacitor and Cl the left capacitor.

There is a second benefit from series capacitors (in magnetic loops), they act a voltage dividers, thereby increasing the sparkling maximum voltage, allowing for higher power, in particular in the case of magnetic loop aerials.

Back to the capacitance story: the butterfly capacitor symmetrical, i.e. both capacitor have the same capacitance. What if the use variable capacitors having different capacitances?
Lets go through this with an example:
Assume that:
Cr = 10pF
Cl = 100pF
What will be the change in 1pF on either capacitor on the resulting capacitance?
  • no change: (10*100)/(10+100) = 1000/110 = 9.091
  • Cl lowered by 1pF: (10*99)/(10+99) = 990/109 = 9.083
  • Cr lowered by 1pF: (9*100)/(9+100) = 900/109 = 8.257
Very obviously changing the higher capacitance has less influence than changing the lower capacitance.

And this is a fact I make use of in my most recent design: a magnetic loop aerial with an asymmetric series of capacitors.

Pictures say more than words:

Fig.1: asymmetric series of capacitors (purple) terminating a magnetic loop

Pic.1: real life look of the terminating capacitance
Fig.2: Dimensions used for the 20m band, blueish stuff being RG213

Pic.2: this is more than half a meter of RG213 dangling out the loop
Speaking of dimensions (finally), I need to add that the loop conductor itself is made from precisely 4m of 14x2 PEx/Al/PEx (out diameter 14mm, wall thickness 2mm).

Of course, a magnetic loop aerial needs a primary loop:
Pic.3: primary loop
Dimensions for the primary have a thumb rule: 1/6 diameter of the radiator when placed very far from objects, 1/5 in average situations and 1/4 when used in doors. Mine is made from 80cm of copper installation wire, i.e. 1/5 diameter of the radiator. Of course is very easily exchanged when going indoors.

Why are those dimensions selected?
As to the loop diameter, having a loop with a generic resonance not much above the future operating frequency allow for small capacitance values to terminate (tune) the loop. Having a low terminating capacitance lower the voltage across the capacitor and broadens the bandwidth of the loop.
The length of 4m of said material, when bent into a circle, deliver a natural resonance at about 15.5MHz. Starting from there, very little capacitance is required to resonate the loop at 14MHz.
The 70cm for the length of the "insert" were a lucky scientific a precise guestimate...

How to tune this loop and why is it asymmetric in capacitance?
Both these question seem unrelated, but they are not! The beauty of this entire design is found in asymmetry actually. Remember the section about changing the larger or smaller capacitors in a series of capacitors? The shorter end of the coax (when inserted into the tubing) acts like a "band set", the longer end like a "fine tune".
Inserting the coax entirely in a symmetrical fashion, the resonance drops to close to 9MHz, tuning here is very fiddly...
Having the coax in asymmetric configuration, the longer end provides relatively smooth tuning.

What is the bandwidth?
Well, I have not yet tested the aerial decently, but, first measurements with an MFJ-269Pro indicated that the loop, tuned to 14.060MHz is good for +/- 20kHz.
Certainly there are ways to calculate the bandwidth, the radiator 12mm has a circumference of 4m. There must be some web-application to evaluate such a loop ( indicating a bandwidth of about 40kHz... (see below).

My plans for the loop are: QRP and PSK on 20m. Hence, I taped down the short end, as to have my band set. Of course, WSPR and QRSS are also in the reach of this loop...
This loop still is in experimental stage. For a more permanent solution, I will install an electrical box over the terminating capacitor, as to prevent water to collect within the tubing. For the same reason I may even drill a small hole into the bottom of the loop, allowing for drainage.

Concerning the dimensions of such a loop, 30m may still be an option. However, I rather see myself building this loop for the higher bands in the near future.

Results from

Antenna efficiency: 68% (-1.7 dB below 100%)
Antenna bandwidth: 40.3 kHz
Tuning Capacitance: 50 pF

Capacitor voltage: 631 volts RMS
Resonant circulating current: 2.77 A
Radiation resistance: 0.223 ohms
Loss Resistance: 0.104 ohms
Inductance: 2.58 microhenrys
Inductive Reactance: 228 ohms
Quality Factor (Q): 349
Distributed capacity: 11 pF

Antenna "circumference": 4 meters

Loop antenna Side length: 0.500 meters
Antenna diameter: 1.2 meters

The specified conductor length of 4 meters is OK.

Conductor length should be between 2.59 and 5.17 meters at the specified frequency of 14.06 MHz.

For highest efficiency, the conductor length for a small transmitting loop antenna should be greater than 1/8 wavelength (greater than about 2.59 meters at the specified frequency of 14.06 MHz).

To avoid self-resonance, the conductor length for a small transmitting loop antenna should be less than 1/4 wavelength (less than about 5.17 meters at the specified frequency of 14.06 MHz).

Input Values:
Length of conductor: 4 meters
Diameter of conductor: 1.2 centimeters
Frequency: 14.06 MHz
Transmitter power: 5 watts

Sunday, March 30, 2014

3 Inch HiFi PVC Pipe - The Alphorn (MLTQWP)

Finally I found some time to tell/show a little bit more about the speaker enclosures I was bragging about so much lately.

Most importantly, why was this so interesting to be posted on a blog concerned with RF. Well, to my very own understanding, the principles behind emitting sound waves it somewhat similar to the principles of emitting radio waves.
Here is why:
Analogy between a quarter-wave vertical and a quarter-wave speaker enclosure
On the left hand side, the above sketch shows the good old vertical quarter-wave antenna driven by a gamma-match. Indicated in blue, the current distribution along the radiator. Of course, this current will emit a magnetic fields (green), which was the purpose of the antenna in the first place.

On the right hand side, you see a loudspeaker cabinet called quarter wave pipe (QWP). In such a pipe design, similar to the gamma-match of the vertical, a driver (in acoustics loudspeaker chassis are called drivers) creates pressure waves (green) somewhere in the middle of the "conductor". Similar to a quarter-wave antenna, the conductor is excited at a resonance frequency. The blue line indicates the air speed, i.e. current, through the pipe. The red arrows indicate sound emissions.
  • The driver emits sound to the front side of the cabinet.
  • The standing wave emits sound at the open end of the pipe.
While the driver emits what ever is present in the AF signal, the pipe predominantly emits sound at its resonant frequency and harmonics thereof. The latter, of course, is a problem! In musical terms, this could lead to a "One Bass Note Samba", something nobody would enjoy, contrary to the "One Note Samba" having quite some more notes than one only.
Consequently, the bandwidth of the pipe needs some severe broadening!

Back to aerials, there are 2 ways to make an antenna broadband:
  1. add more resonators (e.g. log-per, dipole fan)
  2. add Ohmic resistance (e.g. T2FD, Beverage antenna)
In acoustics, both can be done too. As I indicated before, antennas and speaker cabinets have a lot in common!
In acoustics, one can add more resonators by tapering a restrictive volume and add resistance by stuffing said volume with dampening material.

For a HiFi speaker cabinets one needs a very homogenous emission over several octaves, i.e. close to 0Hz up to 22kHz (those are, of course, extremes). A mixture of multiple resonances and some severe resistance is used. Actually, there is an added bonus on the resistive part of things... not only does stuffing material add resistance, it also lowers the velocity of sound within the medium. A stuffed enclosure looks larger to sound-waves than the same enclosure not being stuffed.

PVC piping, at last we leave the theory part of things, is a very convenient stuff to work with. Now we talk about pipe in the sense of water pipe. Have I forgotten to mention that the word pipe could have several interpretations, sewage pipe, water pipe, organ pipe, pipes of bagpipes, etc. OK, pipes, i.e. PVC pipes, fittings and stuff thereof. Here is the B.o.M:
PVC parts

The parts serve the following functions:
  • tubing form the pipe's body, obviously
  • the elbow is the resonator's "mouth", bending pressure waves towards the listener
  • the T-piece is to the driver
  • the reduction pieces will taper the resonator
  • and the end cap will be the end cap, i.e. terminating the resonator.
The elbow, T-connector and fat tube form the lower part of the pipe. This is all 3" piping. The length of the tube it 1m.
The upper part of the pipe is tapered down to 2" and 1.5" PVC tubing. The respective tubes have a length of 50cm each. Obviously, fittings add length to the final product.
Adding 1 additional diameter to the taper, will just add 1 more resonance (and its harmonics). Hmmm, "Two Not Bass Samba", really?!  No, that does not help!
The tubes therefore receive insets. The process of making those is pretty easy. My preferred method is using heating pipe insulation foam tubing. Cut in half, the stuff can be easily be cut diagonally. A result of this is tapering for pressure waves.
Tapering inserts

That's the tapering part dealt with... Those speakers are called TQWP.

However, before putting everything together, we need to address the resistive part of broadening the bandwidth of the pipe and lowering its resonance. There is no photo of this step. The material involved is polyester from an IKEA pillow, the cheapest acoustic stuffing on the market!
I complemented the inserts with the pillow stuffing and shoved it down the respective 50cm PVC tube.

We are nearly finished... there is just one other addition, before the "cabinet" can be assembled. In order to prevent the creations of unwanted harmonics, dampening material (cleaning cloth!) has to be added behind the driver (within the T-piece).

Here is a photo of the finished speaker:
The Alphorn speaker

Yeah, this is a very tall speaker. For obvious reasons, I call this speaker "Alphorn". The image above shows the thing from floor to ceiling, as installed in my attic.
The sound of those speakers is amazing! From classical to hiphop, via kizomba, funk and rock... the sound of those speakers blows me away!
Mind you, those are very cheap 3" drivers...

The drivers came in a stereo set priced less than €40.
Why am I mentioning this? The price for the PVC plumbing parts, used for this project, is actually > €42.The passive parts of the project exceed the price of the active ones... not sure what that means in the context of modern electronics.
Speaking money, the Alphorn speakers sound like speakers in excess of at least 1k€.

Back to theory... the total length of the pipe is about 225cm. 2,25m in quarter-wave reflects 9m of wavelength, which in itself equates to a sound frequency of about 33Hz. This would be the resonance of the empty (non-tapered) pipe.
As said before, the upper part of the pipe is stuffed with IKEA pillow material, hence the real resonance frequency will be even lower. Stuffing enclosures is called "Mass Loading".

There are 2 sharp steps in the taper, one at 120cm and one at 175cm. Those steps add resonances at 62.5Hz and 43Hz, which of course will also have harmonics too.

The resulting speaker enclosures qualify for the MLTQWP... and... they sound amazing!

UPDATE on alternative design thoughts. Thoughts only. On the internet a couple of designs float showing curved structures, involving a plurality of bends and elbows. Similarly to stepped tapers, those bents create reflections and disturb the air-column of the standing wave pretty good. Bending the design to limit height of the speakers seems however a very attractive thing to do.
My gut feeling tells me that 45º bents at carefully selected lengths could work. Maybe two of those just above the feeding T, followed by a reducer and two more 45º bents. Such a design would keep the tallness of the speaker at about 140cm. However, I still believe that such a measure will reduce the amplitudes of the lowest frequencies.

Monday, March 17, 2014

More Pipiness in the Audio Department

More improvement to report on the PVC pipe speaker enclosures (Voigt pipes).

Today I decided to stuff the upper pipe resonator. Stuffing reduces the velocity of sound due to increased density of the resonance volume. Such an increase of density lets the resonant volume appear larger to the sound-waves.
Quick excursion to a previous post: those pipe-enclosures are just broadband quart-wave resonators, having a pressure node at the closed end of the pipe and a velocity node at the open end.
Reducing the velocity of sound inside the pipe will therefore lower the quarter-wave resonance, resulting in more bass.

Poor man's stuffing could be cotton wool (cotton batting). This stuff introduces a severe problem! The pipes will be home of moths!

There is help, however, a real poor man's stuffing: polyester fiber from real cheap cushions. At a modern Swedish furniture store, such cushions are as cheap as €2/pc containing 400g of fibers, which is a lot more than needed.

I loosened up the cushion fiber, in order to create a very light and fluffy stuffing. Mind you, with 2" drivers, you don't want to dampen too much. It took just a few grams to lightly stuff the tapered resonators.
The result was quite pleasing. However, running the converters w/o the upper resonator revealed some adverse effects of the horn (lower part of the pipe/resonator).
To remedy this effect, I dropped a quarter cleaning cloth, cut the long side, into the tube, covering the back of the driver and following down as deep as possible.

Finally, my PVC-plumbing Voigt pipes are pretty close to Voigt pipes made from wood, as found on the world-wide-waiting-network, i.e. stuffing above the driver, damping behind the drivers and a horn like bass mouth.

As announced before, the next step has to be the homebrew low noise amp.

Thursday, March 13, 2014

PVC Pipe Nightmares

Uhh?! What is going, first he told us how cool those pipe are, and now nightmares?! What is going on?!
Very simple, when pushing one inferior variable to the best possible, another one will take the place of being the disturbing one... the one that spoils all the fun and excitement.
Well, this time, it was the stereo that was driving the fun, as I thought.
Although I still believe this is a very nice device, I have to report that this device creates a hizzzzzz under "normal"circumstances. As soon as any of the buttons is tampered with, the hizzz is absent, just to return after a few seconds... VERY ANNOYING!

This "feature" was audible only with my PVC Voigt pipe-type speakers. What do we learn from this?!
  • When using a cheap stereo, use the speakers supplied, so you don't hear the deficiencies!
  • When hooking up a cheap stereo to some decent sound converters, you may be able to hear the defects the respective sound converter has.
  • Dimensions given by your sound-Guru are relative... does a 1.5m Voigt pipe sound better than a 2+m Voigt pipe?! Probably not... shorter pipe, less bass..
Am I happy with my more than 2m long Voigt pipes driven by 2" drivers? Yes I am! I think those are the most amazing cheap speakers I ever listened to!

Remedy for the hizzzzzz problem: build a dedicated AMP (opamps/LM368) in a classical way...