Thursday, May 28, 2015

Bozak Rebuild Project - Part 11

It's been over three years (Yikes! How time flies) since I started this quest to rejuvenate my 1958 Bozak 302-A speakers and slowly but surely things have gotten better. New satellite enclosures, new network designs, new drivers, new inductors, and new capacitors have all improved the original sound that today rivals the best of the best high-end designs. Along the way I have shared what I discovered with you in hopes that your own learning curve for improving your own speakers will be shortened.

This segment of my ongoing saga recaps the current thoughts on the redesign of this classic loudspeaker. I will share with you the driver complement, constraints, and challenges in maintaining a high 1-watt sensitivity loudspeaker while still maintaining excellent accuracy in both the frequency and phase domains.

First, I wanted to share with you why I chose the Bozaks over other models: their high sensitivity. This translates into loud music with modest amplifier power. The Bozaks have a 1-Watt sensitivity of about 97dB/W/m meaning that it takes A LOT less power to drive them to the same SPL as an 87dB/W/m loudspeaker. (I’ll talk about the pros and cons of low and high efficiency loudspeakers in another segment since that’s another can of worms.)

I started with the philosophy of making as few changes to Rudy Bozak’s original 1958 design as possible but rather applying current technology to a dated approach. I initially tried the well-regarded “Pat Tobin Mod” of upgrading capacitors with positive results. However, measuring each individual driver with its original first-order network revealed each driver severely “bleeding” beyond its desired operating band. So higher-order crossover networks seemed to be the solution.

Second and third order networks were tried with mixed results and finally I settled on fourth-order Bessel networks for the M-T and T-ST but a second-order Butterworth from the W-M. Along the way I discovered that my Bozak tweeters had a lot of holes in their diaphragms and these tweeters gently rolled off at about 8KHz. So replacement tweeters with the same high sensitivity were sought out. This proved to be a long process since most drivers in this range were PA speakers that were not what one would consider high-end.

I finally came across the Peavy Versarray that used a planar tweeter with very good sensitivity, the RD-1.6. After many attempts at taming this driver, I abandoned the work. The next choice was the Mundorf AMT 164UM2.1 that is still in the system today. It performed much better than the Peavy however it still had issues with the top octave. Using a forth-order Bessel LPF to limit the HF content turned out to be a good choice but that meant searching for a super tweeter.

Again the search mainly found PA drivers and exotic compression horns that – while they may have sounded good – were beyond the reach of my wallet. Using Panasonic EAS-10TH400A leaf tweeters in a previous loudspeaker with fantastic results I decided to search for something using leaf technology.

I purchased a pair of Bespoke Aria tweeters (these folks use the Monacor RBT-95 true leaf super tweeter) but I never liked the way they sounded. This has been my main efforts over the entire year: taming the Monacor. I had faith that all I needed to do was to persist and persevere and things would work out. But until I made some near-field measurements, I did not completely understand why the sound was so weird. Anyway without belaboring the point, the solution was to cross this super tweeter REALLY high (currently it crosses at about 34KHz) to control the Monacor's contributions below the crossover frequency that really screwed up the sound of the stellar Mundorf AMT tweeter.

Monacor RBT-95 Leaf Diaphragm

Below is a photo of the final crossover network (as of today) and here is the configuration:
  • Super-Tweeter high-pass filter (HPF) uses Russian Teflon capacitors shunted with Corning glass capacitors with wire-wound ground-path (shunt) chokes
  • Tweeter uses a combination of Mundorf, Audyn, Russian Teflon, Obbligato, and Corning glass capacitors with ribbon signal-path chokes in the low-pass filter (LPF) and wire-wound chokes in the HPF. A sort of "Zobel network" is also used to minimize T-ST interactions.
  • The midrange uses Mundorf, Russian Teflon, Clarity, and Corning glass capacitors with a Solen capacitor for the notch filter. The midrange network uses ribbon chokes in the signal path and wire-wound shunt chokes. The midrange uses a notch filter at about 3KHz to help smooth a mechanical resonance. A Zobel network is also used.
  • The woofer (actually two woofers in an isobaric arrangement to improve bass response in a small cabinet) uses a low-loss toroid inductor and Clarity caps for the crossover and the Zobel network uses Clarity and Audyn caps.
  • All signal-path resistors are Mills non-inductive.
  • The midrange and tweeter are T-pad attenuated. All attenuators are better than 1% hand-matched.
  • All capacitor and inductor values in the network are spot-on hand-measured.
  • All impedances of drivers at the crossover points were confirmed via LRC resonant measurements with my HP 465 oscilloscope and function generator.
  • All network LPF/HPF performance was verified with near-field measurements of each individual driver and then its partner to assure minimal driver interaction.
  • All point-to-point network wiring is #14AWG stranded silver-plated OFHC copper, Teflon insulation.
  • W-M crossover is -6dB BW2; all others are -6dB Bessel4 (hence the high parts count for this 4-way system).
  • Each driver has its own network board and speaker terminal pair.
  • The satellite box is now a truncated trapezoidal-pyramid design with curved front edges and adhesive felt on the face
  • Presently each speaker is tri-wired (one amplifier, two channels, six speaker wires total). Wiring varies in material content but all are star-quad.

Crossover Network as of 5-23-15

Remember that in my world things routinely come and go but I can report that I am pretty happy with the way they sound, and that is saying a lot. To give you an idea of what they sound like, give a listen to the outstanding Vapor Sound Joule. The Joule has much better bass control and my Bozaks have a better top octave. The lower mid-bass between the W-M is the other noticeable difference but this difference depends a lot on room resonances and speaker placement.

There is one other change that I am in the process of implementing: better crossover-to-speaker wire terminations. It is well known that ferrous terminations (steel nuts and bolts) will magnetize slightly as the signal passes through. Magnetizing the nuts and bolts steals some of the energy that really should go to the speakers. Using stainless steel versions changes all of that since stainless steel has no appreciable magnetic properties (see below). As you can also see, I continue to use star-quad stranded silver-plated-copper wiring throughout making the biggest difference in the size of the sound stage and other differences in transient response.

Stainless Steel Nuts and Bolts

Also evident in the above image are the numerous holes from countless failed attempts at a HPF design, redesign, re-redesign, re-re-redesign...for the super tweeter. One day I will replace that board and make it at least look better. And some other day I will design and build a better woofer enclosure (it's already on the drawing board waiting for spousal approval).

BTW, many of the ideas I used in this redesign you can also use on any design. Here is a brief summary of generic mods:
  • Take out your crossover network and go through it by using better wire in a point-to-point method abandoning those puny copper PC board traces.
  • Use good wire everywhere and orient the chokes 90 degrees to each other (horizontally, vertically, or laterally) and don’t place two similarly oriented chokes near each other.
  • Use good quality capacitors in the crossover network
  • Use three screws (wherever possible) in a triangular pattern to mount the drivers. BTW, you can test this on your woofers by loosening all but three screws are still in the holes but not touching the basket. Give this one a shot and be amazed.
  • Put felt around the midrange and tweeter drivers and make sure that the screws used on them are as flat with the baffle board as possible
  • Use separate ground runs from the back speaker terminals through the crossover network and then to the driver
  • Try bi-wiring or even tri-wiring and experiment with wire designs of your own. I prefer star-quad with silver wire; you may have a different preference since this is a pretty lively wire and may cause your mid/tweet to sound harsh or metallic.
  • Put your speakers as far into the room as aesthetically possible without breaking up your relationship (1/3 of the way toward the listening chair is a good number)
  • Move your speakers around a little here, a little there, and see if the bass or sound stage improves
  • When finished moving them around, put your speakers on spikes (three if possible; two in front and one in back). If you have hardwood floors, slip three pennies underneath (again two in front, one in back) and be amazed! I call this “Phil R’s patented 6-cent fix.” Catchy, right?
  • Buy a really good power conditioner and plug all of your equipment into it (no sharing of outlets). Isolation transformers are not enough; RF filtering is absolutely essential.

I’m sure there is more but this should keep you busy for a while (maybe three years or so ;). Tweaking is part of the fun of this hobby and every point of the playback chain has the potential to be tweaked. Remember that swapping out speakers will give you the biggest change in your system. If you are not happy with them, start there.

So how does it measure? I knew you were waiting for this. Well this was a surprise to me: it measured - from the same listening position I have made all of my other measurements - rather disappointing since the microphone I used suffers greatly above 6KHz and below 300Hz. Many of the 3,000 or so measurements I made over these three years looked great on paper but sounded absolutely awful. This one shown below measures fair but it sounds truly amazing. To give you an idea of how the sound changed, the oldest surviving RTA measurement (hard drive crash at fault here for eliminating older measurements) using the original four-tweeter design and the Pat Tobin network is shown for comparison. Also shown in green on the graphs are the approximate crossover points for each of the two systems.

The characteristic sound of the original Bozak is gone except for the captivating midrange of the B-209B. The woofer box is now ported so the bass is quite a bit deeper; it also has a slight “tubed” port characteristic. While the flared port is not objectionable, it is another area of interest to me. A tapered port design may be in the distant future. For now, I am pretty happy with the way it sounds. With the exciting dynamics and deep bass my wife is also. So stay tuned and see what pops up out of my head next in Part 12?

See also Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, Part 7, Part 8, Part 9, and Part 10

Yours for higher fidelity,

Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please remember to reciprocate by purchasing one of my eBooks or through a PayPal donation, My newest title is called Where, oh Where did the Star of Bethlehem Go? It’s an astronomer’s look at what this celestial object may have been, who the "Wise Men" were, and where they came from. Written in an investigative journalism style (like that of the Discovery Channel), it targets one star that has never been considered before and builds a solid case for its candidacy.

My other titles include:

Copyright © 2015 by Philip Rastocny. All rights reserved.

Tuesday, May 19, 2015

Tributaries 4K UHDMI Cable Review - Part 3

In Part 1 of this series, we tested the tester to assure that what I assumed about my system was true and it was NOT. Rest assured that now all is now well with my system and, BTW, if you haven't rechecked your own calibration settings in a while you should probably do so.
In Part 2, we saw how a NULL test reveals any differences in photographs and that photographic images of the same images revealed visual differences between HDMI cables. This evidence debunks the debunkers, something most of us already knew but could not prove - no more! I hope that the word finally gets out, that the folks who believe that wire is wire read Part 2, and for anyone to try that experiment for themselves. It is a simple to confirm by even the most casual photographer with a modest camera - but it must be done at night so the ambient light level remains the same. I would love to have a better camera that did not compress the video data so that I could see more differences.
Here in Part 3, I will try to subjectively assign visual differences between these cables. And based on a recommendation from a good friend at, I will refer to these cables by simpler, less confusing names rather than their product numbers. So to refresh your memory, there are four cables under test:
  • UHD HDMI (UHD-010b) I shall call the BASIC cable
  • UHD Slim (UHDS-010b) I shall call the SLIM cable
  • UHD Slim Active (UHDS-020b) I shall call the SLIM-A cable
  • UHD Pro (UHDP-010b) I shall call the PRO cable
The Cables Under Test
The comparisons will all be made against my trusty 1080p SERIES 8 cable. Are you with me? Good. Let's get to it.
Now choosing a video sequence worthy of repetition is a challenge since I must do this all during one setting otherwise my video memory fades and I am unable to make fair comparisons. So for this test, I chose the already well-regarded and well-reviewed movie Avatar 3-D version. I chose to watch the scene where Jake and Neytiri cross a log (scene 11, time: 40:30-41:50) and seeds float down from above. This scene segment is where Jake asks, “What are they?” and Neytiri responds, "Seeds of the Sacred Tree...very pure spirits" (aka the Tree of Souls). The Seeds magically settle on Jake's body and then drift away.
Most mystifying about this particular scene is not only the floating and spinning of these Seeds themselves but also the various camera angles from which this scene is simulated. Even distortions from simulated camera-lens swaps (shorter-focal length and lower f-stop versus longer-focal length and higher f-stop) are faithfully reproduced at the outset of this scene giving you a feeling of looking at a background in the shape of a “curved sphere” rather than the more-frequently-used “flat field.”
With a fresh set of batteries in the glasses and a bookmark (OPPO's AB Replay function) noting the start/end of that scene, I am ready and will make progressive comparisons to two cables. BTW, to assist myself in complete attention to the video, I decided to mute the audio during their review. And to eliminate influences from changes in ambient room lighting, I performed this part of the evaluation at 3:00 AM.
Series 8-to-Basic
This was a tough one. Although I preferred the Basic over the Series 8, I have a difficult time sorting out how to tell you why. These two cables are strikingly similar both revealing full rich colors and fantastic contrast. But the Basic cable edged out the Series 8 in what I may call video fatigue: things just looked a bit more spatial. Specifically, small improvements in detail to the bottom two “arms” of the seed were noticed with the Basic and less defined in the Series 8. And the sense of the size of the forest felt “different” meaning the 3-D perception was a little better. There is not much more I can say other than these two cables are remarkable with only minor differences.
Series 8-to-Slim
Here is where things start to get exciting. With the Slim cable, the blood on Jake’s right upper arm is much better defined and perceived in greater contrast than with the Series 8. Also better defined are the blood vessels in Neytiri’s sclera (the white area of the eyeball) along with the radial lines of the iris (limbus, stroma, and colorette). The animation art of making inanimate things look real by the addition of lighting and shadow can better be appreciated with the Slim cable.

Neytiri’s Eye Detail
Additional detail in both the upper and lower arms of the Seeds is evident with the Slim cable over the Series 8. Background images believed to be blurry in the Series 8 are now clearer with greater-perceived three-dimensionality. The luminous moss on the background logs now hints at some edge detail where with the Series 8 this level of information is completely lost. Jake’s brow wrinkles show depth and convey emotion veiled until now. The dark pigment lines in Naytiri’s face show they are just below the top layer (epidermis) of her skin rather than on its surface.
Series 8-to-Slim-A
OK, now this is getting a little tougher again. The differences noted with the SLIM cable over the Series 8 are also present in the SLIM-A cable. The only additional perceived difference is in thin lines such as the top “arms” of the Seeds at their edges. From inside to tip, the arms widen, then taper to a point, then widen again finally tapering to a single tip. As the seed spins, you can see how thin this line is between the upper and lower parts. Also, the green glowing specs on both Jake’s and Neytiri’s faces are brighter than in the Series 8.

Greater Thin-line Detail
Series 8-to-Pro
It is obvious that the full quality of the Pro cable is beyond the ability of my 1080P playback system to reproduce. The only differences I noticed in addition to the aforementioned improvements were the in Neytiri’s eye lashes. With the Series 8 (or any of the other cables), her eye lashes were blurs or completely unresolved. Some additional contrast was also noticed in the folds of the clothes in Jake’s pants and in Neytiri’s loin cloth. Beyond this, it is difficult to detect any additional improvements with my 1080P system.
Follow-up Observations
After this 3:00 AM evaluation, I forgot to swap the Pro cable out of my system and went back to bed. A few hours later, I transcribed the notes from this review into meaningful and coherent grammar. The entire day passed without watching any television until that evening when my wife put on the latest Survivor episode. I am glad she did.
Because of the limitation in the number of colors used in the Avatar scene, a full appreciation of the Pro cable was overlooked. Once watching other material with the Pro cable, colors – especially the greens – took on a new level of accuracy. When watching the few seconds of introductory scenes to Survivor of the Nicaraguan coast with its already enhanced and over-saturated colors, additional contrast improvements were obviously observed with the Pro cable over the Series 8. There is richness that rivals good plasma television sets (note that I am not comparing LEDs as an equivalent to a plasma screen, just another movement in the “right” direction).

Deliberately Enhanced Green Colors in “Survivor” Episodes
Further observations of other program material show skin tones reproduced more faithfully with greatly enhanced contrast especially in low-light situations. Over the next few days, I really began to appreciate what I was missing with the Series 8 cables. The Pro model outperformed the Series 8 in just about every way you can imagine. It was like buying a new television set where even the commercials became more enjoyable to watch (well, not really “watching” the commercials as much as enjoying the striking improvement in video quality).
For me, this was a revealing adventure in appreciating (and demonstrating) the technological advancements in interconnecting wires. In other words, it is evident that all HDMI cables are not created equal especially given the measured results in Part 2 of this series. HDMI cables indeed are different but which is best? That is for you to decide.
The Series 8 cable was pretty darned good for its time but as it goes with inevitable technological improvements, any of these four new cables from Tributaries would be a welcomed addition to your own system. The level of diminishing returns applies to HDMI cables as it does for any other piece of audio/video gear. Personally, the best bang for the buck came from the SLIM model and additional, more subtle improvements came with the SLIM-A and PRO models. If my video system were 4K-capable, I am certain that even more differences would be noted.
The old saying of “garbage in – garbage out” applies to every link in the audio/video chain. Cables are a “low-hanging fruit” way to improve your existing system. Unlike springing hundreds if not thousands of dollars for a new Blu-ray player, a modest investment in cables can provide more enjoyment at a far more reasonable price point. Determining which cable is best for you is part of the fun of this hobby.
Buying an expensive cable and installing it in your system may or may not give you the results you expect. Much like learning how to listen to music to appreciate its plethora of intricate nuances, you must be similarly skilled in video nuances to appreciate minor changes in color accuracy, clarity, contrast, and dynamic range. Like hearing a floor-standing speaker compared to a sub-compact bookshelf model, some video differences are striking but others can be overlooked.
I do not consider investing in a cable and then finding another more to my liking as a waste of money but rather an investment in education. Without taking the time to personally observe that cable X is visually different from cable Y, how would you know for sure? How would you develop your visual acuity? Reading reviews such as mine can give you an idea where to begin and what to look for but the final call comes down to your system and what you prefer. My wife prefers a lot of bass to “feel” the impact of an explosion in an action/adventure movie; I do not. Being an artist, she is also far pickier about color accuracy than I am. Your taste in video performance is similar where specifications can guide you to a choice but your eyes must weed out the “sheep from the goats.” Know that any of the Tributaries HDMI cables will definitely be in the “sheep” category.
The Tributaries line of audio/video cables are high quality products providing vast improvements to your video enjoyment compared to those supplied with your equipment or available at those “big box” stores. Any of these four reasonably-priced HDMI cables also gives you far more entertainment value than reaching for the esoteric sky. Remember this: When looking to upgrade your system, think of your cables first as a possible weak link in the “source-to-destination chain.” Swap out your cables and then sit back, put on your favorite movie, and enjoy things you may have never noticed before. Any of these four cables are truly “eye candy!”
If your local dealer does not handle these fine Tributaries HDMI cables, you can purchase them directly from the manufacturer in various lengths at Tributaries Cable, 6448 Pinecastle Blvd., Suite 101, Orlando, FL 32809; Phone: (888) 554-2514. See their web site at for information about all of their audio/video cables and any of their other fine products.
Yours for higher fidelity,
Philip Rastocny
I do not use ads in this blog to help support my efforts. If you like what you are reading, please remember to reciprocate by purchasing one of my eBooks or through a PayPal donation, My newest title is called Where, oh Where did the Star of Bethlehem Go? It’s an astronomer’s look at what this celestial object may have been, who the "Wise Men" were, and where they came from. Written in an investigative journalism style (like that of the Discovery Channel), it targets one star that has never been considered before and builds a solid case for its candidacy.
My other titles include:






Sunday, April 19, 2015

Tributaries 4K UHDMI Cable Review - Part 2

As mentioned in Part 1 of this series, it is best to begin at the beginning and test the tester. Saying this another way, make sure that the assumptions you make about your system are valid and that things haven't changed to which you are unaware. In particular, it is a good idea for you to test your own TV with a simple video image that shows you if your set is properly adjusted. The standard test images I used are 1080p test image or a 4K test image (NOTE: these are high-quality PNG-format pictures, not videos).

In Part 2 of this series, I will test the following four cables manufactured by the Tributaries Cable Company of Orlando, Florida, on my Samsung F8000 65" 1080p LED TV:
  • UHD HDMI (UHD-010b), 1-meter length, MSRP $35
  • UHD Slim (UHDS-010b), 1-meter length, MSRP $65
  • UHD Slim Active (UHDS-020b), 2-meter length, MSRP $85
  • UHD Pro (UHDP-010b), 1-meter length, MSRP $80
My current (reference) cables are the 1-meter Series 8 Tributaries HDMI and I have been very pleased with their performance. With time I have grown familiar with the characteristics of these reference cables and I look forward to observing any changes these wider-bandwidth cables may make in the 1080p signal world. Unfortunately, I do not currently own a 4K television so I cannot test their true potential. However, I will report on any improvements noticed in the 1K format. I suspect that - if anywhere - I may observe improvements in 3-D images. But we shall see...

To be consistent, I used my tripod-mounted FUJI S1500 digital camera to photograph the video image on the screen. The object is to use this same image and display it through each of the HDMI cables taking identical exposure images. Then, I will use PaintShop Pro to arithmetically subtract the information between the two images (the Reference Series 8 and the cable under test) and show you the results. What differences there are will be shown as non-black features in the Difference image. Exposure value for all images was ASA64, 1 second, F/5; no other adjustments. Source files for each image is 3648x2736 pixels and 4Mb in size (JPG format). All images were taken at 3:00AM to eliminate any ambient lighting changes.

Now there is some error introduced to the source images due to JPG compression. However if there is a compression error introduced by the camera, a test between two pictures of the same image of the same cable through the same camera with the same exposure should reveal what errors are introduced. Below are two such pictures taken through the Series 8 cable.

This type of test is called a NULL test and it takes into account all forms of change regardless of the source. In other words, what the Difference image shows are the things that have changed between the two images and nothing else. So theoretically the mathematical difference between two pictures of the reference image should be a completely black image (no difference) and in fact it is. This verifies that the software is working as anticipated and the assumption that the JPG compression adds no comparative errors is correct (i.e., identical amount of compression error introduced).

So the testing strategy is sound. Now let's see the difference between the Series 8 HDMI cable and the UHD-010b cable.

Viola! Herein is evidence of subtle differences between these two cables. This is not rocket science folks, just simple computer wizardry. For those naysayers who claim that there are no differences between cables, here is indisputable proof that there are indeed differences. Although the differences are minor, there are differences since the NULL (Difference) image is no longer completely black. What these differences are or how they impact an image remains to be seen, but that is not the object of this test. This test just points out differences and nothing else.

Now let's see what this same process reveals when comparing the Series 8 HDMI cable and the UHDS-010b cable.

My oh my. To me it is evident that there are even more differences here. This is very interesting. So let's see what the next cable looks like compared to the Series 8: the UHDS-020b.

Fascinating. To me, it appears that there are more differences measured between the Series 8 and the UHDS-10b than between the Series 8 and the UHDS-20b. But let us continue and see what happens with the last comparison to the Series 8: the UHDP-10b.

Again there are measured differences between cables and it looks like the cable that shows the most differences is the UHDS-010b, the UHDS-20b measuring second, the UHDP-10b third, and the UHD-10b last.

I could go on and analyze the differences between these four cables themselves but I will not since it is clear that one cable shows more differences than the other three.

Now this is what the data tells me about image differences but it tells me nothing about the image quality or color accuracy. For that, a different set of testing is required - subjective - and these tests will be covered in Part 3 of this series.

Yours for higher fidelity,

Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please remember to reciprocate by purchasing one of my eBooks or through a PayPal donation, My newest title is called Where, oh Where did the Star of Bethlehem Go? It’s an astronomer’s look at what this celestial object may have been, who the "Wise Men" were, and where they came from. Written in an investigative journalism style (like that of the Discovery Channel), it targets one star that has never been considered before and builds a solid case for its candidacy.

My other titles include:

Copyright © 2015 by Philip Rastocny. All rights reserved.

Thursday, April 9, 2015

Tributaries 4K UHDMI Cable Review - Part 1

I have been very busy voicing my system over the past eight months and have not had a lot of time to dedicate to this blog. But that is changing now that the voicing is pretty much done (is anything like this ever REALLY done?). Anyway, it is safe to say that I am once again fairly happy about the sound emanating from my system, something I could not say as recently as last week. But "happy" is a relative term and those of us cursed/blessed with golden ears know that happiness is a moving target (solve one problem, uncover another). And so now that the rig is ready, on to the show!

My experience with HDMI cables after obtaining my Samsung F8000 65" 1080p HDTV as noted earlier in my blog, it became clear to me (pun intended) that as it is with audio cables, video cables can or cannot be properly designed and produced. For those of you who started out in high-end video - like me - and assumed that the video band did not suffer from the same issues the audio band did, we were both greatly surprised when swapping out these seemingly inert pieces of wire. But the truth of the matter is that - as there are audible differences in audio cables - there are visual differences in HDMI cables.

One of the things that drove me crazy about the Monster Cables HDMI I started out with were their huge favoring of the color red. Even their "then top-of-the-line model" was woefully inadequate at preventing simple issues like blooming, bleeding, and other nastiness in what should be distinct and individual color characteristics. Remember folks, HDMI is a digital signal within this cable and supposed to be totally immune from the effects encountered by their analog signals predecessors. Apparently, NOT!

It is unfortunate that theory and reality rarely coincide; if you doubt my words, ask/observe any professional weather forecaster. There are so many variables when creating mathematical models that it is difficult to account for them all in a single model. It is the variables (or assumptions) that are not accounted for that can create errors (i.e., what goes in is not the same as what comes out). Even when many of these errors or assumptions are addressed, there are additional complications beyond the theoretical design created during the manufacturing process (cold solder joints, ground loops, poor or broken shielding, etc.). In other words, a really good design can be totally destroyed by sloppy manufacturing or cheap parts. From the consumer's perspective, sloppy manufacturing is usually perceived as a non-functioning piece of gear but in cables and interconnects it can show up as audible or visual anomalies.

And while we are on the topic of assumptions, I decided to retest the video section of my rig and yes, I found other errors built-into the television receiver itself that I assumed I had already resolved. These errors were introduced at some point over the past year in one of the software upgrades from Samsung and this error escaped my attention until now. You can retest your own display with a standard 1080p test image or a 4K test image (NOTE: these are high-quality PNG-format pictures, not videos) to assure that all is well with your own rig and what I like about the 1080p image in particular is that it cures about 90% of problems for 1080p-format TVs without having to resort to hiring a calibration technician. So with my video system confirmed to be operating properly (test the tester), I could now proceed with the evaluation.

One last note of change in my system that I wish to point out: I moved all of the equipment 7" further away from the front wall so that cable swaps would be easier to perform. Moving the TV and equipment into the room also pushed my speakers this same distance into the room. This minor repositioning had major effects: it opened up the soundstage (L/R size) and greatly increased its depth. (See also the Rule of Thirds.)

In Part 2 of this review, I will cover another breed of HDMI cable whose design addresses issues with higher frequencies required in the 4K video format manufactured by Tributaries. But for now, I highly recommend that you re-check/tweak/re-tweak your own TV with either a 1080p test image or a 4K test image and you can eliminate another assumption that could lead to your own built-in errors.
And without giving away the show, in Part 2 of this review I prove that there is a measurable visual difference between HDMI cables and show you how you can do this same proof yourself.

Yours for higher fidelity,

Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please remember to reciprocate by purchasing one of my eBooks or through a PayPal donation, My newest title is called Where, oh Where did the Star of Bethlehem Go? It’s an astronomer’s look at what this celestial object may have been, who the "Wise Men" were, and where they came from. Written in an investigative journalism style (like that of the Discovery Channel), it targets one star that has never been considered before and builds a solid case for its candidacy.

My other titles include:

Copyright © 2015 by Philip Rastocny. All rights reserved.

Monday, March 16, 2015

Bespoke Aria Super Tweeter Review

I have a super tweeter in my system that crosses over at roughly 8.7KHz. This super tweeter is made by an England-based company (I believe that this company is now defunct) called Bespoke and the model is the Aria, a leaf tweeter in a round plastic cabinet with switch-selectable first-order network crossover points (12KHz, 16KHz, and 20KHz) and switch-selectable attenuation (0db, -3dB, -6dB). I initially bought them because of their high sensitivity (claimed 98dB/W/m) to match with the rest of my system and I personally love the sound of a good ribbon anyway.

The Bespoke Aria Ribbon Super Tweeter
 When I received them, I tried attaching them directly to my system with mixed results: I could hear the promise that these drivers had but was sadly disappointed by the built-in network. Great idea; poorly implemented; mediocre sound. I suspected that a compromise was made with the type of crossover capacitors used to keep the cost down and I was right. After all, how can any manufacturer sell super tweeters for $300/pair retail including switchable networks and use high quality crossover components? Remember that your system is like the links in a chain where the strength of that chain is determined by its weakest link, here the capacitors used in the super tweeter network.

So it was off to the workbench I trotted with brand new drivers in hand to undo the switching features and connect the driver directly to the terminals. While I was at it, I also replaced the internal wiring with some hand-made star-quad wiring (silver-plated OFHC copper, Teflon insulation) I usually use when doing such work and added a bit more internal sound damping. The next step was to measure the performance of the "native" driver (without the built-in network).

Ribbon or Leaf drivers always require some sort of minimalist crossover network to eliminate LF content which could irreparably damage the internal transformer. And with the addition of a 8.2uF Clarity SA capacitor in the signal path (Fc=2.5KHz) I was ready to make a near-field RTA measurement. Placing the microphone centered and on-axis as close to the driver as I could without it touching it, I made the following pink-noise measurement with the REW v5.1 RTA software and my calibrated microphone.

Bespoke Aria Pink Noise, 8.2uF Capacitor

The peak at about 4KHz initially puzzled me however with some quick calculations it appears to be a major cabinet resonance perhaps combined with a mechanical diaphragm/suspension resonance (1128 feet per second/4000 Hz=0.282 feet=3.3384 inches or 86mm; roughly the internal diameter of this tube). Given this resonance, the usable bandpass should be at least an octave above this 4KHz cabinet resonance point (8KHz). BTW, note that the performance of the microphone used to make this measurement was woefully inadequate since it is well known that this type of driver delivers excellent and very linear response to way over 20KHz (i.e., this test also revealed the microphone's upper-limit).

Ignoring the band above 15KHz (the estimated upper limit of the microphone) I realized that such an unexpected peak would require EITHER a very high crossover point with a normal second-order network or higher to minimize the interaction of this peak with the tweeter's bandpass. I opted for a fourth-order network ( more complex, more intolerant to tolerances, but a 360-degree phase shift).

Now comes the task of deciding what style of network design (Butterworth, Bessel, etc.) to use. Many online tools are available for free use to mathematically calculate the component values for crossover points and I encourage you to use any one of these tools for your own designs. One of my favorites is here. While there are words that describe the effects of each type of network on the resulting amplitude and phase of the sound at or near the crossover point, one must consider how this design impacts "voicing" of your driver (that is, the effects of that type of crossover network design on your driver's physical and electro-mechanical properties). Assuming that any network design will operate satisfactorily is an incorrect assumption (you know what they say about assuming anything, right?).

A brief virtue/compromise description of the four most popular crossover designs are presented below:
  • Bessel - VIRTUE: optimally constant group delay in the bandpass (i.e., flat phase, fast settling time). COMPROMISE: Slower initial rate of attenuation beyond the bandpass
  • Butterworth - VIRTUE: optimally flat frequency response and low ripple in the bandpass (i.e., flat amplitude). COMPROMISE: overshoot and ringing
  • Chebyshev - VIRTUE: steepest roll-off rates. COMPROMISE: adds amplitude peaks/dips and even more ringing than Butterworth
  • Linkwitz-Riley - VIRTUE: Uniform amplitude at crossover point. COMPROMISE: adds group delay in bandpass
As you can see, design selection is not a simple task since each type of design has its own virtues and its own compromises; such is the drawback of using passive crossover networks as opposed to multi-amplifier configurations with active crossover networks. The other issue complicating a network design decision is this: What may on appear paper or simulation to be the best choice design could measure exactly as predicted, but in reality the listening results may be absolutely the opposite. In other words, what is sound in theory may not be so in practice. Models can only predict from known variables. Much like forecasting the weather, network-driver, filter-phase or filter-amplitude shifts, and who knows what else dynamic physical interactions are unaccounted for in these theoretical mathematical models. You just have to use them to start and then tweak from there.

For example, here is the near-field measurement of this same driver using a fourth-order Bessel network with a crossover frequency of 8.7KHz. One would predict that not only would the 4KHz peak be resolved because of the steep filter slopes but also that the driver would be uniform above the crossover frequency. However, here modeling and reality deviate as shown next.

Bespoke Aria Pink Noise, Bessel 4 Network, Fc=8.7KHz
While it is true that the network functioned somewhat as predicted, the 4KHz peak is still prominent and now only -5dB down from the crossover frequency. So now what? Obviously this is not going to sound well so what can be done? The process is called "voicing" and another filter is added to tame these unwanted characteristics. Voicing is added when a Zobel network is added to a dynamic driver to tame its rising HF impedance curve and also applied to LF impedance peaks. These filters  "nominalize" the impedance of the driver for the crossover network thereby permitting it to operate as predicted. These same filters can be applied to tame unwanted SPL peaks.

For example, adding a LF L-C filter to the Bespoke can change its SPL characteristics. The graph below shows the change of adding a 0.12mH inductor in series with a 2.0 ohm resistor and then placed across the terminals of the super tweeter (shunt across the terminals).
Bespoke Aria Pink Noise, Bessel 4 Network, Fc=8.7KHz, 0.12mH+2ohm shunt
Well that certainly changed things. The 4KHz peak is now gone but as they say you never get anything for nothing. The SPL is very uniform with the expected HF rise of this type of driver design. However, the acoustic contribution of the super-tweeter into the tweeter's bandpass is now an issue. So what can be said? Passive networks have issues when used in loudspeaker designs.

The other drawback to using passive crossover networks is the interaction between other drivers and crossover components. Remember that adding another driver not only permits that driver to more optimally handle its bandpass, but also introduces other dynamic variables to the existing drivers and network. So by adding a super tweeter and its crossover network to your system, it also changes the performance of the other drivers. In effect, adding a super tweeter can change the entire sound of your entire speaker (including woofer performance), and not just add an extension to the top octave.

So how do you decide what network design to use knowing that it also impacts the operation of the existing system? You must listen to the results and listen very carefully. This means that there is both an art and a science to creating proper overall system design (read you never get anything for nothing; there is always a compromise).

For example, I first tried a Butterworth design and while it sounded pretty good, the super tweeter sounded quite off with an unacceptable upper-midrange glare. I then tried a Bessel design and had similar disappointments where the super tweeter just did not have that pristine clarity I hoped to achieve with this driver. So I was at a crossroads: which is the best compromise? Too much glare or to soft an operation? I chose to eliminate the glare since I find this more fatiguing to listen to over prolonged periods (I listen to my rig a lot). What did I compromise? Bandstop issues.

Another factor I needed to resolve at this same time was sibilance. At some crossover frequencies, sibilance became unacceptably pronounced and less so at others. And of course there is another compromise at play: standard values of capacitors. If I could find a crossover point where an off-the-shelf value was available, the cost of the network components would be kept under control. Inductors can be easily wound or unwound but capacitors must be purchased lower-than-desired values and parallel shunts added to achieve the desired capacitance (if a non-standard value is required). If I selected a point where two or three capacitance values were needed (paralleled), it would raise the cost of the network but it is the only way to get the desired value (another compromise).

My philosophy around building crossover networks is to use lower-cost components to find the neighborhood of the solution and then spring for the big-boy toys in the final design. Iteration, repetition, measurements, and subjective comments all help when making any tweaks to maintain your sanity. Change one thing at a time to keep you from becoming overwhelmed and record all of your data.

Once all experimentation is complete, you should sit down and look at your comments about the various configurations. Nothing is more valuable than analyzing your impressions and correlating what you hear with that design. You typically glean insights to what the configuration is trying to tell you by this analysis rather than by staring at the graphs and numbers. And by all means, proceed slowly, preferably over several days or even weeks. Allow yourself enough time to listen to a variety of music and signal sources to accumulate a final impression.

What did I glean?I observed the rising SPL and correlated it to the rising impedance (Z) of a dynamic driver (similar curves, different Q). I tamed this super-tweeter SPL peak with the same type of L-R network used to tame the rising Z of a dynamic driver. I accomplished this through experimentation and measurement rather than calculations. Finding the right combination took a while but in the end the results paid off big time.

Once the final design took shape, I then replaced the cheap signal-path capacitors with quality versions and viola! Not only were all of the issues resolved, the quality capacitors (here Teflon) allowed the driver to perform at its best. Transient response was improved and neutrality achieved. In other words, the network became transparent and permitted the driver to perform at its optimal peak rather than colored by passive components.

Is the Bespoke Aria a good driver? Yes and no. It needs a lot of help to get it to perform at its best. However, once you do, it is very smooth and transparent. I suspect that other driver choices may be more to your liking since they require less tweaking (voicing). However, their cost is double or more of this driver and by taking the time to tweak it you can gain valuable knowledge about driver and network behavior while adding a high-quality top octave.

BTW, the drivers used in this super-tweeter appear to be available from Monacor in the UK for about GBP 46 each (including VAT).

Yours for higher fidelity,

Philip Rastocny

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