Thursday, May 22, 2014

Speaker Enclosures - Part 1

One of the challenges speaker designers routinely face is overcoming the laws of physics. How they choose to compromise their designs to address physical laws is an individual choice, some resorting to baffling, others to exotic shapes, and still others to exotic materials. All are good choices and none are free; there is always a downside to any choice in any design.

The biggest motivation a designer chooses something over something else is usually cost where if the best way to implement a design is this way but a good way is another less-expensive way, the latter will likely be chosen to permit higher profits and lower consumer costs. Is this bad? No, not at all. It's just a choice and you choose exactly the same exercise in everything you do when considering something to purchase.

For example, my wife loves purses and shoes and all of her life she bought those inexpensive accessories from mass-marketing stores - and she was happy. For her birthday, I sprung for an esoteric purse and that changed everything. When you step up in quality, you get more than a better-made product, you see how others choose compromises that such a purchase does not. Mercedes Benz automobiles, Brietling watches, and Gucci shoes are all examples of how everyday purchases can choose fewer or no compromises when implementing a design and owning such a product exposes the compromises made by inferior designs.

The single-most frustrating law of physics that plagues speaker cabinet makers is the internal resonances (aka dimensional resonances). Much like a tiny room, speaker cabinets are prone to vibrate at certain frequencies based mainly on parallel walls. The best thing one can do is to spread these dimensional resonances out so that they do not all occur at the same frequency, at the same frequency as a musical note, or with multiple resonances at the same frequency. You can measure the internal dimensions of your solid-rectangular speaker (subtract the thickess from the external dimensions) and calculate their existing resonances with the WINISD speaker design software available here. Such software automatically creates recommendations for cabet dimensions based on the volume required.

Such software (and designers) often use recommended ratios for height, width, and depth to build cabinets such as 1:1.14:1.39, 1:1.26:1.59, 1:1.28:1.54, 1:1.44:2, 1:1.60:2.33, 1:1.62:2.63, and 2:3:5. But such ratios are not really all that useful in predicting how many resonant modes will occur near each other (sum together creating a louder resonance), and what musical notes these resonances will exaggerate (over emphasize a note on the musical scale despite the anechoic response of the driver).

For example, plugging internal box dimensions of 20cm, 30cm, and 50cm into a simple resonance mode calculator yields harmonic resonances at 343Hz (near F4), 571Hz (near D5), 667Hz (near E5), and 685Hz (near F5). What this means is that such a box will artificially create louder notes at those points (especially D5, E5, and F5) independant of the driver's anechoic response.Changing the box dimensions to 19x30x48 better distributes the resonances in between the fundamental frequencies of notes on the musical scale. This means that all notes on a cello run will appear more uniform with the 19x30x48 cabinet as opposed to the D5, E5, F5 resonances with the 20x30x50 cabinet.

Other resonances can occur within a cabinet creating other issues above the operating range of the driver. For example, when harmonics of a resonance occur within a cabinet, the harmonic content of that fundamental note will also be exaggerated. So if a cabinet resonates at multiples of the fundamental frequency of D5 (667Hz), multiples of this frequency (2x, 3x, 4x, etc.) are its harmonics and resonances at these multiples change the way the character of that note is percieved (colors the sound and makes that saxaphone or piano note sound less real).

Aside from juggling the cabinet dimensions, what else can be done to minimize these internal resonances?
Logically, the simplest thing to do is to avoid using parallel surfaces (sides). Next to a solid-square box (one major resonance and the most resonant shape you can choose), the poorly-designed solid-rectangle vies for the second-worst shape since there are three parallel sides creating three characteristic resonances between them. But yet the vast majority of speakers built today use solid-rectangular enclosures. Why? In a word: cost. Straight cuts, square corners, and uncomplicated angles are mass-production friendly with low material waste. In other words, it is incredibly easy and highly efficient to build a solid-rectangular enclosure as opposed to a more esoteric shape.

Question: What can you do to improve your solid-rectangular speakers?

Answer: Build a new, properly-designed enclosure with the same volume and put the existing parts into it. For those of you with moderate wood-working skills, you can build another solid rectangle with minimal grouped dimensional resonances using the above calculator or hire someone to build you another cabinet. Tweak the dimensions to retain the required volume while adjusting the characteristic resonances to occur at fundamental frequencies that are not the same as those of musical notes.

For those of you with advanced woodworking skills, you can take this opportunity to build an alternative shape with fewer parallel surfaces. This approach will be covered in Part 2 of this series.

Related ArticlesSee all entries about speaker enclosures in Part 1Part 2Part 3, and Part 4. Also, a related article on the effects of crossover network components on driver performance.

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, 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, 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.

Saturday, May 10, 2014

Onkyo P-3000R Preamp Review - Part 2

In Part 1 of this review, I evaluated the single-ended (RCA) inputs. The performance was stellar and I praised the circuitry for doing what I recalled it did best: sonic accuracy, imaging stability, and front-to-back depth.  However, also included in this preamp are some pretty sophisticated digital components worthy of mention.

In comparing the quality of any two sources, switching back and forth between two sources – preferably within two seconds – is the best way to evaluate such differences. Very few golden-eared folks have adequate long-term sonic memory to recall all of the nuances and subtleties when taking more than two seconds or so to switch between sources but devising a good method that itself does not influence the test results has been an equal challenge.

Some people believe that a Y-splitter is one way to achieve quick results and such a method will allow you to quickly switch between two RCA interconnect cables.  For example, two RCA cables can be connected to two inputs on a preamp (e.g., CD and AUX) and the other ends to the same output on a CD player via the Y-connector.  But I suspect that such a test method introduces errors in the form of ground loops between both cables.  So if the introduction of a Y-splitter is of questionable value when evaluating two sources, how can one do so without such degradation?

Y-Splitter RCA Cable Test

The answer is pretty simple: think light.  The Onkyo P3000-R preamp has four RCA and two TOSLINK optical-digital inputs allowing you to perform simple source A/B switching without introducing ground loops into the test.  So for Part 2 of this preamp’s evaluation, I will use my reference 1-meter RCA interconnects and a 1-meter Better Cables TOSLINK cable both connected to the same source: my stock OPPO BDP-105.

With such a configuration, I am able to play the same track on the OPPO and instantly switch the preamp between the CD and AUX inputs. What this configuration allows me to do is instantly hear the difference between the DACs in the OPPO (ESS Sabre32 Reference) and the DACs in the Onkyo (Burr-Brown PCM1795 192/32). This should be interesting.

P-3000R Rear Panel Inputs

Upon first hearing, I could distinguish very little difference between the two sources, something I did not anticipate. I listened to MP3s all the way up to DSDs and although there WERE differences, they were so slight that I could not consistently identify them from one format to another. Even my wife said that the differences were so small that she would have a hard time deciding which sounded better, other than the TOSLINK cable giving a slightly deeper bass output. From the get go, I knew this was going to take the right source material and more time than originally anticipated.

As my wife heard, the deep bass was a tiny better with the TOSLINK cable than the RCA so I was at an impasse. However, I had planned to make another modification to the woofer's crossover network in my speakers and now seemed like a good time to do so. With a 4-ohm woofer, the insertion loss of a typical large value wire or foil inductor - even with 12AWG wire size - approached a DC resistance of 0.5 ohms. This means that 10% of the power to the woofer is dissipated by the inductor.

I have always wanted to try a high-power toroid coil since the DC resistance in these inductors is about 1/10th that of any other coil type and now seemed like a good time to give it a go. I also wanted to lower the crossover point from 850Hz to 500Hz , a much better upper-frequency limit for a 12" driver. With a  quick check to Parts Express, I found a 3.3mH toroid that with a bit of unwinding would work in my crossover network.

Now unwinding this type of inductor is not as simple as measuring it with an inexpensive RLC meter. Because of the low DCR, higher currents are required to accurately determine the value and so I drug out my oscilloscope, sine wave generator, and a known value capacitor and resistor. It took about 11 turns off of this inductor to get it to the value I needed as determined by the LC tank circuit it created (carefully measure the minimum "null" and read the frequency, then simple math tells you the value).

A Toroid Inductor

I rolled this toroid inductor into the woofer crossover network and immediately I noticed a much cleaner, tighter bass. Considering that the moving mass on these woofers is an incredibly low 40g, this was not expected. But the surprise came when watching Star Trek: Into Darkness.  WOW did that extend the extreme LF response and remove muddiness! So the weak link in my chain was the woofer inductor and now I have never been happier - as is my wife who loves bass, especially explosions and deep thuds.

Back to the TOSLINK/RCA (aka Burr-Brown/ESS Sabre32 Reference) dilemma. Now that the LF extension and clarity is better, I hoped to hear more differences and be able to easily discern them. The large organ recorded by 2-L on the  Kare Nordstoga improvisation proved to be the right choice.

All throughout this 24/96 piece (also available in SACD), wall-rattling bass can be heard that captures the true character of a very large brass pipe. Through the Burr-Brown/TOSLINK version, the bass was present but not full nor did it reveal the true timbre of the largest ranks. Sounding muted rather than forward, this configuration - although pleasant - just did not compare to the OPPO/RCA version. The OPPO extracted more information from the source and left you feeling more like "you were there" listening in a pew somewhere forward of the pipes in the balcony. More depth, more width, and more height all combined to giving a well deserved and decisive advantage to the OPPO/RCA version. While other parts of the frequency spectrum were also different, these differences were not as noticeable nor as obvious as the very deep bass and hall ambience.

As I have mentioned in the past, if you are searching for ways to improve your system, look at the size and shape of the sound stage and then choose what sounds more realistic. For now, I will stick with the OPPO/RCA combo for decoding.

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, 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, it targets one star that has never been considered before and builds a solid case for its candidacy.

My other titles include:

Friday, May 9, 2014

Martin Logan CLS-3 Z-model Modifications - Part 1

Ever since I heard my first pair of electrostatic speakers, I was captivated by their unique appeal. Such openness, such airiness, such detailing, and such beaming. Well...nothing is perfect...but a good electrostat is pretty close.

Later I heard a pair of Apogee full-range ribbons and I was again mesmerized by what a linearly-driven diaphragm could do that dynamics could not. A friend of mine has a pair of Martin Logan CLS-3z driven by four huge Audio Research monoblock tube amps, something that took my breath away. But as all humanmade things go, one of them died. So he sent his boxes off to be repaired and 5 months later with zero work accomplished, he insisted they be returned. Frustrated, he asked me (paraphrasing), "You want to give it a go?"

Never one to turn down a good challenge and to see what was under the hood of the CLS, I agreed and we arranged a trade: I repair his speakers and he builds me a new satellite box. Both he and I were waiting to do something for the other and willing to put that little extra touch on what friends do for friends.

I got the boxes and boy was I surprised. I heard these speakers and they indeed sounded fantastic but once I looked at the circuit board, I was - in a word - shocked. The primary side of the transformers, the side connected to the amplifier via speaker wires, was what one would not consider to be the best design. Because of the high current delivered to these transformers, not one but several parallel traces ran from here to there but instead of converging at a large circular trace, the converged on a single small trace and then to a small circular trace. Now if you don't understand what impact this has, think of using #22 wire as speaker wires and you'll get an idea of this faux pas.

So further investigation showed that resistors were used in the path between this transformer and the speaker terminals on back. What I found unusual was that these were inductive-type resistors, something that most engineers avoided in the high-end realm years ago. So I decided that rebuilding this part of the circuit would be highly beneficial and so I did.

After a few hours of tracing out the circuit, I was very surprised by the simplicity of its design. And as many have pointed out in other DIY groups and forums, there are many things that one can do to just swap out components and replace them 1:1 for higher quality devices. While a definite sonic benefit will be reaped, unfortunately this will only solve half of the built-in issues with this box.

My friend encouraged me to take a no-holds-barred approach at redesigning this unit and so I did. They will be finished (parts still on order) some time next week. After I stuff everything back into place, I'll let you know what we hear.

Here is a picture of the one of the boards before modifications:

Note the use of multiple traces on this side of the board (there are even more on the bottom side). So, let me explain a little about what you see. The large rectangular gold things are resistors and they - along with the red coil called an inductor - electrically connect the speaker wires to the transformer (there are actually two transformers but both are used as one) I spoke of earlier. The black and yellow wires run to these transformers and the speaker wire connections to this PC board are barely visible in the lower RH corner.

See if there is anything else you can spot that could be remedied and when I post the results of the mods in my next article you can see if you are correct.

BTW, what these multiple resistors do is to double the power handling capacity of the resistor but reduce the overall resistance by half. The two top resistors are 2 ohm 50Watt so they actually become a 1 ohm 100Watt resistor. The two on the lower RH corner are 12 ohm 50W so they become 6 ohm 100W. The one on the lower LH side is used when the NORMAL/SOFT switch is in the SOFT position only.

Put on your thinking caps. If you've read all of the things I've written about over the years, you may be able to guess correctly as to what I am about to do. Stay tuned...

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, 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, it targets one star that has never been considered before and builds a solid case for its candidacy.

My other titles include:

·  Extreme Audio 1: House Wiring·  Build an Extreme Green Hot Water Solar Collector
·  Extreme Audio 2: Line Filtering·  The Extreme Green Guide to Wind Turbines
·  Extreme Audio 3: Chassis Leakage·  The Extreme Green Guide to Solar Electricity
·  Extreme Audio 4: Interconnect Cables·  Meditation for Geeks (and other left-brained people)
·  Extreme Audio 5: Speaker Wires·  Althea: A Story of Love
·  Extreme Green Guide to Improving Mileage·  Build an Extreme Green Raised Bed Garden
·  Extreme Green Organic Gardening·  Build an Extreme Green Rain Barrel
·  Extreme Green Organic Gardening 2012·  Build an Extreme Green Squirrel-Proof Bird Feeder
·  Build an Extreme Green Composter·  Extreme Green Appliance Buying Guide

Copyright © 2015 by Philip Rastocny. All rights reserved.