Bozak Rebuild Project - Part 2

Bozak Rebuild Project - Part 2

In Part 1, We saw the starting point from which I planned to improve the basic sound of a vintage pair of speakers. Many folks are familiar with the Bozak name but unfamiliar with their sound so hopefully in this series we can all climb up the better-informed ladder and upgrade a good beginning into a truly contemporary pair of high-end speakers.

I started with quite a mess and a plan of attack to try to fix the sound of these old speakers hoping that from the other side of the grave Rudy Bozak himself would be proud.  Rudy did not like the sound of second-order crossover networks so he chose simple first-order designs.  While there is nothing wrong with this (and it keeps costs down), my ears are more sensitive to the overlap region on either side of the crossover point.  When drivers are not properly limited, the resulting coloration introduced by drivers asked to reproduce parts of the audio spectrum to which they are not intended to do is more objectionable to me than the effects of higher-order crossover networks.  So my first thought was to roll off the midrange more steeply with a second order network.

To confirm that the midrange was indeed the culprit and not the tweeter, I disconnected the woofer and tweeter, and ran another RTA measurement.  The results are shown below.

 Pink Noise with Tweeter Array Disconnected

This graph clearly shows that the midrange driver is still contributing acoustic energy up to 7KHz despite the fact that the crossover network is supposed to roll off these effects at 2.5KHz.  I had found the source of the problem and this was half of its solution. 

Checking the math and the value of the midrange inductor (1.12mH), I was satisfied that the 2.5KHz crossover point was indeed correct and suspected that the sonic contribution above the crossover frequency was from the rising impedance of the driver.  I tried many Zobel networks across the midrange terminals all with little success and then decided to move to a second-order network.  The results of this change are shown in the next RTA image.

 Second-order Midrange Network

I then added a Zobel network to the woofer and things really started getting to sound real.  The timbre of the drums came to life where you could hear the skins stretching as the sticks struck.  The mellowness of each drum became clear and distinct making one wonder exactly what kind of wood these drums were made of.  The over-exaggerated upper midrange was tamed and an edginess that was once predominant was now under control.  The depth of the sound stage improved and I knew I was on the right track.

Things were better but still not great.  The new network improved clarity in the lower midrange but introduced more sibilance, an issue to which my ears are very sensitive.  Life with a smoother midrange was more tolerable than the sibilance so we moved on to the missing top octave.  We’ll see what was involved with that next time in Part 3.

See also Part 1

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 tell your friends and if you are so inclined, buy some of my work at Amazon.com.  My titles include:

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

Copyright © 2015 by Philip Rastocny. All rights reserved.

Apologies to my followers

Some of you may have noticed that I have been remiss in routinely posting to my blog (highly uncharacteristic of my past behavior).  It is not from lack of desire but rather an adjustment of priorities that I have not.

I am an author and I am working on a new book on the subject of choice.  Targeted at young adults, this book will hopefully help youthful minds understand how choices are made and how they can adjust their habbits and patterns so that they can make new and different choices.  Using a holisitc approach of integrating body, mind, and spirit, this book points out examples of when everyone, even agnostics and non-spiritual folks, do this every day and how integrating all three parts of your being aligns itself to make quality choices.

I am almost finished, hoping initially to release it before the holidays. However, it looks like I will return about the second week in January with new thoughts, new ideas, and new observations. Please bear with me until then: I'll be back!

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:

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

Copyright © 2015 by Philip Rastocny. All rights reserved.

Sunrise on Friday December 21, 2012

Well the holiday season is attracting traffic here in tiny Brooksville like hungry children to Christmas candy and we are holding up in our castle until the surge subsides, at least as much as possible. We decorated our tree and had a small party last weekend. But the really big news is happening this Friday morning precisely at sunrise.

At the moment of sunrise an astrological alignment will occur that has not occurred in 24,000 years but coincidentally on the same day as the last day of the current long count of the Mayan Calendar. At the exact moment of sunrise, not a day before nor a day later will this alignment hold true. As you gaze at the first signs of light from our massive yellow solar ball and glimpse its first shape and color, you will be gazing toward the very center of the Milky Way.  Our galactic center located in the constellation Sagittarius is the heart of our pinwheel-shaped galaxy and only once every 24,000 years does this phenomenon occur. While today we are looking just slightly off from that precise point, when the first long count began 5,126 years (13 Baktuns) ago, it was indeed pointing at this exact place.  But as time marches on, so does the wobble of our planet and the precise location of the galactic center has now drifted about the width of your pinky fingernail at arm's length and horizontally to your right.

This 24,000 year event is called the "earth's precession" and is the result of an impact millions of years ago from a very large meteor, possibly the one that devastated the dinosaurs. This impact knocked our spin from pointing stationary (at the same place in the heavens) to one that wobbles (draws a circle through the heavens).  Although we perceive that the north star is at the same place each year, it is not.  Because this precession is slight, over the 5,216 years since the very first long count it is has moved about the width of your pinky fingernail at arms length to your right (not much but undeniable). Like a spinning top as it looses speed starts to wobble before it falls, so does the earth wobble in it's precession. In 18,874 more years (that is on calendar date December 21, 20,886), this wobble will once again come full circle and point this same sunrise event precisely at the galactic center as it did on the day of the first long count.

At about 7:19AM eastern time on December 21, 2012, the sun will peek above the horizon.  Put your pinky finger at the top edge of the sun and horizontally to the right will be our galactic center.  Our local astronomy club and a few metaphysical friends are getting up that day to celebrate this once in a lifetime event.  I invite you to do the same.  Remember, do not stare at the sun too long since doing so will cause blindness. But just as it is reasonably safe to gaze at a sunset, so it is reasonably safe to briefly gaze at a sunrise.

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:

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

Copyright © 2015 by Philip Rastocny. All rights reserved.

Bozak Rebuild Project - Part 1

Most of you know that I have rebuilt a pair of Bozak B-302A speakers being as true to the original design Rudy Bozak used for this model as possible.  Several other folks (Tobin) have modified this stock crossover network replacing capacitors and adding attenuation to the tweeters to upgrade this design.  This modification balances the SPLs between the drivers, something desperately needed as you will soon see.  While doing so retains the original sound and improves clarity, I wanted to take this design a step further and apply additional knowledge that was unavailable in the early 1960s.  Let’s see what I found from the work of others and what I did to improve it.

Original Bozak B-302A Urban Built in 1963

I used the original woofer enclosure (about 5¼ cubic feet), added a second woofer in an isobaric push-pull configuration, and built a truncated pyramid (non-parallel) 1 cubic foot infinite baffle satellite box to house the mid and tweets (added two more tweets per speaker).  I also mounted the stock crossover network board onto the rear of the woofer cabinet to permit quick tuning adjustments, changed all wiring to OF-HC star-quad copper (4 x 12 AWG), and bi-wired the system to the amp.  I finished the speaker tops off cosmetically with a piece of 2” thick granite that helped to mechanically isolate the two enclosures from each other.  Speaker spikes to both the woofer and satellite boxes topped off the mechanical design.

Enclosures for the Bozak Project

Below is a picture of the enclosure and rear-mounted crossover before removing the old speaker terminals.  Internally mounted woofers are down firing, hard-wired to the network, and stock legs moved to the face.  The speaker spikes raised the height of the legs enough for the pull woofer to clear the floor by ½”.

Original Rear-Mounted Crossover Network

Before tearing apart my Bozak crossover network, I decided to make a few measurements with that RTA Pro software I have been telling you folks about to establish a baseline.  I made all of the pink noise measurements in all of the articles that appear in this blog about these speakers from the same position in my listening room.  Below is this reference RTA image showing crossover points with the original crossover, upgraded capacitors to Clarity SA of identical values, and a -16dB T-pad attenuator on the vertical four-tweeter array.

Where This All Begins

While the above graph clearly shows the inadequacies of the room+speaker, I decided to tune the crossover network to the room instead of using the conventional anechoic approach (after all, I would have to adjust the EQ of the system again which seemed like a waste of time).

Despite the huge swings in SPL (+/- 10dB!) and the nasty goings on at 4-6KHz, the system sounded amazingly good.  I used several reference tracks to determine what was right and what was wrong with this design including Dire Straits “Brothers in Arms,” Norah Jones “Nightingale,” Eric Bibb “Tall Cotton,” Cirque du Soleil “Taiko,” and Eric Clapton “Change the World” among others.

To start, the system fell flat over 8KHz with zero sparkle in the top octaves, something I was used to with my previous ribbon tweeters and missed dearly.  Transient response was frighteningly excellent with moments of shock and surprise when explosions from movies rocked the room and the ease at high SPLs was unstrained and exhilarating.  It was a very different sound from the 12” B2 medium sensitivity system I outgrew even with its Dynaudio D-54 midrange driver, one that was very revealing of instrumentation and nuances in musician technique. 

For example, the fingering of Clapton’s acoustic guitar in “Change the World” was brisk and succinct without ringing or edginess.  But voices were brighter and the absence of the top octave very annoying.  Inner detailing of any percussion was totally absent so the first adjustment to the design required the addition of a very high efficiency super tweeter to match the efficiencies of the rest of these amazing Bozak drivers.  But there was something about the midrange that kept me going.  Maybe it was the air of the aluminum cone or the striking accuracy it revealed, but whatever it was, I am very glad I persisted over the past 12 months to improve their character instead of assigning these speakers to the “failed attempt” heap.

Rear View of Satellite and Network

From this starting point, I began the quest to tame the peaks and address the valleys of this radically new design.  The remaining parts of this blog will help you understand what designers go through when tuning a speaker and may inspire you try a few things on your own. 

So fasten your seatbelt and make sure your trays are locked in the upright position.  We’re headed were no one has gone before and doing so with as little engineering-speak as possible!

This series is continued in Part 2.

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:

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

Copyright © 2015 by Philip Rastocny. All rights reserved.