Monday, December 3, 2012

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

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