Tuesday, January 29, 2013

Bozak Rebuild Project - Part 5

Driver materials totally impact the characteristic sonic signature of a dynamic driver. For example, harder materials like aluminum or titanium dome tweeters have a sonic signature that can be audibly recognized once your ear is trained to listen for that signature.  The Audax TW025A28 I am using in this rebuild project is no exception to this phenomenon.
The Audax is a gold-sputtered titanium dome with a sonic signature reminiscent to that of titanium compression horns.  Like these horns, there is a huge benefit of very fast attack and decay times at the expense of an inherent resonance at about 6KHz.  Taming this resonance can prove to be a problem but one way to do it is to cross it over at a pretty high frequency with a second order or higher network.  The results are quite tolerable compared to a lower crossover point or a first-order crossover network.
In working with this super tweeter, I kept running into the sonic signature when trying to cross it at about 8KHz and finally gave up and disconnected the driver. I preferred listening to no top octave at all instead of one highly colored in the 6KHz region.  Of course that decision did not last long and after borrowing a friend’s XTZ Room Analyzer, I discovered more of what was going on behind the scene.
I made near-field (less than ½” away) measurements of all drivers in the satellite box one at a time.  Resting the microphone on a tripod and removing the physical presence of myself and the laptop from the measurements as far as I could, I tested each driver with the same amplifier volume level and moved the tripod and microphone only.  I also disconnected the leads on the other satellite drivers not under test leaving the network components but not the load to the amplifier.  These 5 measurements revealed several things: each of the six B-200Y tweeters had its own contour, and the midrange and super tweeters were very well matched.  You may notice the polyester pad at the rear of the satellite used to absorb wrap-around energy at the rear surface of the box.

Near Field Measurements with the XTZ

The Bozak tweeters are 35 years old and it is amazing that they still work at all.  I understand that the variation in output is attributed to their age and accept it as a compromise in this design.  I want to retain as much of the original components so that other can try what I have done to improve their own Bozak systems.
The first thing I did was to change the super tweeter crossover point from 8.3KHz to 12.5KHz since the acoustic energy contributed below the crossover point was not only significant but also highly objectionable.  Using the two Russian 0.56uF polystyrene capacitors as a starting point and the measured impedance of the tweeter at 12.5KHz (6.5 ohms), I wound a 0.14mH choke and installed the Bessel-2 components into the super tweeter HPF.  I then modified the tweeter LPF network in a similar manner by adding a hand-wound 0.31mH choke to its network configuration (BW-1, 8.3KHz).
After a quick audition, this revision proved to be moving in the right direction.  Sibilance and vocal colorations essentially disappeared and I knew that I would not have to toss my efforts with the Audax driver and start over (as I had throught as recently as yesterday).  Smoothness returned to this spectral region and although there is still some lingering metallic sonic signature to the super tweeter, the level is adequately suppressed with the new network design.
Next was the tweeter work.  Presently the three tweeters are attenuated with a T-pad to -3dB.  I ordered new components and installed the -5.5dB attenuator to the tweeter network.  The levels of the tweeter and super tweeter were now perfectly matched.
I decided to also change the crossover design for the midrange LPF to a Bessel-2 using the stock choke.  Moving the tap to the N102 terminal uses the stock 1.1mH choke.  Coupled with a 4.7uF shunt, the midrange energy above the crossover point was again controlled and the results were highly satisfying.  After listening, I needed to address the individual driver phasing issue and did so this morning.  Polarities are now W+, M+, T+, ST- and the resulting RTA measurement of the entire system is shown below.
RTA Analysis of the New Network

I have not had a chance to do much prolonged listening but from my initial tests this is a really good thing.  Everything above 1500Hz is fantastic with incredible inner detailing and superb accuracy.  Strings are delicate and revealing and vocals are smooth with a natural presence.  The sound stage is well defined and now reaches deep into the rear corners.  The speakers completely disappear and a wall of sound is now produced from about four feet into the rear wall of the room.  This psychoacoustic illusion is only present in the best of the audio playback systems I have ever heard.
My wife is still asleep and will be so for another hour or so.  With the volume of the system at a very quiet level, the smoothness of the Native American flutes I hear now are stunningly accurate, the drums dynamic and revealing, and the voices gentle and sweet.  There is a slight exaggeration in the lower mid-bass region as also observed in the RTA measurement and I will address that issue next.  But for now, the raspy 6KHz region is tamed and tonal balance has returned to my listening room.
So I hope you enjoyed this dissertation and can benefit from what I have tried.  I will make the network pretty once I stop tweaking it and maybe add a final picture of it in Part 6 (although this may never happen, but then again I said that Part 5 would not happen either but here it is).  As I said, I am still writing and I have a life.  Besides audio I also love the beach, astronomy, and bicycling with my wife.  I have another novel and a self-help book in the works so time allocated to my blog is very limited.
Bear with me, keep checking back for updates, and you will see I have much more to share.  Hmmm…what if I do this to the tonearm of the turntable…

See also Part 1Part 2, Part 3, and Part 4
Yours for higher fidelity,
Philip Rastocny

Copyright © 2015 by Philip Rastocny. All rights reserved.

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