Wednesday, March 12, 2014

Bozak Rebuild Project - Part 10

In my previous attempts to upgrade a pair of Bozak B-302a speakers, I fiddled with a second-order crossover network in hopes to improve the sound. Using some trickery, I was able to get the RTA measurements to "look" right but the resulting sound was really off the mark. Testing each driver independently revealed where this network design was and was not working.

In review, the original crossover network from Rudy Bozak used a first-order Butterworth design (BW1). I revised this to a second-order Bessel design (BE2) hoping that the steeper crossover slopes would attenuate the influence of one driver on another (e.g., the midrange on the tweeter and vice versa). While it improved the sound, individual driver in-circuit measurements assured me that steeper slopes were required to truly prevent this interaction.

A third-order Butterworth design (BW3) is currently in place yielding remarkable results and the old Bozak B-200Y tweeters have retired and Peavey RD1.6 ribbons now are featured in this design. Crossover points are BE2@450Hz and BW3@2,348Hz and the sound is quite smooth through 12KHz. Using higher-quality capacitors on the RD1.6-side of the crossover network added another level of refinement that blatantly pointed out the shortcomings of the old Clarity SA capacitors. So the old adage of "you get what you pay for" held true here as it does so often when buying other high-end gear.

The sound of the system is currently captivating with a huge soundstage extending far into the side walls adjacent to the speakers. Front-to-back depth is also surprising and image stability is spot-on regardless of what instrument is playing or where the engineer placed it with her/his mixer's pan pots. Studio and hall ambiance is also easily articulated as is inner detailing, always a plus to any audiophile's ears.

There is a downside to of all this, or rather there is a potential to make future improvements. Presently, the speakers exhibit a certain sterility from this latest round of refinements for which I am having a difficult time tracing down its source. This same issue reared its ugly head in previous upgrades and changes, and  now it has returned. Backing out of changes one step at a time and then returning it to normal takes time and so far I have not discovered the source of this problem (most likely revealed in Part 11). And lastly, even though the RTA measured better at the start of this round of changes, there are certain things that have greatly improved that do not conform to what the measurement shows. In other words, the RTA only tells part of the story.

Below is a summary of how RTA measurements changed through three phases of the crossover network's evolution while moving from the starting point (before), interim 1, interim 2, and final design for this network. Although work focused primarily on frequencies above 2.3KHz, it is interesting to note the influence the changes made to the tweeter had on the woofer.

Here is the "before" RTA graph of the system using all Clarity SA capacitors in a BE2 crossover network on both the W-M and M-T points (6-27-13):

BE2@450Hz and BE2@2,348Hz Clarity SA Capacitors, no HF compensation

The system sounded good up to about 8KHz where the magical tinkles of tambourines and cymbals were just muted. You could hear them but the SPL was just mismatched. Plus, there was an annoying 6-7KHz sibilance making prolonged listening difficult. (Also note the dip at about 500Hz).

Here is the "interim 1" graph of the crossover network using all Clarity SA capacitors in a network using two different network designs: a BE2 on the W-M point and a BW3 on the M-T point (7-1-13):

BE2@450Hz and BW3@2,348Hz Clarity SA Capacitors, no HF compensation

The sound was better in that the top octave started to materialize and the annoying sibilance was gone. The sparkle was still lacking and the smoothness in the midrange had something missing that I just could not put my finger on. (Note the increased dip at that 500Hz point.)

Here is the "interim 2" graph of the crossover network using a variety of Mundorf Supreme capacitors I had lying around on the RD1.6 portion of this crossover network and using a Clarity SA HF compensation capacitor (2-10-14):

BE2@450Hz and BW3@2,348Hz Mundorf Supreme Capacitors with HF compensation
With this really uniform SPL measurement, I was anxious to hear what the system sounded like. I was greatly disappointed. Although things sounded "smoother" they did not sound real. There must have been a huge amount of phase distortion introduced by the HF compensation capacitor below its crossover point that contributed to this very edgy sound. (Note however that the 500Hz dip is gone.)

Here is the "final" graph of the crossover network using a better selection of Mundorf Supreme capacitors (lower parts count) on the RD1.6 portion of this crossover network and shifting the compensation point up just a little (3-10-14):

BE2@450Hz and BW3@2,348Hz Mundorf Supreme Capacitors, higher HF compensation

The sound was initially VERY pleasing with midrange detailing I had never heard from this system. Especially noteworthy is the clarity of the planar tweeter yielded providing very low distortion and accurate reproduction of the timbre of individual instruments. However, the HF compensation point still needs more adjustment and that is where I am today. (Note that the 500Hz dip is still mostly gone and other bass peaks and dips are now present.)

I have experimented with many different component values for this HF compensation point, some with resonating effects causing buzzing and some with other unique colorations. But progress is coming and with each change I hear more and more. When the final level is chosen,I will post it in Part 11 of this series.

Before this phase of changes, I was satisfied with the sound of the system but I knew it had greater potential than the crossover network allowed it to deliver. Two changes made a big difference in overall clarity: first, moving to a third-order network on both the midrange and tweeter legs, and next moving to the Mundorf Supreme capacitors. HF compensation brought sparkle back into the top octave but this is at a price of phase distortion and the final crossover point for this "kick" is yet to be determined.

For now, I am thoroughly enjoying the sound of my system, especially its huge soundstage and definition. Much like removing layers of an onion, each time I settle on a change in design, my system reaches down into the truth of a recording. If a production is weak, I will hear it. Conversely if a recording is superior, it stands out and grabs your attention.

It's nice to dim the lights, put on a small jazz combo, and imagine who may be sitting at the table next to yours in that dingy dark basement. Presently it is raining very hard and the sound from the roof and windows reminds me of the old Leon Russell song "Manhattan Island Serenade" (Carney album). Leon sings of a van broken down on the highway with the sound effects of wet tires rolling by. I think I'll spin this and enjoy the weather.

Will there be a Part 11? Why not? Click this link (link to be updated soon) and see if anything else has percolated to the surface!

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

See the Thiele-Small specifications for the Bozak woofer here.

Yours for higher fidelity,
Philip Rastocny 

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