Tobin Mods to Bozak 302A Urban speakers - definitely worth it!

Thanks! I stated 10 dB knowing it was on the low side, just to be safe.
Pat Tobin

 

Historical notes: Before WWII, Rudy Bozak was speaker designer for Cinaudograph, the leading maker of very large theater speakers. At the Bozak plant Rudy had one of 30" diameter. After the war he put in time with Wurlitzer in the electronc organ division. R.T. Bozak was founded in either '49 or '50, in Buffalo, NY. Very soon they moved to Darien, CN, where the company was located for many years.

In '77 Rudy sold the company to a group that included a former Bozak employee, and the name was changed (briefly) to N.E.A.R. That was the end of Bozak as we know and honor it today. Many of the key people left, including the one artisan who made all the very distinctive and advanced woofer cones. Rudy was technically on staff, given an out-of-the-way office in a warehouse somewhere.

There was another incarnation or two. Then in '82, the company was re-born with family members and some original key people returning. Sadly, Rudy passed away in February '82. I would like to think that he was involved in guiding the rebirth. The products made during this period returned to original quality; I have samples from each of these eras mentioned. Some new products were tried, including a dual-voice coil sub woofer. This reincarnation seems to have lasted until about '86.

Pat Tobin

 

Pat - thanks for the history lesson!

Am I remembering incorrectly or did I also read somewhere (maybe Roger Russell's web site?) that Mr. Bozak was involved in at least some loose way with the McIntosh company's early speaker development efforts?

 

Tony, I think he was, but not much came of it. I have read, somewhere on the 'net, (probably under McIntosh) some much later Mc engineer really trashing the Bozak principles. How many know that at one point, Bozak made drivers and maybe enclosures too for Heathkit?

 

Pat, glad to see you're adding the Bozak history and knowledge for us. You're right, I have seen on ebay a couple times the Bozak components made for Heathkit. They were painted in the light green Heathkit color on the drivers. The label said something like "Bozak made for Heathkit". Talking about the latter incarnations of the Bozak company, I have a brochure from the Bozak Audio Labs featuring their line. I don't think they stayed in business for long. Thanks for sharing your knowledge!! Mike

 

OK, I just can't let things rest with these speakers...the word "obsession" does come to mind.

Latest big tweak -adding an L-pad to the woofer. I came to the conclusion after prolonged listening that even after having L-pad capability with the mids and tweets, in a 12x16 listening room, the bass response on these speakers is just too much...most people complain about lack of bass with their speakers, mine have it in spades - and in a small space, that can lead to a tendency to boom and impart a dark coloration to the sound. Probably more a function of the infinite baffle, cube design of the cabinets, and my small room, than the woofer driver itself.

Putting a final L-pad on the woof now allows attenuation to taste and room size. I'm now getting natural, full bass but without over-predominance, and without an overly dark and soft coloration to music in general. Great speakers just got even better. Truly done with monkeying around now - will not be messing with the upper crossover, leaving at 3300, they sound too good to change at this point.

Between this and getting my MC30 can capacitors and diode up to modern standards this weekend - the amps are now faster and have much improved dynamics - and it was a far easier job than I anticipated - my system has never sounded this good - I'm in heaven!

 

Hey Tony - Here are some comments on Bozak Bass. I know I will be telling a lot things you already know, but this is addressed to all Bozak enthusiasts.

First, we all know that the B-199A 12" woofers are designed for 'infinite baffle' enclosure. That is a major departure from conventional speakers. Thiele and Small pointed out that the woofer and enclosure should be considered together as a system (which can be analyzed using filter design equivalencies, the basis of the TS parameter system). We don't need to go into that here except to note that the great majority of LFR (low frequency radiator) systems use an active enclosure. The enclosure becomes resonant slightly below the natural LF cutoff of the woofer, thereby extending the lower bandwidth of the two as a system. If vented and ported enclosures are designed very carefully, they can be extremely effective.

Rudy Bozak, however, was a purist. He realized that any type of active enclosure degrades LF transient response. That is because of the very nature of a resonant system. He chose to design a woofer powerful enough to reach below 40 Hz with no help. Consequently, an enclosure for the Bozak woofer should be the opposite of active - it should be completely passive - the more 'dead' it is, the better. And the enclosed air volume must be at least 5 cu. ft. for one woofer, 8 cu. ft. for two and either 10 or 12 for four (the Concert Grand systems) - I don't recall which it is.

Inside, it is very important for there to be a free-hanging curtain (reaching side to side but supported only at the top) about midway from front to back. This curtain should be of moderate mass - not just some flimsy angel hair stuff. The purpose of the curtain is to suppress the 'cavity resonance' that occurs in any closed space when excited by LF sound waves. This is often overlooked in home-built enclosures, but its importance can't be over-stressed. In active enclosures, the cavity resonance is tuned to supplement the woofer, but with the Bozak we want no cavity resonance at all. Bozak generally used the same material with which the box was lined. More about that later.

Finally comes the part most often overlooked - panel flex. In a closed box with a very powerful woofer, some really strong air pulses (pressure and rarefaction) take place. Here's the way to look at this: in a single woofer system, the area of the front, back, top and sides may well be 20 times the area of the woofer cone. (Just a guess off the top of my pointed head, without picking up the calculator.) Now let's divide that by 2 to account for the fact that all the panels are, in terms of acoustical physics, "edge-clamped diaphragms" in which only the center portions are free to bend very much. In our rough example, that gives us a panel area (neglecting the bottom) which is free to vibrate, of 10 times the area of the woofer cone. See where we're going here? If the panels flex to 1/10th the displacement of the woofer cone, acoustic radiation from the cabinet panels EQUALS that of the woofer cone! That's why Bozak went to considerable lengths to use very dense wood for the panels, then brace them in all enclosures except the E-300 (the name for the enclosure used in all single-woofer 300 (two way) and 302A systems (three way). There just isn't the room for braces in the E-300 and the smaller panels reduce the severity of panel flex, but don't eliminate it. In the Symphonies, the front panel is braced by a couple of hard wood pieces running between the drivers, plus a front-to-back 2" x 4" brace. In the Concert Grands, there is both a 2 x 4 front-to-back brace and another side to side.

Actually I just snuck this in, because it doesn't directly affect Tony's problem of too much bass. The flexing of cabinet panels reduces bass in the room, because the panels a responding to pressure waves from the rear of the woofer cone, which are out of phase with the desired pressure waves from the front of the cone. But as long as we're talking enclosures, I wanted to do the whole story.

Tony, I don't know what is causing your excessive bass. But I strongly recommend against using any kind of pad (electrical attenuator) between the woofer and the amplifier. That's because it degrades damping factor, DF. Let's talk about that for a moment. Any speaker cone which vibrates also tends to overshoot a little when excited by a strong pulse of audio, and ring a little when audio is suddenly cut off (at the end of a wave). Fortunately, a cone and its motor (magnet system and voice coil) work both ways. They function as an electrical generator as well as a reproducer. When the cone does a little bouncing of its own, a current representing that movement is generated and is seen at the output of the amplifier. This is where the enormous benefit of DF takes place. The actual output impedance (often called the 'source' or 'looking back' impedance) of the amplifier is lower than the load impedance by the damping factor. That is, if the amplifier is designed to drive an 8 ohm load and has a DF of 20, the actual 'looking back' source impedance of the amplifier is 8/20, or 0.4 ohms. So any current generated by spurious vibrations of the woofer cone is shunted by the source impedance of the amplifier, in our example not 8 ohms but 0.4 ohms. This 'stiff' 'looking back' impedance very effectively damps (not dampens, that requires liquid) the tendency of the woofer cone to do it's own thing, over and above what it is told to do. BTW, DF is really effective only on the woofer which has considerable cone excursion. Not at all effective on the tweeters and midrange because of the minuscule cone excursion. Also, the Bozak tweets and midrange are very carefully and effectively mechanically damped, something that can't be done to such a high degree on the woofer.

The big furor about speaker cable size is mostly concerned with not adding resistance between the amplifier and the woofer cone. It's not the minuscule loss of driving current that is the big concern. Rather, the concern is because any resistance between the amplifier output and the woofer DEGRADES DAMPING FACTOR. The addition of a pad would do the same thing, only much worse than a fraction of an ohm of speaker cable.

Also, the DF of the Mac VT amplifiers was around 12 - very moderate.That was considered pretty good in the VT days, altho some amplifiers went as high as 20. A moderate DF like that, in an extremely stable amplifier like the Mac VTs, is ideal for driving complicated crossovers in speaker systems with nasty impedance peaks and dips. A nasty load will cause many amplifiers to misbehave. But in the case of Bozak - nothing is kinder for an amplifier to drive than a first-order (6 dB per octave) xover and smooth drivers without sharp imedance peaks. So it was a happy-enough marriage, altho Rudy Bozak considered a DF of around 12 a bare minimum. I used an MC-30 in the mono years with my Bozak systems and loved it. (I am now restoring my MC-240 to original performance for sale.) But modern SS amplifiers often have a DF approaching 100. That gets reduced by speaker cable resistance, but is still very good. As we all know, the first SS power amplifiers usually left a lot to be desired. But when really good ones began to appear in the late '60s, Rudy Bozak wholeheartedly endorsed them. That's when he began promoting bi-amping of Bozak systems.

In the very late days - after Rudy had sold the company and was no longer in charge, some woofer cones were coated. That added mass to the cone, which lowered the natural resonance and thereby provided much stronger lower bass in a closed box system. I worked on a pair of Bozak LS-400 for three weeks and almost went nuts until I realized that the woofer cones were heavier, which changes everything. In enclosures a little smaller than the E-300, it sounded like the woofers were ready to tear the box apart on loud, low bass. That, of course, was a substantial resonance well below the normal LF cutoff of a Bozak single woofer system. A heavier cone has two evil side effects: it reduces the overall level above the big bass peak, and it decimates transient response. So it's not something I recommend; just something to look out for. In those speaker systems I did add L-pads before the mids and tweets to bring them in line with the softer-speaking (except at very low freqs) woofer.

If I were faced with a condition of too much Bozak bass, I would start by making sure there is a suitable center curtain inside the enclosure. On all Bozak-made enclosures, it should be there. Then I would run a freq. sweep (from oscillator or CD) with a meter across the speaker line. I would want to be absolutely sure that there is no anomaly in the audio electronics system that is causing it. Don't laugh - it can and sometimes does happen.

If that one passes and the enclosure is OK per above, I would consider the room. You mentioned 'small.' Below about 100 Hz, the dimensions of the room can cancel and support bass radiation. Put on a steady LF tone and move around the room. You should hear peaks and nulls, but how severe are they? Important: a room of cubic dimensions is almost hopeless. That's because when a bass wave length is supported in one dimension, it is also supported in the other two, creating a horrendous peak, and corresponding nulls at corresponding bass frequencies. The best rooms for smooth bass response are those of three dimensions which are not terribly close to one another. Acoustical treatment on the walls does nothing to smooth bass response - the waves are so long that a couple of inches of absorptive material can't 'get hold of' a bass wave in order to absorb a goodly part of it. A lot of heavily absorbent furniture in the room will help, but that's about all that will help in an existing room. The best space for speaker systems is a broken space, where the boundaries are irregular and broken by large doorways etc. In extreme cases, such as small recording studio rooms, bass traps can be built, usually above the ceiling. But that's about all that can be done in a cubic room, or one in which two of the three dimensions are very close. The room's effect on LF response applies to 'normal' listening positions. When listening 'near field,' the room is not nearly as much in the picture. That is because the listener is hearing mostly the velocity component of the sound wave, and not nearly as much the pressure component.

I hope this has been helpful to all. Tony, I look forward to further reports.

Best wishes to all, Pat Tobin

 

Pat - thanks for your detailed analysis/thoughts.

While I appreciate the theory about detrimental effect of the L-pad attenuation on damping factor, I'm not hearing problems in actual listening. I'll listen carefully again but so far, it sounds fantastic to me.

If it helps, I'm not attenuating the woofer tremendously - just a touch. I still get great responsiveness, just a little less tendency to boom on recordings with very strong bass content. I am about 90% sure that the problem I'm having is with the room primarily, but I'm basically stuck with that as you mention.

Nonetheless, I will try adding the enclosure center curtain sometime soon (I recall you mentioned that in a prior e-mail to me way back when) to see how that works out and let you know impressions.

Best - Tony

 

OK Pat - I'm listening to the speakers a bit more this evening - I THINK I may be hearing what you're hearing with the attenuated woofs - takes down the volume but the transient response may be a little less stellar - reflecting negative impact on the damping factor. Not real obvious, but I think I hear it. They still sound fantastic by the way - but I think I get your concern better now.

So will try out the center curtain - do you have a recommended source for purchasing the carbon fiber batting material you once told me you like to use for this?

Tony

 

Updates since my last post in July:

1) After extended listening I returned the upper crossover point to the original 2500 Hz. Sounds more natural there - though the B209B mids can produce sounds way up into the treble range, that is not their strong suit. Lower crossover though remains lowered to 400 (from the stock 800) and that, to my ears, is a big improvement.

2) I also tried Pat's suggestion of going to the 1.1 mH inductor coil for the mids, rolling them off more sharply than the gentle stock 6 dB at 2500 Hz. This makes a big difference, removing the trace of over-brightness in the 2500+ range where the mids otherwise in effect double the tweets. Much more natural sounding transition from mids to tweets. Great tip, Pat!

3) After playing with the L-pad relative adjustments between drivers, I no longer have a bass boom problem and no longer have to attenuate the mids and woofs at all. Basically I am still new at this and didn't realize what a profound impact the TWEETS have on the sound of bass! They were too attenuated and it made everything sound like mud down there. Love not having to attenuate the woofs - transient response is much better with them full throttle, probably for reasons Pat indicated above.

4) Oh and I added medium thickness cotton rag throw rugs (obviously without any rubber or other type of backing, just plain cotton) as mid-cabinet curtains, again as Pat suggested, and it does subtly but audibly tighten up the bass. Clearly these enclosures were designed to have such curtains. That Rudy Bozak knew what he was doing.

I've had this current set up now for a couple weeks and haven't had the desire to tweak anything - they are sounding clean, clean, clean - so I think this may actually be the end!

 

Thanks for filling in the details, Pat! I am very, very pleased with this solution thus far (I used the 1.1 mH inductor on my home-brew crossovers).

One other comment - getting the Bozak Urbans (which have rather short, squat cabinets) up off the ground a bit has helped too. I want to make some nice, massive stands for them eventually, but just to experiment I have each of the speakers up on 4x4 wood blocks (two each under the front and back legs, raising the speakers up about 8 inches).

This brings the tweets up to appropriate height for an average height listening chair - getting the maximal output point of the tweets right about at ear level or slightly below and bringing the mids also a bit closer to ear level. Imaging is improved as is overall tonal balance.

Interestingly, this also eliminated the slight residual bass boominess that I was mentioning. Perhaps some of that was related to the woofer sitting so close to the floor and also (maybe?) there was some resonance being transmitted through the bottom of the cabinets and then being amplified/richoceted around in the 3-4" space (with the stock little short legs on the cabinets) between the bottom of the cabinets and the floor??? I don't know, all I know is that bass sounds more natural now too with the cabinet elevation.

Again, I'm sure it'll be even better with appropriately designed, massive stands to raise them up to the same height. Probably will remove the stock legs altogether.

Pat, if you have any suggestions for optimal stand material/construction, I'm all ears! I'm thinking about plain old cinder blocks (painted black to be less obtrusively ugly) with a sheet of BluTack between the speaker cabinet and top cinder block. The stands will be sitting on a Pergo floor layed over a concrete slab foundation - so this should make for a very solid coupling of speaker to foundation, I would think, for very few $$$.

Tony

 

So... Mr. Tobin,
Is there any chance of the B-305s turning into a singing pair of stereo swan's or is it going to continue to be a ugly mono duck?
I rescued the (matched) pair I mentioned earlier from Denton, Tx and they look great (just a touch of water damage on the grills). I would hate to have to part them out but they do have all the right pieces.
pc

 

Guys, fairly new to this forum but have been reading this incredible thread. Some of the detail and sharing of info is incredible. I am in the fortunate position of inheriting a pair of 302a moorish style bozaks. I don't know if there should be a "b" prefix just before the "302a"? Does much of the discussion re: the urbans also apply to these models. Were these just external cosmetic changes or were there internal differences as well? Thanks in advance. And again, I can't tell you how wonderful the info and help is on this site and thread. Norm

 

Not wanting to take all the "FUN" from your
free deal, I would suggest the web as having
all of your answers. Your Moorish set is IMO
a more (and likely the most) collectable style.
Have fun, there is a lot of info out there.
pc

 

Thanks for responding. I have been googling without much benefit. This thread has actually been the most helpful. You said that you thought the moorish style is more collectable. Do you know if it differs from the urbans in any way other than exterior styling? Thanks Norm

 

Pat redid my B-410 CG;s and they sound phenomenal...really added a sparkle plus the ability to taylor the highs with the removable bridge cable..

 

Just another useful tidbit of Bozak info that I recently learned - again per Pat Tobin - here are some key factory parameters for the Bozak B-199A woofer that Rudy Bozak shared with him:

Fas (free air resonance) 39 Hz

Qms 3.13

Qes 0.776

Qts 0.622

Vas 6.37 cubic feet

As DIY speaker enclosure designers will know these numbers are very useful in calculating box volume and, if you decided to experiment and go ported, port size. These numbers are HARD to come by - Bozak never published them in any literature I've ever been able to find. Pat's sharing them is GREATLY appreciated!

BUT - in considering the idea of ported enclosures with these drivers, it's worth STRONGLY emphasizing that according to Pat, Rudy Bozak was NOT a fan of ported enclosures at all and expressly designed the Bozak woofer to give plenty bass without the help of the now more standard bass reflex design.

As Pat summarized:

"In the days when all other large speakers were 'more efficient' than Bozaks, Rudy pointed out that, at 40 Hz, the Bozak woofer was far more efficient than anything other extant. All the other woofers pooped out down low and depended on the support of an active enclosure. Rudy eschewed active enclosures because all of them degrade transient response. So he designed a woofer that was very powerful 'way down low, for use in a passive enclosure that gives the woofer no help."

Certainly my own experience with this woof in its original sealed Bozak Urban enclosure would support Bozak's conviction and Pat's comments - mine give very nice, realistic, finely detailed bass (you can hear the growl of upright bass) with phenomenal transient response - all without a port.

 

Normo, only styling differences. The Moorish style was offered in the 302, Symphonies, and Concert Grands. I got lucky and found a pair of the Symphony Moorish low boys.

 

Normo, I forgot one more the B305 also. Sorry

 

Well, my first post should be a Thank You to all, especially Tony for this fabulous thread, and Pat, for sharing his vast knowledge so freely. I brought home my first pair of Bozaks last Wednesday, a well preserved pair of 1963 302A Urbans. I had been looking for anything Bozak after Rybeam (from previous post) had brought a pair out to my house as part of small audionut get together. We had them hooked up to a Kenny 9600 that another guy brought, and to say the least I was quite impressed. When I got mine home, I wasn't expecting them to sound very good with the original caps, so I just grabbed the handiest amp I had on the shelf (a little KLH model 52) and a turntable to test them out. At first the highs were very grainy as expected, but after a few LPs they smoothed out enough to tell me I was going to like these. I spent the next couple days searching the web for info, and finally found this thread. At this point I am just planning on the basic mods as detailed in Tony's schematic, and changing the tap on the mid inductor as described by Pat. Luckily mine allready have the AL cone mids and tweeters. Thanks again for all the sharing of info and experiment results, Jay

 

Just thought I would post an update. I finished my crossovers using the Dayton Precission caps and Mills resistors. I am really impressed with these to say the least. I've been using a PP EL84 intergrated amp, and Shure V15III cartridge for power. My brother stopped by yesterday and I put on a Chet Atkins and Doc Watson album, and I really don't think it had ever sounded better to my ears. My feeling is that I was hearing just what was on the vinyl. No more and no less. The imaging and transient response capabilities of the Bozaks really shined with the acoustic flat picking style of Doc, and the ocasional electric guitar of Chet. It was one of those special moments, where you could close your eyes, and swear the preformers were in the room with you. These old Bozaks have definately found a home. Thanks again for the info, Jay

 

All,
I was pointed towards this most excellent thread ! I have been hunting around for a set of Bozaks for the last couple of months (to no avail as of yet...They all seem to be on the opposite side of the country !). When I am finally graced by the magnet gods - I will be more than preped to dive right into 'em !

Thanks again,
Steve

 

I am new to this hence why I am reviving this thread. I understand the schematic but what are the resistor wattage values needed for this mod?

Thanks, Earl

 

I have recently rebuilt my Bozaks from the 1960's, which originally had 1 B199a woofer each to have two woofers. At the time I could only find B207a and B207b's, can anyone tell me what the difference is between the three types? Can they be used interchangeably?
- Thom