Questions about Bozak B-305s

[Adam White]

Hello! I picked up a pair of Bozak B-305s recently. These were built (out of solid tropical mahogany) in Subic Bay in the Philippines in November of 1960 (presumably by a service member stationed there). Somehow they made their way back to Jackson, Mississippi and they’re in great shape! I just recapped them last night with Solen 5uf caps and they already sound better. There are a few things about them that I think could improve:

1) I don’t think the internal insulation scheme is correct. From all I’ve read and seen, the insulation should be pretty simple… insulate the 6 sides of the box and hang a curtain (so that it’s not touching the sides or the bottom) behind the speakers. I’ve read descriptions of this, but this YouTube video seems to nail it (time stamp 8:19):

Anyway, mine look like this, which I don’t think is at all correct:

I don’t know if there’s supposed to be a divider between the 199s, but I don’t think so. There’s certainly not currently a curtain. So, that’s issue #1… easy enough to solve if the advice is… “yeah, do what that guy did in the YouTube video (which is my inclination).

2) I read very mixed things about the Tobin mods… what does seem helpful is to lower the crossover point from 800hz to 400hz. The duplication of frequency in that range between the 199s and the 209 is pretty noticeable. I have read every forum I can find and I can’t find a simple (preferably pictorial) explanation of which inductor tap I would need to switch. If anyone can break that down for me, I would appreciate it!

@drbiggles and @Retrovert, I have learned a lot from you both in other threads. Anything you could add to my situation would be appreciated!

Here are a few more shots:

 

[fjdds]

From what I've learned from others, hanging curtain of cotton batting, available from upholstery shops, in general. Not sure what that material is over the supports. Fiberglass?

Re: the Tobin mods. I'd live with them for awhile as is. You can always try the Tobin mods later, and determine if you prefer the sound or not. As always, what's best for your ears/room/equipment, will determine what's the best for you.

I'm sure those smarter than me will chime in.

 

[Retrovert]

Congratulations on that Bozak score! Beautiful cabinets!

Lining and Curtain

Your analysis of the brokenness, alas, is correct, and so is that of @fjdds.

Sorry to be the bearer of terrible news: the thick felt partitions are not a Bozak. That, alas, is something else, and the partitions likely do not work very well because the felt is so dense without loft. I see ripping out mistake in your future.

In summary: the Bozak is an infinite baffle which loses the backwave, and contains both a lining and hanging curtain, both made from tufflex or cotton batting. Many times integrators and hobbyists built speakers and did not install the cotton-batting lining/curtain and instead typically lined with fiberglass or felt, or stuffed with fiberglass, or both. This serious error, alas, is commonly seen. Stuffing with fill to delay the backwave will smother the drivers and ruin the sound because the B-199 is designed to operate without back-pressure.

If either the lining or curtain are missing the speakers will have poor low-end response as well as sonic aberrations.

You're on the correct track for ripping out the felt and building a thick lining and hanging curtain. I must do that with my B-305, as it still has the original kumsul. Bleh.

Crossover

TL/DR: the crossover cannot be Tobinized.

The B-305 you have is a 16 Ω system — it was created long prior to the development of the 16 Ω B-199 parallel-wired in the dual-woofer Symphony to yield 8 Ω — which is the core problem. You simply cannot Tobinize the existing crossover as Tobin's clever trick won't work.

Some history.

The Bozak crossovers were intended to work across the line by adding inductor taps. Remember, as inductance increases the frequency decreases, and as inductance decreases the frequency increases. Which is why woofers and midranges use large inductors and RF uses small ones.

The sole 16 Ω system — possibly aside from non-standard, full-custom units built by integrators — was the B-305. Everything else in the Bozak line was 8 Ω. This is key! The crossover had to work at 16 Ω and 8 Ω.

The Tobin modification, as I've elsewhere explained, leverages the fact that the inductors were dual use, i.e. both 8 Ω and 16 Ω, so using the incorrect tap (i.e. the 16 Ω tap with an 8 Ω system) lowers the crossover point by a factor of 2. This is why the 16 Ω B-305 crossover points cannot be downwards shifted: the inductor, by design, is used at its maximum value for 800 Hz in a 16 Ω system. But doubling the inductor can work for for the 8 Ω speakers, on both woofer and midrange.

The formulas, which I have elsewhere posted, are that f ~ R / L. The ratio between impedance and inductance must be maintained for the same crossover point.

So if R is 8 Ω, doubling L will halve the crossover point. But when R itself is doubled to 16 Ω, L cannot be doubled to halve the crossover point because the inductor already is at the maximum value, i.e. that for 800 Hz.

To run at 400 Hz you need new inductors for both the woofer and midrange. To avoid saturation I suggest air-core and lower gauge.

BTW: using the half tap will upwards move the woofer and midrange crossover point to ≈ 1,600 Hz which will sound terrible. As you discovered!

Midrange

Is your midrange aluminum cone or paper?

If paper, many users on AK have been suckered by unscrupulous sellers into purchasing 16 Ω midranges for their 8 Ω systems, so you might be able to find a pair of the aluminum cones in BarterTown. The 16 Ω variant is not useful for the 8 Ω systems and thus goes for a fraction of the cost of its 8 Ω sibling.

I also suggest reading about my flower-pot modification, which is a midrange bell to prevent cross-modulation between woofer and midrange.

Tweeters

Are your tweeters aluminum cone or paper?

Either way, you might be interested in my super-tweeter modification.

Dual Woofer

A dual woofer system in that cabinet may not be the best approach because the drivers may not converge in your space and the amount of bass may be overpowering. I was going to modify mine to be a single woofer system. Basically a B-302 in a giant cabinet.

Making such a change would permit you to drop to 8 Ω and simplify life. An 8 Ω midrange would be required for such a change.

Not sayin', just sayin'.

 

[Adam White]

Love all of this @Retrovert, thanks for the detailed response! I recapped the crossovers and some 2 inch cotton batting is being delivered on Friday… so I’m going to fix the insulation issue before I do anything else to them. One thing I noticed in the YouTube video that I referenced is that the guy didn’t insulate the motor board. Is it ok to not do that? That insulation is not cheap, so if I don’t have to I won’t. But your wisdom on this topic would be appreciated. I’m loving listening to them already. I’ll be happy with any improvement that proper insulation brings.

To answer your questions on the drivers. I think both 209s are aluminum, but one speaker has 200xs and the other 200ys. I honestly don’t have much of a problem with the mids and highs (I can’t hear crazy high anyway). If I can find two sets of 200ys at some point, I might switch them out. I’ll have to look for your super tweeter mod.. that sounds interesting as well.

Thanks,

Adam

Congratulations on that Bozak score! Beautiful cabinets!

Lining and Curtain

Your analysis of the brokenness, alas, is correct, and so is that of @fjdds.

Sorry to be the bearer of terrible news: the thick felt partitions are not a Bozak. That, alas, is something else, and the partitions likely do not work very well because the felt is so dense without loft. I see ripping out mistake in your future.

In summary: the Bozak is an infinite baffle which loses the backwave, and contains both a lining and hanging curtain, both made from tufflex or cotton batting. Many times integrators and hobbyists built speakers and did not install the cotton-batting lining/curtain and instead typically lined with fiberglass or felt, or stuffed with fiberglass, or both. This serious error, alas, is commonly seen. Stuffing with fill to delay the backwave will smother the drivers and ruin the sound because the B-199 is designed to operate without back-pressure.

If either the lining or curtain are missing the speakers will have poor low-end response as well as sonic aberrations.

You're on the correct track for ripping out the felt and building a thick lining and hanging curtain. I must do that with my B-305, as it still has the original kumsul. Bleh.

Crossover

TL/DR: the crossover cannot be Tobinized.

The B-305 you have is a 16 Ω system — it was created long prior to the development of the 16 Ω B-305 placed in parallel to yield 8 Ω — which is the core problem. You simply cannot Tobinize the existing crossover as Tobin's clever trick won't work.

Some history.

The Bozak crossovers were intended to work across the line by adding inductor taps. Remember, as inductance increases the frequency decreases, and as inductance decreases the frequency increases. Which is why woofers and midranges use large inductors and RF uses small ones.

The sole 16 Ω system — possibly aside from non-standard, full-custom units built by integrators — was the B-305. Everything else in the Bozak line was 8 Ω. This is key! The crossover had to work at 16 Ω and 8 Ω.

The Tobin modification, as I've elsewhere explained, leverages the fact that the inductors were dual use, i.e. both 8 Ω and 16 Ω, so using the incorrect tap (i.e. the 16 Ω tap with an 8 Ω system) lowers the crossover point by a factor of 2. This is why the 16 Ω B-305 crossover points cannot be downwards shifted: the inductor, by design, is used at its maximum value for 800 Hz in a 16 Ω system. But doubling the inductor can work for for the 8 Ω speakers, on both woofer and midrange.

The formulas, which I have elsewhere posted, are that f ~ R / L. So if R is 8 Ω, doubling L will halve the crossover point. But when R is 16 Ω, L cannot be doubled because it already is at the maximum.

To run at 400 Hz you need new inductors for both the woofer and midrange. To avoid saturation I suggest air-core and lower gauge.

BTW: using the half tap will upwards move the woofer and midrange crossover point to ≈ 1,600 Hz which will sound terrible. As you discovered!

Midrange

Is your midrange aluminum cone or paper?

If paper, many users on AK have been suckered by unscrupulous sellers into purchasing 16 Ω midranges for their 8 Ω systems, so you might be able to find a pair of the aluminum cones in BarterTown. The 16 Ω variant is not useful for the 8 Ω systems and thus goes for a fraction of the cost of its 8 Ω sibling.

I also suggest reading about my flower-pot modification, which is a midrange bell to prevent cross-modulation between woofer and midrange.

Tweeters

Are your tweeters aluminum cone or paper?

Either way, you might be interested in my super-tweeter modification.

Dual Woofer

A dual woofer system in that cabinet may not be the best approach because the drivers may not converge in your space and the amount of bass may be overpowering. I was going to modify mine to be a single woofer system. Basically a B-302 in a giant cabinet.

Making such a change would permit you to drop to 8 Ω and simplify life. An 8 Ω midrange would be required for such a change.

Not sayin', just sayin'.

 

[Retrovert]

Until now I had not watched the video, which I briefly spot-checked.

I am both confused and concerned by your comment about "insulation". In the context of the video I conclude you are using UltraTouch which is for insulation purposes. No no no no.

Do NOT use UltraTouch! It is NOT cotton batting. It is dense and has no loft. Biggles ruined his Bozaks with that and ripped it all out. This has been extensively debunked.

I suggest avoiding all "insulation" products, especially the "recycled fabric" type. Only use cotton batting for upholstery purposes.

 

[Adam White]

@Retrovert… sorry, inappropriate use of the English language there. I didn’t mean insulation. I am using this stuff (or will be in a few days): https://www.atsacoustics.com/recycled-cotton-batts.html

I noticed the guy in the video didn’t cover the motor board and I wondered your thought on that?

 

[Retrovert]

Wrong material.

Not fluffy.

That looks to be an UltraTouch clone, filled with fail.

 

[drbiggles]

Bozaks require proper batting, period. They're picky, and it's one of the many pitfalls that can cause their performance to flatten. How do we know? Read a Bozak post where a poster says, Bozaks aren't my cup of tea. Or, Bozak? I don't get it. If your hairs don't all stand at attention, and you end up weeing your pants, you did it wrong.

Biggles

 

[Adam White]

So I’m returning this stuff: https://www.atsacoustics.com/recycled-cotton-batts.html

And looking at this stuff: https://fabricwarehouse.com/cotton-...-zT-iZQhC0BmECGt4rpDJ_bJKKdDmiKQaAqnyEALw_wcB

Any thoughts? My only concern is it hanging properly and not tearing under its own weight.

Bozaks require proper batting, period. They're picky, and it's one of the many pitfalls that can cause their performance to flatten. How do we know? Read a Bozak post where a poster says, Bozaks aren't my cup of tea. Or, Bozak? I don't get it. If your hairs don't all stand at attention, and you end up weeing your pants, you did it wrong.

Biggles

 

[drbiggles]

Cotton upholstery batting! That's it! Use 3M 77 spray adhesive, and or little squares of cardboard then a staple or two to hold in place. That's for sides top bottoms. Line back of curtain with scrim sprayed with 3M 77 to hold it. You'll figure it out, you see what we're saying.

Biggles

 

[Adam White]

Awesome, I’m glad I’m on the right track! When you say scrim… is that just a thin piece of fabric to hold it together? One more question and I’ll leave you guys alone. Should the motor board (the baffle the speakers are mounted to) have batting applied to it? Thanks for all of the help!!!

Cotton upholstery batting! That's it! Use 3M 77 spray adhesive, and or little squares of cardboard then a staple or two to hold in place. That's for sides top bottoms. Line back of curtain with scrim sprayed with 3M 77 to hold it. You'll figure it out, you see what we're saying.

Biggles

 

[drbiggles]

Yeah, on the scrim. Use it sandwiched in between two pieces. No on the motor board, not necessary. You're close!

Biggles

 

[rhodus]

I just acquired these. Need all the help I can get