Bozak B-4000 Questions

[Retrovert]

Anyone have thoughts on incorporating an external tweeter, (in my case, a Morel), to the inside of the Symphony without major surgery to the cabinet? Perhaps replace one of the existing y tweeters with a soft dome, wired/ and band passed as it is now?

Disclaimer: it's late and I may have butchered the math, so I'm showing all the work.

Bozak Cult Rule 5: Loudspeakers are competing forms of millenarism seeking to immanentize the audio eschaton, so experimentation upon Bozaks is not merely encouraged, but respected, even when it fails, as true knowledge must be painfully acquired.

If I correctly recall, the tweeters are something like series wired to yield 32 Ω pairs, then two pairs are parallel wired to yield 16 Ω, and two pairs of those four are parallel wired to yield 8 Ω:

(16 Ω + 16 Ω) || (16 Ω + 16 Ω) || (16 Ω + 16 Ω) || (16 Ω + 16 Ω)
= 32 Ω || 32 Ω || 32 Ω || 32 Ω
= 8 Ω​

It is not possible to simply remove a tweeter without rewiring the connections, because the Impedance will change from 8 Ω so the crossover will no longer be at the proper point:

(16 Ω + 16 Ω) || (16 Ω + 16 Ω) || (16 Ω + 16 Ω) || (16 Ω)
= 32 Ω || 32 Ω || 32 Ω || 16 Ω
= 4 Ω​

Padding that out will sound terrible.

So obvious (pass 1) solution is to drop down to two tweeters (16 Ω || 16 Ω = 8 Ω). Naah, that's silly, for we at Bozac Cult are clever (pronounced "crazy") and can do better.

Four tweeters can be made close enough:

(16 Ω + 16 Ω) || 16 Ω || 16 Ω
= 6.4 Ω​

which permits a 1.6 Ω padding resistor to yield 8 Ω, albeit with a slight loss in power (impact upon damping factor for a tweeter should not matter).

But let's see if we can do better so we don't get kicked out of Bozak Cult for not thinking it through.

Observe that five tweeters may be used:

(16 Ω + 16 Ω) || (16 Ω + 16 Ω) || (16 Ω)
= 32 Ω || 32 Ω || 16 Ω
= 8 Ω​

Which permits a super-tweeter to be added without an impedance change, but at the cost of losing three tweeters in total. Not to worry. Those B-200 tweeters needed an L-pad anyway, so now they'll need less attenuation.

Et voila! The crossover need not change, nor does the super-tweeter bandpass.

Different impedances could be for the super-tweeter with a re-designed band-pass for the super-tweeter — two 8 Ω tweeters in series = 16 Ω, so halve the capacitor and double the inductor — but the load line would shift in a tube amplifier. Distortion would result, but it's high-frequency where humans poorly hear so very likely not significant.

 

[fjdds]

Your so good at this!!! Every time you post, it's like a master class.

That makes perfect sense, as you said, the Ys have to be cooled off anyway.

Is the following a dumb idea:

-Unscrewing one of the Ys from the line array, (the easiest one to get to) but leaving it connected as is and somehow fixing it internally so it doesn't move around. Then, using that location to install the supertweeter using it's own, wiring, bandpass and l-pad, just moving the l-pad to the outside of the cabinet near the one for the Ys.

At least the microbes on the inside will have some nice treble.

 

[Retrovert]

I suggest not leaving the tweeter inside the cabinet because the pressure in the cabinet from the woofer will modulate (move) the tweeter's cone and ruin the driver.

I am looking into my crystal ball. I see... I see.. I see a pair of diagonal cutters in your future.

The general rule is that drivers must be within 1/4 wavelength of each other to avoid comb effects, so, yes, position the super-tweeter at the top.

The Symphony takes space to converge and the super-tweeter will be no different. But since frequency becomes more directional with shorter wavelengths, likely best to position the super-tweeter higher up, closer to the ears than the knees.

 

[fjdds]

I am looking into my crystal ball. I see... I see.. I see a pair of diagonal cutters in your future.

LOL.

But technically, barring ruining the driver, would that method work?
Thanks

 

[fjdds]

Or, leaving everything as is, and trying to figure out a way to cut another hole in the baffle.

 

[Retrovert]

But technically, barring ruining the driver, would that method work?

I don't exactly understand the question.

Are you asking about the mechanical issues?

The woofer's backwave contains energy which — prior to its dissipation in the batting and muffler region created by the curtain — is sufficient to move the tweeter's cone in and out, since the tweeter driver has a sealed cone and sealed back, so a pressure differential therefore exists from the front to the back. (I am ignoring the small air leaks which are necessary to prevent the tweeter cone from moving like a barometer to reflect changes in the internal/external air pressure.)

The backwave is how the woofer's mechanical energy is converted into tweeter cone movement. As the backwave dissipates the tweeter cone then returns to its rest (neutral) position, and radiates into the cabinet the energy which moved the cone, but at a different frequency, thereby causing distortion. Think of this as the back pressure moving a cylinder in and out.

It is no different than cone movement arising by using a magnetic field developed in the voice coil working in attraction/repulsion to a fixed magnetic field (either a permanent magnet or electromagnet) in a conventional speaker.

The flower-pot cover for the midrange prevents the midrange and woofer from cross-modulating, and thus reproducing sound at the wrong frequency, i.e. distortion.

Is distortion audible from leaving a small tweeter in the cabinet? Likely not. Measurable? Likely not. But definitely present? Yes.

 

[fjdds]

Understood.

I was just trying to figure if there was a way to get the supertweeter inside without having to re-wire everything, or cutting the baffle. I just don't like the look of the external tweeter.

No shortcut here, I guess. Short of putting a small padded flowerpot on the tweeter, LOL

Time to pull out the cutters.

Thanks for your time.

 

[Retrovert]

You're welcome.

 

[rider33]

when I first got my Symphonies I thought they sounded lovely tho a bit dull on top. So I added a simple, separate soft dome on top. That helped but ever sounded quite right to me, likely because I just inserted a cap to limit it to above 8k vs a proper 4 way crossover. Next up I replaced the crossovers which helped, a lot, what I should have done to begin with. Much better but still not the last word on articulation. That led me down a rabbit hole of tweeter replacement but three things kept me from doing that: 1) desire not to mess up the cabinet in any way (do no harm), 2) it seemed like it was going to get expensive, fast given the number of drivers involved, and 3) it might fix the high-high end but in the process harm the beautiful way the oh-so-nice midrange driver blends in with the lower treble. Since the very large room I have these speakers in also has a bunch of other equipment I came up with a simpler solution: in listening to something that seems to need more definition, I turn on the ginormous Paradigm BP Eclipses sitting next to them. They are on a separate amp and thus have separate level control and oddly, bring to the party what the Symphonies lack. 'Not a solution for everyone for sure but before attempting major surgery you might want to try hooking up that other set of speakers you have sitting in a closet somewhere and see what you think. It's a hobby after all, have fun with it as you never know...

 

[Rybeam]

Lol love these deep discussions on Bozaks. Just know I love my late '70s double cabinet sized, recapped as original Symphony's.

 

[fjdds]

If anyone is interested, I re-built the crossovers for my Symphonys. (non-Tobin, with b800 mids) . Same as my previous iteration, but a little more organized.

As per Retrovert, bandpass on the 200y tweeters, and a bandpass on an external supertweeter (Morel).

(Image of main XO was not yet completely connected.)

Not the prettiest build, but works VERY well. Used Dayton polys with .1 and .01 bypass caps on each leg of the crossover.
Will finish up the soldering and shrink wraps after I fully test.

Vader, our crazy kitten, was enjoying Eva Cassidy.