Bozak DMS 2500; Specs? Opinions?

[Retrovert]

Hold the phone guys. I am not sure anyone is in a position to critique these speakers considering they only are 1/2 functional. How about first simply replacing the tweeters and then assess things. You aren't into them for very much so acquiring some correct tweeters should still be doable.

Uh, we are discussing the accuracy of the low end. Nobody needs to hear the speaker to tell you that bass-reflex is not as clean as infinite baffle. Nobody needs to hear it to say that. That's just how bass-reflex works! The tweeter is irrelevant for the issue of boomy bass.

The OP doesn't like the boomy bass-reflex sound. That's why stuffing it improves the quality. Filling that cabinet with cotton batting instead of fiberglass will be even better.

Or lining the cabinet and making the port aperiodic, as the port change will help to reduce the cabinet resonance peak. Have you read Jordan's paper on that?

Otherwise I guess my opinion of these which is pretty high must be completely wrong. The one that has some that are functional.

Ummmm, didn't you like your Bozaks just fine with the original paper-cone midranges and tweeters? As I recall, I had to repeatedly argue with you to persuade you to upgrade those. I don't remember, so correct me on that if it's wrong.

It's a bass-reflex cabinet, not an infinite baffle. Ports cause group delay and resonance artifacts. Some can hear it, some can't, some just don't care. It doesn't matter what the tweeter does, the bass response is not as clean by definition.

 

[Drugolf]

What I am implying Retrovert is that there is no baseline to base (no pun intended) an opinion on yet with this particular situation. I do have an opinion because I have these speakers and they are not boomy. So my take is lets get them performing as they were intended first and then assess what if any modifications are worth it. If JamVal want's to bother tracking down new tweeters and make the investment of $ and time on these (might not be worth it) and then replace at least the cap, I think we can then see what he's got there. (and I was wondering when you would pick up on the fiberfill stuffing and suggest the cotton)

If others that have these speakers chime in and all agree "yes, they are bloomin' boomy" then I guess I would be the one that would need to listen to them a bit more carefully.

And no, I never argued about the mids and tweeters on my Grands. I never argue with folks about this stuff. I was simply asking questions and taking advantage of all the great input made available by you and biggles and others on the subject as I made my way through the process. I believe with mine since I already had the paper cone mids and X tweeters, I wanted to get the other aspects working correctly first (crossover and that damn cotton) so I too could experience the speakers and the poor drivers first hand so I could then eventually grasp the significance of the improvements once I replaced those older drivers. Was in no major hurry and good lord, with the grands we are talking multiples of each to track down and purchase. I am still not done - need a few more tweeters.

In fact with all that I have learned about Bozaks in working on the grands and reading the various threads in which you and others give input, one of my biggest take-aways is that Rudy never really got the tweeter side of things right. How many times have all of us just wanted to cut to the chase and use a more modern tweeter along with those great other Bozak drivers to finally take care of that last frequency range? Well, here we have what may be a version of that evolution with the DMS2500 that incorporates a tweeter that might just get us close to what we want. I have always likes the Peerless that are in my 2500's

BTW, I could do some measuring with my 2500's

 

[Retrovert]

What I am implying Retrovert is that there is no baseline to base (no pun intended) an opinion on yet with this particular situation. I do have an opinion because I have these speakers and they are not boomy. So my take is lets get them performing as they were intended first and then assess what if any modifications are worth it. If JamVal want's to bother tracking down new tweeters and make the investment of $ and time on these (might not be worth it) and then replace at least the cap, I think we can then see what he's got there. (and I was wondering when you would pick up on the fiberfill stuffing and suggest the cotton)

Ok. Let me explain this in a different way.

What you are proposing will waste the OP's time on a project doomed to fail, ab initio. My suggestion is to use the same cabinet and driver, the only change being eliminating the port, removing stuffing and adding lining. My suggestion to ditch the fiberglass or stuffing is not silly anti-fiberglass hatred, it's that fill serves a different purpose in a bass-reflex cabinet, and the infinite baffle must be lined to remove the backwave. I have rock wool in my AR-3 cabinets where it is required for proper functioning of acoustic suspension.

Your claim appears to be that an improved tweeter will improve the overall sound of a bass-reflex cabinet and nobody should comment on that cabinet until the tweeter has been replaced. I'm arguing that bass-reflex has reduced accuracy in the low end, among other issues, and that the tweeter isn't the source of these low-end problems. Bass-reflex is a compromise: it delivers far greater low-end response than would otherwise be possible in that cabinet size. It has a cost: boomy sound, resonance (one note bass), ringing, increased excursion at the lowest end (which can damage the speaker), etc. The OP has already demonstrated to his own satisfaction that stuffing the port removed such artifacts.

Beyond all of this, Rudy Bozak's drivers were not designed for bass-reflex. Just weren't. That means the ratio of Fs/Qes is all wrong, being too low. One can't just start stuffing drivers into cabinets for which the driver was not designed and expect great results.

That "Bozak" bass-reflex cabinet, furthermore, is, on its face, poorly designed. The port is front-facing (done so it can be placed on a bookshelf close to a wall) which increases port noise (chuffing) instead of diffusing it, and it is not chamfered. Most mass-market speakers use a plastic insert to get better transition. I doubt that port was simulated like the modern designs, and I also doubt it is optimally configured.

Rudy Bozak would have disowned that speaker. Bozak's ported designs like the B-201 and its ilk were a compromise to put the B-199 in a too-small cabinet in order to compete with the smaller bookshelf units that were eating Bozak's lunch.

I do own, BTW, transmission line cabinets which sound much better because the transient response is similar to sealed cabinet. But that's beyond the scope of this discussion.

In fact with all that I have learned about Bozaks in working on the grands and reading the various threads in which you and others give input, one of my biggest take-aways is that Rudy never really got the tweeter side of things right. How many times have all of us just wanted to cut to the chase and use a more modern tweeter along with those great other Bozak drivers to finally take care of that last frequency range? Well, here we have what may be a version of that evolution with the DMS2500 that incorporates a tweeter that might just get us close to what we want.

That's not really fair.

Rudy Bozak's tweeters are not merely excellent for their time, but exceptional; they were, and likely remain, about the best one could do with a rigid metal cone. None of the cone tweeters of the 1950s and 1960s were very good by modern standards, and all have problems at the high end. It wasn't until the advent of the soft-dome tweeter that higher frequency response was achievable in a single driver without using an electrostatic or AMT, dipoles and ribbons have a number of issues beyond the scope of this writeup, or the compression drivers in horns.

I have long suggested for purists the idea of using a band-pass filter with cone tweeter limited to the vicinity of 8k Hz to maintain the driver blend and balance, and then transition to a soft-dome tweeter for the higher air.

 

[Drugolf]

The only ting I am claiming is the OP has no idea what this speaker is actually supposed to sound like. That can not be debated.

The reason Rudy never got the tweeter right (at least to the level we all will be happy with it today) is simply because he ran out of time. Otherwise I am sure he would have. I agree that isn't necessarily fair, but we all run out of time on things eventually. For it's day and related capabilities in sound reproduction, no doubt what they had was okay. Although Rudy seemed to continue to refine what he had a fair amount along the way with them as all designers do.

 

[JamVal]

@Retrovert - Regarding the crossover. The value of the inductors has been bugging me, did I read them so far wrong? I'm a bit OCD, but have zero knowledge of crossover design and the related calculations. I found a calculator on line, but couldn't find one that looks like what I'm dealing with.

The Woofer branch on mine looks like a first order - just one inductor in series with the woofer.
I took a photo of this inductor ( had to use a mirror and then invert with photoshop):

What does it mean?

The tweeter branch looks like a second order - 4.7uF cap in series and an inductor in parallel, but also a 3 ohm resistor in series. Another smoke and mirrors photo:


I also wanted to double check the value of the tweeter cap: yes 4.7uF


I was hoping @Drugolf could tell me something about the crossover on his version. His date of manufacture over a year before mine with a different tweeter. Could a different crossover design account for his satisfaction with bass SQ versus my dissatisfaction?

 

[Retrovert]

The calculators won't explain what is going on.

I've keypunched the formulas. Use them. Work the math by hand and you'll get a much better feel than using black-box calculators.

The full-range needs an inductor to implement a low-pass filter: only low frequencies are passed, higher ones are attenuated at 6 deciBels / Octave.

The tweeter needs a capacitor to implement as a high-pass filter: only high frequencies are passed, lower ones are attenuated at 6 deciBels / Octave. An inductor after the tweeter steepens the slope to 12 dB / Octave, removing lower frequencies near the Fs for the tweeter.

Given:

R is Nominal Speaker Impedance (Ω)
C is Capacitance (Farad)
f is Frequency (Hertz)​

Inductor Formulas:

L = R / (2π x f) (Henry)
f = R / (2π x L) (Hertz)
R = 2π x L x f (Ω)​

Capacitor Formulas:

C = 1 / (2π x f x R) (Farad)
f = 1 / (2π x C x R) (Hertz)
R = 1 / (2π x C x f) (Ω)​

L is inductance in Henry, so to convert from Henry to milliHenry multiply by 1,000, to convert from milliHenry to Henry divide by 1,000.
C is capacitance in Farad, so to convert from Farad to microFarad (uF) multiply by 1,000,000, and to convert from uF to Farad divide by 1,000,000 or multiply by 0.000001.

The capacitor value is clearly specified:

C = 4.7 uF = 4,233 Hz @ 8 Ω​

But that is for a nominal 8 Ω speaker. The specifications for the Audax show 5.8 Ω, with an Fs = 1,090 Hz.

Ok, let's recompute the capacitor filter to be running at 8.8 Ω (5.8 Ω nominal + 3 Ω resistor):

C = 4.7 uF = 3,849 Hz @ 8.8 Ω​

At 11 Ω that's 3,790 Hz.

None of these points are correct, as per the calculations below set forth, but hold that thought.

The inductor values are:

L8 = 0.63 milliHenry = 2,021 Hz
L1-L2 = 0.509 milliHenry = 2,501 Hz​

I think the "2" on the second inductor is part of the label, i.e. "L1-L2". 2.509 would be a huge inductor, crossing at 507 Hz. So that clearly makes no sense.

L1-L2 is used on the midrange-woofer and L8 is a second-order for the tweeter to remove any lower-frequencies near Fs, as this will cause distortion in the tweeter.

So if the midrange/woofer is crossed at 2,501, the tweeter picks up far too high at 3,849 Hz!

In order to have the tweeter crossover work at 2,500 Hz the value must be:

R = 1 / (2π x C x f) (Ω)
= 1 / (2π x (4.7 * .000001) x 2500)
= 14 Ω​

so something isn't working. Just isn't working.

The math cannot lie.

That's not to say I didn't make a mistake in the calculations. Do check them. I show the formulas.

 

[Retrovert]

I was hoping [USER=191105]@Drugolf could tell me something about the crossover on his version. His date of manufacture over a year before mine with a different tweeter. Could a different crossover design account for his satisfaction with bass SQ versus my dissatisfaction?[/USER]

Not so much. The tweeter has ZERO effect on sound quality of the bass/midrange driver, as long as it is not shorting something out.

People have different opinions on the sound of bass from infinite baffle vs. bass reflex vs. acoustic suspension vs. transmission line vs. etc. This accounts for Drugolf finding the bass to be perfectly fine while you find it boomy and resonant.

I often hear one-note bass coming from cars with bassheads nodding up and down in gleeful bliss as if they have found the One True Sound. Whatever. That just makes my teeth rattle and my heart go into fibrillation, and I'm three lanes away.

I don't like bass reflex, but I do like the longer transmission lines and aperiodic ports. It's all about how the reflected backwave interacts with the cone, and whether or not a resonance peak exists for the cabinet inside the normal listening region.

Edit: fixed typo pointed out to me.

 

[Retrovert]

The only ting I am claiming is the OP has no idea what this speaker is actually supposed to sound like. That can not be debated.

The tweeter won't affect boomy resonant bass form a bass reflex.

The reason Rudy never got the tweeter right (at least to the level we all will be happy with it today) is simply because he ran out of time. Otherwise I am sure he would have.

He did solve the problem as best it could be solved.

The Z tweeter was a curvilinear improvement on the Y tweeter, but it was evolutionary, not revolutionary. Rudy had improved the cone driver. Yet it, too, had ringing issues which is why it was externally damped with a latex coating and internally damped with a latex plug which, over time, degenerates. So he did solve the problem, but his management team was inadequate and the company couldn't make the tweeters with good quality control. (This comes from a former employee who watched it all melt down.)

This is why we avoid the Z tweeters.

 

[Retrovert]

There's another possibility. Just occurred to me.

I tracked down the actual Audax specs from the datasheet, not the lies from the replacement driver. It says 6.8 Ω @ 4,000 Hz.

Now we need a leap of faith.

Suppose L1-L2 is TWO inductors: a full-value of 0.509 milliHenry and a half value at 0.2545 milliHenry using a middle tap. Many Bozaks had tapped inductors.

That creates a full-range crossover point at 5,005 Hz.

If the tweeter is 6.8 Ω using a 4.7 uF capacitor yields 4,982 Hz.

Which matches!

But, again, only if that tap exists and only if the 3 Ω resistor is not used.

So, scratch all of this.

But it sort of works.

 

[JamVal]

@Retrovert - Thanks for all the advice and knowledge.

I have the same opinion on ported / bass reflex speakers. In general I don't like them, in particular the older/vintage versions.
For example I had a set of Marantz Imperial 7G. I heard good things about the Imperial series 6's, 9's. But Imperial 7 was the dog of that series. Lucky for me I found them at the curb in front of my neighbor's house.

So back to these Bozaks.
The conclusions I'm drawing from the wisdom collected here:
1. Obviously they need a set of tweeters.

2. Crossover. Even keeping the same values, the 4.7uF cap needs to be replaced, and the ferrite core inductor for the woofer should go. But if this inductor is a center tap, does that explain why it has a ferrite core. The tweeter inductor core is air. If I'm going to work on the crossover, maybe I should rethink the whole thing. I'm out of my league here.

3. I don't like the boomy bass, and I agree that the tweeter selection won't change that. So I should close off the port, dampen the box with cotton. There's no bracing in these, do you think that would help?

4. I know this is not the forum to discuss value, but I want to keep that in mind, in case I don't like the results.

My logic says to decide on a tweeter. Going with the Audax replacements or the Peerless would put me at or over my perceived value breakeven point. And these two are the only ones that look like they would be a drop in replacement for the tweeter baffle opening. Anything else I've seen would need some custom mounting plate to cover the oval opening.

Or maybe mount a replacement tweeter in the port hole and leave the disconnected Audax tweeters where they are. That would solve plugging the port.
Something like this:


Tweeter Recommendations?

 

[Retrovert]

@Retrovert - Thanks for all the advice and knowledge.

You're welcome.

I have the same opinion on ported / bass reflex speakers. In general I don't like them, in particular the older/vintage versions.

Me, neither.

You might want to see if anyone near you has a pair of the mid-sized Bozak B-302 three-ways. I think you'll find it has the cleanest bass and clearest midrange you've ever heard. They are, however, a bit on the larger size if one is used to bookshelfs.

2. Crossover. Even keeping the same values, the 4.7uF cap needs to be replaced, and the ferrite core inductor for the woofer should go. But if this inductor is a center tap, does that explain why it has a ferrite core. The tweeter inductor core is air. If I'm going to work on the crossover, maybe I should rethink the whole thing. I'm out of my league here.

The inductance of an air-core inductor basically is a function of the number of turns. Adding a core increases inductance without requiring additional wire. It is a cost-saving change.

So the ferrite slug merely increase inductance without adding wire; it has nothing to do with whether or not the inductor has any taps.

I'd check out where the B-801 was crossed. That was a B-800 in a box with two B-200 tweeters. The box was about the size of a traditional bookshelf.

3. I don't like the boomy bass, and I agree that the tweeter selection won't change that. So I should close off the port, dampen the box with cotton. There's no bracing in these, do you think that would help?

A few issues.

Cabinet ringing can be damped by stiffening the walls, which is why bracing is normally added. Those cabinets, however, are MDF or something similar, which means it is difficult to screw into. You'd likely need to epoxy the cross-members. Or add some additional mass to the inside walls.

Before going to all this trouble, try the simpler approach of closing the port and completely lining the interior with thick batting. That should kill the backwave and help with the ringing.

Try it and see.

4. I know this is not the forum to discuss value, but I want to keep that in mind, in case I don't like the results.

If the experiment fails, and you decide you have no interest in using the B-800s in another cabinet, you can trash the cabinet and sell the drivers. Transfer price is generally about $100.

My logic says to decide on a tweeter. Going with the Audax replacements or the Peerless would put me at or over my perceived value breakeven point. And these two are the only ones that look like they would be a drop in replacement for the tweeter baffle opening. Anything else I've seen would need some custom mounting plate to cover the oval opening.

What's your budget on this? Any soft-dome tweeter should be fine if you're crossing high enough. Going down in frequency usually is the problem.

Parts Express has many inexpensive soft-dome tweeters. Check the reviews. If you find the tweeter isn't to your liking, send it back and try another. No harm done.

If you like the sound you can always have a new cabinet cut out of 3/4" birch plywood and build the speaker you like.

Or maybe mount a replacement tweeter in the port hole and leave the disconnected Audax tweeters where they are. That would solve plugging the port.

Yeeeeah. I wouldn't do that. You want the drivers to be as co-located as possible for best convergence. This is why a coaxial mounts the tweeter in front. It's all about making a point source.

Just epoxy a piece of plywood over the port on the interior. Nothing fancy. Make sure it's a good seal. If you have access to a router you could cut a circular plug and install that. Always better to have a smooth front on the baffle. Someone on AK might even cut you plugs if you pay for mailing.

 

[twiiii]

DM series went totally against everything Rudy Bozak developed earlier. None of his systems were ported, the bass was produced by 12 inch B-199 woofers in infinite baffles. The b-800 BC was a full range used as a mid range speakers, excepting in a commercial speakers tilted toward disco use or in large directional column speakers. Bozak never used a domed tweeter. Near Audio designed and built these speakers.

 

[Retrovert]

DM series went totally against everything Rudy Bozak developed earlier. None of his systems were ported, the bass was produced by 12 inch B-199 woofers in infinite baffles. The b-800 BC was a full range used as a mid range speakers, excepting in a commercial speakers tilted toward disco use or in large directional column speakers. Bozak never used a domed tweeter. Near Audio designed and built these speakers.

In 1960 Bozak offered the Sonata No. 1, numbered either B-801 or B-2000, which was a full-range B-800 with a pair of B-200X tweeters (mounted vertical or horizontal) in the same cabinet as a three-way with a B-199. (I have a pair in storage. TINY cabinet for the B-199, so I'm going to install the B-800.) So there was a full-range system in a sealed cabinet using the B-800.

The Sonora B-201 put a B-199 in a ported cabinet. This was prior to the company's breakup. It uses the B-450 midrange which is not very good and Rudy Bozak reportedly never liked it.

While Bozak when Rudy ran it never used a soft-dome tweeter, the B-200Y doesn't really go all that high and it starts getting spikey. Something needs to pick up the slack. I've suggested using a band-pass on the B-200Y to cut it at 8k to 9k and switch to some tweeter which will run from there. That way the midrange-tweeter merge isn't affected, as those drivers were designed to cleanly mesh.

 

[Retrovert]

In case this is useful, attached is the 1986 specification sheet for the Audax HD12X9D25 Tweeter.

Not my scan, so, as the kids say, it is what it is.

 

[Retrovert]

In another thread I posted the dimensions for the "Sonata No. 1" cabinet aka the B-2000 or B-801 and thought it might be of interest.

The cabinet has dimensions 23 H x 14 W x 12 D.

So that may provide you some idea of what Bozak considered appropriate for a B-800 to obtain a good bass response.

 

[JamVal]

In another thread I posted the dimensions for the "Sonata No. 1" cabinet aka the B-2000 or B-801 and thought it might be of interest.

The cabinet has dimensions 23 H x 14 W x 12 D.

So that may provide you some idea of what Bozak considered appropriate for a B-800 to obtain a good bass response.

These DMS2500 are 22 H x 10 1/2 W x 8 1/2 D. Maybe they thought the port would help compensate for less cabinet volume.

I'm not convinced I want to spend any money on these. Time on the other hand is something I have a lot of.
So today I decided to make use of those Pioneer paper cone tweeters. They measure 7 ohm on my DMM. So for now I'll leave the 3 ohm resistor in the xover network.
I'm also convinced that I won't like the bass regardless of what tweeter is used. I made some mounting plates for the Pioneer tweeters as the hole in the baffle is oval and too wide for the tweeter frame. I put a gasket between the mounting plate and the baffle, and another gasket between the tweeter and the mounting plate. Attempting to get a good seal. I did a little research on aperiodic enclosures and came across a Scanspeak Aperiodic Vent and Dynaudio Variovent. Without putting any science to a design I plugged the ports with some foam wrapped around 1 1/2" of A/C Pipe insulation and plugged the inside diameter of the pipe insulation with more of the same foam. Seems to be a happy compromise between the open and closed port. I think I may need to put a gasket under the woofer rings, as I feel some air around them while they're playing. They're not much for looks right now, just listening to them to see if they are worth a set of tweeters. Oh, and some caps.

The bass has tightened up considerably.

 

[drbiggles]

YES. Nice action there pally boy.

Biggles

 

[Retrovert]

Nice experimental work!

Using my phone so apologies for the terseness and less-obsessively edited content.

The two cabinets--yours and the ancient Sonata--are comparable in volume. The port was intended to deepen the bass to compete with speakers having a larger woofer. Miserably fails, of course, if one at all values sonic accuracy.

I expected the aperiodic to be an improvement. Here is a bigger one: remove the stuffing (fiberglass or polyester) and line the cabinet's interior with cotton batting to kill the backwave. That will create an infinite baffle like other Bozaks. Try it with and without an aperiodic port. Stuff the port with filter scrim or maybe shredded long-fiber wool.

I suggest a first order crossover with an inductor for the woofer and a capacitor for the tweeter. Not expensive.

If you would like some help on that PM me. It is a worthwhile project and won't cost much, under $30 for crossover parts.

 

[Retrovert]

Almost forgot.

Try sealing the junction between the tweeter and baffle. Any leak is detrimental as it permits the backwave to interfere. You may use any dense foam insulation for that purpose. Easier and less messy than silicone or aacrylic caulk, or Mortite.

Also, if your cone tweeters are not sealed on the rear you should add domes (look up the flower pot mod I describe for Bozak midranges) to prevent cross-modulation of the drivers. That will clean up the sound.

 

[JamVal]

Almost forgot.

Try sealing the junction between the tweeter and baffle. Any leak is detrimental as it permits the backwave to interfere. You may use any dense foam insulation for that purpose. Easier and less messy than silicone or aacrylic caulk, or Mortite.

Also, if your cone tweeters are not sealed on the rear you should add domes (look up the flower pot mod I describe for Bozak midranges) to prevent cross-modulation of the drivers. That will clean up the sound.

Yes, I did make gaskets from dense foam sheet, two sets, one cut to the size of the tweeter mounting plates, and another for the back of the tweeters themselves. The Pioneer Cone Tweeters that I'm using have a sealed back, they are the FB cone tweeters that were used in the Pioneer CS-77A, 88A, A500. I'm going to make a set of gaskets for the woofers also.

As far as the cotton batting? What thickness do you recommend. I have some that is either cotton or wool maybe 1/2" thick that came out of a similar sized set of Coral B-300 (21 1/4" H x 13" W x 11 1/2 D) that I got for free. Long story, they weren't salvageable. All the drivers appeared to have been bayoneted - the guy that gave them to me had them in his bunker while serving in Vietnam.