Bozal 302a: build crossover to accommodate 16 ohm mid-range?

[shadetreehif]

Calling all impedance whisperers,

I recently inherited a single bozak 302a with aluminum cone tweets and paper mid. I would like to do the Tobin crossover mods, which assume an aluminum mid. So I ordered a b209c from an auction site. Turns out, the mid I received is 16 ohm. My question is: is it possible that I could build a new crossover to accommodate the 16 ohm driver while retaining the soul of the original speaker, with Tobin crossover points? Thing is, I haven't been able to source an 8ohm example, and reconing is a bit daunting.

Thanks,

Shadetree

 

[Retrovert]

Sorry to be the bearer of bad news.

You cannot use that midrange in an 8 Ω system. While you could change the band-pass components to accommodate, your amplifier will see a varying load. (The crossover does not instantly switch from one driver to another, and the gradual slope will cause increases and decreases in impedance.) If the amplifier is a tube amplifier (output transformers) this will cause reflected impedance issues, shifting the load line and causing distortion.

Was it advertised as 16 Ω? Many sellers are, ahem, less than honest, and don't reveal the non-standard impedance. If so, you can grieve it with the auction site. Otherwise, the seller may accept returns if you pay postage both ways.

The 16 Ω midrange was only used on the 16 Ω B-305 (single midrange) and the 8 Ω Concert Grand (dual midrange wired in parallel to yield 8 Ω) , so it has very little utility outside of those units.

 

[shadetreehif]

Sorry to be the bearer of bad news.

You cannot use that midrange in an 8 Ω system. While you could change the band-pass components to accommodate, your amplifier will see a varying load. (The crossover does not instantly switch from one driver to another, and the gradual slope will cause increases and decreases in impedance.) If the amplifier is a tube amplifier (output transformers) this will cause reflected impedance issues, shifting the load line and causing distortion.

Was it advertised as 16 Ω? Many sellers are, ahem, less than honest, and don't reveal the non-standard impedance. If so, you can grieve it with the auction site. Otherwise, the seller may accept returns if you pay postage both ways.

The 16 Ω midrange was only used on the 16 Ω B-305 (single midrange) and the 8 Ω Concert Grand (dual midrange wired in parallel to yield 8 Ω) , so it has very little utility outside of those units.

After reading a bit about bozak crossover design, it sounds like the networks were made to accommodate system upgrades, i.e. doubling up of drivers, drivers of alternate impedance, etc, in that the inductors support multiple taps. So if I were to tap into the midrange inductor at the full value tap, I am thinking that would present a 16 ohm load. On the crossover itself the note reads n102 for 16 ohm. So if that's correct, then I would be ok, after moving the jumper, for preserving the crossover values with the 16 ohm mid-range (like that used in the 305}, but I would forfeit Tobin's mod for circumventing the Tweeter overlap with the mid-range and would instead need to cross the Tweeter in at 3500hz. I'm a humanities person with a smartphone in one hand and soldering iron in the other, not a student of physics, so I welcome all commentary. Side note, the irony is not lost on me that the title of this thread mistakenly refers to a muffler rather than a speaker.

 

[Retrovert]

After reading a bit about bozak crossover design, it sounds like the networks were made to accommodate system upgrades, i.e. doubling up of drivers, drivers of alternate impedance, etc, in that the inductors support multiple taps. So if I were to tap into the midrange inductor at the full value tap, I am thinking that would present a 16 ohm load. On the crossover itself the note reads n102 for 16 ohm. So if that's correct, then I would be ok, after moving the jumper, for preserving the crossover values with the 16 ohm mid-range (like that used in the 305}, but I would forfeit Tobin's mod for circumventing the Tweeter overlap with the mid-range and would instead need to cross the Tweeter in at 3500hz. I'm a humanities person with a smartphone in one hand and soldering iron in the other, not a student of physics, so I welcome all commentary. Side note, the irony is not lost on me that the title of this thread mistakenly refers to a muffler rather than a speaker.

No, it doesn't work that way.

The Bozak crossovers were intended to work with a common system impedance for the entire speaker. Either 8 Ω or 16 Ω not a mix. You cannot move jumpers like that. This is 100% guaranteed to not work as you expect.

The only 16 Ω system was the B-305. Everything else was 8 Ω. The Tobin modification, as I've elsewhere explained, uses the fact that the inductors were dual use, and using the incorrect tap (i.e. 16 Ω tap with an 8 Ω system) drops the crossover point by a factor of 2. This is why one can't shift the 16 Ω B-305 crossover points but can do it for the 8 Ω speakers. This drops the woofers from 800 Hz to 400 Hz, and the midrange from 800 Hz to 400 Hz.

I don't know where you're getting the 3,500 Hz number instead of 2,500 Hz.

 

[shadetreehif]

No, it doesn't work that way.

The Bozak crossovers were intended to work with a common system impedance for the entire speaker. Either 8 Ω or 16 Ω not a mix. You cannot move jumpers like that. This is 100% guaranteed to not work as you expect.

The only 16 Ω system was the B-305. Everything else was 8 Ω. The Tobin modification, as I've elsewhere explained, uses the fact that the inductors were dual use, and using the incorrect tap (i.e. 16 Ω tap with an 8 Ω system) drops the crossover point by a factor of 2. This is why one can't shift the 16 Ω B-305 crossover points but can do it for the 8 Ω speakers. This drops the woofers from 800 Hz to 400 Hz, and the midrange from 800 Hz to 400 Hz.

I don't know where you're getting the 3,500 Hz number instead of 2,500 Hz.

Hmm. I was afraid you might say that. So it looks like I can either do a partial mod with the taps to shift handoff points with my existing paper mid, or build a cabinet and add another 199a combo. I reckon I'll try the former, then the latter if it sounds like it's needed. Thanks for the guidance.

 

[Retrovert]

Hmm. I was afraid you might say that. So it looks like I can either do a partial mod with the taps to shift handoff points with my existing paper mid, or build a cabinet and add another 199a combo. I reckon I'll try the former, then the latter if it sounds like it's needed. Thanks for the guidance.

Yeah, I suggest you not build a 16 Ω system. That is a major undertaking to fix the wrong midrange choice. Expense would be even greater. Those dual-woofer cabinets won't converge in a small space. I suggest you read Biggle's Symphony experiment. (That is dual 16 Ω woofers wired in parallel.) Never converged in his living room so he sold off all the parts.

The paper-cone B-209 is not a terrible driver, it just isn't anywhere near as good as the aluminum-cone version. If you have paper-cone midranges are you sure you have aluminum cone tweeters? The shiny dustcaps are the B-200X with paper cones.

Why are you writing "partial mod". What part of the modification are you not doing?

Don't forget to use the flower pot modification to prevent cross-modulation.

Did the eBay seller list the impedance on those drivers? If not, you have grounds for a return.

 

[twiiii]

The issue is the woofers and the tweeters are now 3 db more efficient than the midrange. Because of the impedance difference. 1010102 crossover has a mid coil with two taps so it will accommodate the HF roll off of the mid. and you can reconfigure the crossover to get the correct value to roll off the bass freq to the mid by selecting the value of caps to be used. The tobin mod will configure the tweeters. But the mid still going to be an issue. Now there is a Bozak owners group on the net, and maybe if you contact them some of the collectors might have a need for 16 ohm mids for their Concert Grands and B-305's and be willing to work out a deal. Audio Classics stocks raw Bozak drivers, so you might give Steve Rowell a call, and maybe you can work something out.

You could bi amp to match the the woofers to one amp and then the midrange to another amp and add another pair of tweeters to each system to get the 16 ohm value you need. The Owners group will have a diagram to show the values necessary to change the tobin mod to 16 ohms.

You also might learn something by contacting BoB Betts on the group or on Facebook. He was a design engineer that worked personally with Rudy Bozak to configure new Bozak designs and develop the 200Z tweeter, which negates the need for the tobin mod. You also have to decide if you want mids with Ceramic magnets or Alnico magnets. I prefer the older magnets, especially with direct coupled SS amps.

 

[Retrovert]

Yikes! The OP just wants to get his speakers up and running, and you want him to do a new design?

The correct, and far, far, simpler solution is for the OP to buy 8Ω midranges and be done with it. The complexity from trying to fix this with a bi-amp, plus a crossover redesign, plus extra drivers (which cost almost as much for the four tweeters as would two midranges!), is unmanageable for someone who isn't an electrical engineer. I'm an EE and I could easily do all of this, and I have spare amplifiers, yet I wouldn't do it.

While bi-amping is possible, the tweeter impedance would be wrong. So a tri-amp would be required, unless, as you noted, an extra pair of tweeters was added. But it would then be too bright and power would have to burned to fix this. And filters would need to built for the amplifiers! It's just not a viable solution! It even costs more than doing it the right way, even without the cost for a second amplifier!

I feel like I'm in the story of Ms. Peterkin whose coffee had salt in it. Yeah, that.

The aluminum tweeters can be adjusted to match a paper midrange with an L-pad with the wires brought to the rear of the case. But, again, if the OP will just purchase the correct midrange so that's going to come out in the wash.

This is why I continually post warnings, over and over, to never purchase the 16 Ω midrange and to always look for markings in the listings because some unscrupulous sellers remove them thinking by the time the customer figures it out it will be too late. I doubt anyone is going to swap. The number of dismantled Concert Grands and B-305s is very high, and I doubt anyone is short a 16 Ω midrange. That's why eBay is full of them.

No need to pester Bob with this. He can't fix the mismatch in driver impedance. Nobody can. It's the wrong driver! Bob has been really helpful and kind to us Bozak owners for help we need, and using up his limited time on this is really not the best use. This project should not go forward as an 8 Ω and 16 Ω hybrid. That simply won't work out.

 

[Retrovert]

For those of you whose childhood did not include this story, here it is for your education and entertainment.

"The Lady Who Put Salt in her Coffee", The Peterkin Papers by Lucretia P. Hale. (1886; 2nd edition)
http://digital.library.upenn.edu/women/hale/papers/salt.html

This was Mrs. Peterkin. It was a mistake. She had poured out a delicious cup of coffee, and, just as she was helping herself to cream, she found she had put in salt instead of sugar! It tasted bad. What should she do? Of course she couldn't drink the coffee; so she called in the family, for she was sitting at a late breakfast all alone. The family came in; they all tasted, and looked, and wondered what should be done, and all sat down to think.

 

[shadetreehif]

I like the analogy. I think what I'll do is carry out the flower pot and Tobin mods on my 302a, give it some listening time, then decide whether to send the 16 ohm mid back or build a cabinet for a 305, with a second woofer wired in series for 16 ohms. I have only the one speaker, as my grandfather purchased it in the days before stereo.

 

[Retrovert]

I like the analogy. I think what I'll do is carry out the flower pot and Tobin mods on my 302a, give it some listening time, then decide whether to send the 16 ohm mid back or build a cabinet for a 305, with a second woofer wired in series for 16 ohms.

I suggest you send the 16 Ω midrange back. Your resale value on that frankenspeaker will be zero. It's a big undertaking to avoid a simple return.

If you have one cabinet, and like the sound, you can purchase a pair and have a surround setup (left/center/right) that will kick ass for a whole lot less than any other three-speaker setup.

Check Craigslist for a pair of B-302s. You should be able to do that for about $200 to $300 with the aluminum cone drivers for midrange and tweeters.

I have only the one speaker, as my grandfather purchased it in the days before stereo.

Funny you should mention only having the one.

I have a single B-305 that will be internally partitioned into two separate cabinets and used as a center channel, omnidirectional, and subwoofer for my home theater setup. (My living room is small, so a dual woofer in one cabinet won't properly converge.) The left/right are the small three-way B-313 which is too small to deliver subtantial low-end bass. I'm limited to maybe 55 Hz, but that's fine for apartment living, and the omni will dig plenty deep for movies.

The B-305 came from the grandson of the original owner, who'd purchased it after returning from the Korean war. He only had the one, and after he upgraded to a different stereo setup had used the B-305 as a surface to stack things on for many decades. After his death it needed a new home. It traveled a long way to get to me.