Adding Zobel to existing crossover?

[Bratwurst7s]

James,

A Zobel network is designed to create a constant impedance (primarily resistive) load in a reactive network. Your woofers, with their large motors represent a highly inductive load - so the Zobel network was added to make the woofer appear as though it is a resistive load - and not reactive (inductive).

From the top; voltage and current in ANY electrical system are not always "in phase". This means that you can have high voltage and no current or high current and no voltage. This difference between the voltage and current phase is referred to as "power factor". These two values, voltage and current become out of phase when the electrical flow encounters reactive elements (capacitors and inductors). Your crossover network is by definition highly reactive.

To make the situation even more difficult the speaker motors themselves (big windings) are also reactive (primarily inductive). So given the large motor on your woofers the Zobel was added to adjust the power factor of the incoming audio signal so that whatever power is being pushed to your speaker has a unity power factor (voltage and current completely in phase). Thus making your crossover and motor network more efficient.

Therefore adding a Zobel to the mid-range elements of your crossover will NOT make the mid-range "less" pronounced - if anything the mid-range will get LOUDER - for the power factor should be, if you design your Zobel correctly, enhanced and therefore pass a more powerful signal to that driver.

Do not add a Zobel to your network. That will not serve your purpose (which seems to be to attenuate the mid-range signal). My suggestion is that the only way you can lower your mid-range signal is to add a "real", i.e. a resistor of some value (I would go higher than 10 watt capability by the way - if nothing else parallel the two 10 watt resistors). Look for around one or two ohms to start your experiment.

Or you can be cute - as I have done myself - and add a lower than sufficient resistor thereby creating a volume dependent load, i.e. the resistor will heat as you increase the signal and the resistance will therefore INCREASE - so you will load the signal more heavily at higher volumes. Works quite well with the one caveat that you may burn out the resistor at a ridiculously high volume (ceramic resistors simply crack when heated too much - and the resistance then goes to infinity or "open circuit").

Let me know if this helps or you need more advice.

Thanks Reddog, that helps. The thing is that my goal is twofold+. My ears are telling me that my mids aren't just a bit too loud, but I feel that they are getting a bit too much into the high range. I realize that that is more properly addressed by adjusting the crossover point, but had hoped that by flatening their inductance curve that I might be able to avoid making any changes to the crossover point, something that I would rather avoid with my non-skills in that regard. Then I could address the mids loudness as a second step with padding. And when I ordered the zobel parts I also ordered some 1.8, and 1.0 ohm 10 watt MOXs also, so that I would be prepared if that is necessary. Is that really taking the wrong path?

Let me ask this: After I replace my 10" woofers I had been thinking about replacing the iron core inductors in the woofer section of the crossover with some air core coils that have less DCR. The stock inductors measured out at 3.4mH and 0.9 ohms. I had hoped that by lowering the DCR on the woofers inductors it would raise their efficiency, and then I wouldn't need to pad the mids as much, if at all. Is there any sense to that, or am I trying to think too much.

James

 

[Bratwurst7s]

There's lots of info on single and dual driver systems, but asymmetric drivers make my brain hurt. No idea on how they interact or how to calculate for them. A single driver system with an asymmetric passive radiator is common and presents no problems, but I'm thinking asymmetric sized drivers probably need their own separate chambers to work properly, and even then the crossover design might be complicated or not realizable due to the electrical interactions.

I've tested t&s parameters of lots of drivers and it's pretty easy. Common signal generators don't have the output capability, so you need any audio power amp between the generator and driver. The FG-5000 is uncommon here, but it appears to have the standard 50 ohm output. Just hook it to any amp or receiver. You can figure out Vas using either a quick test enclosure, or with the added mass method. Both work, but I prefer the enclosure method.

Well, it's definately making my brain hurt. I've been searching for several months for info online, and I have a 7th ed. of The Loudspeaker Cookbook (which only addresses symetrical woofers). If I'm understanding what I've found correctly (big if), the method is to measure each driver separately, add the results, and divide by 2, and supposedly the woofers will just share the results as they wish. The original setup is a 4 ohm resistor and 66uF of caps (2 x 33uF). I most likely will not try to change this, but it's an interesting excersize to try to understand it.

I don't recall seeing any other speakers with asymmetric woofers. and Speakerlab did it from the start with the Sevens, and in a single enclosure. I'm not even going to attempt to understand calculating a crossover point with these things, I'm sure that my little brain would melt down and run out my ears like hot wax.

The FG-5000 is a nice unit, I've been playing with it a bit at work with some old PA speakers just to see how it works. And I bought one of those little Lepai 20w amps to bring in and use for testing.

Looking at the You-tube vid of DATS it works with the added mass method, I'm looking forward to buying DATS to play with, but that will probably sometime this summer.

James

 

[bazz_77]

Let me ask this: After I replace my 10" woofers I had been thinking about replacing the iron core inductors in the woofer section of the crossover with some air core coils that have less DCR. The stock inductors measured out at 3.4mH and 0.9 ohms. I had hoped that by lowering the DCR on the woofers it would raise their efficiency, and then I wouldn't need to pad the mids as much, if at all. Is there any sense to that, or am I trying to think too much.

James

Well, it works just like that, but.. If you decrease inductors resistace from 0.9 to something like 0.2, you get only about 0.5 to 1dB more sensivity. That's not that much.

 

[Bratwurst7s]

Well, it works just like that, but.. If you decrease inductors resistace from 0.9 to something like 0.2, you get only about 0.5 to 1dB more sensivity. That's not that much.

Thanks. No, it's not much, but it just seems like a good idea to raise the woofers sensitivity as much as I can and not have to pad the mids down as much. And I'd heard that going to air core inductors with low DCR will add some punch and clarity to the woofers, so it just seems like an attractive way to go.

I am actually very happy in general with my new mids. They have amazing clarity, but just a touch too much up high.

James

 

[bazz_77]

Only thing is that 3.4mH air core coil with 0.2 DCR would be huge (and expensive). High quality cored coils are just fine.

 

[ConradH]

You know, the entire problem goes away if you slap together an active crossover (or buy some nice boards from Marchand) and ditch the passive crossover entirely. Tuning becomes vastly easier and you can make minor adjustments after listening for weeks, then go back if you want to compare. The asymmetric woofers would both see a low impedance source and interact only by physical pressure. Even that disturbs me a bit since one woofer can drive the other and the physical phase might differ between a high mass and low mass driver. My head hurts. It might actually make sense to purposely limit the drive to one of the woofers so the displacement volume between the two is equal for equal signals, but now I'm just rambling- way beyond anything I've actually messed with.

 

[Bratwurst7s]

Only thing is that 3.4mH air core coil with 0.2 DCR would be huge (and expensive). High quality cored coils are just fine.

Yes to both huge (and expensive). A Mundorf 2.5mm 3.3mH would give me 0.25 ohms DCR for about €88 each, and 3mm would get down to 0.19 ohms for €154 each.

But I've read of some people having some pretty nice results from them. In any case if I go that route it won't be till the end of the year, too many other expenses stand in the way.

James

 

[Bratwurst7s]

You know, the entire problem goes away if you slap together an active crossover (or buy some nice boards from Marchand) and ditch the passive crossover entirely. Tuning becomes vastly easier and you can make minor adjustments after listening for weeks, then go back if you want to compare. The asymmetric woofers would both see a low impedance source and interact only by physical pressure. Even that disturbs me a bit since one woofer can drive the other and the physical phase might differ between a high mass and low mass driver. My head hurts. It might actually make sense to purposely limit the drive to one of the woofers so the displacement volume between the two is equal for equal signals, but now I'm just rambling- way beyond anything I've actually messed with.

I have read what people have been saying about going active but haven't really considered it. Perhaps it's time to start looking in that direction. What does a good quality active crossover cost? Ballpark figure.

The asymmetric woofers that Speakerlab used in the Sevens have always been somewhat controversial, but they do seem to work. They deliver some serious bass, or at least mine did until I screwed up and replaced the 10" instead of refoaming them. They are still not bad at all, but not as good as I want them to be at the moment.

If you are interested you can see the background to all of this here:
http://www.audiokarma.org/forums/showthread.php?t=545406

James

 

[DavidF]

"...The only original driver in the entire box is the 12" woofer. .."
James

Practically speaking, I would think you are on your own in terms of what you mentioned above. Assuming the original xover was designed for specific drivers tuned to interaction among those drivers. I would be less concerned about the effect of the zobel in the circuit than with what has changed overall from substituting in different drivers.

Back to original question adding a zobel could possibly change the xover point. A single inductor intended to roll off the mid's high end will be effected by the rising impedance toward the effect of lessening the roll off (higher impedance requires more inductance value). Assuming a single pole (inductor in series) the actual response will roll off about 6 db per octave but start to flatten out with increasing frequency/impedance. A zobel tuned to the mid driver to offset the rising impedance and the resulting roll off will stay more like 6 dB per octave. Therefore, less overlap with the tweeter and that may reduce the xover point.

 

[tubed]

Here's a pic of the Speakerlab 7 crossover schematic:

 

[Bratwurst7s]

Practically speaking, I would think you are on your own in terms of what you mentioned above. Assuming the original xover was designed for specific drivers tuned to interaction among those drivers. I would be less concerned about the effect of the zobel in the circuit than with what has changed overall from substituting in different drivers.

Back to original question adding a zobel could possibly change the xover point. A single inductor intended to roll off the mid's high end will be effected by the rising impedance toward the effect of lessening the roll off (higher impedance requires more inductance value). Assuming a single pole (inductor in series) the actual response will roll off about 6 db per octave but start to flatten out with increasing frequency/impedance. A zobel tuned to the mid driver to offset the rising impedance and the resulting roll off will stay more like 6 dB per octave. Therefore, less overlap with the tweeter and that may reduce the xover point.

Thank you David.

So, from what you and the others are saying, it looks like that while there may be a lowering of the crossover point, it shouldn't be dramatic, but I should be aware that the mid's efficiency may rise, possably making the mid louder, meaning that I might need to pad the mid more than if I hadn't added the zobel.
And when I replace my underperforming replacement woofer with one that more closely matches the original one, along with raising the woofer section's efficiency by lowering the woofer inductor's DCR I can minimize the need to pad the mids by a small amount.

Is that a reasonable summation of things? If so I can live with that, it takes me a step or 2 in the direction I want to go.

And yes, you are quite right about my being on my own now. They've become somewhat of a Frankenspeaker. But what the heck, I'm now living in Dr. Frankenstein's homeland. Seems like a natural. On the other hand, the mid that I chose (Visaton MHT-12 ribbon) is actually better than the original, and does a better job of working in the 3800hz crossover range. I'm very happy with it, and still have the original ribbon tweeters if I need them. Sadly the original mids and upper woofers are long gone.

James

 

[Bratwurst7s]

Here's a pic of the Speakerlab 7 crossover schematic:

Thanks tubed, but that's for an earlier generation of the 7 with horns. I have a 3rd gen 7 with the ribbon tweeters and 6 1/2" polycone mids and the crossover is quite a bit different. Nonetheless, I appreciate the help.

James

 

[tubed]

Thanks tubed, but that's for an earlier generation of the 7 with horns. I have a 3rd gen 7 with the ribbon tweeters and 6 1/2" polycone mids and the crossover is quite a bit different. Nonetheless, I appreciate the help.

James

Oh good, I was kind of freaking out
I still have never seen nor heard a pair of Speakerlabs in my parts:scratch2:

 

[Bratwurst7s]

Oh good, I was kind of freaking out
I still have never seen nor heard a pair of Speakerlabs in my parts:scratch2:

Well, if you ever make it over to Munich let me know. You can spend some time listening to mine and seeing this fine city. And there's this thing called the Oktoberfest...

James

 

[tubed]

Well, if you ever make it over to Munich let me know. You can spend some time listening to mine and seeing this fine city. And there's this thing called the Oktoberfest...

James

I love beer.

 

[Bratwurst7s]

Update, and for my purposes, closure for this thread. My comments will be brief but I will go into more detail in the previously linked S7 rebuild thread.

First, I realized that I I hadn't done a good enough job of sealing my drivers and made & installed new seals yesterday. This went a long way towards balancing my speakers, the mids were no longer so large in relation to the lows and highs. A small mistake with a large effect, now corrected.

Today I installed the zobels on the mids. Magic! It brought me everything that I had hoped for. This was not a small improvement. The mids and tweeters don't seem to be competing with each other anymore. There is a more homogenous whole speaker sound now and the sound stage is dramaticly improved. I'm very happy.

Many thanks to all of you who commented. You improved my understanding of how this works and I'm grateful. Oh, and to Tubed: Yes, I like Spaten bier also. Such a nice blond foam head.

Cheers,
James

 

[Bratwurst7s]

Perhaps I spoke too soon about closure. Darn it ConradH, you put a bee under my bonnet and it will be at least a year if not 2 before I can do anything about it.

I've read here and there on these forums about active crossovers and bi-amping but never before seriously considered it. Now you have me wondering what it would be like. If I'm understanding correctly I would:
1) Install double binding posts,
2) Remove the woofer portion of the existing passive crossover,
3) Connect the woofers directly to the 2nd set of binding posts,
4) Connect the remaining mid/high portion of the passive crossover to the 1st set of binding posts,
5) Buy a pre-amp, active crossover, and 2 power amps,
6) Enjoy the benefits.

That right?

Am I right in guessing that the woofer zobel would also be discarded, it's duties being taken over by the active crossover?
And the new mid zobel that I just installed, would I need to remove it? Or would it still be working usefully in the remaining mid/high portion of the passive crossover?

Cheers,
James