New DIY MTM Towers designed by Dennis Murphy and Paul Kittinger

[mdrake]

Crossover Question

I was thinking about building these but I had a question about the crossover for the Fountek. It shows a 50 ohm resister across the terminals of the tweeter.
Why would the output of the tweeter be cut by so much? Wouldn't this have the same effect as placing duck tape over the tweeter?
I must not be understanding something. Can someone help explain the use of the 50 ohm resister in parallel with the tweeter?

What would be the downside of taking it out?

Thanks,
Matt

 

[jinjuku]

The downside is the X-Over not working correctly. Just build it as indicated.

 

[mdrake]

I found this on the Fountek site.
http://www.fountek.com.au/products/JP30/FountekRibbon_XOV2_4KHz.jpg

Looks like you can just remove the parallel resister or lower it to suit your tastes. Has anyone done this?

Matt

 

[Swerd]

I was thinking about building these but I had a question about the crossover for the Fountek. It shows a 50 ohm resister across the terminals of the tweeter.
Why would the output of the tweeter be cut by so much? Wouldn't this have the same effect as placing duck tape over the tweeter?
I must not be understanding something. Can someone help explain the use of the 50 ohm resister in parallel with the tweeter?

What would be the downside of taking it out?

The 50 ohm resistor is in parallel with the tweeter, so it doesn't cut the tweeter output as you suggest, it goes to ground. There is a 2 ohm resistor in series with the tweeter that actually lowers it's output to match the output of the two woofers. Together these two resistors in an L shaped arrangement (hence the name L-pad) work together to keep the load impedance presented at the input of the L-pad constant.

If the 50 ohm part of the L-pad were taken out, I'm not entirely sure what would happen, but I see no reason to do that. Circuit design is not my forte, so this is best explanation I can give.

Perhaps a better explanation would be that Dennis Murphy designed it that way. It would take many years of experience before I would be comfortable questioning one of his crossover designs .

 

[Swerd]

I found this on the Fountek site.
http://www.fountek.com.au/products/JP30/FountekRibbon_XOV2_4KHz.jpg

Looks like you can just remove the parallel resister or lower it to suit your tastes. Has anyone done this?

Matt

It says "to increase the ribbon level to maximum of around 95 dB, remove the R2 47 ohm resistor altogether."

If you do that, you will have an excessively bright sounding speaker. Note also that the crossover shown by Fontek is a 3rd order filter acting at 4 kHz. The crossover in the ER18 MTM is 4th order and acts at about 2.5 kHz. They seem to have little in common other than the L-pad.

Dennis worked on this crossover so the overall frequency response curve is as flat as possible. You can mess with it if you want, but I say if it ain't broke, don't fix it!

 

[mdrake]

The 50 ohm resistor is in parallel with the tweeter, so it doesn't cut the tweeter output as you suggest, it goes to ground. There is a 2 ohm resistor in series with the tweeter that actually lowers it's output to match the output of the two woofers. Together these two resistors in an L shaped arrangement (hence the name L-pad) work together to keep the load impedance presented at the input of the L-pad constant.

If the 50 ohm part of the L-pad were taken out, I'm not entirely sure what would happen, but I see no reason to do that. Circuit design is not my forte, so this is best explanation I can give.

Perhaps a better explanation would be that Dennis Murphy designed it that way. It would take many years of experience before I would be comfortable questioning one of his crossover designs .

Thanks for the reply!
I am trying to better understand the reasoning behind the design and then how to tweak it to better fit my ears.

Matt

 

[Swerd]

Thanks for the reply!
I am trying to better understand the reasoning behind the design and then how to tweak it to better fit my ears.

Some people do find that a speaker with a truly flat frequency response curve sounds brighter than any speaker they've previously known. It is possible to tame the tweeter response a bit more by adding a 1 or 2 ohm resistor across the tweeter's terminals. It's easy to do this without removing the crossover board. That way you can modify things a bit after first listening to the speaker as designed.

An alternative is to build the dome tweeter version. That 1" silk dome tweeter is very smooth. It's output, while clean and strong in the crossover range, does roll off sooner than the ribbon tweet does.

I've never encountered anyone who wanted to make the tweeter output hotter on one of Dennis's designs.

 

[mdrake]

Thanks for the help! I thought the resister in parellel is what reduced the output, since it is basically going across the tweeter terminals??

We built a recommend min 7 xover with the resister in place last night and it sounded like we threw the tweeter away. There were no highs, so we took it out. This experience made me double check the xover on this project.

Very, glad to know that Murphy's designs still have some tweeter. As I like my music a little bright.

 

[GranteedEV]

I was thinking about building these but I had a question about the crossover for the Fountek. It shows a 50 ohm resister across the terminals of the tweeter.
Why would the output of the tweeter be cut by so much? Wouldn't this have the same effect as placing duck tape over the tweeter?
I must not be understanding something. Can someone help explain the use of the 50 ohm resister in parallel with the tweeter?

What would be the downside of taking it out?

Thanks,
Matt

hat is a shunt resistor. You would mess up the transfer function if you changed it.

 

[mdrake]

hat is a shunt resistor. You would mess up the transfer function if you changed it.

Thanks, so if I need to my options would be to vary the value or add an lpad.
Is this correct?

Matt

 

[adwilk]

Some people do find that a speaker with a truly flat frequency response curve sounds brighter than any speaker they've previously known. It is possible to tame the tweeter response a bit more by adding a 1 or 2 ohm resistor across the tweeter's terminals.................

Would you not recommend making sure the listener has a good bit of time with the speaker before making this adjustment? I'm not talking speaker break-in.. just the listener getting used to "flat" response? It really is surprising to most...

 

[TLS Guy]

Thanks, so if I need to my options would be to vary the value or add an lpad.
Is this correct?

Matt

What you need to do is leave the crossover circuit alone! You have no clue what you are doing.

Since this is a long thread, here is the crossover circuit to remind everyone how it looks.

Now this crossover is unusual and complex, and designed to smooth the response of the ribbon.

It is a highly modified third order crossover.

A third order crossover would have the 16 mfd, 6 mfd caps and the 0.4 mh inductor. The two ohm resistor in series with the choke is changing the Q of the filter, and the 22 mfd cap with the 0.4 mh inductor is creating a resonant circuit to deal with a response irregularity in the tweeter.

Now the 2 ohm and 50 ohm resistors are forming an L-pad to match the tweeter level to the woofers. So that is one reason you don't need to fiddle with them. The other function of that L-pad is to correct impedance so the filter works properly. If you go removing that 50 ohm resistor you will change the crossover frequency.

The 50 ohm resistor is not across the speaker terminals as you say. There is a 0.1 mh inductor in series with the output and the tweeter.

This is to pull down the rising HF response of the ribbon. Pretty much all ribbons suffer from this problem that needs correction.

If you want a lousy speaker don't built that one, build something else.

The world is awash, in lousy speakers to buy and build.

 

[mdrake]

Thanks for the great explanation!!!!!!! I will build it, as it is drawn. I would like learn all I can on crossovers, any recommended reading?

Matt

 

[Swerd]

Mark

Thanks for your explanation. I needed some help like yours.

 

[Swerd]

Thanks for the great explanation!!!!!!! I will build it, as it is drawn. I would like learn all I can on crossovers, any recommended reading?

A simple quick online read is here Crossovers 101, written by Dennis Murphy himself. It just touches on the subject. There are a few more short articles of interest here Audioblog.

To learn more, I recommend a book, Speaker Building 201 by Ray Alden.

You had mentioned adding an L-pad. The crossover already has one, a fixed L-pad. Perhaps you mean a variable L-pad.

I see two reasons to avoid using a variable L-pad:

1. An L-pad can only reduce output. It cannot increase it. A variable L-pad allows you to easily select how much quieter the tweeter response will be, but it cannot increase the output like you say you want.
   
   
2. Variable L-pads all suffer from oxidation on the various exposed contacts. Sooner or later, they will become noisy, scratchy sounding, "full of static". At their worst, they suffer complete drop out of the signal.

Hope this helps.

 

[TLS Guy]

Mark

Thanks for your explanation. I needed some help like yours.

Thanks, I have looked at the Fountek response.

This crossover is designed to take care of the broad based peak below crossover. The unit has to be crossed over at 2.5 kHz. The response rises by 5 db at 1.5 kHz, and this must be taken care of, as it is well within the crossover slope region. The response rises above 6.5 kHz and the 0.1 mh inductor addresses this.

Sensitivity of the tweeter is high, 94 db, so hence the L-pad to control output.

The tweeter is frail and the manufacturer has warnings attesting to this.

The tweeter only handles 17 watts, which is lower than most dome types. So if power is increased to the tweeter, damage would likely occur.

If the OP is planning a diet of highly compressed pop music, then I think he should build the version with the dome tweeter.

I know Dennis has a fondness for ribbons, which I do not share. We need data on the response over time of the current crop if ribbons. In my experience they are renowned for changing spec over time. Distortion is higher, and they are harder to integrate with the other units.

I know there is a vogue for them, and I frankly think a lot has to do with visual appeal. However when you peel back the onion, there are a huge number of dome tweeters around, with excellent frequency response, that are a lot less trouble to deal with and far more robust.

 

[GranteedEV]

17 watts into a 95db driver would produce 107db@1m at tweeter frequencies. Not sure who would ever even approach that above 2.2khz LR4 or whatever dennis has this crossed at.

Compared to that, the excellent dayton dome is rated at 100 watts, yet with that input would produce only 1 db more as it is only 88db sensitive, and 100w into any tweeter is surely into compression territory, regardless of manufacturer ratings. The fountek also looks more conservatively spec'd, as peak power handling is 235% of continuous. For the dayton, peak power handling is 150% of continuous.

 

[TLS Guy]

17 watts into a 95db driver would produce 107db@1m at tweeter frequencies. Not sure who would ever even approach that above 2.2khz LR4 or whatever dennis has this crossed at.

Compared to that, the excellent dayton dome is rated at 100 watts, yet with that input would produce only 1 db more as it is only 88db sensitive, and 100w into any tweeter is surely into compression territory, regardless of manufacturer ratings. The fountek also looks more conservatively spec'd, as peak power handling is 235% of continuous. For the dayton, peak power handling is 150% of continuous.

All things won't be equal though.

I bet the power of that Fountek, will be markedly reduced below 2.5 kHz.

Ribbons and high power have and always will be problematic, with the power they handle rapidly declining with frequency.

I know that the crossover is rolling off below 2.5 kHz, however this is the point were the energy and power in program material is rapidly escalating.

These are issues where specs only get you so far. I do know that it is the power below crossover that gets ribbons into trouble. The high sensitivity I agree helps you.

However the energy spectrum of pop music is rapidly changing, as the use of the synthesizer capabilities of Abelton Live rapidly spread.

It is now easy for any clown in a studio, to put as much energy in any part of the audible range as he chooses, and add harmonic distortion by the bucket load to increase the kill rate!

 

[David LR]

Very interesting, this crossover discussion. I purchased the RS28F dome version of the kit from Meniscus. I seem to be the only one building the dome version. I was initially set to order the ribbon version but I let myself be talked out of it by the Meniscus rep. I think he was concerned about narrow dispersion issues with the ribbon. In any case, the RS28F seems to be highly regarded, with Murphy as the crossover designer, I'm sure they will be more than fine. I have completed the basic MDF shell of my cabinets , I guess I should start a separate thread. Lots of these ER18MTM on the go right now.

Regarding the crossover, it's very striking to me looking at the numbers of components used in the woofer sections vs. tweeter sections. In the dome version, the woofer section has only TWO components, while the tweeter section contains 6 components (if we are counting the two combined 24.7uf caps as one). I know less than nothing about crossover design but this seems unusual. Is Dennis paying very close & particular attention to the tweeters here ? Would appreciate an informed insight here. Thanks

David

 

[GranteedEV]

Very interesting, this crossover discussion. I purchased the RS28F dome version of the kit from Meniscus. I seem to be the only one building the dome version. I was initially set to order the ribbon version but I let myself be talked out of it by the Meniscus rep. I think he was concerned about narrow dispersion issues with the ribbon. In any case, the RS28F seems to be highly regarded, with Murphy as the crossover designer, I'm sure they will be more than fine. I have completed the basic MDF shell of my cabinets , I guess I should start a separate thread. Lots of these ER18MTM on the go right now.

The NeoCD3.0 itself has fine vertical dispersion. Certainly no less than that of the mtm speaker it is in. Might have a higher crossover point though, i suppose. Higher crossover points do compromise overall dispersion.

Regarding the crossover, it's very striking to me looking at the numbers of components used in the woofer sections vs. tweeter sections. In the dome version, the woofer section has only TWO components, while the tweeter section contains 6 components (if we are counting the two combined 24.7uf caps as one). I know less than nothing about crossover design but this seems unusual. Is Dennis paying very close & particular attention to the tweeters here ? Would appreciate an informed insight here. Thanks

For starters, the slopes are probably assymmetrical. this is done to ensure good driver phase integration. So the woofer can roll off early (probably starts where the baffle step of the box first kicks in) and slowly, while the tweeter rolls off a bit more steeply. They'll both be -6db at the crossover point and track in phase over a decent bandwidth, but they may not follow the traditional slopes you probably have in mind. While it'd still be justifably an LR4, it may not officially be one. The main reason for this is that woofers's voice coils are a deeper acoustic center than a flush mount tweeter's voice coil.

Second the tweeter at least in the ribbon circuit (I did not really look at the dome circuit) is much more sensitive, and needs to be padded down.

Beyond that you sometimes need to correct the native frequency response or impedance profile of a driver before you can get the desired end result... drivers have resonances plain and simple. this of course is very specific to every individual loudspeaker design... you work with the drivers and their flaws.

Sorry i couldn't be of more help than that.