My RS180 MTM Design

[Jeff B.]

Re: My RS180 MTM Design

I modeled the RS28A and RS28F using some frd files I worked up from another project. The tweeter placement is close with my model being 1.5" off center and your actual design being 1" off center. So you should attribute any differences under 5Khz to the 1/2 inch difference in offset and the diffraction caused by the woofers mounted in the baffles, but the top 2 octaves should be accurate. While it does look to be a drop in around the xover region, it is about 1.5db to 2db hotter with the RS28F from 6khz on up. The frd files for both tweeters were created from the RS28A and RS28F measurements done by John Krutke. See results below:

RS28A Vs. RS28F with xover applied


Another forum member built them and implemented a zobel to bring the top end down to match the RS28AS, you can see some measurements he did here in post #36.

Chris

The current RS28a's are almost 2dB more sensitive than early versions. Plots I have seen of current tweeters (A and F versions) overlay each other almost perfectly except for the response above 15kHz. By the way, I don't get quite that much rise at the top of my RS28F.

[czag]

Re: My RS180 MTM Design

Yes. I believe it was Paul Carmody who measured both and found they had identical roll-offs. The original data indicated otherwise. From his measurements it looks like the regular RS28a would work fine. In fact, from all I have seen the RS28F should too, since I applied no response shaping to the tweeter's top end in this design.

I modeled the RS28A and RS28F using some frd files I worked up from another project. The tweeter placement is close with my model being 1.5" off center and your actual design being 1" off center. So you should attribute any differences under 5Khz to the 1/2 inch difference in offset and the diffraction caused by the woofers mounted in the baffles, but the top 2 octaves should be accurate. While it does look to be a drop in around the xover region, it is about 1.5db to 2db hotter with the RS28F from 6khz on up. The frd files for both tweeters were created from the RS28A and RS28F measurements done by John Krutke. See results below:

RS28A Vs. RS28F with xover applied


Another forum member built them and implemented a zobel to bring the top end down to match the RS28AS, you can see some measurements he did here in post #36.

Chris

[Pete Schumacher]

Re: My RS180 MTM Design

How audible would you think a difference in crossover point between 1700 on the woofers versus 1800hz on the tweeter be?...if I used a tweeter that was pre-set by design to roll off at 24db/octave at 1800 and cannot be made to go lower? This is sort of a goofy question but I have a friend who has a pair of RAAL 70xrs and we were considering repurposing some old speaker cabinets for fun. YES..RS-180s are not Seas or Scan Speak but it may be a fun experiment none the less.

If the phase is tracking well, the slight dip in summation at the XO point is likely to be inaudible.

[billschulte]

Re: My RS180 MTM Design

How audible would you think a difference in crossover point between 1700 on the woofers versus 1800hz on the tweeter be?...if I used a tweeter that was pre-set by design to roll off at 24db/octave at 1800 and cannot be made to go lower? This is sort of a goofy question but I have a friend who has a pair of RAAL 70xrs and we were considering repurposing some old speaker cabinets for fun. YES..RS-180s are not Seas or Scan Speak but it may be a fun experiment none the less.

[Pete Schumacher]

Re: My RS180 MTM Design

I will second Paul Carmody's assessment of the A/F versions. I have both here, and am able to drop either one into a current build, and the response plots with XOs applied is identical, save the top octave.

[gimpy]

Re: My RS180 MTM Design

Thx, Applesauce. I checked that link out, but the pricing, shipping, customs (if I read/understand it correctly) is kind of cost prohibitive, especially this time of year, for me. It totals about $160 or $170, I think.

These will be used in a 50 liter, curved speaker that I started about in another thread. It was originally for the Nat P's, but I think the footprint of the curved speaker will not give me enough space for that XO; therefore, the interest in this design when I came across it. I do already have the bracing/skeleton cut out for that shape and size. (I assume I can increase the volume with no tweaking of the crossover?).

Jeff, if you think the rs28a is good for the original design you show (no audible differences), then, I think that is the way I will go and what I will get.

Please let me know if my thinking is wrong and I need to change something.

thx,

[Jeff B.]

Re: My RS180 MTM Design

I can't help you with summarizing the substitutions. (I do know that the information is all here in this thread, because I've been following it since the beginning, and I think the RS28A was belatedly deemed a drop-in replacement for the RS28A-S, but someone else should chime in to confirm...)

That said, there are two RS28A-S still available at Solen:

http://www.solen.ca/pub/cms_nf_catal...d=2104&nobut=1

I bought mine from there (six months ago when there were four available - not exactly hot sellers!) with the sole intention of building exactly this speaker as Jeff laid it out to begin with. I haven't got around to it yet... But soon, I hope. Soon!

Yes. I beleive it was Pau Carmody who measured both and found they had identical roll-offs. The original data indicated otherwise. From his measurements it looks like the regular RS28a would work fine. In fact, from all I have seen the RS28F should too, since I applied no response shaping to the tweeter's top end in this design.

[Applesauce]

Re: My RS180 MTM Design

But, PE does not have the tweeter Jeff used on these originally. Is there a drop in tweeter that can be used with the original crossover design? I have read this thread 3 or 4 times now and, I am sorry, but I do not understand all the talk about the substitution parts, etc. It is just over my head. I need the design to be on paper so that I can follow it point by point. Even then, I make mistakes.

I can't help you with summarizing the substitutions. (I do know that the information is all here in this thread, because I've been following it since the beginning, and I think the RS28A was belatedly deemed a drop-in replacement for the RS28A-S, but someone else should chime in to confirm...)

That said, there are two RS28A-S still available at Solen:

http://www.solen.ca/pub/cms_nf_catal...d=2104&nobut=1

I bought mine from there (six months ago when there were four available - not exactly hot sellers!) with the sole intention of building exactly this speaker as Jeff laid it out to begin with. I haven't got around to it yet... But soon, I hope. Soon!

[gimpy]

Re: My RS180 MTM Design

After reading this post, I have thought about building a set of these speakers. I had originally planned on building a set of curved Nat P's, but think that these might be a little easier to build (the crossover).

But, PE does not have the tweeter Jeff used on these originally. Is there a drop in tweeter that can be used with the original crossover design? I have read this thread 3 or 4 times now and, I am sorry, but I do not understand all the talk about the substitution parts, etc. It is just over my head. I need the design to be on paper so that I can follow it point by point. Even then, I make mistakes.

Is there a printed design somewhere with current drivers and tweeters that I could use?

Sorry, for my confusion,
thx.

[Jeff B.]

Re: My RS180 MTM Design

It looks like Jeff did work out a crossover for a pair of 4 ohms in series (see posts 64, 68 and 69). It also looked like ontariomaximus was going to build a pair. I was wondering if he (or anyone else) did and how it worked out.

I'm also interested if anyone did a Thor-alike with different drivers.

Maybe Jeff or ontariomaximus could weigh in on this if they are monitoring this thread.

I never build them with two 4ohm drivers in series, I just scaled the crossover assuming the same basic frequency response of the woofer, adjusted for the change in sensitivity. It should be pretty close though.

Jeff

[JCC]

Re: My RS180 MTM Design

It looks like Jeff did work out a crossover for a pair of 4 ohms in series (see posts 64, 68 and 69). It also looked like ontariomaximus was going to build a pair. I was wondering if he (or anyone else) did and how it worked out.

I'm also interested if anyone did a Thor-alike with different drivers.

Maybe Jeff or ontariomaximus could weigh in on this if they are monitoring this thread.

[greywarden]

Re: My RS180 MTM Design

Unless Jeff has the crossover worked out for a 4-ohm-in-series version, the corssover will have to be reworked. As to how do they sound? Jeff has commercially-sold designs with his name on them, I wouldn't worry too much about how they sound, haha.

http://www.salksound.com/continuum%20-%20home.htm

http://www.salksound.com/archos%20op...20-%20home.htm

http://www.salksound.com/soundscape%2012%20home.htm

To start with... no telling how many others he's had his hand in.

[JCC]

Re: My RS180 MTM Design

Has anyone built this design with the RS 180-4s in series with the crossover mods discussed by Jeff B. above? How do they sound?

The reason I ask is that I have some original Thor TL cabinets that I need to do something with and I thought of putting those drivers in the cabinets. The reason for considering the 4 ohm version is that they have a higher Qts and lower Fs than the 8 ohm version that I thought might work better with the TL cabinet. I also like Jeff B.'s simple crossover design. Has anyone compared the sound with something more complex like the Modulas or Natalies.

My alternative is to do somethign with Usher 8945s but they are more expensive.

[scottq]

Re: My RS180 MTM Design

Hi Jeff (or someone else that understands ),

I'm struggling a little understanding all the relationships of the network... I think I'm lost in the simplicity.
Simple in number of parts, but very finely tuned to mate with the complex impedance and frequency response of the drivers.

The system sensitivity plot shows ~90dB @2.83V (correct?). Yes

Sensitivity is 94.1 dB @2.83V for the two RS180S-8's in parallel (per Jeff's post). Is this the low frequency sensitivity or the mid-band sensitivity? I'm assuming mid-band, based on what I see in the response curves.
Sensitivity is ~90.5-91 for the RS28AS-4 @2.83 V, pretty flat out to 11k. Sensitivity is usually a low frequency or average parameter.
When designing crossovers you have to deal with the actual responses which can vary quite a bit, sometimes several dB above the rated sensitivity at some frequencies.

Here's my understanding (confusion?):
1) With no pad on the tweeter, that means everything should be operating at it's nominal sensitivity. But, I'm only seeing a 3-3.5dB difference between the woofers' and tweeter's sensitivities (see above, ~90.5 vs 94.1) for baffle step. The sensitivity appears to drop off a few dB below 1kHz on the woofers. So, is this the ~6dB total difference that's being stated for baffle step? I'm not sure where the 6dB is coming from.
2) I see that the shelf is dropping ~6dB from 100Hz to 1000Hz, but what is creating it? If my calculations are correct, the 1.2mH coil would appear to begin its influence at ~460Hz (-3dB center) for a flat 3.5 Ohm impedance; This is close to the -3dB point on the shelving rolloff. Obviously, impedance isn't flat on the drivers; is this changing impedance what's causes the shallower (only 3dB/octave) shelving rolloff?
The baffle step compensation is coming from the 1.2mH inductor and its initial roll-off. Your thinking is correct.

3) Is the notch filter responsible for the sharper rolloff after the XO center?
The 20uF cap and 0.025mH coil are forming in the trap for the cone breakup (by my calculations, centered ~7.1kHz, which matches with the notch in the transfer filter). I'm think I'm square on this.
The notch filter has been tuned so that its corner behavior matches that of an LR4 crossover. Remember, the notch filter includes the 1.2mH inductor. Without it there is only a drop in the impedance due to the cap and small inductor, but no change in the response.

4) Why is the crossing point (1.7kHz) -9dB (in the filter transfer function graph)? Shouldn't it be -6dB for 4th order? Wouldn't this create a -3dB dip in this region? Or is this accounting for the peaking in the RS180's acoustic response in this region?
Yes. You are trying to understand things by looking at only the transfer function of the filters. Although the transfer function shows the effects of the complex impedance of the drivers on the circuit, it does not show how it mates up with the natural on-baffle frequency response of the drivers. The final results are shown in the first post in the thread. The filter accounts for baffle step, rising and peaking response in the woofers, and the baffle diffraction bump in the tweeter's response.


Thanks much in advance... I'm going in circles

Jeff, thanks much for your confirmation (and time)! Looks like I did eventually put the pieces together; I was hoping that I was on the right track. Not having things printed out in front of me, I was going back and forth between the transfer function plots, the Dayton spec sheets to see driver responses, the crossover, and a few calculator windows.

Thanks again,
Scott

[Jeff B.]

Re: My RS180 MTM Design

as an uninformed observer of this site, I have been following this section closely. I would like to do a bi-amp of a single Dayton RS180(8 or 4) and found Jeff Bagby's Denhams response shaping network. Does anyone have a network like this worked up for a single RS 180?

If you use an RS180-4, then you can use the same network on the woofer. If you use the 8 ohm model then the values would need to be scaled to match, but that wouldn't be hard to do.