designing OB RIBBON mid and tweet speaker

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I am in the process of researching options for my next speaker build. I have settled a bit on the idea of doing an open baffle/dipole design for ribbon mid and tweet that will be based on the design parameters of the ACR Isostatic RP200 which uses same mid/tweet as rp9 that I have. I have a feeling I may prefer sound of a sealed enclosure for the woofers. If the woofers I chose would work best in a ported enclosure, so be it.

Here is what I am working with currently.

I purchased the Fostex RP9 speakers at swap meet for 30 bucks. They appear to be brand new in the box, so using the mid and tweet will possibly save me money in the long run for total cost of components. The woofers were not usable due to age and some cone damage (Fostex use to be close to where I live, and I have a suspicion these were refurbished or new old stock someone got when they closed down). I will need to find other woofers.

The tweeter is of unknown model number. I have a suspicion it is an ft7rp without the front bezel or faceplate, possibly. It is a super tweeter.
The midrange is the fs21rp.


My current idea is to make an open baffle that will hold the mid and tweet above the woofer enclosure. I am listening to them with original 3-way 1st order crossover and the upper high midrange is a tad overbearing (as shown by peak in fs21rp response graph I will post below). I hope a good crossover may alleviate this :( I took out the bad woofer, and am using a woofer in it's place that is not working well but is better than nothing.

I would like to stay as close to RP200 or other rp series designs as possible, as they use the same midrange tweeter combination and I have some of the design parameters and crossover points making my possible crossover points and woofer selection a littie easier. I like the idea of dual 8", or maybe dual 10". Slender and tall speaker.

I would like to go active crossover in future for all crossover points I may use with exception of tweet to mid. I think it will make amplification easier for my first time in active crossover foray.

I would like to crossover the dual woofers to 80 hz, or possibly 100 hz depending on woofers chosen (as my receiver has built in crossover point here). I feel that going with seperate large powered subwoofer(s) may allow for a little punchier bass yet more clarity from speakers?

The RP series all crossover the tweet and mid at around 700 and 5000. If they also have the 12 inch, it is crossed over at 150 hz!!!! I would like to use these values since they seem to be decent speakers and they know how to design :D Unless you see a better idea from graphs attatched.

I am not stuck on woofer selection so if you look at the attatched pics and info, please share your ideas. I would like the woofers to stay Fostex if possible, just to make the project closer to RP series original design.

Please note that the 12" used in the higher RP models is still available new. The 8 inch woofer and the 7 inch are not. Have not looked for a 10 inch as Madisound does not carry a Fostex 10" (Not that I have to buy from them, just easy reference for me).

I am open to the idea of going completely open baffled, but this will make the project more likely to happen at a later date due to me wanting EQ and active xo's. Not to mention I would need more help in designing since I am completely new to open baffle designs!!!! I still think sealed woofers may be my best bet for starting out as a beginner. For a little more SPL.

I also need to figure out if I can just put the mid and tweeter on front of baffle, or if mounting from behind with nice chamfer or bezel would sound better.

I also wonder if a waveguide would work with ribbons? If anyone thinks so and can help me work on some designs, I would be grateful :D
See attached pics for Ribbon waveguide idea ;)

On with the pics and stats for ideas.
First picture is of the rp9 that I got. It has an 8 inch woofer for scale reference...

The rest are RP series pictures I found online.
I also threw in a couple of other pictures I found interesting, although using different ribbons altogether.

Here are a couple of interesting links. The first link has full stats for the fs21-rp.
Fostex FS21RP

I don't know german. But the numbers and graphs I CAN read hahaha.

Here are some pictures and a little info on the RP series.
Magnat Sound-Studio 1979 - 1990

Sorry for long post. Just getting some thoughts I am having out there to see if anyone may be able to help at all.
Thanks for your time.


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I will be using for 1 seated position most likely.
Stereo only.
I don't know if I will have a dedicated real wall for dipole reflection???
I would like the dual woofers to go to 80 hertz flat in sealed enclosure (the fostex 8's seem to start to roll down a little at 150 hz :( is that bad for not being in enclosure?). 10's will do it no problem but would make cabinet wider.
I then would like the sub to go to 20 flat if possible. I think 30 is ok since many people say most music stays above 30 and 20 is hard to do. But if I can do flat to as low as possible with sub that would be great :D 15" or 18" for sub is fine :D May only have 300 watts plate amp to start with for sub.

I am also interested in buying some basic DIY measuring equipment in the future. Maybe an RTA to start with? This would help me learn to tune and experiment with speaker set ups.

Here is a cool picture I found. I like it a lot :D
Looks like an enclosed mid tweet version of what I want to do... Looks to have amp for tweeter... Maybe overkill for me? I dunno :D


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If you're after an "almost" full range speaker,follow the ACR300 design.
All speakers can be found in the second hand market/ebay.
Why go for the the open back/dipole design?You have to have a very good control on the speaker placement in your listening space (IF it fits) and you loose some dbs
from the low section of the RP-21 (exactly where it needs it).
These RP-21 units are really fast and transparent.
I would cross at 750Hz to keep some space from the 650Hz break and surely more agresively than 1rst order!
A really fast midwoofer (aerogell?-ceramic?) would help the mating.
To be honest, it would be easier for me to add sound absorption or deflectors to help with room modes, than it would be for me to arrange the speakers to fit the reflective needs of the dipole/OB set up. Now you have me thinking. Thanks for bringing that up. I may not be ready for dipole. But I can always rebuild enclosures when I live somewhere with more room!!!

Also, I have read the rp21 will perform better in an enclosed cabinet as far as lower midrange is concerned, as you mentioned. It may be able to handle more power and be crossed over lower if I do so. Will not know for sure until I am able to get to the stage where I can measure output and what not :(

Here is what I found on fs21rp info.
I can't find the website, but I found a post where a guy was talking stats in a forum and he posted that the midrange was in an enclosure of 15L or so. I can't remember the number!!! If it is 15L that is about half a cubic foot. Which about matched the pictures I have seen of the enclosures people put these in!
I don't know how to use T/S parameters for a midrange. Anybody got some input based on the graph of this midrange? :)

Taken from this website. Note what part of the page they are using this for as I don't speak the language :D hahaha. I hope this is the T/S parameters!!!
Fostex FS21RP

Resonanzfrequenz Fs 623.0 Hz (+/-0.7%)
DC-Widerstand Rdc 7.18 Ohm (+/-2.0%)
Mechanische Güte Qms 3.537 (+/-48.6%)
Elektrische Güte Qes 68.347 (+/-17.5%)
Gesamtgüte Qts 3.355 (+/-47.3%)
An externally hosted image should be here but it was not working when we last tested it.
Wirkungsgrad Eta (1m, 2.83V, Halbraum)
91.07 dB (+/-0.34)

I found the info on the enclosure! If you look at the spec sheet for the rp200, you can zoom in and see the total enclosure volume for that design is 71 liters. They break it down to the drivers, and its 28, 28 and 15 :)
Cool. So now I know that if I go with the sealed enclosure on the mid, it's 15 liters. Awesome!

Now to decide on dual 8's or 10's??? 8's may give me better mid bass and upper midrange. Not to mention there seems to be a large selection of them out there! 10's will extend lower and may not need a subwoofer if I decide to play the speakers alone. I am not sure now :D

I am thinking the rp300 design would be kind of useless for me, as I plan on using a subwoofer, and that design has a crossover of 150 which is very close to being a passive subwoofer. I think the dual 8's would give me nice punch and spl while providing good lower midrange (look at rp400!). If I tune the ports properly, could a ported enclosure give me good smooth response and good transience if I use an 80hz high pass with the subwoofer? Also, do I need to do WTMW if I cross over at 700 or so? I read you can get midrange blur if two woofers are mounted below midrange :( If I do a regular enclosure, this will not be an issue if I have a woofer on upper part of front baffle as apposed two both woofers close to each other on lower front baffle in TMWW configuration........

Fostex does not make a 10 inch woofer that I can find. I still think it might be nice to stay Fostex for the woofers. Thus the push for the 8's. I am also liking the idea of a paper woofer for a warmer sound. Don't know if that will match the ribbons, or will it? I don't have enough experience with ribbons. All my car audio has been carbon or polypropylene. But my home audio has been mostly paper woofers with soft domes tweeters! I just got a (cheap off craigslist v20) set of Boston home speakers with metal tweeters and not sure I like them. Reminds me of JBL titanium :( I am not sure I understand the benefits of going with ceramic woofers with the ribbons. The ceramic woofers Madisound offers are SOOOO expensive! Not sure it's worth it. I may need some 'splainin' :D Anything that will give best sound with reasonable budget is wanted. Not trying to be cheap, yet not trying to buy very expensive stuff.

Ok. I will stop for now. Need to sleep on this for a bit. Been researching for weeks. Too bad it doesn't show :(
The sound of a good OB is worth it all the way!

A good idea came into mind, it is attached below. I don't insist on the proportions.

It combines a TL and a OB. Actually the original idea is this one: Enclosure Design and Construction for the Lowther DX Series of Drivers, but upside down.

I have tested it upside down and adapted to another woofer and the results regarding the low frequencies is great. With the opening at ear level the perception for very deep and convincing bass is very strong.

p.s. don't mention my woodwork skills - I am just testing the idea...


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I you are going to use subs anyway (i thought that you said "music only")
then no need to go under 100Hz.Twin 8" will give you omph and speed.
Try the Hivi golden cone ones that will match the color of ribbons!
Cheap too.
For a chamber resembling the shape of the RP-21 you only need about 15cm depth.
Stuff the 2/3 rear part with Acustastuff or similar and you're good.
Don't opt for too warm 8" cones.They have to be fast and have good
strong magnets to start/stop on par with the planars.
Unless you can find the Fostex FS-41RP (he,he,he) that would cure your problems...
I listen to a bit of "electronic" music, so I figured the sub would add a bit of depth, even with speakers that can go somewhat low. I don't need the bass too loud, just want it to be smooth and deep. Plus I figured a separate sub would ad more power to the system for clarity and depth at louder volumes if need be?
Judging by the response graphs on the Fostex 8" I think it will do the higher midrange just fine. The cone on it is not heavy, either. I think I am narrowing the price range to just over 100 dollars to under 200 for each woofer. At least for the quality I feel may work well.
Here are just a few of the woofers I have found that may work while looking through

This is the cheapest, and seems to have a heavy cone.

This Eton and the Fostex are in the higher price category for me.
On the Fostex, I don't like how the bass starts to roll off at 150 hz, but this woofer may produce better upper mid and will work well with subwoofer? Will it work well in a sealed enclosure?

This is the woofer I have been really looking at. It is around 135 bucks and looks to have a decent sized dip in response starting at 650 hz, which may work well with the midranges. It is also designed for a sealed enclosure, which I like.

Yeah, finding a "new in the box" set of FS-41RP's would be interesting. Probably not realistic to look for them, and may not be able to afford them if I do :D

Lastly, I will most likely make the upper section of enclosure for the mids and tweets modular, so that I can swap between open baffle and sealed. I can make up for some of the SPL loss or frequency response changes between enclosures with EQ... With a modern digital EQ I can save presets, so I could swap enclosures and test between the two if need be :)
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