Jim Shearer's Fostex FF85K and Eminence Alpha 15A Open Baffle Speaker

[MJK]

If you are going to bi-amp, I would recommend the Eminence Alpha 15A. I believe the Alpha 15A is of better quality construction compared to the Goldwood. You could go OB or H frame.

Since you are biamping the level match to the full range driver is not so important. You coud easily use any of the Fostex or Alpair drivers and even the Jordan driver. The Fostex FF85K is an excellent option, from the stand point of performance and price, if you decide to go that route.

[mr ed]

Thanks Martin, very good tip and bonus of alpha being cheaper
Not having heard a ff85k nor the f120a I can only hope the difference is small versus the cost difference in a situation where the bass is not an issue. In the end you get what you pay for typically. so I should not over speculate here.

On another note, I tried to follow your instruction on down loading mathcad so I could purchase your work sheets. I have some pc issue that will not allow me to run it. I am some what computer handicapped
So I will have to stick to your tried and true formula for now.

Cheers
Ed

[MJK]

Ed,

My recommednation would be to stay away from the F120A. I have only heard it sound great in one situation (Jim's Metronome with my tube amp), in an OB my pair of F120A drivers sound mediocre at best. If you are going to spend that much money you might consider the Jordan JX92S or maybe the Alpair 10 or 12. The best buy would be the Fostex FF85K in terms of performance/cost. If you start small with the FF85K, you could always change to a bigger driver later if you felt the need and just make the hole bigger.

[ChestofFur]

I'm in the process of building this speaker as well. I've ordered the crossover parts
per the original open-baffle design, although I did go with cheaper inductors. A pair of fountek FR88-EX are on the way, as well as a pair of Aurasound NT1-204-8D 3/4" tweeters. I was going to go with the fostex ff85k but it was difficult to find and shipping from the one place I found it was too expensive. I'm hoping the fountek works well.

I'm planning on placing the tweeter close to the fountek and crossing it over very high (at 9khz) just to smooth out the top end. I have no experience building crossovers but the electronics repair guy down the street says he can make a crossover for the tweeter. Any tips about adding the tweeter would be sweet.

For the woofer I'm going to use the hawthorne audio silver iris 15 inch drivers that I already have. The specs look almost identical to the alpha 15a.

[ChestofFur]

There are some similar designs on the net that use very efficient full-range or mid-range drivers and I'm wondering how they get those to work. Martins design
says that for this type of design to work the woofer should be efficient and the full-range not so efficient.

Here's a design that seems to fly in the face of what (I think) Martin is saying
Endorphine from Kingston Kitchen

Anyone feel like dropping some knowledge?

ps. Terryo, thanks for the earlier advice on crossover components. I decided not
to take it but only because it was over my head.

[MJK]

Originally Posted by ChestofFur

There are some similar designs on the net that use very efficient full-range or mid-range drivers and I'm wondering how they get those to work. Martins design
says that for this type of design to work the woofer should be efficient and the full-range not so efficient.

Here's a design that seems to fly in the face of what (I think) Martin is saying
Endorphine from Kingston Kitchen

Anyone feel like dropping some knowledge?

He has set the minimum efficiency of the bass driver as 97 dB/W/m. Maximum Qts is listed as 1. The low pass crossover is defined as 2nd order between 300 to 400 Hz. Seems kind of like the designs I have been favoring. Not sure what the efficiency of the mid and tweeter are to get a feel for the combined response.

The problem I have with this site is that he does provide many interesting and creative looking designs but without any initial or final measurements, documentation of the design trade-offs, or any real description of the engineering that he performed to arrive at the design. There is no way to assess if his design is high performance or is just average, if it meets your specific design objectives, or if it is even worth building. A lot of creative thinking but in my opinion not enough hard information to confidently build any of the designs.

Maybe that is just me.

[planet10]

Originally Posted by mr ed

Not having heard a ff85k nor the f120a I can only hope the difference is small versus the cost difference in a situation where the bass is not an issue

Ed,

When you take away the need to produce bass, there is no contest in mind. I'd pick the FF85 (even ignoring price). You give up some efficiency, but as Martin pointed out early on, that can mean more extended bass (with a passive XO anyway).

Note: my comparisons are based on treated drivers, not stock.

dave

[mr ed]

Originally Posted by ChestofFur

There are some similar designs on the net that use very efficient full-range or mid-range drivers and I'm wondering how they get those to work. Martins design
says that for this type of design to work the woofer should be efficient and the full-range not so efficient.

Here's a design that seems to fly in the face of what (I think) Martin is saying
Endorphine from Kingston Kitchen

Anyone feel like dropping some knowledge?

I built these and they do not work.
Now with that statement I have to say the Saba drivers are incredible and can reproduce music with amazing clarity and detail. They can be wonderful run by 1 or 2 watt set amps, But bass integration is difficult because they are so efficient.
The crossover recommendations in that link are off, I dont know what he smokes
I may for kicks try Marin's Hframe and turn down the Saba via active cross over, but I think ultimately to follow Martin's guidelines for fullrange drivers will yield best results.
Thats my 2 cents from living with them for about a year

Ed

[mr ed]

Originally Posted by planet10

Ed,

When you take away the need to produce bass, there is no contest in mind. I'd pick the FF85 (even ignoring price). You give up some efficiency, but as Martin pointed out early on, that can mean more extended bass (with a passive XO anyway).

Note: my comparisons are based on treated drivers, not stock.

dave

You are confirming my speculation David. I see no reason to spend the doe on the 120's, a smarter bet would be to buy your modded drivers.

Ed

[Garytr]

Hi,
I have been following the FF85K OB with great interest. I have 8 of these but have not heard them yet.I know some of you are very familiar with these. I have never made an OB, but want to give it a try, I used to make magnetic planers and loved the sound.
I have three main concerns I will put out : 1) A large baffle may give a diffuse/foggy sound stage. 2) The FF85K has little excursion, a small diaphragm, giving limited output. 3) Losing more efficiency with BSC.
My proposal, use three of them in MTM, (Mid-Full-Mid) above an OB sub, rolling off the treble of the outer two to avoid combing and baffle step losses. The narrow baffle should image well. The added output would be welcome as well. I realize there are many considerations here, but any thoughts?
Gary