kmaier said:
Right, this is the standard and what I strongly implied/assumed, though didn't specifically state, here: http://www.diyaudio.com/forums/showthread.php?postid=1387387#post1387387
GM
Correctomundo! You want to raise the driver's in-box Fs (Fc) at least 50%, so if the driver's measured free air Fs = 100 Hz, Fc = >150 Hz, then Vas = Vb*((Fc/Fs)^2-1), ergo you can see why an accurate Vb is essential for good, repeatable accuracy.
Based on this formula, if we assume a Fc/Fs = 1.5 and use one of the published D5 Vas, then the test box's Vb + diaphragm air load = 5.4951/((1.5^2)-1) = 5.4951/1.25 = ~4.396 L (~0.155243 ft^3). In his suggested 1 ft^3 box, 7-8 could be tested at a time and averaged out.
Anyway, there's more than one way to derive Vas, so I'd sure like to know what procedure he has in mind that requires a box Vb that swamps the driver's Vas, i.e. a nearly IB test box. Accurate efficiency measurement? Or is this a 'worst case' for the field coil drivers and slugs would be used to reduce the bax Vb as required? If so, seems like several test boxes would be quicker over the long term.
GM
Based on this formula, if we assume a Fc/Fs = 1.5 and use one of the published D5 Vas, then the test box's Vb + diaphragm air load = 5.4951/((1.5^2)-1) = 5.4951/1.25 = ~4.396 L (~0.155243 ft^3). In his suggested 1 ft^3 box, 7-8 could be tested at a time and averaged out.
Anyway, there's more than one way to derive Vas, so I'd sure like to know what procedure he has in mind that requires a box Vb that swamps the driver's Vas, i.e. a nearly IB test box. Accurate efficiency measurement? Or is this a 'worst case' for the field coil drivers and slugs would be used to reduce the bax Vb as required? If so, seems like several test boxes would be quicker over the long term.
GM
Great idea about playing it into the box backwards, somehow I missed that. I agree, calculating cone volume is a lot easier. Error will be smaller too, there's no geometric formula for a spoked speaker basket. It's cold here now, so all the woodworking fun stops till the weekend when it warms back up.
I have been getting to know Quarter-wave mathcad sheets the last couple days. Very cool! Worth $25 to check it out. Martin is very helpful in helping a neophyte get started. Thanks Martin!
Rich
I have been getting to know Quarter-wave mathcad sheets the last couple days. Very cool! Worth $25 to check it out. Martin is very helpful in helping a neophyte get started. Thanks Martin!
Rich
Feastrex in the News
High praise from a Stereophile magazine reporter:
http://blog.stereophile.com/ces2008/010808feast/
A useful write-up in Six Moons:
http://tinyurl.com/ysl8x9
-- Chris
High praise from a Stereophile magazine reporter:
http://blog.stereophile.com/ces2008/010808feast/
A useful write-up in Six Moons:
http://tinyurl.com/ysl8x9
-- Chris
Finally completed the BVR enclosures and have them playing... great drivers... and a very nice enclosure design... thanks! Here's a link to some pics:
Regards, KM
An externally hosted image should be here but it was not working when we last tested it.
Regards, KM
Thanks Chris... a bit of a challenge with the Oak, but ended well. Here's some intial feedback (also on AudioCircle):
First things... they weigh in at a scant 40 pounds each... but the bottom chamber under the port is empty. Having the spikes is nice, easy to level and have firmly on the ground... down side, 40 pounds on 4 spikes... not enough weight... lead shot is on the agenda. Have spent a few hours listening to them but also did numerous hours of break-in on the drivers which settled out the T/L parameters so they maintained high consistency before they were installed.
Initial listening impressions:
1- first thing that hits you... very quick transient response and is probably more transparent than any cone driver I've heard.
2- they are very sensitive.... play quite load with less than 1/8-watt. (moved the tap from 8- to 16-ohm on my 45 amps)
3- very extended high-frequency range, the sizzle on cymbals and the metallic air from brushes is crisp and clear, like it's right there.
4- BVR enclosures have a good bottom end... no, they don't cover the last 1 or 2 octaves fully, but they are solid and punchy, you feel it.
5- musical... vocals, brass, orchestral, percussion... so far everything sounds excellent (sans a bad recording).
6- excellent control and large (cone) excursion easily allow low to high SPLs with no strain, grain or shout.
So, that's it for now... will report more once I get more time with them and get the shot loaded for better stability. So, initial verdict:
Very happy camper so far... they are very impressive drivers, they may have no rival.
Regards, KM
First things... they weigh in at a scant 40 pounds each... but the bottom chamber under the port is empty. Having the spikes is nice, easy to level and have firmly on the ground... down side, 40 pounds on 4 spikes... not enough weight... lead shot is on the agenda. Have spent a few hours listening to them but also did numerous hours of break-in on the drivers which settled out the T/L parameters so they maintained high consistency before they were installed.
Initial listening impressions:
1- first thing that hits you... very quick transient response and is probably more transparent than any cone driver I've heard.
2- they are very sensitive.... play quite load with less than 1/8-watt. (moved the tap from 8- to 16-ohm on my 45 amps)
3- very extended high-frequency range, the sizzle on cymbals and the metallic air from brushes is crisp and clear, like it's right there.
4- BVR enclosures have a good bottom end... no, they don't cover the last 1 or 2 octaves fully, but they are solid and punchy, you feel it.
5- musical... vocals, brass, orchestral, percussion... so far everything sounds excellent (sans a bad recording).
6- excellent control and large (cone) excursion easily allow low to high SPLs with no strain, grain or shout.
So, that's it for now... will report more once I get more time with them and get the shot loaded for better stability. So, initial verdict:
Very happy camper so far... they are very impressive drivers, they may have no rival.
Regards, KM
Nice work
KM,
Very nice job...Love the oak.
I think your report on the qualities of the Feastrex are spot on.
I too am running them with a 45 amp with a 16 ohm transformer from Magnequest and they seem to do just fine.
I did hook up my subs for a few days but I didn't like the way the sub seemed to mess up the sound of the Feastrex...Mushy
(Lowthers are still in the corner)
Phil
Santa fe
KM,
Very nice job...Love the oak.
I think your report on the qualities of the Feastrex are spot on.
I too am running them with a 45 amp with a 16 ohm transformer from Magnequest and they seem to do just fine.
I did hook up my subs for a few days but I didn't like the way the sub seemed to mess up the sound of the Feastrex...Mushy
(Lowthers are still in the corner)
Phil
Santa fe
Phil,
Thanks for the kudos... yea, oak looks nice. Due to the simple fact that they can reach high SPLs without break-up I'll be trying some higher powered amps once I get them built... both a 2A3 based amp (only another 3 dB) and a WE300B based amp which will push up towards 10 watts, or another 6+ dB output. I'm curious how linear the response is once you get larger excursions.
As for subs, I've had the same situation with the Quads... nothing really seems quick enough and makes the bottom end feel loose, sloppy and muddy. However, I still plan on finding a way to get matching subs at some point. Hopefully before the end of this year I'll get a prototype TL for a NOS pair of Kef B139 drivers done. I'm hoping these will match either the Quads or the D5nf drivers.
Regards, KM
Thanks for the kudos... yea, oak looks nice. Due to the simple fact that they can reach high SPLs without break-up I'll be trying some higher powered amps once I get them built... both a 2A3 based amp (only another 3 dB) and a WE300B based amp which will push up towards 10 watts, or another 6+ dB output. I'm curious how linear the response is once you get larger excursions.
As for subs, I've had the same situation with the Quads... nothing really seems quick enough and makes the bottom end feel loose, sloppy and muddy. However, I still plan on finding a way to get matching subs at some point. Hopefully before the end of this year I'll get a prototype TL for a NOS pair of Kef B139 drivers done. I'm hoping these will match either the Quads or the D5nf drivers.
Regards, KM
Drawings Posted
Mr. Teramotos's mechanical drawings of all but the D9 MA driver are now posted at the Lotus Group Website:
http://www.lotusgroupusa.com/Feastrex1.htm
Scroll to the very bottom.
We should be posting the D9 MA drawing soon.
Thanks,
Joe Cohen
The Lotus Group
Mr. Teramotos's mechanical drawings of all but the D9 MA driver are now posted at the Lotus Group Website:
http://www.lotusgroupusa.com/Feastrex1.htm
Scroll to the very bottom.
We should be posting the D9 MA drawing soon.
Thanks,
Joe Cohen
The Lotus Group
Quick deep bass is an oxymoron though. The 139 isn't the greatest LF unit in the world any more (if it ever was) but it's a good unit for TL loading. One one the problems with it was that people would insist on running it up high (sometimes past 1KHz ) which it flat isn't suited to, even when you fudge its natural problems with a myriad of notch filters. XO at or below 100Hz to your main units & you're in business though. Personally, I'd prefer a couple of low Q 15in JBLs or classic Altecs, as you can do more with them, but it does a decent enough job like that.
) which it flat isn't suited to, even when you fudge its natural problems with a myriad of notch filters. XO at or below 100Hz to your main units & you're in business though. Personally, I'd prefer a couple of low Q 15in JBLs or classic Altecs, as you can do more with them, but it does a decent enough job like that.
That's my point. A longer wavelength a) takes longer to produce & propagate (otherwise it would be a short wavelength) and b) naturally has a slow decay time. It's not easily damped in-room.
WRT the drivers themselves, one of the reasons I generally favour low Q drivers is their better damped behaviour, so on balance I like as powerful a motor as I can get. The B139 isn't remarkable either way in this respect IIRC. Amplitude is also a consideration -lower the amplitude, lower the stoke distance so the quicker it will be reproduced, with obvious caveats of course.
WRT cabinets, given that a horn is impractical for extremely low audio frequencies (with a couple of exceptions like TD's double-tapped design) a TL or EBS cabinet of some kind is generally my choice of monopole source. Dipoles are nice & (nominally) ~30% less energy goes into exciting room-modes according to Linkwitz, but flip side is they need bucketloads of power to compensate for their natural roll-off, so YMMV as ever.
WRT the drivers themselves, one of the reasons I generally favour low Q drivers is their better damped behaviour, so on balance I like as powerful a motor as I can get. The B139 isn't remarkable either way in this respect IIRC. Amplitude is also a consideration -lower the amplitude, lower the stoke distance so the quicker it will be reproduced, with obvious caveats of course.
WRT cabinets, given that a horn is impractical for extremely low audio frequencies (with a couple of exceptions like TD's double-tapped design) a TL or EBS cabinet of some kind is generally my choice of monopole source. Dipoles are nice & (nominally) ~30% less energy goes into exciting room-modes according to Linkwitz, but flip side is they need bucketloads of power to compensate for their natural roll-off, so YMMV as ever.
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