I just finished these speakers today. They are a sealed cabinet, Qtc = .7, for feastrex D5nf. It was originally built in white pine boards, when i was on my Altmann inspired "raw wood to get into heaven" kick. Unfortunately, it worked well on the first time I tried it and poorly on every time since then. Now I finally understnd why. I got lucky. It is hard to make a resonant box work. It is easier to make a dead box work. These were unlistenable in pine. So I wallpapered it with another layer of 3/4 MDF. It still range a little on the rap test (singing rap into the driver hole.) So I glued in some braces so max unsupported span is <6" as B&W research suggests. Voila! Dead. A few handfuls of pink FG, and I have a hifi Feastrex speaker. It sounds OK!
I had been using a big vent reflex (BVR) horn design by Planet-10. I really like that speaker, but it was made out of 100% MDF. I think the coloration is a combination of mild horn midrange echos and mild phase distortion but moreso the resonance. It worked well with voltage source amp, like Sol's i60. But Sol advised me to switch to current source amp for use with single driver speakers. That was a result of the first appearance of feastrex at a g2G 2 years ago. I had ruined those first pine boxes by bracing and stuffing them to "make them better." Tat combined with a bad match with Cary SLI80 a voltage source amp, Sol said he would design a current source amp for me.
Then I saw a post on AC by a guy announcing the publication of his book about current source amps in hifi. i got the book for Christimas and enjoyed it. Sol read it and gave it his blessing, the guy seems like a whack job to normal brainwashed audiophiles who like things just they way they are now, thank you. But his claims matched what Sol told me about current amps a year before.
The problem with current source is that it cannot control the driver AT ALL. It wants a pure flat impedance curve. It can only be used on single driver speakers, no passive crossovers, unless they are designed specifically for current source amp. Drivers should have a low mechanical Q so their resonance is low and it can damp itself. There are few speakers like that nowadays. Acoustic suspension was like that, but that's out of favor now with reflex better understood. Reflex is double bad for current source amp because you have the driver resonance 110Hz on feastrex D5, plus you have the resonance of the port playing low the notes with air spring energy in the box. All this screws with the impedance plot and the current source amp chokes, making big loose flabby bass. But the mids...
OOOOOOhh the mids and highs are ALIVE! Dense, thick tone, presence, wicked beautiful sound. So after hearing traces of it with the CS amp Sol designed for me I sought to optimize the system for the amp. The voltage amp was better in the bass because it can control things with reflex ports of the planet10 BVR design. But it didn't have the midrange.
So I pulled out these sealed boxes, fixed the pine, put a zobel network on the amp to flatten the driver's impedance in the high freqs and now I have it tuned in pretty well. It sounds great. It sounds much better with the CS amp than it does with the VS amp as it should, single driver speakers should. I still need to scan the impedance with my woofer tester 3 to see what the actual impedance looks like, electrical impedance in the top freqs, and mechanical impedance in the lows are all visible on one graph with WT3. The zobel can be tweeked to flatten the highs, but the bass impedance must be flattened with stuffing to a point, then an aperiodic port must be used like a shock absorber to control the spring of the air inside the box. It is a port blocked by air resistive material (fiber). The amount of fiber or size of the hole is adjusted until the impedance is flat. Scan Speak aperiodic vent does this:
https://www.madisound.com/store/product_info.php?cPath=404_116&products_id=1323It is a tricky skill to dial out the low freq resonance with aperiodic port. Don't really know how well it will work. But my resonance is at 110Hz, so it needs to be addressed. But thankfully, in the small box, the bass is already much better controlled than it was in the BVR with CS amp.
Eventually I want to make these Q.7 speakers in a sphere shape for minimal diffraction. I heard some of those at RMAF 08, and loved the sound. There is a certain free, open, not there sound of a sphere. But they aren't easy to fabricate.
Here are the pictures. The dark spots are stains made fron clamps. I think it is just from the MDF, but it could be from rubber feet on the cheap Home depot clamps. I will round over corners and paint them to match the green fireplace. I made some stands to go with them. They put the driver centers at 39", my ear height. They are 27" tall, but hopefully they will work with other normal bookshelfs that come to visit now and then. They are hardwood ply, glued and stapled together, filled with sand in the center column. Spikes are coming, as is a 1.5" inch thick top platform, and finishing.
Overall I think the sound is great. The bass rolls off slowly, so there is some bass, but it is only 3.5" driver so you can't expect too much. Bass is musical, but there is no impact. I will try it with a sub tomorrow. But Feastrex are notoriiously difficult to mix with a sub. At 50Hz maybe not too bad.
The little silver amp on top is the current source amp, c25, 25wpc based on IC chip. The i60 is below that. Sonos, TT, Tuna, CDP also visible.
That's all I can type tonight. Here's your audio porn Rob: Thanks for pushing me to finish. Now push me on the acoustic treatments...
