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Old 26th May 2014, 06:41 PM   #1
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Default "Fait accompli" MK II

I guess I should wait with this post, but I really have to share this because the sound is so intensely satisfying.
It all started when I found an add for 4 Seas 25tv-ew, 10" alnico-woofers. Much like the two I had in my previous attempt, except those are ferrite. The guy asked $90(!) for all four of them, and they where in perfect condition. So what to do, buy of course. The idea I came up with, was a narrow open baffle with the Aucharm/Diatone P610 (of course) and a couple of Vifa XT25sg90-04 wired in series, one forward, one backward, and behind this an open baffle sideways with two woofers in different direction in push-pull. The woofers are 16 ohm so the would be in parallell. The Seas woofers being what they are, need no inductor and the tweeters would just have an 8.2 uF preceeding them. Said and done. The result? Utter failure. Thin, bottomless, a disappointment. As I could have guessed, no real surprise, but I wanted to try. Anyway, this is where the fun starts. I had the drivers. I also had my old attempt, the "Fait accompli" with a 10" in a 33 liter aperiodic box firing downwards. So what if....I took the boxes opened them, put one woofer in the bottom, firing upwards, into the box, the other one firing sideways, wired them push-pull, with the bottom one being out of phase. Put as much stuffing back in as the drivers allowed, closed and wired the two things together, the two 10" woofers push-pull in a 33 liter aperiodic box, 2 x 16 ohm in parallel, with the 6.5" Aucharm p610 and the two 4 ohm Vifa tweeters in series ( dipole, as the backward tweeter is reversed) in a narrow (20 cm) open baffle. And I can´t get my fill of listening to this arrangement. For the first time I truly experience the concept of soundstage. Open, lifelike, three-dimensional sound. The music, with every detail is in my room and the speakers are auditively invisable. But, it looks like hell though with the bassbox and the tall open baffle with its empty bassbaffle. I do have to find a way to merge these units together in an aestethically acceptable manner. ( and I do have a small fear of loosing the said soundstage if I do so, maybe it is a result of the sound being divided in two units per channel?) But the fact remains, 2 woofers, the fullrange and 2 tweeters, with only one capacitor! And a sound that leaves nothing more to wish for....
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Old 27th May 2014, 01:48 PM   #2
DeonC is offline DeonC  South Africa
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Pics, please.
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Old 27th May 2014, 02:40 PM   #3
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OK. As you have gathered I´ll win no prizes for the design (yet). Hope the pics will tell the story.
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Old 27th May 2014, 06:16 PM   #4
system7 is offline system7  United Kingdom
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This thread seems to be named with some irony. How can a "Fait Accompli" have a Mk II version?

For all that, it's a labour of love, with some good points. The SEAS 10" driver is kinda nice.

And the small XT25 has some good qualities, including a much better sound than your average soft-dome. But implemented very badly here IMO. You can't cross it over on first order filter at 2.5kHz. It just has terrible Fs resonance problems without a serious tweeter filter and tends to distort at the first hint of midbass.

This is definitely what Lynn Olson calls a LGWAG material speaker that won't go loud comfortably: Can you have sparkling treble but without sibilance

A SEAS 27TFFC is the usual weapon of choice here if you don't use a wildly expensive SEAS T35. But I would also consider this Monacor HT22/8 as well on 4.7uF and some resistance around 8R:
HT-22/8

Sorry to rain on your parade, my friend, but the Dynaco A25 approach relies strongly on a tweeter that can handle lower than average frequencies. So I hope you find my comments helpful.
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Old 27th May 2014, 06:42 PM   #5
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The irony wasn´t intentional, but now that you mention it, it´s kind of amusing. Didn´t really think of that. Anyway, I see your point, I guess I have to turn up the volume to neighbourannoying levels to check. However, you´re forgetting that the main player here is the full range. Both the woofers and the tweeters have slightly lower SPL and are there merely to support it. And I know from my single driver efforts ( with this driver), that the tweeter isn´t crucial. In this design the woofers are the really needed ones as I won´t get any real bass off the p610 in an open baffle. The tweeters are just there to give some air( which they surely do, I might add), so if I do find it to be a problem I´ll just swap the cap for a slightly smaller value.
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Old 29th May 2014, 04:33 AM   #6
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So far, no problems. However, I don´t really have the amp-power to verify this distortion-problem. I only have 2x8w single ended 807 and 2x25 2021 tripath. And with both these amps full volume stays clean. Then again, as mentioned earlier, the tweeters are somewhat hidden behind the full range. As for a MK III, the solution is of course not a smaller cap, but using the scanspeak 83200 (XT25tg30-04) instead. Does anyone have an opinion on this (crazy) idea I woke up with today. In a MK III I was thinking of using the alnicoversion of the full range, but that driver has a little higher SPL so I was thinking of putting an attenuator (L-pad) on it so that i can dial in the right balance....But L-pads are normally used on tweeters, what would the drawbacks be of using it on a full range?
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Old 31st May 2014, 01:18 PM   #7
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Well, well, my, my... It dawned on me that my aperiodic box was to small for the two drivers. Not that I had any complaints about the sound, but theoretically....Starting to think about resizing, my mind sort of drifted towards transmission line. Now, feeling a bit lazy and not wanting to start the whole process of building again I remembered the various "sonotube" subs I´ve seen. Found something similar locally (paperfibertube for casting concrete pillars, 25 cm in diameter, 1.25 m in lenght) Filled them up with some polyesterfilling from cheap IKEA pillows, 700 g in each. So now the whole thing looks even worse, but sounds even better. ( I wonder if it might even be a true full range now, i.e. down to 20 Hz. Well maybe that´s hybris, but we are shure talking sound in the high end of the scale. Bassdrum-kicks have never been more present and true to life. I mean, there is really low bass, but it´s not the syntetic powered subwooferbass. It´s tight, detailed and natural. And most important - only present when there actually is some basscontent in the music, that is where subwoofers often go wrong, adding bass when there´s not supposed to be any. And all this still with just one cap. (!)
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Old 31st May 2014, 06:54 PM   #8
system7 is offline system7  United Kingdom
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You could load smaller images IMO. They take about 10 seconds to load...

I just realised you are using a quite remarkable 6" fullrange driver here. The Aucharm/Diatone P-610 is exotic and expensive to say the least. In fact it strongly resembles my cheapie retro Sony E44 bass, which is a really nice driver too.

Click the image to open in full size.

The Sony has a remarkably good mylar cone tweeter too.

Click the image to open in full size.

Sony used a single 2.2uF cap too, but I prefer a proper crossover.

I think it was Lynn Olson who said to trust your ears with drivers. Some very simple ones can sound much better than the expensive offerings from people like scanspeak.

I do wonder if you could use those lovely SEAS A25 drivers more effectively though. They seem wasted as subwoofers.
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