Hi Lusk, and anybody with else with the patience, it's been a long day in the chariot for a recently-dissected specimen (I meant me, but the speakers too).
Quite a few changes, quite a few loose ends sorted:
The new 3.9 mH C-coils went in, as inductors for the 8" woofer. These have done the job of raising the sensitivity of the upper 3-ways, better matching the bottom 15" woofers.
I put the Scanspeak tweeter back in its little L-frame to return for duty as part of my Centre speaker, in the 7.1 system at the house.
Fitted the other SB tweeter back in the 3-ways, so they're now symmetrical again.
Put the 12 + 15 R = 6.67 R resistor parallel pairs in, as tweeter attenuation. This worked: it sounds more like a proper working system with the symmetry. May need to go back to 6R8 if it stays this bright, but will wait till it's played in a bit more.
Fitted new 39 uF Cross Caps to the bottom 4th Order passive LP filter. It had been a bit of a bird's nest with paralleled caps, so is now one level neater.
The upper 3-way is still based on Troels' SB 3-way crossover. This remains all 2nd Order, with no HP woofer filter, other than acoustic roll-off by the sealed 40 litre box. Main changes to the design are:
- SBA TW29RN tweeter, instead of cheaper SB29RDC-C000-4.
- SBA SATORI MW16P-8 mid, instead of smaller SB12MNRX25-4.
- Troels Gravesen/SEAS JA8008 woofer (94 dB, as opposed to 88.5 for the original SBA SB23NRXS45-8).
- 8" Woofer C-coil has much lower DCR, and nearly double the mH, due to using the JA8008 driver. Also to cut its mid-range output to suit XO to mid.
- Mid gets 50% more uF in HP section to lower crossover point to woofer. Also a CR across its LP coil, to tame upper roll-off and XO point.
- Attenuation adjustments, and placement of resistors within filters.
- And of course addition of the bottom SBA SB42FHC75-6 15" woofer below, in its 240 litre vented box. It has 4th Order passive filter.
Today I re-wired the whole caboodle to run as a 4-way passive again, all off Linc's Bel Canto Pre 1 and the Parasound A 21 power amp. (I think the A 21 enjoys having something to do for a change).
Sorry, pictures might be a better way of presenting all that, but I'm not skilled in doing hybrid photos with text added.
The upshot of all that is a pretty nice speaker system. I ran a few of the usual CDs through - Prokofiev, Stravinsky and Mahler, but look forward to getting the Server plugged in to deliver some Hi-Def files. Just in case there was a good drum track, I played some bits from Abbey Road, but it didn't have what I expected. But quite a shock - I've never heard such separation, such clarity, such a raw '60s sound, and all the lyrics so clear. Just goes to show how attitudes, presumptions, and memory, change over 2 years - I really need to go back to previous speakers to get this in contrast. I've created something pretty different, and I'm getting to accept that it's a big step up from what I've had in the past.
The voicing sort of works, with, as I mentioned above, maybe a bit too much brightness from the tweeters. The prototype tweeter 'wings' are still on, which does brighten the treble.
What bothers me is a kind of 'cuppy' sound from the mids. I had this complaint with the original 4" mids, but maybe this EBP thing, mentioned in my previous post, might just have the clue. Both drivers are sealed in accordance with WinISD - correct volume for Qtc of 0.707, but both sound wrong to me. I've checked their EBP numbers, and guess what? - they're both about 140, which is what Per said needs vented enclosures!
The new AT 8" would be expected to have as little bass as possible, so I can keep the sealed (it would reduce to about 30 litres) box and not have to make an expensive HP filter for it.
Another puzzle is that the 15 H 52 06 13 SDKA mids that I'm talking about with Per have an EBP of 130, and he says 4-5 litres sealed would suit them. I have a note here saying "Glasswolf Pages", so I probably need to look that up again and try to work out something. For this mid, Troels uses an aperiodic vent in 4.9 litres, so I can build that in, with some difficulty (vent might have to be out one side).
Fitting a vent and doubling the inner box size for the present mids would be a major rebuild. So I'll keep considering the AT drivers, which would need a rebuild, but would be (more?) permanent. Yeah, that sounds good. Give me more of this transparency thing I keep hearing about, please Universe?
So @lusk, please spell it out for me simply, if you can advise on EBP - and anything else that comes to mind of course.
As usual, I welcome comments or advice from all readers. If you're shy, please PM. It could save a lot of dollars!