Originally Posted by
RMC
Hi Eduardo,
Its generally a good idea to put a protection capacitor in series with the compression driver in case of electronic crossover mishandling or malfunction for example.
You didn't mention which 18" woofer you're actually using, but since you previously mentioned the EVM18B S2, I'm assuming here this is the one.
The 12 db/oct 500hz xo in 2446 specs is indicated as minimum. I would crossover a little higher to make some meaningful room for the protection capacitor. Below 700hz the compression driver's response falls rapidly anyway. Since the woofer is a musical instrument driver it has rising response which you can take advantage of by crossing over just a little higher such as 700-800hz. The 2446 sure won't mind.
The rule of thumb for blocking capacitors is to locate it about an octave below crossover frequency to minimize interactions, so moving xo a little higher you can now put a good quality blocking capacitor for 400hz roll-off.
Btw, as i recall JBL never made an 18" woofer version of the 4530. To my knowledge there isn't a JBL blessing on such design. They could easily have done one if they wanted to having the 2240, K151 and E155 drivers, all three suitable for horn loading as per JBL. But they didn't.
I remember reading years ago in one of the Pro Audio magazines a reader question asking about scaling up or down the 4530 cab. The Engineer who replied (maybe from JBL) said it wouldn't be proper. There would be issues with the horn's throat and/or mouth making it less than optimal. Possibly why JBL never made one...
I know many guys have made some of their own brew anyway. But that doesn't make it an approved design per JBL engineering standards. No doubt it will still produce sound, then you can enjoy it, however i'd suggest to avoid beating on it too hard. A 4530 type box doesn't go deep, its rather made for bumped up bass around 60-100hz...
Note the EVM woofer has Qts of 0.36, this is very high compared to drivers JBL suggested for use in such rear loaded folded horn, having Qts of 0.17 to 0.21 (maybe stretchable a little). My guess is that the higher Qts EVM may further increase the cab's bumping bass by design, which may not be that bad here when looking at woofer response curve showing LF level being lower than the rest. Indirectly might be sort of a "balancing act" then.
Richard
TO Short Circutz2
Simply a precision, the EV SH... cab you mention uses the EVM 18B Pro-line rated 400W, whereas the poster here has indicated using the EVM 18B Series 2 rated 200W. Just don't want him to think that he can go 400W...