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Old 23rd September 2012, 10:49 AM   #11
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It is impossible to say what impact changing the amp would have, possibly your DIY amp is not that good if you had those results.

Those jack connections will also be balanced just like the XLRs, it's not a problem to connect either.
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Old 23rd September 2012, 11:44 AM   #12
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Always, always use a DC blocking cap even with an active crossover for the mid and tweeter. Maybe, some mids are robust enough. Depends on what they cost and if you are willing to take the risk. A good $20 poly cap is a lot cheaper that a tweeter.

Be sure the gain of all the amps is the same. That goes for the sub amps too. Other wise the balance will change when you change volume.

A couple other advantages of active: It is a lot easier to add extra filters to help the bass along in your room, BSC and of course, dealing with the driver impedance is then the amp's problem, not the crossover. Active usually allows 24dB slopes, which I far prefer for a sub as I can raise the crossover to as high as 80 Hz and still not localize the sub. (If I have a plate amp, I add in it's crossover for a very steep cutoff) Using a typical "sub crossover" I have to drop to about 60 Hz.

There are plenty of adapters for RCA to phone or RCA to XLR. Piece of cake. I just make my own cables as I like to keep things short and tidy. I use a cheap Bheringer in one of my systems. I really need to upgrade to a DIY one of these days. Most any discount music supply store has the adapters cheap.

I was amazed the difference in sound when I replaced a pair of plate amps with a used Parasound on the subs. It should not have made a difference, but it was striking.
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Old 23rd September 2012, 11:45 AM   #13
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Oh yea, this thread should be moved to multi-way not shouldn't?
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Old 23rd September 2012, 01:24 PM   #14
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Moved to multiway. I hadn't even noticed! Good tip on the gain of the amps, something I had not thought about, and would have had me scratching my head when I finally get my active crossover done for my "subs"!

The gain on the two amps I was planning to use is quite different, I figured I'd just attenuate the higher gain one (as I can't easily change it due to a very compact p2p construction that makes changing the gain resistors almost impossible. I might need to make the BA3 sooner than I thought...

Tony.
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Old 23rd September 2012, 06:55 PM   #15
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Thank you for the tip!
So those DC blocking caps, how does they work, what's their deal in the cross-over?
Is there a formula to calculate their value regarding the cut-off frequency of each driver? I'm sorry, this is defenitely a 'noob' question, but I'm quiete new in cross-over networks..
And what do you mean with 'sub', tvrgeek? Do you mean subwoofer or just the woofer (eminence delta 15lfa in my case)?
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Old 23rd September 2012, 10:19 PM   #16
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basically a cap in series with the mid and tweeter will protect the speaker agains DC (should the amp develop a fault, or pass through any DC from the source. The cap will be a first order crossover, so needs to be chosen to roll off below the active crossover frequency.

You will need to know the impedance of the speakers to choose properly. (or you could just go bigger). here is an online calculator to help Crossover Design Chart and Inductance vs. Frequency Calculator(Low-pass)

I'm not sure how far below the active crossover frequency you would want to put the passive crossover point, to avoid having much effect on your active acoustic slope., I'll do some spice simulations later today to get an idea

edit: I did a quick sim using a 6 ohm resistor as the load. note that the lower the impedance the bigger the cap you will need. note that the sim is overly simplistic but gives an idea of how the cap will affect response. I'd be tempted to say 100uF for the tweeter, but that seems a bit extreme, perhaps tvrgeek can chime in as I have no practical experience in doing this (having only done passive crossovers).

see attached pic.

Tony.,
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File Type: png crossover_sim.png (25.5 KB, 95 views)
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Last edited by wintermute; 23rd September 2012 at 10:31 PM. Reason: add sim.
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Old 24th September 2012, 03:24 AM   #17
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Here is a comparison with different impedance loads. A speaker presents a complex load that a resistor cannot simulate, however the graph shows the importance of knowing roughly the impedance of the driver. If you measure the dc resistance of the voice coil that should give you the worst case impedance, size based on that and you should be ok.

Tony.
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Old 24th September 2012, 03:41 AM   #18
Pano is offline Pano  United States
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If you really want to do this right, you need to measure. A mic, a soundcard and some free software will get you there. Oh, and some help on this forum.

For really finding what works, the Berhinger DCX2496 digital crossover is great. You can keep it (and upgrade it) or move on to a passive crossover as you see fit.

You need to measure first, for sure. Very nice looking set-up, BTW.
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Old 24th September 2012, 04:38 AM   #19
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To aid in your calculations:

If your JBL 075's have genuine JBL diaphragms in them the actual impedance throughout their working range is right about 10.2 Ohms.
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Old 24th September 2012, 12:49 PM   #20
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Ok, I think I understand.

So for the JBL tweeter (2402), I found a DC impedance of 6.25ohm on the spec sheet.
If I would do an active crossover at 4500hz (this sounded quiete good when I tested) and the minimum crossover frequency of the jbl 2402 tweeter is 2500hz (spec sheet), I thought of a DC blocking cap that cuts off at 3500hz.
The calculator (first order cross-over at 6db/octave) gives me a result of a cap with the following value: 7.268571428571429 f
Could that be correct? Or do I need a second/third/... order crossover?

So, I have to find out the DC impedance of the midrange driver too and to calculate the value of this cap too, isn't it?

Is there a difference when putting the cap in series with the driver if I put it on the + or the - of the driver?

Yet another question: are digital cross-over much better then analog cross-overs? I found some analoge X-overs for a reasonable price, but those aren't as advances as the digital behringer suggested...
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