Crossover question

[jtrosky]

So I have an under-seat subwoofer (JBL BassPro SL). It's built-in crossover has a 12dB slope - and the internal crossover is not "defeatable". I'm running everything active via a DSP. Right now, I have the crossover dial on the sub itself turned up high (around 150hz, I think), just so that I can control the crossover via the DSP instead. However, if I have the subs crossover set to 150hz on the sub itself (again, 12dB slope) and then setup a 24dB crossover for the sub channel in my DSP - at say 80hz, does the slope actually become a 36dB crossover, even though the crossover freq in the DSP is well below the crossover freq set on the sub itself?

Basically, what I'm asking is this: In this situation, where the sub has an undefeatable 12dB crossover slope, should I really be setting the slope to 12dB in the DSP if I actually want a 24dB slope?

Thank you!

[jtrosky]

Anyone? I know you more experienced guys out there know the answer to this easily. :-)

Although, I guess I could just measure the results with the DSP set to both 24dB slope and 12dB slope and find out the answer myself, huh? ;-)

[Weigel21]

May in fact be quicker and easier to do just that.

It may cause some sort of unwanted phase shift by using a 12dB and 24dB filter together.

[BigAl205]

I've never run across this personally, but my thinking is the crossover slope for a low-pass would be whichever is lower in frequency. IOW, if the processor is crossing it over at 80hz with a 24db slope, the 100hz crossover on the amp won't even see much of a signal TO filter out. Any signal left would just be cut another 12db.

[papacueball]

There was a discussion about this on MSS. I think the best thing to do in this situation would be to use both crossovers at the same frequency with 12 dB slopes, avoiding an unwanted phase shift in the 150 Hz range.

[Smitty]

Is 150Hz the highest it will go?

[jtrosky]

Actually, I stated things incorrectly. The max crossover frequency is 120hz, so it's currently set to 120hz.

I like the idea of setting the sub's xover freq to match the DSPs xover freq and just set the DSP use the same 12dB slope as the sub - thereby getting a 24dB slope - that makes sense! The only potential "issue" is that it will be hard to be "exact" on the sub xover since it's just a dial without many markings, but I should be able to use measurements to get it close enough.

Thank you all for the help - I really do appreciate it!

[jtrosky]

Ok, another unrelated question regarding this under-seat sub... While reviewing the subs documentation to see the crossover range, I noticed this in the specs:

Input Sensitivity:
- Line-level input: 0.2V -2V
- High-level input: 2V - 20V

Right now, this is not a problem being that it's connected to a DSR-1 DSP (low voltage RCA outputs), but when I replace my DSR-1 with a Helix DSP.3, which has 6V line-level outputs (I think), what happens being that this under-seat sub says it only supports .2V - 2V on the line-level inputs? This really goes for any device with low-level inputs - what happens when the signal coming to the device has a higher voltage than the line-level input supports (which is only 2V in the case of this under-seat sub)? Does the device (under-seat sub in this case) just lower the voltage down to it's max supported voltage?

Do you think I will have issues connecting this under-seat sub to my Helix DSP.3?

Thanks again for all of the help - I appreciate it.

[Jdunk54nl]

In the output area of the helix software, you will have to turn your gain down on those channels to make it max 2v. Then set gains on sub amp accordingly. IIRC doubling/halving the voltage is +-6db, so to cut by 1/3, it should be around -9db..Someone correct me if I am wrong. But I would personally attach a DMM to the output and check the level to be sure.

[Souths1der]

I'm with Big Al up above, if you have that sub maxed out, there may not be much signal left to filter. But if you're worried, I would just use the built in crossover and leave the Helix in bypass for your sub channel, but invert the polarity. Set the crossover on your sub to 80Hz. Change the highpass on your woofers to 80 12db slope in Helix.