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Old 9th January 2014, 05:47 PM   #21
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The high-pass will definitely be needed - the QSC filters off somewhere ~10Hz, but it'll still be putting out a lot of power at 5Hz if you ask that of it.
You could just put an 18Hz high-pass filter on the input to the 3-way board. I see no reason that shouldn't work.

I'm sure you'll like the QSC amp, and FWIW you could build it into the subwoofer enclosure. It'd need a seperate compartment from the driver(s), and suitable ventilation**, but should be reasonably easy to do. The only downside would be the weight of the finished product. The amp ain't light, and neither are subwoofers. I suppose that one's up to you.

**it'd probably be easiest to leave the front uncovered, and put a hole at the back of similar size to the fan inlet. If you put the amp and driver(s?) on the front face, a bit of speaker cloth would cover everything nicely. I'd probably put the amp at the bottom of the enclosure for stability.

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Old 10th January 2014, 11:36 AM   #22
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When I was cleaning out the 850, I notice alot of variable dials inside, then I look at the spec and apparently there is already sub sonic filters (thats what its called, now I know)
"selectable 30Hz, 50Hz, out"

See specs on this page...
QSC Power Amplifiers - USA 850

The 850 manual says on page 3 under DC fault protection
"The USA Series limits the response below 20Hz to protect the amp"

See this thread also QSC USA 900 Amp Question...

Not keen on touching those variables but I suspect they are filters.
Not sure how I would measure either, maybe just into a 8ohm block and measure with computer...

Also, I've decided to go for a slightly more robust peerless woofer, 10 or 12 unsure, I need to see how much wood costs . I have some lying around at my dads...
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Last edited by giro1991; 10th January 2014 at 11:43 AM.
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Old 10th January 2014, 12:05 PM   #23
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Do not touch those variable resistors.
Chances are they're for bias current. If it works, and there's no obvious distortion, there's no real need to touch them. You could maybe try to reduce the bias to get the amp to run slightly cooler, but the gain would be so minimal that I wouldn't bother: the engineers at QSC are, I suspect, much more experienced than either of us.

Dip switches? Are you sure?
I've had my QSC open (mostly out of curiosity), but didn't spot any. I'll go and have another look to confirm, but I suspect that website is inaccurate.

Edit - just opened mine up. No dip switches in here.

Chris

Last edited by chris661; 10th January 2014 at 12:23 PM.
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Old 10th January 2014, 03:24 PM   #24
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You would think a subsonic high pass filter would be easy to find online, where can I get one?

EDIT Found DIY KIT on ESP
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Last edited by giro1991; 10th January 2014 at 03:32 PM.
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Old 10th January 2014, 03:38 PM   #25
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Quote:
Originally Posted by giro1991 View Post
[SIZE=3]
Because QSC is bridgeable at 8ohm would it not be a good to use all the power available, as opposed to making one channel redundant?
I wont be driving the system to beyond 90dB (or 80dB considering how small the room is).

I'm unsure on final Low Cut frequency because I live in an
end terraced (✓) bungalow (✓) but multiple neighbors quite close.
I don't not know what 25hz sounds like never mind 20hz.
Bridged to 8 ohm should be OK. If you look at car woofers, you can also find dual 4 ohm coils. That might be good, 1 coil per channel. Then later if you want you can add another of the same sub, rewiring each for 8 ohms.

I personally would not put the subsonic low cut filter on the sub. Every filter degrades transient response somewhat. The one time you need something like that is a ported box, if you are playing music with significant content below the port tuning frequency. That is pretty rare, assuming you tune down to 25 or 20 Hz. And, if you want to be playing 16 Hz organ pipes and crazy movie demos, then you really just should use a sealed box anyway. (Sorry, in this view I can't easily see what your final box idea was, if you said)
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Old 10th January 2014, 07:38 PM   #26
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Sealed boxes almost always imply a Linkwitz Transform (or similar), which would, left unchecked, throw out a lot of power at very low frequencies. At the very least, a first-order filter ought to be added circa 15Hz, just to make sure movie effects don't send the coil smashing into the backplate. It only need happen once...

giro, one driver or two?

If you're set on the Peerless drivers, if might be worth going for 2x10" over 1x12": that'd give you a better range of form factors, slightly higher SPL (from increased cone area), at a similar price IIRC.
If they're 8ohm a piece, I'd consider either one per channel on the QSC, or paralleling the drivers and bridging. That's only if you need more power out of the amplifier: if the amp has no problems pushing the drivers to Xmax with one per channel, there's no need to eek out the extra power - the driver displacement is the limiting factor.

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Old 12th January 2014, 11:58 AM   #27
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I'm thinking about trying a PrecisionDevices 1550 in 6th order bandpass
looks like it will work ok, and is not overly expencive
and the benefit ? ... 'wasting' the money on a top quality driver instead of cheap junk ?
but will it make any difference ? ... I don't know
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Old 13th January 2014, 03:43 PM   #28
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appreciate the help chaps

Probably will use Elliot Sound PCB 99
15 delivered from oz for the PCB (not sure if its populated, probably not).

I will go for one woofer for now and perhaps get another If I feel I need more definition.
not sure if I need 12" though it would be nice to match the performance of the 400 from bk, which looks like a 12" paper cone.

Not sure what to make of tinnitus' suggestion?

I read that the lowest musical note is 27hz on grand piano so I think 25hz cutoff is what I need. Does Win ISD take into account the subsonic filter?

I found this on the peerless 830845 (paper). Does it matter that the response is not flat in this SPL graph? I'm assuming it can do flat to 25hz (but at lower level)
Click the image to open in full size.

Not keen on that blue curve though I'm sure REW software could finalize this.
I was hoping to d this project over 2weeks but it seems it will probably take 2 a month or so depending on how fast things get shipped out.

Cheers
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Old 13th January 2014, 04:21 PM   #29
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You can make your own circuit boards using stripboard.

Here's some reading about sealed subwoofers:
ESP - The Linkwitz Transform Circuit
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Old 14th January 2014, 02:21 PM   #30
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Ah makes sense now thanks for the pointer...
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