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Old DIY Box Question

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2K views 8 replies 2 participants last post by  Shakermaker 
#1 ·
Hey all, I have recently run into issues with my DIY sub I built 12 years ago (details below). Just the last few months I had started to notice distortion/flutter/chuffing at around 35hz or so while whatching movies and playing test videos on youtube. I've noticed that the cone has seperated about 1/3 of the way around the cone. I've glued it back on hoping that it'll work the same. If not, it's a 12 year old Cerwin Vega that I can just replace.

My main concern is upon reading up abit more (something I don't think I researched properly years ago) is my sub box too big? At the time, I recall using probably WinISD. Of course being that long ago, I don't have any idea where or if I saved the design files. I have no idea what I 'tuned' it to or why I made it this large.

Outer measurements of box (made with 14mm mdf): W 490mm D 450mm H 820mm - Approx 180 litres (6.3 ft3)

Inner is filled with 3/4" poly-fluff batting.

Ports
Two Flared 200mm x 140mm (100mm to 90mm width end to end)

Speaker Specs: 12" Cerwin Vega

Diameter: 12”
No. of Voice Coils: 1
Sensitivity (1W at 1m): 94 dB
Power Handling: 400 Wrms
(cont. unclipped power per EIA RS-426B)
Frequency Response: 20 ~ 500 Hz
Nominal Impedance: 8ohms
Voice Coil Diameter: 3.0”
Mounting Diameter: 11” (279.4 mm)
Mounting Depth: 5.625” (142.9 mm)


Vega-128 Thiele-Small Parameters
Free-Air Resonance: fs 24 Hz
Total Q: Qts 0.362
Mechanical Q: Qms 3.89
Electrical Q: Qes 0.399
Compliance Volume: Vas 3.08 ft3
Maximum Excursion: Xmax 1.50" (+-0.75”)
38.0 mm (+-19.00 mm)
Voice Coil DC Resistance: Revc 6.4 ohm
Volume Displacement: .13 ft3

Amp: 250w Sub Amp (Cross over and 180 phase etc built in)

Thanks for your help/advice!
 
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#3 · (Edited)
Hey Mike. The internal volume would roughly be 116 litres (taking into account sub volume displacement and the 19mm mdf + 70mm worth of insulation). I'm not sure of the amps total displacement but do recall it didn't protrude further than the insulation I'd used.

The ports are circular, 200mm in length and start as 140mm wide (form the openining of the flare) to 100mm then to 90mm at the end. The rear of the ports are not flared. See my crudely drawn MS paint diagram below. (Not to scale).

http://i.imgur.com/IMj9Bbj.jpg

The layout itself is below, again not to scale and poorly drawn.

http://i.imgur.com/vylF9y6.jpg

I have a feeling the ports may be too close to each other. (Measurements are taken from the inside of the ports ie: at 100mm start of port. Speaker measurements taken from start of rubber rim). The box itself is covered in 2-3mm speaker fabric.
 
#4 ·
Your cabinet is tuned way too high ( around 36 hz) for watching movies. Movie low frequency content goes down to 20 hz and lower so the playing content below the tuning frequency destroyed the subwoofer.

What is the brand and model number of the sub amp? I need to find out if it has a subsonic filter.
 
#5 ·
I've unscrewed the plated amp and had a look at the layout, the closest I can come to finding a model number and or make is 15-A2284-02. I bought this from Jaycar (Australian based electronics retailer) around 1999/2000. From a few Google searches I understand that this amps are actually made by Digitech. I did ask the store if they still sold these amps and they don't. I think they may have stopped selling (or making) these back in 2009.

There's a site that lists (I can't post URL's yet) a '350 watt Jaycar : AA0508' and is similar what mine looks like. However, the circuitry may be completely different. The site mentions mention at the top that:

Most also come with +4dB or +6dB boost at 35hz, which may or may not be a good thing depending on your application
Which, with my current box isn't ideal.

I do have pictures of the circuitry if that helps you see if there's a filter involved? I'm still digging through old manuals warranties etc to find the manual (if any) for this.

Would increasing the port lengths with a pvc elbow or two help matters? Or is this box doomed.
 
#6 ·
Your current box size and ports are fine, you just need to extend the ports.
Assuming 4 db of boost in the Jaycar amp, orange is what you currently have which is a 36 hz tuning frequency.
Green would be if you tune the box to 21 hz.
This can be accomplished by adding 645 mm to each port for total length of 845 mm for each port.

Text Slope Plot Line Font
 
#7 ·
Thanks for the replies mike.

Is there any optimal design for the extension of the ports as per my paint diagrams below.

A: http://i.imgur.com/eOZ7K2p.png
PVC elbow with pipe to extend to the base of the cabinet. (Width continues as 90mm as from end of flared port.

B: http://i.imgur.com/l21ZJW4.png
PVC elbow with pipe to another elbow joint facing driver. Width continues as 90mm as from end of flared port.)

C: http://i.imgur.com/zUKAY8P.png
PVC elbow with pipe to extend to the base of the cabinet with another flared port attached at the end. (width flared out from 90mm to 140mm)

Hope that makes sense?
 
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