DIY Budget subwoofer design

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I have access (~25 euro) to a Fenton 902.426 driver (8" Woofer with Kevlar cone | Tronios) and I'm considering building a vented box around it. I have modeled a 41L box (300x400x500) in speakerboxlite, for a vented enclosure with a port tuned around 28-30Hz, built using 18mm MDF. My plan is to do a bottom firing or side firing, with the port in the front. I will match this with a yet to be defined class D amp.

As far as I can tell I will get a inefficient, but usable, subwoofer, with an F3 around 28Hz, and a almost flat response (some EQ/XO will be required). The max SPL will not be that high, but I think it is adequate for the scenario.

The subwoofer will be used to complement other speakers (dual drivers with a port) in a HT scenario, which go down to 80Hz. These are 50W speakers, but due to neighbors and kids, the volume never goes that high. I'm looking for extending the frequency range and some stronger and deeper bass.

Although I'm a long time lurker of this forum, this will be my first attempt at the DYI aspect of subwoofers. I'm a bit uncertain, and would like to have your opinion on this build. Can you help?

Thanks in advance!

Edit: Link to speakerbox calculations: Project short info
 
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I have access (~25 euro) to a Fenton 902.426 driver [...] I have modeled a 41L box [...] My plan is to do a bottom firing or side firing, with the port in the front. I will match this with a yet to be defined class D amp.

The Fenton range is of unknown quality*. I'd suggest treating the specs with caution: have a plan B in case the (already quite high) Qts is higher than advertised.

I'd also go with side firing unless there's something that really compels a down firing layout (like a woofer-destroying puppy in your home).

The low sensitivity is not strictly bad news. A cheap amplifier that would be unacceptable for hiss / high noise level in an efficient system might be fine when driving an 85dB woofer.





*I'd like them to be good, cos a local store has them at decent prices.
 
Fenton seems to be the same as tronios, QTX, Skytronic, which places the brand in the budget segment. I have seen "good enough" reviews for the brand: it works but will not meet audiophile standards :p

With VAS at 23.6L, QTS of 0.482 and actual volume of 40L I think i'm already giving some slack for higher QTS. Additional stuffing can also help.

The only backup plan will be to select a speaker from a catalog. However, the price will skyrocket. I miss parts-express in western europe....
 
Hi All,

FYI:

b:)
 

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Is there a way to simulate/calculate the effect of stuffing the interior?

Some real tests, arguably better than a simulation:
Data-Bass

Bjorno's calculations look good. I had to zoom in to make them legible :)

If eq is a given, you might consider using 2 drivers in sealed configuration.

When I sim two drivers in 40 litres (sealed), it appears to have the same excursion limited SPL (in the 30-80Hz range) as the 40 litre ported option.

Pros (2 drivers sealed)
-ability to mount them on opposing walls (for vibration cancelling)
-less harmed by TS parameters being off

Cons
-greater cost
-less heat dissipation
 
Awesome. Thanks bjorno, hollowboy.

I will try the ported approach to keep the design simpler and because I may be optimizing an non-existing problem (according to my ears and living room). Also, I can get a MDF board cut to my specification with no extra cost, which reduces the burden of changing the enclosure.

The simulation provided by bjorno states the driver dimensions are actually off, is this correct?
 
Thanks.
I already built the subwoofer and and just waiting for an opportunity to assemble the electronics, post some pictures and actually test it.

The final enclosure had 290x400x462 mm, and was built from 16mm MDF. This had to do with the space available in the living room, the size of the panels commonly available and the cost. The speaker in the front, with a round port below.

The interior and all exposed faces were "sealed" with several layers of a solvent based product, and the exterior was covered with two layers of a very thick primary, sanded to 600 grit. Instead of painting, I found a black vinyl with a good looking mate texture and used it. It looks nice and was _very fast_ to apply.

A brief test up to 50W provided very nice results with a clear and deep bass (it seems to fit the purpose). However at this level of power, many problems can go undetected.

I plan to add a TDA 7498E Amp, and a NE5532 LPF, which are around in some storage box.
 
Just reporting on the design:

Powering the Fenton with a red TDA7498E at 24V in BTL mode just proved to be a bad idea, as the TDA output pins rapidly dissolved. This actually happened at half power (if so), after a handful of stronger bass beats.

Any suggestion for a alternative class D amp? I'm considering a design such as the L15D, based on the IRS2092, powered at 50-60v.
 
Update

First prototype implemented:

- 5532 based Pre-amp and Filter
- L20D Amplifier
- 500W SMPS
- Currently connected using speaker levels (Right channel) from another Amp.

Amp and Power supply are outside in the back, while Pre-amp is inside the enclosure.
The output is very reasonable with no perceived distortion (perceptual), and capable to putting some deep bass in the living room.
It extends the existing speakers very well adding frequencies up to 20-30Hz as expected.


I generated 100ms impulses, with 10Hz of separation (top line), and recorded the output from the Speakers (middle line), and subwoofer (bottom line). Microphone is not calibrated, nor expected to be linear. The subwoofer complements the speakers by adding quite a few frequencies and starts putting tones at ~20Hz (with audible attenuation).
It should be noticed that the speaker and subwoofer lines are delayed by about 300ms (I was using bluetooth)

TODO:
- Seal the enclosure
- Adjust port length (currently it is 100mm)

Current issues:
- There is a very small, barely audible 50Hz hum.
 

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Where are the build pics???

Not much of the build process as this is a standard enclosure.
I will add some pictures of the current state.

In the meantime, temperature images of the L20D and SMPS.
Based on this I will probably increase the wattage of those resistors.
 

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Some photos of the current prototype, still missing many things.

The back plate is still PVC and will convert it to steel or aluminum after the positioning is correct.

No internal bracing as it will be used for comfortable levels.
 

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Yep. Not sure if the components will go inside the enclosure or I add a protection to the outside. Don't like the +100ºC of those resistors, but the amp + SMPS should be very efficient (>90%).

The current approach will be to leave it in the outside, then enclose it and monitor the temperature. Probably I will also add a small fan (6v) because the resistors are near capacitors and that is a recipe for failure.
 
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