Converting a 3-way boombox to 2.1 setup

[OrangeBug]

Hey Guys, first post here, excluding my intro post.

I recently came across a pair of JVC G-Max boombox speakers. They're 3-way bass-reflex type speakers. I'm working on converting them into a pair of bookcase speakers with separate sub. I used them last year at school with an old home theater reciever and very much liked the sound, although the dual 120W subs were overpowering, so I had to cut the bass way down.
Here's some specs of the speakers from the user manual:

Speaker: SP-MXGA77
Type: 3-way bass-reflex

Speaker units:
Subwoofer: 13.5 cm (5 3/8 in.)
Main Woofer: 16 cm (6 5/16 in.)
Tweeter: 5 cm (2 in.)

Power handling capacity:
Subwoofer: 120 W
Main speaker: 60 W

Impedance:
Subwoofer: 6 Ω
Main speaker: 4 Ω

Frequency range:
Subwoofer: 30 Hz — 70 Hz
Main speaker: 70 Hz — 20 000 Hz

Sound pressure level:
Subwoofer: 73 dB/W•m
Main speaker: 86 dB/W•m

Dimensions (approx.):
290 mm x 317 mm x 336 mm (W/H/D)
(11 7/16 in. x 12 1/2 in. x 13 1/4 in.)

Mass (approx.):
5.5 kg (12.2 lbs) each

Because the drivers are from a boombox, I've been unable to find the original driver specs, so designing a proper speaker cabinet from scratch is difficult, if not impossible. I decided to reverse engineer the existing cabinets by measuring the internal volume, and recreate it in a smaller bookshelf sized speaker. While using only one of the subs in the old cabinet as my subwoofer.

So here's my problem:
I calculated the volume of the mid/tweeter part of the original cabinet, which came out to about 300 in^3. Because the mid is 6" the best box dimension I've come up with is 8"x12"x3". The depth is what worries me. Am I doing something wrong?
The original cabinet had an internal wall separating the mid from the woofer. I measured only the mid side of the cabinet. Does the larger void on the sub side effect the mid, and do I need to account for it in the new cabinet?

I can post pictures if it helps.

[sreten]

Hi,

You have the active biamping c/o specs ?

You cannot simply remove half the subwoofer because "its too loud",
That is irrelevant with active bi-amping. Your chucking away 6dB
of bass capability, and with a 5" driver that is inevitably limited.

rgds, sreten.

[jerome69]

Hi ,

It is better to keep as is, a 3 ways. A 2.1 gives less good sonic result.
The problem it is an active system. if you go to passive you need to add a new subwoofer which can do 86dB...

I think it's better to built or to buy a new speaker. There are a lot of nice projects on the internet.
See Speaker Kits: Madisound Speaker Store


Regards.

[OrangeBug]

Sorry, I should have mentioned that the original amp is dead, the boombox was rescued from a flooded basement.

Each of the 2 cabinets has 2 sets of wires, 1 for for the mid/tweeter, and 1 for the sub. I had the 2 sets of wires wired in parallel into the audio receiver, as it didn't have a sub output. Because of that setup the sub was receiving full spectrum audio, and was "too loud" (I had it plugged into a TV for awhile, and you could here it thumping from across the street). I ran the audio through a 3-band EQ and cut the bass down almost completely to compensate.

I'm planning on getting a cheap t-amp to power the speakers this fall, because the receiver is to big to drive down to college with (don't worry, once I have an actual income again I'll buy a decent amp). Which is also why I want to rebuild the cabinets, as an attempt to make then a little more compact.

[jerome69]

One simple solution i see is to add a simple inductor on the woofer to lower high frequency output. for a 4ohms woofer L=10mH.
10mH 18 AWG I Core Inductor

[OrangeBug]

Yes, thank you, I've looked into using an inductor as a low pass filter for the sub.
However may I move us back to my original question:

Originally Posted by OrangeBug

...Because the mid is 6" the best box dimension I've come up with is 8"x12"x3". The depth is what worries me...

Also I think I may have posted in the wrong section as my setup doesn't really have a physical crossover. Mods feel free to move it if you wish.

[jerome69]

It is the good section. Ok i have some problems with your unit, not SI. 300in^3 ~ 5Liter is not a lot for a 6" but sufficient when use as a midrange. A 6-7" usually works in 15-35Liter BR box. I don't think you can reduce the volume of the box easily. 5 Liter is good for a 4".
Second we don't know what are the active corrections ? In active you can reduce a lot volume of speakers because equalizations are easy.

I think to make a complete reverse engineer, it will better to measure T/S parameters and to know active corrections.

Hope this helps.

[sreten]

Hi,

The "thump" is caused by allowing the the sub to overlap the bass mid,
it has nothing to do with how "loud" the sub is, because it isn't, its
sensitivity is a lot lower than the bass/mid.

The bass mid may be booming on its own if the actual design calls
for it to be high pass filtered. Seems you are not using the sub well.

Get an AV amplifier that somehow you can connect the sub out
to two spare channels for driving the subs and play with set up.

http://resources.jvc.com/Resources/0...7/22048ien.pdf

rgds, sreten.

[OrangeBug]

Originally Posted by sreten

...Get an AV amplifier that somehow you can connect the sub out
to two spare channels for driving the subs and play with set up...

I'm thinking of getting another amp for the sub an plugging it into the sub-out of my soundcard, I can do internal EQ to cut out the high end.

I'm working on measuring the T/S parameters of my speakers, from this website: T-S Parameter Calculator

I still need to go back and get a more accurate measurement of the resonant frequencies.



Also, to apologize, I realize that a lot of what I've talked about here would make any decent Audiophile cringe. But money is very tight at the moment, and I'm leaving for college in 4 weeks, so I'm just trying to do a quick project before I leave. And don't worry, I'm not modifying the original cabinet in any way. So if after I build new cabinets, and realize "wow these suck", I can always go back to the original 3-way speaker cabinet.

[OrangeBug]

well this is annoying, I came back today to finish working on the t/s parameters, and all my ohm measurements don't match anymore. Yesterday I was able to go back to any frequency and receive the same measurement every time. I'm also not convinced my setup is outputting constant voltage (outputting from my sound card) across the entire spectrum, which may also be skewing my results.
T-S Parameter Calculator

While writing this I found some posts about AudioTester, so I'm looking into building the wiring harness for it. Hopefully it will yield more accurate results.