Passive sub, stumped already

[DIYNick]

Hello experts. I bought a Jamo passive sub very cheap to mess around with and bolster the sound from my full range driver set up

I'm powering it from a spare autocostruire 2020 t amp board, but I'm confused. There is a vibration inside which I thought could be loose driver bolts. (which it wasn't although I did get a few turns on the bolts). On opening the box the two drivers for each channel face each other on the same baffle. One with it's rear to a sealed box and the other with it's rear to a ported box. Surely this cancels most of the sound out?

1) how does this work
2) Output is low as well, do I need a more powerful amp?

Thanks in advance, nick

[6L6]

The drivers are in an "Isobarik" arrangement, and as one of the drivers is connected in reverse, the cones move with each other.

The 2nd driver is acting like a much bigger box, which keeps the actual box small, with high WAF.

[DIYNick]

6L6, ok thanks for explaination, now I see. I need to check the wiring as it was loose and disconected when I opened it up. Any idea how efficient such an arrangement is? Does it need a big amp to get it working correctly?

Nick

[picowallspeaker]

Nick ,have you seen the statistical distribution of energy in the audio spectrum ?
Most goes to bass ...depending on which kind of music ,of course .
so the amplifier is asked to deliver current at the very low frequencies.Power in / power out equals efficiency ? If you just put a steep highpass filter before the sub .. Power ( sound SPL ) delivered by a pair of woofers in P-P ,but this is applyied to any kind of driver ,it depends of how they behave in their useful bandwidth ,so if you cross them right you reach optimum efficiency

[DIYNick]

Picowallspeaker, ok I understand the principle of what you're saying, my ability to apply it may not match this! On removing the bottom of the unit there is on each channel input an inductor and a capacitor. Would this be the low pass filter?

I have now reversed one of the drivers so + and - are the opposite to the other driver. (although this goes against the colour coding) output has increased now.

I still have a vibration/rattle at certain frequencies, which is annoying. Can't see anything obvious yet that maybe causing it.

[revboden]

Photos would help, what kind of drivers are they? What brand size etc?

[Brian Steele]

Originally Posted by DIYNick

There is a vibration inside which I thought could be loose driver bolts. (which it wasn't although I did get a few turns on the bolts).

Check to see if the inductors in the x-over board have come loose.

Originally Posted by DIYNick

On opening the box the two drivers for each channel face each other on the same baffle. One with it's rear to a sealed box and the other with it's rear to a ported box. Surely this cancels most of the sound out?

That sounds like a "clamshell" isobaric 4th order bandpass system. See the following link for an example of such a system: ElUglito

[DIYNick]

Photo attached of current/original condition with bottom access open.



Divers are approx 8" overall dimension, so about 7" across the cone. (can't see a brand, it has 'Jamo' stickers on).

Wiring could be ratling as is doesn't seem to do it now I have opened the box

Comments welcome.

[Brian Steele]

Originally Posted by DIYNick

Photo attached of current/original condition with bottom access open.

Curious design choice. I'd be concerned about leakage between the two volumes because of where and how they implemented that separation panel.

[DIYNick]

Brian,

Mmmmmmm....yes, I did think about that, i'm a bit reluctant to perminantly seal it, until at least i'm happy with the sound. I have rapped foam around the inner cables, secured with zip ties for now, this has improved matters, just a slight rattle at higher volumes/certain frequency.

Do you think the inductor/capacitor arrangement is a low pass filter then?