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Discussion Starter #1
Hi all,

I had an old JBL Synthesis S400 Stereo Subwoofer Amplifier (2 x 200W @ 4 Ohms) lying around and I’ve been experimenting with 2 Mordaunt Short 309i active Subwoofers (175W @ 4 Ohms) that I picked up cheap as they had blown amps so I’m using them as passive by just feeding the drivers direct from the JBL amp. Seeing as the whole lot cost me very little it sounds great and nicely matches the rest of my Mordaunt Short 7.1 system.

Question is - would I be wasting money by using the existing (ported) enclosures and upgrading the drivers? As a quick look there are some Dayton 10” 4 Ohm 200 RMS 400 W max drivers that could fit the bill.

Currently I’m very pleased with the sound but during particularly prolonged / heavy film sequences when I have everything cranked up I can tell the subs are being pushed to their limits. As I am bypassing the 309i amps (and therefore the notch filters, crossovers and gain) there is only limited level reduction available from my Denon AV receiver the subs can get a bit overworked in those parts of movies. I could put some attenuation in somewhere but I’d rather spend the money by improving things rather than restricting.

Then a 2nd question - I actually have a 3rd 309 subwoofer with a working amp so if I wanted to go mad I could split off the Denon Sub out and feed the 3rd sub as an active. I’m not going to really benefit from much more loudness but could it even things out further? Room size is 16ft x 15 ft x 9 ft.

Any thoughts appreciated - just let me know if you need any more info.
 

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There's more to consider when replacing a sub driver then just the size. It has to be designed for the alignment - ported or sealed - you have to match the sensitivity and load on the amp, the TS parameters need to match the cabinet volume, you have to configure limiters, etc. The cabinet dimensions, alignment, amp and DSP programming all work in concert and if any are not matched the results will generally be pretty bad. Or worse, it could be terminal for a driver that gets pushed past its design parameters. You can download something like WinISD and model everything to see if the enclosure/driver/amp combo you're consider would work, so that's an option. That can be a bit tricky but it's perhaps worth considering.
 

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Discussion Starter #3
Thank you Jman.

I've started a bit of digging but need to delve deeper - not sure how I will fare with WinISD - that's all new to me and I'm on a Mac so am already up against it, but will persevere.

I won't claim to fully understand the TS side of things but I have found all the relevant TS details of the 10" Dayton drivers and will try and work back from there - interestingly they indicate an optimal enclosure vol of 0.75 cu ft for ported (which my enclosures are) which seems a bit small compared to the Mordaunts which are (at a quick calc) 1.4 cu ft for lower powered drivers.

If it gets too complex I may take the angle that I'm already flying by seat of pants having bypassed the Mordaunts amp with the Synthesis amp which is stretching the low powered drivers already and therefore do I just go with the lesser of two evils and hook up the more powerful drivers on the basis they are less likely to fail than the existing ones. But that's a last resort.
 

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I won't claim to fully understand the TS side of things but I have found all the relevant TS details of the 10" Dayton drivers and will try and work back from there - interestingly they indicate an optimal enclosure vol of 0.75 cu ft for ported (which my enclosures are) which seems a bit small compared to the Mordaunts which are (at a quick calc) 1.4 cu ft for lower powered drivers.
You're right, something sounds off. 0.75 is a bit small for a sealed sub with a 10" driver, for ported it's surely too small.


If it gets too complex I may take the angle that I'm already flying by seat of pants having bypassed the Mordaunts amp with the Synthesis amp which is stretching the low powered drivers already and therefore do I just go with the lesser of two evils and hook up the more powerful drivers on the basis they are less likely to fail than the existing ones. But that's a last resort.
It sounds like you have a desire to go the DIY route. If so, do you have to keep those cabinets? DIY Sound Group has some flat pack sealed subwoofer enclosures that you can add your own driver and amp to and build something yourself. Done this way you can finish them any way you'd like.
 

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Discussion Starter #5 (Edited)
You're right, something sounds off. 0.75 is a bit small for a sealed sub with a 10" driver, for ported it's surely too small.
Here's the link showing 0.47 cu ft
https://www.soundimports.eu/en/dcs255-4.html

And here's a link to an even more powerful 10" driver and this one claims only 0.47 cu ft for a ported enclosure ????..... or am I misinterpreting something ?
https://www.soundimports.eu/en/rss265ho-4.html

Thanks for the DIY Soundgroup link, I'll take a look but to be brutally honest the route I'm kinda hoping to go is the quick, lazy route and hope for reasonable gains - normally I would relish jumping in and going over the top on something like this however I am already totally saturated getting a high end soundproofed music recording studio finished and any leftover spare time getting my head round high end video production which I have just got hooked on to compliment the audio work I do. Within my family "Dad's little projects" are infamous so something like this sub project is going to have to go in under the radar :grin2:. Once my time gets freed up I quite fancy delving deep in to building a more optimum sub to get more refined results however for now I'm just a bit uneasy about hearing the current subs flapping away at times and would be happy to drop £100 - £200 on a couple of decent more powerful drivers as a quick fix to use the MS sub cabinets that fully match the rest of the speakers.
 

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Here's the link showing 0.47 cu ft
https://www.soundimports.eu/en/dcs255-4.html

And here's a link to an even more powerful 10" driver and this one claims only 0.47 cu ft for a ported enclosure ????..... or am I misinterpreting something ?
https://www.soundimports.eu/en/rss265ho-4.html
If you look at the F3 spec for vented you'll see 40Hz and 47Hz which is way too high for a subwoofer. That tells me the box is not optimal and needs to have more volume and port area.

Thanks for the DIY Soundgroup link, I'll take a look but to be brutally honest the route I'm kinda hoping to go is the quick, lazy route and hope for reasonable gains - normally I would relish jumping in and going over the top on something like this however I am already totally saturated getting a high end soundproofed music recording studio finished and any leftover spare time getting my head round high end video production which I have just got hooked on to compliment the audio work I do. Within my family "Dad's little projects" are infamous so something like this sub project is going to have to go in under the radar :grin2:. Once my time gets freed up I quite fancy delving deep in to building a more optimum sub to get more refined results however for now I'm just a bit uneasy about hearing the current subs flapping away at times and would be happy to drop £100 - £200 on a couple of decent more powerful drivers as a quick fix to use the MS sub cabinets that fully match the rest of the speakers.
Totally understand. My girls are grown and on their own now, but I do recall those days.
 
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