S752 - External Amp With 40V Speaker Level Inputs To Improve Stock Underseat Woofers

[ghtyr]

I feel like the individual audio system is nearly perfect for me, I would just like to squeeze a tiny bit more sub bass out of it. I've optimized my audio settings which made a huge difference but I'm still looking for a tiny bit more sub bass. I've read that the stock woofers are actually really good and could benefit from simply adding an external amp to power them.

Would simply running the wires that go to the stock underseats from the stock amp into the speaker level inputs of a Hertz DP 4.400 (accepts 40V input) and powering the underseats from that yield any improvements?

I know lots of people use a LOC but I'm wondering if I can get away with not using one if the amp can handle 40V input, is this an ok way to go?

I guess the root of the question is are the stock underseat woofers the limiting factor and no external amp would be of any benefit OR is it the stock amp/eq/rolloff that is limiting the stock underseat woofers and a bit more bass could be squeezed safely out of them using the Hertz amp?


Thanks so much for the guidance!

[Redd]

Easiest way is to replace the underseat subs with an aftermarket plug & play solution. Several brands like Eton offer BMW-specific options.

[Herofmine]

I did retrofit F01 HK amp and B&W SUB from F90/G30, sounds amazing and way better compared to stock individual audio in our cars

[ghtyr]

Originally Posted by Herofmine

I did retrofit F01 HK amp and B&W SUB from F90/G30, sounds amazing and way better compared to stock individual audio in our cars

Doesn't that amp have less power? Is it a trade-off to get the B&W subs to work?

Sounds like those subs are a good upgrade, did you have to unglue from the F90/G30 enclosures and transfer over to your stock enclosures? Did you have to add spacers for the increased depth of the woofers? Wouldn't it be better to leave the individual audio amp in and then swap over the B&W woofers?

[ghtyr]

Ya so it looks like the displacement of the underseats is the limiting factor for low bass performance (flat driver with low xmax and small enclosure). I'm going to pursue only utilizing them for midbass and running a trunk subwoofer instead.

Thanks for the help!

[Thumbie]

Originally Posted by ghtyr

Ya so it looks like the displacement of the underseats is the limiting factor for low bass performance (flat driver with low xmax and small enclosure). I'm going to pursue only utilizing them for midbass and running a trunk subwoofer instead.

Thanks for the help!

If what you are looking for is more punch in the bass, this will be the most effective solution.

[ghtyr]

Got a Recoil RED1800.5 which will do two bridged channels of 460W RMS @ 4ohms and a sub channel at 600W @ 4 ohms. I also got the Recoil PLC44PRO LOC which will take 40V input.

Going to keep the stock underseats and cross them over @ 80Hz to a Dayton RSS390HO-4 sub. I'll mount the sub on a baffle to the back of my seats to fire through the ski pass.

I modeled the driver for 600W in a sealed box 15.5 cu ft to match the trunk volume approximately and set the HPF for 25Hz and the lowpass to 80Hz (attached screenshots from winisd). This driver advertises xmax @ 12mm but it has been found to actually run closer to 17mm before any distortion or danger. The way I had it set up in my theatre it was modeling to hit 16mm and it measured super clean and never had any issues, so if I don't like the highpass at 25Hz I have room to play with and lower to 20Hz before I hit 16mm in the model. Its absolutely not an IB sub but the model looks good and at the end of the day if i don't like the sound I'll just build the rest of the sides of a small sealed enclosure around the baffle.

Sending 460W@4Ohms to each of the underseats is a bit much so I'll have to be careful with the gains on those. Luckily the stock woofers are 7ohm so I'll likely be sending 230ishW to them, still a bit too much. Limiting them to midbass and keeping gains down should help keep things safe.

I'm just going to cut the wires from the factory amp to the underseats so I can intercept with the LOC, send to amp with RCA's, then run speaker wire from the amp back to the other end of the cut wire that goes to the underseats. I don't care about a harness as I'm never selling this car.


Is this an ok plan or is there anything I'm not considering here? Any red flags? I already have the amp, LOC, and sub (actually have two of them), so ideally I'd like to use those unless there's something glaringly wrong with them I am unaware of.

Really appreciate the input folks!

[ghtyr]

Just want to doublecheck my research and math, is it correct that the stock under seat woofers can safely take 140W each and are 7 ohm woofers (only advertised as 8 ohm)?

So I would want to target amplifier output voltage that matches their max power rating of 140watts @ 7ohms: sqrt of (140*7) = 31.3V?

And looks like the Recoil amp ch5 was measured on the dyno to hit 575W @ 4 ohms before clipping/distortion, so for that channel I'll target 47.9V?

Also, what is the best way to ensure the rest of the car speakers aren't damaged while I'm setting the gain of the sub channel with a 40Hz tone @ 45 to 50 clicks of the volume knob? Or does the factory amp eq/crossovers protect them adequately?

[ghtyr]

Ended up just setting gains by ear with a track I was familiar with. At some point I'll likely bring in my calibration mic and laptop with REW to fine tune it, but it sounds pretty damn good right now.

I am absolutely blown away by the bass improvement with this setup! Still need to clean up some of the wire routing and the baffle is still a rough prototype as there's a few changes I'm thinking about before building a nice clean final version and ordering the carpet to wrap it.


On a totally unrelated note, does anyone else hear that loud ringing?

[Thumbie]

Dang that looks awesome.

[RocketyMan]

Not sure what else I can add to this thread. But +1 on adding a replacement sub in the trunk. "There's no replacement for displacement." And this is true in surface area and volume displaced. The underseat subwoofers are mid-bass at best.

In my experience, the replacements like Bavsound are overpriced and underwhelming. So I don't recommend this at all.


As far as my setup, I just tied the high-line levels to the input of my amp. Didn't need to do much about crossover, but any gain is controlled by my remote switch i placed in the front seat location. I have a low pro-file 12" box that is tune-vented. Rockford fosgate mono 500w. Works perfect.