[guitarguylp]

I'm picking up a 2017 340i tomorrow with the HK system. I do plan on adding a subwoofer. I know that the underseat woofers are a bit odd at 7 ohms, and am thinking about tapping the wires to get a signal for my line out converter, and adding a capacitor inline going to the woofers as a basic low pass filter to allow the subwoofer I'm installing to handle everything below about 65 Hz. All that would be required is a 350uF capacitor for each woofer.

Has anyone done that, and how did it work?
Anyone have any pros, or cons to this plan?

I'm going to use a LC2i LOC, and most likely a visual ice sub box with either a Dayton audio, or a TC Sounds sub

[Billfitz]

Forget the caps, they'd be ineffective and expensive. Turn down the bass control, compensate for it with a sub amp that has bass boost. The H-K aren't odd at all, 7 ohms is their DCR, not impedance.

[guitarguylp]

The total price for two capacitors will be about $12. Low frequencies take a lot of power, so it would be easier on the stock amp, and I could use the bass control without worrying about over powering the underseat woofers. I am also sure that those underseat woofers probably can't do much below 80 Hz in such a small enclosure.

For the price of the mod I might try it, and do some measurements both ways, and see if it helps.

[Billfitz]

I wouldn't use caps that inexpensive, but then I wouldn't use them at all. At best you'd get a 6dB signal reduction at 50Hz, and it's not going to reduce the power draw on the amp anywhere near as much as turning down the bass EQ. As for what the underseats will do this is the measured response of my Hi-Fi system:



The response below 100Hz looks high compared to a home system, but that's because it has to be to mask road noise, which is mainly below 100Hz.

[Arthurrs]

What amp were you planning to use for the trunk sub?

[guitarguylp]

Originally Posted by Billfitz

I wouldn't use caps that inexpensive, but then I wouldn't use them at all. At best you'd get a 6dB signal reduction at 50Hz, and it's not going to reduce the power draw on the amp anywhere near as much as turning down the bass EQ. As for what the underseats will do this is the measured response of my Hi-Fi system:



The response below 100Hz looks high compared to a home system, but that's because it has to be to mask road noise, which is mainly below 100Hz.

Thanks for the graph, I will have to play around with it first. Those $6 each caps are proper crossover caps. The -6 dB at 50Hz isn't much acoustically, but power wise from the amp that will make a difference.

That graph is better than what I would expect a factory system to be capable of. That 250Hz bump might be fun to try to EQ out. My last system was in my F22 228i with all Focal speakers, Match UP7BMW, JL500/1 and a JL 10TW3 sub. That was a killer system. I found out that the Match UP7 won't work in a 3 series HK system, so I am going to see if I need to upgrade the amp for the fronts, or just add the sub.

I tend to overthink the specs since I'm an electronics engineer, and amature musician. Sometimes the on paper is quite different from what we hear.

I'll be reusing my JL 500/1 with a LC2i to bring the level down to an acceptable range for the amp.

[Billfitz]

The dips and bumps are boundary reflection sourced, they don't exist in nearfield measurements. That being the case you can't effectively EQ them out, as their frequency and depth change at each listener position. BTW, I'm a professional acoustical engineer and musician.

[Arthurrs]

If it were me, I'd wait until I could put a DSP/Amplifier in place of the HK amp and have a bunch more options for active processing, before trying to cobble together a fixed passive filter network between the under seat woofer and trunk sub. Great choice for trunk sub amp, I had one in my former F30, worked amazingly well!

[NealfromNZ]

Originally Posted by guitarguylp

Thanks for the graph, I will have to play around with it first. Those $6 each caps are proper crossover caps. The -6 dB at 50Hz isn't much acoustically, but power wise from the amp that will make a difference.

That graph is better than what I would expect a factory system to be capable of. That 250Hz bump might be fun to try to EQ out. My last system was in my F22 228i with all Focal speakers, Match UP7BMW, JL500/1 and a JL 10TW3 sub. That was a killer system. I found out that the Match UP7 won't work in a 3 series HK system, so I am going to see if I need to upgrade the amp for the fronts, or just add the sub.

I tend to overthink the specs since I'm an electronics engineer, and amature musician. Sometimes the on paper is quite different from what we hear.

I'll be reusing my JL 500/1 with a LC2i to bring the level down to an acceptable range for the amp.

Get a Mobridge da1 and you’ll be able use the Match 7 via Toslink. The DA1 emulates a HK amp and mixes it down to a two channel digital toslink output .