Summing of 2 high-level stereo feeds into 1 stereo channel

Pug · 23 Aug 2020 at 11:16

Hi all

this is a car audio topic but I think HiFi brains may be more beneficial to me solving it. In short I think I need to combine two stereo feeds into one combined feed, this is called summing? I don’t want to use a simple RCA Ycable as I suspect interference/back feeding but would be happy to be proven wrong.

Basically I want to fit a subwoofer to my BMW OEM stereo. I have done this numerous times, so “know what I am doing”. The last two BMWs I upgraded all work, but I want to improve the sound, as the subs are only getting a low-frequency feed (duh), and I’d prefer to creep into the low-mids as well as sub frequency.

background: the only way to install them is to take a high level speaker feed and convert to low-level for the amp to drive the sub, that’s all pretty normal. However in BMWs you can only take a feed from the speaker wires after the oem amp has processed it. In my cases the car has an oem subwoofer, and so taking the oem sub feed and sending it to my sub amp works ok. However, I think I’m only getting the pre-crossed (filtered to sub bass) and I’d prefer a slightly higher low-mid bass (or sub amp adjustable) crossover.

I assume (need to check) that a door speaker has the sub frequencies already filtered out, so no good taking a feed from those instead (would they maybe get the full range even though a sub is oem fitted?).

If that’s true, the last option i see is taking the oem sub and door speaker feeds, combining them and then feeding that to my sub amp.

And it’s this summing/combining I need help with. Firstly is my theory ok? Secondly there seems to be a total lack of affordable product to do this. Wiring diagrams seem to suggest building a summing box like this just requires a few resistors, but summing boxes are £hundreds.

any tips/advice or feedback to steer me better?

thanks

hornetstinger · 23 Aug 2020 at 17:20

When in doubt, C4.

Pug · 24 Aug 2020 at 16:55

Heh not sure that's the right answer I needed!

Would appreciate any other insight

BinnsY · 24 Aug 2020 at 21:39

Do you know what outputs your current head unit uses to get to your current amp? If it's something relatively standard I'd have thought it would be simpler to simply bypass the factory amp completely, replace it with your own, and then run new speaker wire to the existing speakers / your new sub? 5 channel amps can be picked up for not a huge amount of cash assuming you've got the time / ability to run the new speaker wire.

jpaul · 25 Aug 2020 at 08:51

- head units digital out I thought, to amp in boot.

An aftermarket dsp(miniDSP £150?)+amp, would be mega-bucks, the high end solution.

Pug said:
If that’s true, the last option i see is taking the oem sub and door speaker feeds, combining them and then feeding that to my sub amp.

With any low mid range take-off&filtering, you obviously run the risk of boosting/double-counting those frequencies, which would then go to both original mid-ranges & sub.

if you had a sufficiently powerful phone as the music source (wireless not BT) then could the phone extract the base and send that independantly to a (wireless) active sub,
that's too contrived.

Pug · 25 Aug 2020 at 16:30

Yep optical/digital from h/u to amp, then crossed and analogue out from amp to speakers.

So if I understand you correctly you're saying the low-pass may drop off @ 90hz and the the mid may start at 70hz, causing a 20hz overlap? And so doubling of that 20hz?

That's a good point I hadn't considered... Though if my aim is more mid/bass that may be a good(ish) thing, and might manageable via gain on sub. Or it'll sound trash, but would it damage anything?

As for the phone idea, I like creative solutions but... I'll probably stick with Harmon Kardon sound as-is thanks though

jpaul · 26 Aug 2020 at 07:57

Pug said:
So if I understand you correctly you're saying the low-pass may drop off @ 90hz and the the mid may start at 70hz, causing a 20hz overlap? And so doubling of that 20hz?

yes

can't see any online analysis of the cross-overs frequency and roll-offs used in the bmw hk system - or, maybe you can determine that from replacement speaker specs.

now found the link of dsp folks I'd bookmarked https://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4-hd
if I update to a newer bm, current one has an alpine with adjustable cross-overs, parametric eq .... , then that was a potential purchase.

Pug · 26 Aug 2020 at 08:08

jpaul said:
yes

can't see any online analysis of the cross-overs frequency and roll-offs used in the bmw hk system - or, maybe you can determine that from replacement speaker specs.

now found the link of dsp folks I'd bookmarked https://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4-hd
if I update to a newer bm, current one has an alpine with adjustable cross-overs, parametric eq .... , then that was a potential purchase.

Just bought this F80 M3, and don't think I can get a better spec audio anyway! It does have an EQ, which I've obv meddled with but just always enjoyed more punch.

I am having a nightmare finding a cable or box, it's a super-simple affair by all accounts, wires joined via some resistors, basic but doesn't exist anywhere. Wiring diagrams exist, but I'm not so confident...