supra0083

Many people have been adding a subwoofer to augment the factory Bose system. I am interested in what people use as input to their aftermarket amplifer for the subwoofer.
1) After the Bose amp with an LOC into line level input.
2) After the Bose amp w/o LOC into speaker level input.
3) Before the Bose amp with an LOC into line level input.
4) Before the Bose amp w/o LOC into line level input
5) Before Bose amp w/o LOC into speaker level input

I tried 5 but not working well for me, bass is weak and my current setting is 4 and is working well but wondering should I do 3 to avoid overtaxing the HU. This is why I setup the poll. Any comments are also welcome.

forbidden

The blows system uses a differential line voltage on the front and rear preouts. IF you tap in before the blows amp on the rear preouts, you are asking for trouble as the rear preout voltage is a really low voltage as compared to the front preout voltage. Verify what lines you tapped into.

supra0083

I was using the Rear Preamp signal before I read forbidden reply. I then tried the Front Preamp signal. It was better but still not what I am looking for, so I had to switch back again. Maybe my Amp (Profile HA700M) just don't do too well with speaker level input. In the manual, it stated that it can take up to 7V in the RCA input but it does not qualified the 7V. It seems it is okay to use the rear preamp signal as an line-level input. I think I saw it somewhere that the signal is about 5V from the HU.

I don't unstand too well on the difference of line-level vs speaker level beside the line-level is normally in the 2.5V RMS range while speaker level can go up to 10V. My question would be:-
1) Do they have the same DC component?
2) Are they both differential signal?

I am not an EE major...maybe someone can shred a light more on the technical difference between the two. I am betting on G8rboy experience that feeding directly into the line-level seems okay for him for 2 years.

MrWigggles

Jesus man. I have shown voltage waveforms from an oscilloscope time and time again that clearly shows this is not the case. I know you have been part of those threads and I know you have seen those traces*. (Edit, I see one place for possible confusion, the front and rear signals before the Bose amp both offer plenty of voltage. I have measured this before and I would be happy to measure it again.)

THERE IS PLENTY OF VOLTAGE going to the Bose amp almost as much as after the Bose amp. And yes it is differential, but so are speaker level signals. The good thing about tapping before the amp is that you are getting a pure flat signal from the headunit before the Bose amp adds its wild eq.

I have done both of the following hook-ups without issue:

"5. Before Bose amp w/o LOC into speaker level input."

"3. Before Bose amp with an LOC into line level input."

The only one of the above that should be avoided is:

"4. Before the Bose amp w/o LOC into line level input"

That configuration will effectively ground the two negative signals coming from the radio (being differential they are NOT 0 Volts like normal line level RCA signals but instead inverted versions of the positive signals).

Still bare in mind that the HU is pretty durable. My install was initially done with the #4 configuration and it survived (others have not survived).

Can I guarantee that you will have perfect results with whatever amp you choose? No. Some amps have shitty speaker level inputs or there could be some weird grounding issue that makes one of the four good configurations (1,2,3, and 5) better than an other. And heck maybe the Bose equlization will make your aftermarket sub sound better so you actually want the EQ (but I doubt it).

-Mr. Wigggles

*https://www.rx8club.com/showthread.p...t=oscilloscope and https://www.rx8club.com/showthread.p...t=oscilloscope

supra0083

Mr. Wigggles. Thanks for the clarification. So my understanding is that RCA is not differential and the -ve is chasis ground while speaker level is differential. I understand I am not doing the techically correct thing by doing 4 but it is working fine for now. I think I would either buy an LOCB.2 later and put it near the HU or just wait until the HU gone dead and get the CD/MP player HU instead (Mine is currently a 6-Disc CD changer).

One side question, my amp speaker level takes 5 inputs:- 2 pairs for speaker signals and a ground. The manual indicate hooking that to chassis ground. Can anyone guess what that ground supposed to be used for? Why they cannot just use the "battery" ground? I put it to chassis ground when I hook it up to the speaker level input. I don't seem to recall seeing this on some other amp that takes speaker level input.

MrWigggles

Supra,

I would guess that the 5th wire of your speaker level inputs is to get the chasis ground potential at the HU. The chasis ground at the HU is going to be a slightly different potential than chasis ground at the amp.

To a certain extent this 5th wire is overkill because if the speaker level inputs to your amp are differential like they should be then you shouldn't need the fifth wire since any common mode noise should cancel out. However, if your amp went the cheap route and is only reading the positive of each signal pair, it might need that fifth wire ground.

Try leaving the fifth input unattached, then trying jumpering it to your amps normal ground and then finally try attaching it to the ground where the Bose amp is.

-Mr. Wigggles

supra0083

I end up using the Dave Navone NE-7V after the Bose amp. I think I am pretty happy with this setting. One thing interesting is that Dave assure me that doing 4 would not hurt my HU. I did not further ask why. Anyway, thanks for all the input.