Thanks guys for your help.
I took out Turbo1 and the sound quality is much better now. Of ocurse the volume is lower but I think I just need a better quality unbalanced-balanced transformer to have it working the right way. As you can see in the guide I sent the left- and right- are connected together. Is there anything wrong with it ?
So you are on the right track! good news!
Balanced means differential input/output.
Unbalanced means that the (-) signal lead is ground/shield.
Balanced is better in a car. You cannot get ground loops. (unless you connect shield in both ends... Big no-no. You have to have cables with two signal leads per channel and one shield connected in one end only). Also any voltage introduced in chassis (generators etc.) does not affect the signal as it affects both + and - to the same amount. Its called common mode rejection. You can still get noise if you use poor cables. So its better with better cables.
Problem is when you mix balanced and unbalanced.
If you have balanced input/output theory means no reference to ground and its like the birds on the 16kV high voltage train line. You can connect -right and -left and ground and in effect get an unbalanced input _or_ output. No current. Bird does not burn. And this is what your link suggests that you do, 35!
In real life its very often a limitations to this.
If your problem 35, was that Bose does not handle a grounded (-)input very well I suggested a way round it (or rather to find out if that is the problem). To decouple the DC with a cap on the (-) lead. Last note was just an attempt to be clear that this ought to be done separately on each channel.
Now, since the turbo seemed to be the problem I do not think the problem was ever a balance/unbalance issue. Since you did not get noise but poor quality I should say that your turbo is not up to your standard. Get a better preamp.
Did this make it clearer or not? :-)
bes51659 thanks again. It seems clear right now. The problem is I do not know much about preamplifiers and all this stuff so I'd be glad if someone could recommend which preamp I should consider. I do not want spend much money since I'm getting a new car soon. Another question is can I get really good sound from a PC ? At the moment I have Creative X-MOD USB as a sound card, but I don't think it is the best choice. Maybe I should think of changing it to some high-end USB device.
Sorry, can't help you there. I've not installed in car yet, so I still live in the pretens that gain is sufficient and that I won't need a preamp.
Sure a PC can sound good! About the board you've got I got a bit suspicious about "award winning new tech to restore lost info in mp3's" and "turn your hifi int xtreme-fi".. I like info like 24-bit DAC, 100dB S/N, etc. :-)
There is some interesting sw to measure your car coupe and compensate for timedelays, and digital filters that you run as batch jobs on your music and therefore has 0 phase error etc! Its a jungle, but its going to be fun to dig into that. Plenty of info on mp3car.
I saw that there are some carputer people have bought fosgate 360. Why in gods name when you got a PC at hand. Its only a bunch of filters at $1000. - At last a reason to motivate a PC heavy investment to the girlfriend :-)
The loss of gain you are experiencing is because you are only using half of the balanced input to the amp. This translates to about a 6db loss. Your pre amp (Turbo1) can make up this gain, but because you are still only using half of the input signal you are overdriving the amp into distortion when you make up the gain you are loosing due to being unbalanced.
If you use balancing transformers (one for each powered speaker) you should get your gain back with clear sound.
Make sure your transformers are 0db gain or up to +12 db gain. Some transformers (like inexpensive Direct Boxes) will drop your gain up to 20db.
Using outboard DSP (like I'm doing here: 1996 300zx TT - Carputer w/ sound system upgrade ) is far more affective than using the PC or even batch processing your media (that sounds like a nightmare).