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Discussion Starter #1
Jaak if you are out there you may be able to answer this:

I am looking at the schematic for the connection between the head unit and the Bose amp. I have heard you make references to these lines being balanced. From the schematic it would appear that this is the case. Have you ever tested the lines to see if the lines marked (-) are actually inverted signals of the (+) lines. I was wondering if the (-) lines could be ground lines and the shield around the bundle is just a redundant shield. I have seen this done on VW's. I am looking to replace the Bose amp. If they are balanced then that means I am going to have to construct something from op-amps to convert the signal, unless of course you have a simple solution to offer. I saw the cable that dookie used for his conversion but I would like to use a more cost effective approach. Any suggestions? Sorry I started a thread for this but I was not allowed to message you directly. Anyway someone else might get something good out of this.
 

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Hi Generator,

I wonder why you can't PM me? Weird... I'll have to go check the settings again. You can also email me at muran0 @ rogers . com without the spaces...

There's three ways I can think of to convert the signal. One is Opamps as you suggest and this would be the most efficient and flattest way.

Another is using an audio transformer, but it would have to be a high quality one so you don't lose the lower frequencies and you still keep it reasonably flat.

The other way is to feed the pair of wires into a resistive divider which is really not the best way, but would work. There would be signal loss, that you might be able to overcome with the gain control on the input of the amp. It's ugly and may be noisy, but you might even be able to get away with a couple of 4k7 resistors off the balanced pair, with one going to ground and the other going to your input. If the gain is too low, you could perhaps take it down to around 1K. My concern is keeping the Bose from driving into a short. The input impedance of your amp would become part of the resistive divider.

I wince just thinking about doing it this way, but it might actually work OK, in spite of being a bit ugly.

If someone makes a reasonably priced decent converter, that would be the best solution for the least amount of hassle.

Hmmm... The more I think about it, the more the op amps make sense. It's so much easier to go from balanced to unbalanced with an op amp, then the other way.

And the cost would be very small. You do have to build the circuit however.

Using an 8 pin DIP packaged dual op amp that's got excellant frequency response with a few resistors and it would be done. If you wanted to eliminate DC offsets that might (but shouldn't) be present, a few coupling caps would be needed.
 

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Did a quick google search and found a good article from Electronic Musician, on building a circuit for this. Unfortunately, they don't show the pictures, which is really annoying!

http://emusician.com/ar/emusic_build_em_level/
 

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Discussion Starter #4
Thanks Jaak:

I started looking into building a set-up from op amps but I found that the really high quality op amps require weird input voltages and some require plus and minus supplies. I am not building a special power supply for this project as it will only be another source of possible noise. The single supply op amps out there had some issues with the audio spectrum (linearity perhaps). I will still look around. I think Maax may have a good one. While I was browsing around though I found these guys who make line converters especially for BOSE setups. Appparently Bose has to do everything different. They have lower than normal feed/output from the head unit to their amps. Could be a problem if someone is not aware of this. Gains would have to be set excessively high on outboard amps to compensate. (I remember someone posting a question about why he had so much background noise in his setup) Here is a link if anyone is interested.

SOUNDGATE LINK
 

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Yes, op amps need a split supply, so that is more circuitry that would have to be done. If done correctly, it would be quieter than the Murano's supply, but it's the hassle factor.

I remember the noise thread and while it's a concern, there's something wrong in that setup that's causing the noise. There shouldn't be any noise issues off balanced lines if the circuitry is designed and installed properly. The whole point of balanced complementary lines is that any common mode noise (What's not the desired signal) will be cancelled out.

I agree, the least amount of trouble to buy something, and unless it was grossly overpriced, that's what I'd be doing.
 
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