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Discussion Starter #1 (Edited)
Hello,

Have been looking at Jomor's posting (really cool) on modifiying the original Blaupunkt head unit in order to tap pre-out RCAs. Am wondering if its possible to connect the pre-out to the existing Bose amplifier unit? (by using existing connection)

Anyone knows if the Bose amplifier has built-in attenuation circuit since itss taking speaker output from the Blaupunkt.

Would the sound be too soft if the pre-out is connected to the Bose (if there's attenuator)?

Thanks & Happy New Year!
 

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Hmm... interesting idea.. bypassing the hu's internal amp chip and the attenuating circuit inside the Bose amp, makes good
sense. But i m not sure if its that simple to implement. Apart from the fact that the Bose system is using specific equalizer settings,
there is also a rumour that it has additional equalizing filters inside the amp. There are some things that should be considered
in such modification, like if the attenuating circuit should be completely isolated by cutting tracks on the pcb or if those
filters (if they actually exist) should be bypassed or not ( in case better speakers are gonna be used ) etc.

But I m not sure if its worthed the trouble..

Happy New Year
 

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Discussion Starter #3 (Edited)
OK, finally did the pre-out mod, by removing the amplifier IC as well. So now my HU has 4 channel pre-out conected to the "speaker out" harness on the HU.

When connected to the Bose, volume need to be cranked up to 60 or more for decent sound level. Therefore, there is a attenuation circuit in the Bose. And there's also audible little buzzz noise (what's the possible cause?) when the engine is running.

However, intitial listening result is that the sound quality is much much better with the pre-out, definition is like "10 times better", highs' resolution are better, better transient, bass is not "boom-boom" but tightly controlled. Vocal is warmer and more real. There's no background noise when engine is off.

Meanwhile, will try to figure out how to eliminate the buzz & the attenuation circuit in the Bose, failing which, will start looking for another amp.....

cheers.

note for picture:- there are other connections on the other side of the PCB too....
 

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colins said:
OK, finally did the pre-out mod, by removing the amplifier IC as well. So now my HU has 4 channel pre-out conected to the "speaker out" harness on the HU.


You should fit RCA connectors for the pre-out at any case. The pre-out is "unbalanced", meaning it has Left +, Right + and a common ground. The hu's speaker output has not common ground, meaning the Left - and the Right - are not the same, keep that in mind when figuring out your amp's
circuit and making your connections.


ABout the buzz, its a common problem. You should:

1) Ground the HU at the same spot where the amp is getting grounded ( this cancels the ground loop and it also improves sound quality)

2) Use high quality SHIELDED RCA cables to pass the signal from the hu to the amp. MAKE SURE THIS CABLE IS AWAY FROM ANY OTHER 12V WIRE (fuel pump supply, lights or wiper wires, the amp's power supply itself etc )
 

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Discussion Starter #5
jomor said:
You should fit RCA connectors for the pre-out at any case. The pre-out is "unbalanced", meaning it has Left +, Right + and a common ground. The hu's speaker output has not common ground, meaning the Left - and the Right - are not the same, keep that in mind when figuring out your amp's circuit and making your connections.
Thanks for the advice. But the pin-out at the speaker connectors now has common ground for all the -ve pins. Since the HU uses the TDA7573A IC (double bridge config), so am using "pin#9" for all signal grounding.

I will try to do a proper HU grounding and see if the buzz (or rather whine) goes away.

And another "phenomenon" is that there's a small "pop" when brake is applied i.e. brake switch is triggered.....arrrggghhhh....
 

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Of course you have common ground now at your hu's output. Its the amp's input i was actually refering to, it is supposed to be expecting the TDA's output, which isn't with common ground. Thats not necessarily a problem but its a strong suspect for increasing the noises you get. It is known that combining balanced with unbalanced signals is a noise-sensitive configuration. Either way, do use a shielded cable for your low level signal, even if you solve the noise problem just with proper groundings.

If I may, I d like to insist on using RCA connectors for the hu's pre-out. Making DIY modifications is fun for the DIY-er, but sometimes they can also be a pain in the the next owner's arse. Think that he wont be able to do any upgrade to the system unless he's very familiar with electronics or he completely replaces all devices. So it would be wiser to use the standard way of making the connections since this will help the next owner identify what changes you ve made, and also it will help you use a high quality shielded cable for your low level signal. It will also give you the ability of instant and easy upgrade of the amp if you ever decide to replace the Bose amp with something better. If you decide to use rca connectors, there is no reason
to drill the hu's case and fit case-type rcas, you can always pass coaxial cable from a hole and use cable-type rca jacks, like the older head units had :)
 

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Discussion Starter #7
Points taken, thanks.

Did a temporary 12v+ & -ve partial-isolation (direct to battery) and notice that the noise reduced by approx. 50%. Will do a full isolation later & see how it goes.
 

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Discussion Starter #8
Finally connected the permanent 12v+ & -ve directly to the HU, and the noise was virtually eliminated (there's still very soft "whine" but not audible with music on). It should be due to the pre-out (originally speaker) cable that's not shielded. ?

After dismantling the Bose, found out that it does NOT have a attenuator circuit. However, on the HU there is a "booster in" connector that the Bose sends 12v to the HU to indicate its presence. By removing the connector, the HU's pre-out is back to normal level and there's also EQ & Loudness selection.

When the HU detects the Bose is connected, it limits the pre-amp level.
 

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colins said:
When the HU detects the Bose is connected, it limits the pre-amp level.
Yes, it is also mentioned at the workshop cd, the booster-in connector disables the equalizer ( using fixed settings optimized for the bose speakers ) and lowers the speaker output level.
 
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