C33 Business Radio Preout Tap

[objectgroup]

Hello, this is my first post here on the forum.

I'm looking to find a way to bypass the internal amplifier in a C33 so that I can send the line-level signal directly to an aftermarket amplifier without using a Line Output Converter (cleaner signal path = better SNR). I've seen examples of people tapping preouts on other BMW heads and wondering if anyone has done it on a C33, or could point me in the right direction.

I took a few photos of the boards near the back of the unit, where I presume the amplifier is. In addition to large & small electrolytic caps, I see a few opamps that appear to be coupled (or decoupled?) to the thick rear aluminum heatsink panel with some white grease-type-sludge. Photos included below of the unit and the breakout board with the molex harness connector.


IMG_9609.jpg

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breakout/molex board

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main board jumper location

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top of breakout/molex board

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bottom of breakout - labeled jumper contacts


I'm handy enough with an iron/multimeter if I need to be, just looking for a clue on where to start since I can't seem to find a schematic. Thanks in advance for any clues/advice.

[MarcoZandrini]

I think it'd be a lot easier to buy an amp that has speaker level inputs.

[objectgroup]

I have that, but would prefer to do it the way that gives me the best fidelity/SNR

[DennisCooper]

I have that, but would prefer to do it the way that gives me the best fidelity/SNR

The flaw in your thinking is that the oem head units that were designed and made over 20 years ago werent built or specified for any 'good' or 'better' levels of audio quality and performance. On top of that, they had the OEM Equalization curve/response built into those IC's. The C33 unit I think has various iterations over different BMW models but I think they're all radio / cassettes? - if so, then cassette is all but a dead medium today. The AM/FM radio stage isn't so good by todays quality and standards either, so if you do find where to get the audio signal prior to the on-board amplification stage, you'll still most likely have the oem EQ curve/response aspect to overcome and even with that gone, then the 'Sound Quality' will be better like you are thinking, but won't be as good as perhaps as what you are wanting !

Most owners/forum members won't mention or know what 'signal to noise ratio' is, as you've mentioned it, then the oem units don't really have a 'good SNR', more a 'useable' level. The end result is that you'll just have louder 'contained' sound quality instead of having an 'improved' sound quality.

Similarly, there's not much call for what you are wanting to do as there's excellent processors that easily take the speaker level outputs from oem head units and cleverly remove or reduce the oem EQ curve and stop and greatly reduce distortion as well. You'd potentially be all connected up within 10-20 minutes or so by using one and it's the route I'd say to take, but I understand they 'relatively' expensive.

If you are happy and do find the oem head unit modified the way you are thinking is good enough for you then absolutely fine and no need to take notice of what I or anyone else says to you !

Cheers, Dennis!

[objectgroup]

Thanks for your thoughts Dennis.

When I mention signal-to-noise ratio, I'm not referring to the internal specs of the C33, but line-output converting an already amplified signal to line level then re-amplifying it. This is what the processors you mention do and these necessarily cause a reduction in signal-to-noise ratio. The idea I'm after is to bypass any signal-degrading elements so that I don't have to try to compensate with extra circuitry. I know that people have tapped preouts on the C33, I've seen mention of it on the forums, however I haven't found any posts that describe the specific solders to tap for preouts.

I don't have any info on the fixed-eq circuits in the C33. I have seen LOC units that compensate for bass rolloff like you mention, but this doesn't seem like the right solution to me. EQing once and then re-applying corrective EQing to get a 'flat' signal sounds like a recipe for phase distortion.

For context, I didn't pick the C33 because it's an audiophile head. I picked it because it 'looks stock' in my e28 and works with a GROM unit for connectivity to phone devices. I'm replacing a clean-sounding but imho very ugly Pioneer head with line outs. I'm sure there are better sounding heads out there (nakamichi, etc.), but I still prefer the look of the BMW head.