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Old 24th February 2003, 10:24 PM   #1
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Default DAC for a car?

I tried the search but came up empty so I'll ask away...

I have the schematic manuals for both the top of the line Alpine car CD player and the top of the line Pioneer CD player. Looking through the schematics, both use the PCM1704 DAC chip. Doing some research, I came upon this forum.

I am interested in building an external DAC that could run in a car... so the big limitation is power supply voltage. I have looked at a bunch of different DAC plans, but seemingly all require a higher level of voltage than can be attained with a 12VDC source. I did find a couple power supply projects that can supply up to -48VDC, so I am thinking it should be possible (especially considering the alpine and pioneer units... but they don't show voltages).

I am a CS major and have built a bunch of vacuum tube devices (preamps and amps mostly), but have not gone into the digital domain before. The filtering and jitter control and _____ all kinda have me thrown for a loop. I am really unsure from what everyone talks about if delta-sigma is the way to go or not... then there is the whole unsampled or not I have been reading about. I am not out to break new ground here... more like take a great sounding proven design DAC and modify it for use in a car on a 12VDC power source.

I would greatly appreciate anyones help on this matter. Thank you very much in advance.

Mark
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Old 26th February 2003, 12:49 AM   #2
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The PCM1704's are awesome DACs... I can't believe they're in a Pioneer deck - or an Alpine for that matter.

Could you email me the schematics that you have? Or else maybe post them as I'm sure others will be interested.

As for improving sound quality in the decks, I would recommend going over the analog stage. Perhaps altering opamps in the filters and/or removing anoying muting transistors.

Improving the digital side could prove to be a real pain. Improving jitter performance can be obtained by better clocking circuitry - Elso's and Halojoy have posted some as I recall that have both gotten great responses from those that use them.

Also, great improvements can be made by making your own power supply for the system. An independent digital power supply (DACs, Decimation filter, etc) and two independent power supplies for the +/- analog sides. DC-DC converters could be an option here for higher voltages needed.

Well, those are just some things off the top of my head. I'm sure others will chime in as well.
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Old 26th February 2003, 01:49 AM   #3
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Tieftoener: thanks for the reply. The model Alpine that I own does not use the 1704's... only the $3000 model Alpine 7990 and the Pioneer P9 do. I am looking into building an external DAC that can be used in a car. That way if you take the digital output of your current cd player (could be the stock one in your car), you can enjoy the sound of a high end DAC and output stage. I know I could get an inverter to get 120AC, but those delivery a TON of noise to a car audio system... hopefully some of these schematics will help. Unfortunetly voltages are not shown so a little more detail is in order. I am thinking maybe a non-oversampling DAC with decimation would be great. One of the nice things about car amplifiers is that you can adjust the input sensitivity for different input voltages... so if I only get 1 volt out, its no big deal. I plan on having this mounted inches from the amp... might even just direct connect them with solder and wire. Here are two pictures from the manual... sorry for the large pictures, but at least the details are there. It seems the 1704 has a strong following... maybe I'll go with it?

Click the image to open in full size.

Click the image to open in full size.
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