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Old 2nd July 2004, 04:46 AM   #1
ksprul is offline ksprul  United States
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Default AC3 - Dolby digital project

I've been trying to find a good starting point for a home grown digtial decoder. My basic plan is to take an exsiting unit and replace the DAC with spdif outputs for each channel pair . I'm then going to feed these into exsiting wadia DAC's that I have.

I picked up a Technics AC500d which i thought would be a good starting point based on reviews. However my initial lisiting tests have me worried that its decoding is not up to the job.

I built a simple cs8402 based spdif interface and connected it to the DAC (cs8401) l/r outputs. It works however when i A/B test it against a recent okyo decoder it sounds very very poor. No high frequency and limited bass.

I'm starting to think that the quility of the decoding software is not great and therfore I was thinking about giving a Rotel RDA975 a try. Does anyone have an experince with these decoders? Anyone else had any luck making them work in this configuration?
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Old 2nd July 2004, 08:22 AM   #2
Cobra2 is offline Cobra2  Norway
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Default Technics SH-AC500

I have the Technics, and find it ok for surround & DTS, but as a 2ch DAC, it is almost useless...
Some improvements I have done:
OpAmp (IC451 & IC452) is main ch. Try OPA2406.
IC453 is for SUB, not critical, IC501 & IC552 is SUB, Center LS & RS, I used OPA4134.
There is a lot of DC-blocking caps; L & R; C453+C454+C461+C462,
C.ch; C503+C505+C509+C515,
LS+RS ch; C553+C554+C555+C556+C559+C560+C565+C566,
try swapping with bi-polar 10uF/16-25V....
I felt that some more gain would help, and fixed it by swapping R453+R454(8,2K) with 4,7 or 5,6K (can't remember exactly...).

Some PSU updates will probably also help...

Arne K
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Old 2nd July 2004, 01:00 PM   #3
ksprul is offline ksprul  United States
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Thanks for the follow up Cobra2

I'm totaly bypassing the analog stages. I'm pulling the digital output from the dsp and feeding it into an spdif format converter.

I guess my initial conclusion of the dsp software being dated or just not that great is correct

any one built anything on a RDA975?
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Old 2nd July 2004, 01:14 PM   #4
ksprul is offline ksprul  United States
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Thanks for the follow up Cobra2

I'm totaly bypassing the analog stages. I'm pulling the digital output from the dsp and feeding it into an spdif format converter.

I guess my initial conclusion of the dsp software being dated or just not that great is correct

any one built anything on a RDA975?
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