I have a Philips CDC935 changer that uses the SAA7341 as the decoder. I have replaced the LM833 op-amp with a (socketed) OP275 and a separate power supply, and all is good so far.
When I look at the block diagram of the SAA7341 that is in the service manual, it IMPLIES that the last stage of the chip is little on-board op-amps (called "summing op-amps") that provide some gain and "de-emphasis". I wonder if these could be bypassed and a better quality op-amp inserted instead? My choice would be some FET-input device, such as AD712 or a later derivative of same. I gather that many people have done this sort of thing on other decoders.
Have I got the right idea about these "summing op-amps"? It seems that their inputs are pins 47 and 49 (left) and 54 and 56 (right) and their outputs are pin 43 (L) and 60 (R). Which input is plus and which minus? Can I just leave the "de-emphasis" components in place, or should they be replaced also? They seem to be just 4.7 nF and 1.8k ohms in series between the plus and minus inputs (if I have the right idea about the inputs!).
If I go ahead with this mod, it might not be possible to go back if trouble arises. The pins on the SAA7341 are very close together.
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David Dalton dalton_95014@yahoo.com
When I look at the block diagram of the SAA7341 that is in the service manual, it IMPLIES that the last stage of the chip is little on-board op-amps (called "summing op-amps") that provide some gain and "de-emphasis". I wonder if these could be bypassed and a better quality op-amp inserted instead? My choice would be some FET-input device, such as AD712 or a later derivative of same. I gather that many people have done this sort of thing on other decoders.
Have I got the right idea about these "summing op-amps"? It seems that their inputs are pins 47 and 49 (left) and 54 and 56 (right) and their outputs are pin 43 (L) and 60 (R). Which input is plus and which minus? Can I just leave the "de-emphasis" components in place, or should they be replaced also? They seem to be just 4.7 nF and 1.8k ohms in series between the plus and minus inputs (if I have the right idea about the inputs!).
If I go ahead with this mod, it might not be possible to go back if trouble arises. The pins on the SAA7341 are very close together.
---------------------------------------------------------------------------
David Dalton dalton_95014@yahoo.com