That is very interesting work you have done. Thank you for sharing. I am curious about one thing. Is the op-amp output voltage really 2.3V as shown in both the before and after schematic? Also, on the schematic shown there is only a -14V power supply. Is there a +14V power supply shown in the other channel or?
The op amps used are DIP8 dual channel types and the -V is on pin 4 and the +V is on pin 8. Here is a link to the 4580 datasheet: http://www.njr.com/semicon/PDF/NJM4580_E.pdf
One of the changes you made, according to your revised schematic, was the removal of R63. Without R63, I would expect that the op-amp output to be at or close to 0V DC. If that is true and depending on the magnitude of the audio output signal, C09 could become reversed biased during the negative swing of the audio signal. If so, that may not be such a great thing. It may be that the original designer purposely created an offset at the output to prevent C09 from becoming reversed biased. Just a thought.
I find it really odd that Toshiba would specify a worst case scenario on the schematic. It would be desirable to have a positive voltage on the output in order to bias the electrolytic capacitor. Is it possible that the purpose of R63 was to create a positive bias on the output?
That's a good point. Thanks. I think that the best solution is to replace all electrolytic capacitors in the phono stage (except the ones connected to +14 VDC and -14 VDC) with film capacitors even though the 10uF and 4.7uF film are much bigger than the original electrolytics but they can fit. So capacitor polarity won't be a problem.
