Well, that's subjective. Initially I had the impression sound improved significantly: better clarity, a bit lower noise. Measurements didn't confirm any drop in noise floor. Unfortunately at the time I did the mod didn't have a second D6C with me to compare them side by side. So it's hard to say how much is placebo effect and how it's a real change. That's why it's best to test for yourself and draw your own conclusions.
Yes, that's a good idea. I am curious as to why your experiment with the OPA failed. The only possibility I can imagine would be if you inadvertently subbed a single-opamp version (OPA1641) which has a different pinout than the 4558, 5532, or the OPA1642.
@Valentin thank you for great tutorial. Can I use this test tape for Dolby levels? It's 400Hz instead 330Hz in your instruction. Or it doesn't really matter exactly what frequency for Dolby?
Yes, the test signal at position 4 (400Hz, 200nWb/m ANSI) can be used for adjustments at levels mentioned in this tutorial. Alternatively, test signal at position 5 (315Hz 160nWb/m DIN) can also be used with levels mentioned in the service manual. Frequency is not critical (different standards use different frequencies), the fluxivity is what matters in regard to Dolby levels. In practice any fluxivity can be used as long as you convert the values so you know the levels you need for that particular fluxivity.
