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.
@Valentin Hi, 47K one for left and one for right channel? Like in my photo? Dolby switcher must be turned off? Without any adjustments I get 140mV for left and right channels in dolby level. I use trueRMS multimeter. And one more question about adjustment record levels. I download Audio sweep gen app for windows set 330Hz 0db and with vol max on PC I have 950mV I tried vol down at 86 it 734mV and at 87 it 797mV I think 734mV close to 750mV but what rec level I should set on d6C 0db on peak meter? It's my first time for that sort of adjustments sorry for my dump questions
Yes, the 47k load resistors are one for each channel. Depending on specific test setup it can be just one in parallel with meter input since only one channel gets measured/adjusted at a time. Dolby needs to be OFF and EQ set on the tape type you're using (most are TYPE I). 140mV seems way off. What test tape are you using ? And how many LEDs light up on the VU-meter ? In regard to record levels, use lower inputs no need for 0dB. Even 100mV RMS is enough to set the levels. It's better to use lower levels, cause this way you can compare 10kHz response with 330Hz easily when setting the bias.
@Valentin thanks. I change connection in line out with only 1 resistor in one channel at a time, and now it seems work right. For dolby I get both left and right channel 340-342mV it's not constant and slightly changes. I think it's ok and no need to adjust here. Maybe someone has already adjusted them before me I don't know. Test tape in attachment. P.S. What are the values before calibration usual?
Yes, it's normal to slightly change (a couple of mV) nothing to adjust there. Varies, some are close to the spec (315-320mV), some are way off (below 300mV), some have significant imbalance (for example L - 270mV, R-330mV).
