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).
@Valentin hello, need your advice) I try to adjust record level. First I check playback level with manual, it 250 mV in both channels at 315 Hz 0dB. But when I'm in record mode (at lower level as you advised) from line in I get 104 mV L and 100 mV R in line out. And I adjust record level according this. But now I thing is this correct? 1. why they not equal, I check again at 315 Hz 0dB and they equal. 2. in my situation I need adjust to 104 mv and 100 mV, or maybe L and R 104 mV or maybe both 100mV? P.S. On my tc-d5 pro II this difference more significant L 103mv R 84mV, but at 315 Hz 0dB L=R 470 mV.
Don't try to correct L/R imbalance by using this adjustment, because you'll end up making levels off. And certainly don't re-adjust the playback levels to compensate for this imbalance, you'll just make things worse. Some L/R imbalance is inherent because of the stereo rec level potentiometer, you just can't avoid that. For perfect L/R balance you need a unit with independent L/R adjustments, such at the TC-D5 you mentioned. So if if L is 100mV and R is 110mV, adjust for same values on the output. Or if you really want to compensate do it half-way, like adjusting both to 105mV. A couple of mV are not going to be a make or break kind of difference here, since you're going to get a lot of variation from formulation to formulation anyway. On TC-D5 Pro II you haven't adjusted the rec level pot (external one) correctly. Note that pot has 2 wheels, one for R one for L Adjust them so levels are equal. Also note we do this adjustment for Dolby encoding, so we're really not interested in correcting imbalances before the Dolby encoder. Rather we are interested in having a good correlation between input and level AFTER the encoder.
Now I understand, thanks. One more question) When I set levels for type I and check it on type II, levels a higher about 20 mV. Should I lower the levels or leave it as is?
It depends on what you want to achieve. One thing to keep in mind is the difference in bias and levels between different tape types (I, II, IV) is higher than what changing the setting can actually achieve. So at the end of day you can achieve either a compromise adjustment or a better adjustment for one specific tape type and one forumulation. If you set the levels for TYPE I, usually TYPE II will saturate early limiting the achievable signal-to-noise ration on TYPE II. It's something I have noticed myself. But if you intend to mostly use TYPE Is, this is the setting to go for. If you set the levels for TYPE II, TYPE Is will have lower levels, potentially rendering audible Dolby mistracking (muffled sound, breathing artifacts). This is also true for bias, most TYPE I need lower bias (higher 10kHz levels) than what the tape type setting is offering. Which again, if you set bias for TYPE I, TYPE II will saturate early having too much treble from the lower bias setting done on TYPE I. If a compromise is what you want, set it half-way. But keep in mind in the end calibration will only be perfect for 1 specific tape.
