Thanks for that Terry . Marantz used 7812 regs for + -12 v but they also use number of discrete regs as well for the lower voltage sections like the dacs .
Yeah - I'm asleep, doh - Homer Simpson moment, haha!
It does have discrete final regs for R and L channel and they look pretty good. So well done Marantz on that count.
I always assumed that the first op amp on the dac board was a dual differential current to voltage conversion ,with the small transistors at the end of the board switching in the discrete filter for de emphasis . Then on to the second dual differential op amp amplifying stage , and lastly on to the single op amp as a buffer , but could be wrong .
Anyway I had considered using the trim pot on the single op amp to null offset , but wanted a baseline .so the best capacitor available would be the best base line to compare direct coupled .
1st 2 OPA's are I-Vs for each phase with de emphasis. There is also some low pass filtering on this stage.
2nd 2 OPA's are for more low pass filtering (2nd order) These are C701/702, C703/704. Again each phase is separate.
3rd single OPA is the DIF to SE converter / OP driver.
I think they prolly use the 3 stages to get the 3 orders of low pass filtering. The low pass filtering removes noise from above audio band.
There are a lot of views WRT how much low pass filtering you need. Most of these early players had 3 orders of filtering. I think for
multibit DACs you don't need much.
With the Killer DAC we ended up using none at all. From memory Steve said it did some damage to the sound so I explained how to remove it.
The low pass filtering does have a pretty big effect on the sound - but with a SS opamp circuit (as opposed to valve) I think you still need some.
Tweaking the LPF on a circuit like the 94MK2 is pretty much beyond simple tweaking as you have to run the RC values through a filter program.
I may need some tips on the correct way to get the best results for biasing class A.
I know Steve would like me to leave it alone.
Steve will want to clone it and sell it - Haha (sorry Steve, couldn't resist
) but in all honesty if you have the player in a
fine state of balance you do have to proceed with caution doing any further mods that are major.
but there is still a bit to do to this player particularly things I have done in the past that i know will make it better.
One thing I did want to do is to showcase the real potential of Marantz's tda1541 , 94 mk2 and what it is capable of without changing the basic circuitry too much except for the clocking improvements .Even the NOS is not a major fundamental change to the original design .
The stock clocks are crap. So just about any after market clock will improve it to some degree.
The Valab clocks are actually not bad.
However the chip that generates the I2S signal (that feeds DAC) corrupts the clock timing quite a bit. I've got scope shots of before / after
re clocking the I2S signal and the 'before' looks pretty messy in comparison.
You can do all the usual oscon etc mods around this chip and it will make a small difference but it's still pretty messy.
The re clocker takes the I2S signals re alignes them perfectly to the clocks rising edge. It also spits out very clean wave forms.
However - it does change the sound quite a bit, so if your player is tweaked to a finely tuned balance then other things may or may not need
to be adjusted. I've tried a few and I always prefer the re clocker but it's definitely cleaner - and that's not always to everyones taste.
T