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Reel to Reel / Neil Young Auction
« Last post by zenelectro on December 10, 2017, 11:37:27 AM »
Looks like Neil is offloading some nice gear.

Quite an assortment of stuff including a host of Lionel model trains of which he apparently owns a part share in the company.

ATR102 1/2"

https://www.julienslive.com/m/lot-details/index/catalog/243/lot/98626/?url=%2Fm%2Fview-auctions%2Fcatalog%2Fid%2F243%2F%3Fpage%3D19%26http_referer%3Dhttps%253A%252F%252Fwww.gearslutz.com%252F

T
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Other DACs / Re: Marantz 94 mark 2
« Last post by brenden on December 08, 2017, 01:26:19 PM »
Welcome  from me too John.
     You should have fun modding your 94 mk2  but it is also full of frustrations at times .If you need help with anything ,let me know .
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Other DACs / Re: Marantz 94 mark 2
« Last post by zenelectro on December 07, 2017, 09:27:27 PM »
Hi everyone.

I also have a CD94 mkii and will be modding soon.

What I never see in reviews about the CD94 MKII is what happens when you substract the signals from 2 DAC where one had an inverted input signal.


Hi John, welcome 

The two signals are opposite (one dac signal goes up +, other goes down -) and are summed in the differential to single ended circuit.

As such only signals that are the same will be cancelled. The call this common mode signals, or common mode rejection. 

Quote

As the clock component at 22.1 kHz is equal in both TDA-1541a and the signals are substracted the clock frequency component is eliminated and you keep the output signal 2x.
Am I right here? ;D


Not quite.

Both DAC's outputs are stepping in opposite directions, so there is no cancellation of the 44.1kHz sample rate (if running zero oversampling).
As stated before, only signals (or noise) that is the same on both DACs will be cancelled. 

Quote


That is why the MKII sounds better. In theory you don't even need a filter because the clock signal is almost not there.


This is not the case - unfortunately. As pointed out above, the 44.1kHz signal 'staircase' of zero oversampling will not
be cancelled at all.
If you still run the digital filter this will be at whatever sample rate the DF runs at, it's 4 x OS in CD94's case = 192kHz.

There's no free lunch unfortunately.

Quote

I will implement a lot of these mods.

Thanks for your explanation.

John
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Other DACs / Re: Marantz 94 mark 2
« Last post by maessen on December 07, 2017, 04:30:05 AM »
Hi everyone.

I also have a CD94 mkii and will be modding soon.

What I never see in reviews about the CD94 MKII is what happens when you substract the signals from 2 DAC where one had an inverted input signal.

As the clock component at 22.1 kHz is equal in both TDA-1541a and the signals are substracted the clock frequency component is eliminated and you keep the output signal 2x.
Am I right here? ;D

That is why the MKII sounds better. In theory you don't even need a filter because the clock signal is almost not there.

I will implement a lot of these mods.

Thanks for your explanation.

John
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Other DACs / Re: Marantz 94 mark 2
« Last post by brenden on December 04, 2017, 03:39:35 AM »
Ok , this is a photo of the  servo board.  Notice some silmic 2  super golds , some auricaps  and other copper foil caps .  Now ,   if its not in the amplifying section or around the dacs  these areas are ignored by many .  These circuits are supplied by the same power supplies  as  the other sensitive  audio and dac circuits   meaning the capacitors are  basically in parallel with the other areas I spent time upgrading . Any inferior  capacitor  here will also  affect our music making  circuits  . Out went the cheap  electrolytics and mylar bypasses  and in went the good stuff . These circuits will be noisy so top quality bypasses  are important in my opinion . My guess is also that the servo could be more accurate  with lower noise   parts  , but at the very least  the power supplies have consistent   top  quality without weak  noise generating caps .  Maybe one day I will  look at building a separate power supply for  some voltages in there  to take it off the sensitive lines .
    Even worse is the ground system in these boards , which traverses from the dac board then through  the  decoder  board   then through  the  servo board  .  Noise will  potentially be breaking into the system all over  the place with this long and winding  ground system .
    I have previously  separated  the  grounds from each board    but  reinstalled them  because  while I am modding the boards it was much  quicker and easier   to unplug  them  , but there was a clear  sound improvement   by using  a more direct  star type ground  . It is sensitive to the type of wire used so need to experiment a bit .but  we need  probably 22  -24 g  and maybe a couple of runs .
   
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Other DACs / Re: Marantz 94 mark 2
« Last post by brenden on December 03, 2017, 11:40:43 PM »
Yes , I was mistaken regarding the 12 v  regs too .  I had a look at the diagrams last night  and was going to post them today .  It makes life easier  when these nice goodies are already there like decent  discrete regs .  I always find  better results with nice bypasses to electrolytic  caps even in good regulated supplies  and even with well  regarded  electros .
    Nice post Zen .  . Some of your suggestions  give me more  to think about. 
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Other DACs / Re: Marantz 94 mark 2
« Last post by zenelectro on December 03, 2017, 10:17:14 PM »
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.

Quote

    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.
Quote

   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.

Quote

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





 
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Other DACs / Re: Marantz 94 mark 2
« Last post by brenden on December 02, 2017, 10:43:11 PM »
Hi Terry , this is the first op amp  after the dac  and the de emphasis circuit .you would understand it much better than me .
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Other DACs / Re: Marantz 94 mark 2
« Last post by brenden on December 02, 2017, 04:48:04 PM »
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 .
    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 .
   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  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 .
    I want to highlight that this eighties technology  could cut it without resorting to major  redesigning  and also to highlight the importance of the supporting components as the major limiting factor in much audio equipment , not just CD players .
     
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Other DACs / Re: Marantz 94 mark 2
« Last post by zenelectro on December 02, 2017, 11:31:06 AM »
I have boxed in the Duelund output capacitors underneath the player . There  isn't  enough  room  to fit them inside . To  access underneath I simply need to unscrew the box section .the caps are affixed to the  inside of box with double sided tape with some insulation over the caps and on the  cd base plate.
   None of this can be seen unless your eyesight is level with the underneath  of the player .
    This will be improved once modding is finnished with better and shorter wire and neater insulation .
   I cut a small slot in the baseplate  for the wires  so the base plate can be removed without unsoldering wires .
  The wires tuck in neatly to the space at the end of the box   once fitted .
  The box was the lid part of an electronic  box from Jaycar . Perfect for this application .

Wow - you have done a lot of work. I wish we could have spoken earlier - read on.

With a 2 DAC / differential to single ended arrangement such as the 94MKII, the output circuit can be direct coupled with no caps required.
This is always the best way to go if possible - it saves money and no cap is better than any cap in signal path - even Duelunds.

However the circuit needs to be a proper DIF -> SE design and it needs a small trimpot to null any offset at OP. This can be done pretty simply with 3 opamps.
The standard circuit uses 5 opamps per channel and is pretty complex. This is mainly because it incorporates Fet switched de emphasis which you don't need.

The opamps should be biased into class A - but the right amount needs to be tweaked.

The standard regulators are either 317 or 78xx type which don't filter or block rubbish coming from other parts of the player above a few kHz very well.
Once you get to 100's of kHz they almost stop working all together. This is one reason why your PS cap changes make so much difference, they are
doing most of the filtering work of the regs.

For regulators I use very simple discrete series pass or shunt are even better. These regs work right up into the MHz range at filtering incoming rubbish.
They also have much lower noise than 317 / 78xx regs - but I'm not totally convinced the random noise is a big issue.
It's also best to use separate regs for R and L channel.

All this can be made on a relatively small ground plane covered board with room for caps, resistors and with opamp sockets.
The baseline performance is a long way above what the standard CD94 gives you.

Then it's a case of tweaking resistor and cap types, opamp types and amount of class A bias to get everything sounding best or to your taste.

T
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