Marantz CD94 DAC circuit question - diyAudio
Go Back   Home > Forums > Source & Line > Digital Source

Digital Source Digital Players and Recorders: CD , SACD , Tape, Memory Card, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 27th March 2008, 12:52 PM   #1
oshifis is offline oshifis  Hungary
diyAudio Member
 
Join Date: Mar 2004
Location: Budapest, Hungary
Default Marantz CD94 DAC circuit question

I am about to modify my Marantz CD94 (not MkII). I already bypassed the SAA7320B/P and installed DIP sockets for the opamps. I just quick replaced the NE5534s to OPA134s for now. The next step will be to get rid of the second opamp, that is an unity gain filter. For the I/V opamp I intend to use OPA627A and I want to try biasing it to Class A. Studying the schematics I came across something interesting: there is a resistor at the output of the DAC. It is connected to an +5.6V Zener diode through some resistors, so I thought it was for supplying the 2mA idle bias to the TDA1541A chip. However, the calculated current is only about 0.6mA. What could be the real purpose of these resistors? I attach the part of the DAC that I am talking about.
Attached Images
File Type: jpg cd94_dac.jpg (86.2 KB, 250 views)
  Reply With Quote
Old 28th March 2008, 01:23 AM   #2
diyAudio Member
 
Join Date: Dec 2003
Location: Sydney
To bias DAC out and keep (close to) 0 V DC at OP amp inputs.

The unity gain filter is also a buffer.... you may need it...

OPA627A output stage is ALREADY biased in class A. What you intend to do will shift the output symmetry and introduce uneven clipping...

Cheers,
Boky
  Reply With Quote
Old 28th March 2008, 07:09 AM   #3
oshifis is offline oshifis  Hungary
diyAudio Member
 
Join Date: Mar 2004
Location: Budapest, Hungary
Quote:
Originally posted by Extreme_Boky
To bias DAC out and keep (close to) 0 V DC at OP amp inputs.

The unity gain filter is also a buffer.... you may need it...

OPA627A output stage is ALREADY biased in class A. What you intend to do will shift the output symmetry and introduce uneven clipping...

Cheers,
Boky
Shouldn't that be 2mA? That would need about 2.8k from the 5.6V zener voltage, or a FET CCS. In the actual circuit there is 2.7k + (10uF capacitor to GND) + 4.7k + (2 x 1.21k) = 9.82k. This makes 0.57mA. Also part of the output AC current flows through the 2.7k resistor, which seems a waste. Not to mention the 10uF capacitor that comes into the picture at low frequencies.

I read about biasing the OPA627 in class A in another thread and there was a consesus that it is a good thing to do:

The OPA627 really sings
  Reply With Quote
Old 6th April 2008, 07:55 PM   #4
diyAudio Member
 
Eric Juaneda's Avatar
 
Join Date: Dec 2006
Location: Avignon, France
Hi oshifis,

By adjusting this resistor you can reach 0V on the output analog stage. After, you can take of the 10F (C590, C591) output capacitor.
I do it on My CD 94.

Eric
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
WTB: Marantz CD94 NickC Swap Meet 0 22nd November 2006 07:34 PM
Marantz CD94 skipping, advice? clintyboy Digital Source 1 25th October 2006 10:07 PM
Marantz CD94 or CD99? commstech Digital Source 2 29th July 2004 01:25 AM
Marantz CD94 lucpes Digital Source 1 3rd May 2004 02:10 AM
Marantz CD94 difference between MK1 & MK2? kasra Digital Source 5 11th September 2002 09:12 AM


New To Site? Need Help?

All times are GMT. The time now is 11:14 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2