TDA1305 / TDA1549 bypass opamps - diyAudio
Go Back   Home > Forums > Source & Line > Digital Source
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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 17th May 2011, 09:40 AM   #1
diyAudio Member
 
Join Date: Feb 2006
Location: Germany
Default TDA1305 / TDA1549 bypass opamps

I just bought a mini USB DAC with TDA1305T Chip inside from China (which is on the way to me now) (review: USB DAC using ancient technology)

As I've seen on lampizators webpage, the TDA1305T has got identical structure as TDA1549. There's shown a trick on lampizators page how to bypass the internal opamps of the DAC Chip (see attachment file).

I wonder If anyone here has already tried this modification. As I don't have much electronic background knowledge, I've got some questions about it:

- Why does the PIN VDDC 10 has to be cutted ?
- Are the PINS 5 + 6 connected to + or to - signal input of preamp ?
- which output values (mV and mA) does the DAC Chip itself have after modification ?

Thank you in advance for help.

Best Regards
Horst
Attached Images
File Type: jpg 751_diagram_after_mods.jpg (59.4 KB, 808 views)
  Reply With Quote
Old 18th May 2011, 10:14 AM   #2
erin is offline erin  Australia
diyAudio Member
 
Join Date: Nov 2006
Location: http://www.makeitpossible.com/
Why do you want to bypass the op-amp?
Are you connecting it to a valve output stage?
If you are not connecting to a valve you dont need to do anything.
  Reply With Quote
Old 18th May 2011, 11:54 AM   #3
diyAudio Member
 
Join Date: Feb 2006
Location: Germany
Yes, the reason for bypassing the opamps is that I'd like to get the pure (unamplified) analog output signal from the DAC itself to connect it to an discret analog pre-amplifier (DIY-Version from a friend of mine).

My friend had some very good results with the same mod for a TDA1547 based CD-Player. The Diagramm of TDA1305T says that it should offer the same possiblity.

As I've read some tweakers said that they got a distorted signal after bypassing the internal opamps. So I hope, there's someone who can give me a good advice how to get best result (swictching of the circuits CEXT1 and CEXT2 and then taking I-out signals from PIN 23 and 24 ?)

Have a look what lampizator's result was:
DACMAGIC Cambridge tubed DAC player TDA1305
  Reply With Quote
Old 18th May 2011, 12:03 PM   #4
diyAudio Member
 
QSerraTico_Tico's Avatar
 
Join Date: Oct 2004
Contrary to the TDA1547 the R-IV is still in the circuit.
Perhaps you can disconnect the analog supply to the TDA1305?
  Reply With Quote
Old 18th May 2011, 03:19 PM   #5
diyAudio Member
 
Join Date: Feb 2006
Location: Germany
Quote:
Originally Posted by QSerraTico_Tico View Post
Contrary to the TDA1547 the R-IV is still in the circuit.
Perhaps you can disconnect the analog supply to the TDA1305?
Hmm ... are you sure ?
look:
http://digilander.libero.it/paeng/a__tda1547_tweak.htm

here's a detailed tweaking description of TDA1547.
The opamps are also inside the DAC Chip.

What would happen if the analog supply of the opamps would be capped ?
Does it still give out a signal when it's capped ?
  Reply With Quote
Old 23rd June 2011, 03:44 PM   #6
diyAudio Member
 
Join Date: May 2011
jockel77, I'm wondering if you made any progress in your quest to bypass the internal opamps of your 1305? I too have a TDA1549 and thinking of bypassing the internal opamps and connect the current output directly to a valve stage. However, I too have very little electronics knowledge so currently searching for the ways to do it.

cheers!
  Reply With Quote
Old 26th June 2011, 08:49 PM   #7
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
Simple answer is that it does not work, of course :

Philips_tda1549

"After intense testing i concluded that the signal is distorted. So I just reversed to the power to leg 10, and signal from legs 4 and 7....."

Removing the power supply to an opamp does not equal to removing the opamp altogether.

Pity. Would have been a nice idea to play around with.


Patrick
  Reply With Quote
Old 27th June 2011, 11:53 AM   #8
diyAudio Member
 
tiefbassuebertr's Avatar
 
Join Date: Jul 2005
Location: D-55629 Schwarzerden
Quote:
Originally Posted by jockel77 View Post
I just bought a mini USB DAC with TDA1305T Chip inside from China (which is on the way to me now) (review: USB DAC using ancient technology)

As I've seen on lampizators webpage, the TDA1305T has got identical structure as TDA1549. There's shown a trick on lampizators page how to bypass the internal opamps of the DAC Chip (see attachment file).

I wonder If anyone here has already tried this modification. As I don't have much electronic background knowledge, I've got some questions about it:

- Why does the PIN VDDC 10 has to be cutted ?
- Are the PINS 5 + 6 connected to + or to - signal input of preamp ?
- which output values (mV and mA) does the DAC Chip itself have after modification ?

Thank you in advance for help.

Best Regards
Horst
Interesting question. Unfortunately I cannot disconnect the line between the output switches and the inverted input of the internal OP-AMP so as the 2K2 resistor. Thus the influence of the input load of this internal I/V stage is still the same.
  Reply With Quote
Old 5th March 2014, 11:12 AM   #9
diyAudio Member
 
Join Date: Feb 2014
If you cut PIN25 and try to grab the current from PIN24, you have to consider that Rconv2 is still in parallel with the signal line thus some current is still flowing trough it. The amount of that current is unknown because we don't know the impedance of the OP1 when it is powered off. In particular you cannot know if that impedance is linear, this probably introducing distortion. My suggestion is to use OP1 as a good I/V converter (as it is) and to apply the valve 'flavour' in a second stage. Enjoy!
  Reply With Quote
Old 6th March 2014, 08:10 AM   #10
Chivvyp is offline Chivvyp  United Kingdom
diyAudio Member
 
Join Date: Apr 2006
Location: London
I tried it with a TDA1549 using the attached schematic (came from another thread on this site iirc) which was reported to work.

In my case it didn't work and seemed to destroy the dac chip, quite possibly due to my soldering rather than an error in the schematic.

Regards

Pete
Attached Images
File Type: gif tda1549 internal opamp bypass.gif (9.4 KB, 396 views)
  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
To bypass or not to bypass... rsumperl Tubes / Valves 8 9th September 2010 04:51 PM
Want to bypass opamps in Sony Cd player ChopperCharles Digital Source 5 9th March 2008 06:45 AM
I2S signals from TDA1305 Skorpio Digital Source 2 19th February 2007 02:21 PM
Help with TDA1305 free_mnet Digital Source 8 18th March 2006 07:57 PM
FS: Power Chip OpAmps and other OpAmps & D to A Converter dtm1962 Swap Meet 4 12th January 2006 10:07 PM


New To Site? Need Help?

All times are GMT. The time now is 08:15 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki