COD - Current Output DAC - Page 14 - diyAudio
Go Back   Home > Forums > Commercial Sector > Manufacturers > Twisted Pear

Twisted Pear Superior quality electronic kits

Reply
 
Thread Tools Search this Thread
Old 20th July 2012, 03:51 PM   #131
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by Zero Cool View Post
with a 90 ohm IV resistor there is about 1v of DC offset out of the DAC. you need the caps to block this DC from a following Buffer stage or use some form of compensation like a servo etc. The distortion is very low using a resistor. I would bet an active stage would have comparable results but a different harmonic structure to the distortion.

I know the active vs passive thing has been debated to death here. but I can tell you first hand a passive IV stage with a jfet buffer sounds WONDERFUL! that is of course MPO on the subject. and YMMV but it is an easy thing to experiment with and see what works for you!


Zc
Thanks. I saw somewhere that someone varied the I/V resistor of some other kind of current output DAC and got much worse distortion readings with higher resistor values. Let me go search on the forum for something about this. I initially did a Google search and got a few results, but nothing good, hence my question here since I have this DAC and COD board.

I have a schematic for a passive I/V stage with jfet buffer. No coupling capacitor is required since the DC offset is so small that the CRD in the circuit compensates just fine. The I/V resistors are 127 ohms in this circuit, for about 1.3Vrms output.

I was playing around with a Zen type of active I/V stage using bipolar transistors but the input is very sensitive to DC offset. I put a blocking capacitor at the input (no resistors at all) and it seemed to work just fine with no loss of bass response.

I have read that the current output DAC's will output the same AC current regardless of the load impedance up to the maximum voltage they can put out, but I have not seen a specific distortion vs. impedance curve anywhere.
  Reply With Quote
Old 21st July 2012, 01:54 PM   #132
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
I did some searching on finding a distortion vs. I/V resistor value chart or something like that with no luck. I don't think it's a linear relationship, but that's just a hunch. I do know that if the resistor is too large then the "compliance" limit will be exceeded such that the voltage output will be clipped. People keep repeating this mantra that a current output DAC must see the lowest possible load impedance but no one puts up any proof. I did a lot of searching Google and reading various websites with no luck so far. The DAC datasheets are no help, since all they say is "do it our way to achieve our results".

So, how low is low enough? 9 ohms? the Zen (Sen, Cen, etc.) only manages 15 ohms. If you use bjt's instead of jfets you can get less than 1 ohm input impedance, according to my simulations.

Also, there is a zero field transformer input circuit that can achieve very low input impedance, depending on the transformer. The Lundahl LL1517 has a winding resistance of some 9 ohms. With the zero field circuit, the input impedance can be made to be 9 ohms and the frequency response can be extended to 1 Hz. The advantage with a transformer is ground isolation. My simulations show this circuit to have high noise though, but I think that's a software issue.
  Reply With Quote
Old 21st July 2012, 02:21 PM   #133
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Further reading has shown that for the antique TDA1541 DAC, a passive I/V resistor of about 60 ohms is the limit. I don't know about the PCM1794 though.
  Reply With Quote
Old 22nd July 2012, 01:41 AM   #134
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
There's some spec floating around that the DAC pins shouldn't see more than 25mV but that's probably only with the really antique DAC's.

One way to do this is to do a passive I/V with a really low value resistor, and then use a normal gain stage after that. I put the RLC filter in with the I/V resistor before the gain stage. S/N is around 120-125dB depending on what kind of gain stage is used.
  Reply With Quote
Old 1st November 2012, 06:30 AM   #135
cotdt is offline cotdt  United States
diyAudio Member
 
Join Date: Oct 2005
Send a message via AIM to cotdt
Testing the pcm1794

It seems like 50R resistor is the way to go for passive I/V for this DAC. I'm going to try this with the Ventus as buffer.
  Reply With Quote
Old 17th December 2012, 07:42 PM   #136
diyAudio Member
 
Join Date: May 2009
Interesting... I was experimentig with higher and lower values for I/V. I have a AD1865 and found the lower values being better for my taste. I am just curious has someone tried less then 1ohm resistor for current to voltage conversation. I know you need a phono or mic input stage but it would be interesting to try....
  Reply With Quote
Old 17th December 2012, 09:32 PM   #137
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
I think you need about 50dB of gain. So, you can work from there.
  Reply With Quote
Old 18th December 2012, 07:41 AM   #138
diyAudio Member
 
Join Date: May 2009
Yes. thats a lot. I would need to make two gain stages for it.
  Reply With Quote
Old 22nd December 2012, 04:15 PM   #139
staki is offline staki  Switzerland
diyAudio Member
 
Join Date: Dec 2007
Location: 2504 Biel
Hi,

If I want to use two COD boards in dual mono configuration, what is the best solution to feed them with i2s input signals (from a USB to i2s converter) ? Just wire both PCM terminal blocks in parallel ?
Will there be a shift between the left and right data bits ?
  Reply With Quote
Old 22nd December 2012, 04:17 PM   #140
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
Quote:
Originally Posted by staki View Post
Hi,

If I want to use two COD boards in dual mono configuration, what is the best solution to feed them with i2s input signals (from a USB to i2s converter) ? Just wire both PCM terminal blocks in parallel ?
Will there be a shift between the left and right data bits ?
You just wire them in parallel, you set one board for left and one for right. I found it best to connect each board to the source with separate runs.

Zc
__________________
It's not about what you can BUY...It's about what you can BUILD!
  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
Output Current from PCM56P K? georgehifi Digital Line Level 30 16th May 2009 07:50 PM
Reduce DAC output current ? Bernhard Digital Source 1 10th January 2006 10:46 PM
Transformer Current output ocool_15 Parts 4 3rd December 2004 07:19 AM
Figuring out Output current. mjarve Solid State 4 19th September 2004 10:29 PM


New To Site? Need Help?

All times are GMT. The time now is 11:12 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