What is the tradeoff to run a DAC single ended? - diyAudio
Go Back   Home > Forums > Source & Line > Digital Line Level

Digital Line Level DACs, Digital Crossovers, Equalizers, 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 14th September 2010, 11:17 PM   #1
DougL is offline DougL  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Wheaton IL.
Blog Entries: 30
Default What is the tradeoff to run a DAC single ended?

I have a DAC based on the PCM1794.
I have a passive analog section using a transformer.
I was looking at the Zen I/V and wondering if I could use a single output and either:
A. Ground the unused input.
B. Place a small resistor to ground.
C. Place an "active load" on the pin and not use the output.

This was just a thought, it would avoid a balanced to Single Ended conversion, and allow a more standard volume control. The question is what do I give up for this?

Thanks in advance.

Doug
__________________
Scienta sine ars nihil est - Science without Art is nothing. (Implies the converse as well)
Mater tua criceta fuit, et pater tuo redoluit bacarum sambucus
  Reply With Quote
Old 15th September 2010, 01:36 AM   #2
diyAudio Member
 
Join Date: Jan 2008
Location: Virginia
You give up to 6dB of signal/noise ratio...
  Reply With Quote
Old 15th September 2010, 02:56 AM   #3
Pano is offline Pano  United States
diyAudio Moderator
 
Pano's Avatar
 
Join Date: Oct 2004
Location: Kona, Hawaii
Blog Entries: 4
How are you doing a passive output stage? Doesn't the PCM1794 need an I/V stage at the output?
I could see maybe a transformer after that.
__________________
Take the Speaker Voltage Test!
  Reply With Quote
Old 15th September 2010, 03:47 AM   #4
DougL is offline DougL  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Wheaton IL.
Blog Entries: 30
Quote:
How are you doing a passive output stage?
Step up transformer and resistor. Started with a Edcor 1:2 and a 1k resistor, liked it so much I picked up the RAKK passive line stage, which is a Lundahl LL1674 and a 3K resistor.
__________________
Scienta sine ars nihil est - Science without Art is nothing. (Implies the converse as well)
Mater tua criceta fuit, et pater tuo redoluit bacarum sambucus

Last edited by DougL; 15th September 2010 at 04:04 AM.
  Reply With Quote
Old 15th September 2010, 04:24 PM   #5
DougL is offline DougL  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Wheaton IL.
Blog Entries: 30
Quote:
You give up to 6dB of signal/noise ratio...
I think can live with that. I was afraid that the output had a lot of second harmonic that needed the other output to cancel, or some other show stopper.

Doug
__________________
Scienta sine ars nihil est - Science without Art is nothing. (Implies the converse as well)
Mater tua criceta fuit, et pater tuo redoluit bacarum sambucus
  Reply With Quote
Old 15th September 2010, 05:13 PM   #6
jcx is online now jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
Default do you have a circuit or just playing with "audiophoolish" ideas?

transformers do diff to single ended

modern monolithic DACs use balanced output to reject large high frequency digital noise current in their substrate - its impossible to get decent S/N without differential output

balanced circuitry also gives even harmonic cancellation

the PCM1794 has large output DC current bias - your I/V has to balance this current with low Vos from gnd - otherwise you get nonlinearity as you start to have signal current shunted by internal protection diodes

the Zen I/V may not have low enough input Vos for the PCM1794, doesn't have DC current balancing so the circuit needs some modifications to be compatible with this chip

where ever you put it, your transformer must not have any net DC current in the windings - blocking C absolutely mandatory


any or all of these ideas will perform measurably worse in distortion than the datasheet 5534 I/V - and much better op amp I/V is possible

Last edited by jcx; 15th September 2010 at 05:23 PM.
  Reply With Quote
Old 15th September 2010, 07:29 PM   #7
DougL is offline DougL  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Wheaton IL.
Blog Entries: 30
Quote:
do you have a circuit or just playing with "audiophoolish" ideas?
I will leave you to judge that. I currently have a step up transformer and a resistor acting as my analog stage.
Quote:
transformers do diff to single ended
Yes, I have heard that.
Quote:
modern monolithic DACs use balanced output to reject large high frequency digital noise current in their substrate - its impossible to get decent S/N without differential output
Good to know.
Quote:
balanced circuitry also gives even harmonic cancellation
Yes, but is it significant?
Quote:
the Zen I/V may not have low enough input Vos for the PCM1794, doesn't have DC current balancing so the circuit needs some modifications to be compatible with this chip
What sort of modifications are you talking about?
Zin was quoted as 15 ohms, and the common source input should be self biasing, especially since the outputs are not DC coupled. It certainly might benifit from 2 pairs of FET in parallel.
Quote:
where ever you put it, your transformer must not have any net DC current in the windings
Agreed.
Quote:
blocking C absolutely mandatory
That's one way of accomplishing it.
Quote:
any or all of these ideas will perform measurably worse in distortion than the datasheet 5534 I/V - and much better op amp I/V is possible
Measurements aren't everything. I should put together an op-amp I/V and compare for myself.
__________________
Scienta sine ars nihil est - Science without Art is nothing. (Implies the converse as well)
Mater tua criceta fuit, et pater tuo redoluit bacarum sambucus
  Reply With Quote
Old 17th September 2010, 04:37 PM   #8
jcx is online now jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
Default too good to be true?

The Zen I/V does sim ridiculously well, IMD as low as -100dB – but who knows how good the jfet models are or how well any particular current out DAC performs with nonzero input impedance and offset

jfet parameter match dependance could be explored - I really expect the complete 2nd harmonic cancellation seen in sim with idendical models is impractical with real world device variation

There is offset of -124 mV at the I/V input with the nominal models and Nelson suggests a low Ohm pot V offset adjustment for mere mortals who don’t have hundreds of the fets to select the Idss matched pairs to get the 1 mV Vos he claims in his article, balanced I/V requires good matching of 4 fets

I show a sim with AC coupled xfmr (it could give selectable balanced or single ended output) to the degree that the caps match there is some psrr but filtered/regulated power is still a good idea

Cap physical size may be a challenge if you insist on film caps and low frequency response

Xfmr ratio, frequency response and Cload modification of the response are all details that need to be addressed

the current sources aproximate the PCM1794 +,- Iout, I show bias current cancellation R but not Vos adj
Attached Images
File Type: png zen_iv.png (21.9 KB, 334 views)
Attached Files
File Type: txt zen_iv.asc.txt (5.7 KB, 22 views)

Last edited by jcx; 17th September 2010 at 04:39 PM.
  Reply With Quote
Old 17th September 2010, 10:24 PM   #9
diyAudio Member
 
Join Date: Apr 2003
Location: Eastern Pennsylvania
Quote:
Originally Posted by DougL View Post
I think can live with that. I was afraid that the output had a lot of second harmonic that needed the other output to cancel, or some other show stopper.

Doug

Actually, you only give up about 3dB in SNR.
__________________
Ken
  Reply With Quote
Old 21st September 2010, 06:03 AM   #10
DougL is offline DougL  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Wheaton IL.
Blog Entries: 30
jcx,

Thanks for the simulation. Very interesting.

Doug
__________________
Scienta sine ars nihil est - Science without Art is nothing. (Implies the converse as well)
Mater tua criceta fuit, et pater tuo redoluit bacarum sambucus
  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
IRF610 I/V convertion from Pass Labs D1 dac - single ended schiller Digital Line Level 5 15th October 2009 05:33 PM
Single Ended, Single driver: In-room distortion measurements Gerrit Boers Full Range 4 27th November 2007 09:12 AM
Run opamp on single supply Ryssen Parts 1 15th March 2004 01:28 AM
Anyone play with the Zen Single-Stage Single-Ended Class A? Rino odorico Pass Labs 13 29th August 2002 10:01 AM


New To Site? Need Help?

All times are GMT. The time now is 04:38 AM.


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