dac I/V convertion with very low distortion - Page 13 - 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 11th August 2012, 04:21 PM   #121
diyAudio Member
 
Join Date: Apr 2003
Location: Eastern Pennsylvania
Quote:
I'm wondering if a simple DC Hfe match via multimeter is good enough, or if I should do something more sophisticated.
I should think that a long-tailed pair test rig would be a good solution. Build a simple test rig which connects the transistor(s) under test as a differential pair with a current source tail. Place matched resistors as loads in the collector circuits. Then drive the base inputs of the differential pair together with an audio frequency signal from a common source. The A.C. current gain matching can then be readily ascertained from taking an A.C. voltage meter reading from between the differential collector load resistors. The poorer the gain match, the greater will be the difference voltage amplitude. A perfect device match would result in a zero volt reading. Handle the devices umder test with a pair of plastic tweezers or pliers so not to heat via your hands and fingers, which would affect device gain.

Of course, matching devices within the actual circuit would probably be best. A concern here is the issue of how to temporarily and repeatedly mount devices for test on the final PCB without damaging the board.
__________________
Ken

Last edited by Ken Newton; 11th August 2012 at 04:34 PM.
  Reply With Quote
Old 11th August 2012, 08:01 PM   #122
diyAudio Member
 
Join Date: Oct 2006
Quote:
Originally Posted by Ken Newton View Post
I should think that a long-tailed pair test rig would be a good solution. Build a simple test rig which connects the transistor(s) under test as a differential pair with a current source tail. Place matched resistors as loads in the collector circuits. Then drive the base inputs of the differential pair together with an audio frequency signal from a common source. The A.C. current gain matching can then be readily ascertained from taking an A.C. voltage meter reading from between the differential collector load resistors. The poorer the gain match, the greater will be the difference voltage amplitude. A perfect device match would result in a zero volt reading. Handle the devices umder test with a pair of plastic tweezers or pliers so not to heat via your hands and fingers, which would affect device gain.

Of course, matching devices within the actual circuit would probably be best. A concern here is the issue of how to temporarily and repeatedly mount devices for test on the final PCB without damaging the board.
I'm old enough to remember these things called "transistor sockets" for small xstors.;-)
The easy thing for you young'n's would be to use an 8pin DIP ic socket for the diff pair, cutting off or otherwise blocking the fourth pin on each side to prevent mis-inserting.
  Reply With Quote
Old 11th August 2012, 09:41 PM   #123
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
Quote:
Originally Posted by regal View Post

The predited THD in this design is incredibly low considering that the best a common base I'V input gives is -80db thd (the first bjt on its own.) It is fascinating to imagine this can be overcome without global nfb, but I think your design must have incredibly well matched subsequent transistors, have you tried simulating by varying the BJT models Hfe a little so as to predict real world? My experience is that with open loop common base I/V no matter how much help is given to that initial BJT, just don't measure well nor sound that great (I'm being subjective).
As the doubt about the need of matched transistors appears very often, i post here the same circuit with bc327 for pnp and bc547c for all npn, they are very different and the thd is 0.000008% (7,8ma/100k), its no need for matched transistors. hope this end the subject, and we can move on.
Attached Images
File Type: jpg bc547c.jpg (422.9 KB, 330 views)
  Reply With Quote
Old 11th August 2012, 09:56 PM   #124
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
Quote:
Originally Posted by MiiB View Post
How doy you trim away the ofset at the output..??
As i say before (post 108) just increase 6.2ma in the upper current source use a pot in the R3 position, this also serves to trim the output offset. You can add a servo for better offset control .
  Reply With Quote
Old 11th August 2012, 09:58 PM   #125
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
Quote:
Originally Posted by Joachim Gerhard View Post
The increase in 2nd harmonic may be even good sound wise, although distortion of this small magnitude is not likely audible anyway.
  Reply With Quote
Old 11th August 2012, 10:02 PM   #126
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
Quote:
Originally Posted by regal View Post
As far as the balanced output issue, I've never seen a discrete design that gives more CMRR than it adds in 2H and 3H. IC's just completely outperform for balanced to unbalanced conversion, next are transformers, so typically it just doesn't pay to build in a discrete bal-unbal conversion in a DAC analog stage. Probably the smartest is to leave a balanced output to the poweramp where if the output is p-p one can take advantage of having both phases. Or use a good multibit DAC.
That is also my opinion.
  Reply With Quote
Old 11th August 2012, 10:06 PM   #127
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
Quote:
Originally Posted by Joachim Gerhard View Post
I designed a simple MC phono pre-pre with a floating supply based on the principle shown.
I will build it over the weekend.
Keep us informed , hope you like the end result.
  Reply With Quote
Old 11th August 2012, 10:08 PM   #128
diyAudio Member
 
Joachim Gerhard's Avatar
 
Join Date: Mar 2009
I have drawn up the circuit diagram. It should work.
  Reply With Quote
Old 11th August 2012, 10:38 PM   #129
diyAudio Member
 
Joachim Gerhard's Avatar
 
Join Date: Mar 2009
This distortion cancelation seems to be the nature of this circuit.
  Reply With Quote
Old 12th August 2012, 04:02 AM   #130
regal is offline regal  United States
diyAudio Member
 
Join Date: Jan 2004
Location: MD
Quote:
Originally Posted by MiiB View Post
The offset current from the DAC-chip makes app 2V offset at the output ..but the circuit does not performance vise seem to run into trouble... In practical terms the offset needs to be addressed...could maybe be done by inserting at counter current at the input, the simple solution would be to use a transformer, but I'd rather not, they cost, and though simple, not very pure.
Problem with a countercurrent at the input is it adds noise to the whole deal (bottleknecks the design), a transformer can really sound great on the output of a DAC stage but this has such good performance I wouldn't want to fog it up with iron. Best bet would probably be a folllowing this I/V with a diamond buffer or other buffer that that can be setup to null the offset with a servo or (preferrably) trimpot. This would allow the use of more/better filtering in between if one prefers.

Quote:
Originally Posted by smms73 View Post
As the doubt about the need of matched transistors appears very often, i post here the same circuit with bc327 for pnp and bc547c for all npn, they are very different and the thd is 0.000008% (7,8ma/100k), its no need for matched transistors. hope this end the subject, and we can move on.
Sorry about the doubt, soured by years of unimppressive results with open-loop BJT I/V's. It really looks like you have conquered the BJT I/V compromises many of us have been dealing with for years. Thanyou for sharing, please keep us updated as you build it and don't lose patience with our paranoria . Do you plan on a breadboard first or starting with PCB's ?
  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
ultra low noise low distortion preamplifier needed neazoi Solid State 13 11th October 2011 01:03 PM
IRF610 I/V convertion from Pass Labs D1 dac - single ended schiller Digital Line Level 5 15th October 2009 04:33 PM
Distortion+low volume.. GeirW Tubes / Valves 5 25th November 2004 12:04 PM
Best low-noise low-distortion buffer? borges Solid State 7 9th December 2003 12:40 PM


New To Site? Need Help?

All times are GMT. The time now is 01:01 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