dac I/V convertion with very low distortion - Page 3 - 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 7th August 2012, 05:19 PM   #21
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
Quote:
Originally Posted by dirkwright View Post
This is very similar to Dr. Leach's moving coil head amp:
Moving Coil Cartridge Head Amps

So, you will achieve lower noise and lower input impedance by removing the emitter resistors R1, R2. If you use current mirrors as suggested by Dr. Leach, then a further improvement should occur. You can lower the input impedance even more by doubling up the input BJT's. I have a similar circuit that has about 1 ohm input impedance.

The DACs that you like have DC current on their output pins which will upset the balance of your circuit. The PCM1794a for example puts out -6.2mA of DC current. Something has to be done about that as well.
Hi dirkwright,
I don't think they are similar.
If i remove R1 and R2 , it will be difficult to control the current variation due to temperature, doubling up the input BJT's is a valid choice, and i think will lower the noise.
  Reply With Quote
Old 7th August 2012, 05:26 PM   #22
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
Quote:
Originally Posted by xslavic View Post
Wouldn't this be better
Simple and convenient,if dual mon o then xlr
Its a very good solution , have you already construct that? don't you have problems with stability (low phase margin)?
  Reply With Quote
Old 7th August 2012, 05:57 PM   #23
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by smms73 View Post
Hi dirkwright,
I don't think they are similar.
If i remove R1 and R2 , it will be difficult to control the current variation due to temperature, doubling up the input BJT's is a valid choice, and i think will lower the noise.
OK. I think that using the current mirror version will regulate the current, won't it? Also, the matching of the transistors plays a major role in this circuit.

Last edited by dirkwright; 7th August 2012 at 06:03 PM.
  Reply With Quote
Old 7th August 2012, 05:59 PM   #24
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
i'm going to try to explain how this topology works.
Its very simple, look at this image, we have a current source I2 , this current is divided by the input and output, if the input current increase 1ma the output must decrease 1ma, its similar to a differencial stage, there is no current gain, so this topology is more like a current mirror than a amplifier.
Attached Images
File Type: jpg base1.jpg (32.2 KB, 661 views)
  Reply With Quote
Old 7th August 2012, 06:00 PM   #25
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by abraxalito View Post
Where did you read that? It might have been true for the old style multibit DACs - they relied on laser trimming of their resistor networks. That's one reason for why PCM1704 is so much more expensive than say PCM1792. But CMOS DACs such as the PCM179x series are designed not to need trimming - that's one of the big advantages in manufacturing for S-D type DACs.
I read it in a paper by Analog Devices. They may have been referring to an older DAC architecture, I don't know.
  Reply With Quote
Old 7th August 2012, 06:07 PM   #26
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by smms73 View Post
i'm going to try to explain how this topology works.
Its very simple, look at this image, we have a current source I2 , this current is divided by the input and output, if the input current increase 1ma the output must decrease 1ma, its similar to a differencial stage, there is no current gain, so this topology is more like a current mirror than a amplifier.
It's just a common base amplifier. There's nothing mysterious about it.
  Reply With Quote
Old 7th August 2012, 06:12 PM   #27
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
Quote:
Originally Posted by dirkwright View Post
OK. I think that using the current mirror version will regulate the current, won't it? Also, the matching of the transistors plays a major role in this circuit.
this topology is not like Dr.Leach. I dont see how to use the current mirror
Are you sure that you understand how it works?

Mouser sells some dual nxp transistores that have 98% matching hfe and vbe for under 0.2 euros
  Reply With Quote
Old 7th August 2012, 06:31 PM   #28
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
Quote:
Originally Posted by dirkwright View Post
It's just a common base amplifier. There's nothing mysterious about it.
the input is a common base amplifier, and of course that there is something mysterious ... the way that q7 and q8 are connected, without them is impossible to achieve such low distortion.
  Reply With Quote
Old 7th August 2012, 07:02 PM   #29
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 think this topology is also very interesting for a Transimpedance MC phono stage.
The only thing that worries m is noise into low impedance, say an MC cartridge that has 6 Ohm DC impedance. On one hand the ist the R9 4kOhm resistor. On the other hand there is Q2, Q5. The Base-Emitter resistance is in the noise loop even when the bias to that transistors is made very low impedance. What whould happen if R9 will be shunted by a big Electrolytic ?
Hi Joachim
Unfortunately, i don't know anything about MC phono stage, but if you tell me what you need, i will be happy to help.
You can shunt r9, but i really don't see why.
  Reply With Quote
Old 7th August 2012, 08:34 PM   #30
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by smms73 View Post
the input is a common base amplifier, and of course that there is something mysterious ... the way that q7 and q8 are connected, without them is impossible to achieve such low distortion.
Q7 and Q8 provide bias to the bases of Q6 and Q3, and also appear to receive a feedback signal on their collectors. There's nothing magical about it, though I may not completely understand it.
  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 01:45 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