dac I/V convertion with very low distortion - Page 57 - 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 6th November 2012, 01:50 AM   #561
diyAudio Member
 
Join Date: Apr 2003
Location: Eastern Pennsylvania
Quote:
Originally Posted by smms73 View Post
...any noise that the servo generate will be filtered by C2 and C3 (I have already test this, and work very well). So this I/V converter will not have output capacitor.
Sergio,

I very much like your use of a current mirror to simultaneously implement both the current source and servo. Killing two birds with one stone; as we like to say in the U.S. . Particularly beneficial is the embedded RC filter of the servo that not only filters the servo amplifier's intrinsic noise, but also any power supply noise coupled via the servo amplifier. In addition, it places an additional pole in the the overall servo transfer function which has a cutoff frequency more than four times lower than that of the active filter pole. Very nice.
__________________
Ken

Last edited by Ken Newton; 6th November 2012 at 01:55 AM.
  Reply With Quote
Old 6th November 2012, 07:07 AM   #562
regal is offline regal  United States
diyAudio Member
 
Join Date: Jan 2004
Location: MD
Quote:
Originally Posted by MiiB View Post
I have more faith in the BJT version.. I have made phono-stages with Jfets, they always sound nice, but also wailed and cloudy compared to the Bjt's

Like your Servo- very much, the same way I designed one in for the paradise phonostage--there we use the OPA134.
I would make a jfet current source instead of the two resistors setting the current. and then insert the servo with two 100 kohm resistors on each side of the CCS.
I would then use the 6.2mA countercurrent to trim the offset to 0 before engaging the servo.
Then you know the servo is just dealing with thermal drift.
I agree if we can make a Jfet I/V with much less complexity, IMHO simplicity is the only reason to used a Jfet Non-NFB I/V with the rare except of DAC chips like the Ti PCM179x that have high output imp.
  Reply With Quote
Old 6th November 2012, 09:21 AM   #563
diyAudio Member
 
Join Date: Jan 2011
Location: Germany
I use version 2. If you use version 1, I would change R4 into 1Meg for lowest possible DC offset from the OPamp itself.

OPamps:
OPA627B
OPA827 (SMD only)
OPA134
AD820


A servo should only take care of the last few mV of DC offset for temperature drift and component drift over time. The major DC offset fix should be a trim pot somewhere.

/S

Quote:
Originally Posted by smms73 View Post
The Dc servo will be one of this, the first is recommended by Bob Cordell and only use 1 capacitor. Can you guys recommend a jfet opamp with good dc precision for this, no need for low noise.
  Reply With Quote
Old 6th November 2012, 09:25 AM   #564
diyAudio Member
 
Join Date: Jan 2011
Location: Germany
One kg of BC327:s - LOL!
Yes, they are costly (but not expensive). Think of them as tubes
I have also bough from Spencer. Never fakes from him.

When you design the ESS9018 stereo and mono mode version, if you can, try to get Zin of the IV converter < 1 Ohms up to 100+ kHz, please.


/S

Quote:
Originally Posted by smms73 View Post
As the pcm1794 has a high output impedance i thought about using the 2sk170/2sj74 pair in the input. I have bought 4 matched pairs from Spencer Cheung, they are not cheap, it cost the equivalent of 1 Kilo gram of BC327 . but i am very curious to try them, and this project seems to be perfect for them. So there it is, the version that I going to build.
I have found an elegant soluction to connect the dc servo, instead of connecting the servo at the input as usual, the servo will be connected to the input of current sources, any noise that the servo generate will be filtered by C2 and C3 (I have already test this, and work very well). So this I/V converter will not have output capacitor.
In the schema is absent the ccs of 6,2ma for balance the pcm1794 dc_offset.

I will do also a version with BC327/337 transistors in the input. And i am working in a I/V for the ESS9018 .
  Reply With Quote
Old 6th November 2012, 04:50 PM   #565
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
Quote:
Originally Posted by Ken Newton View Post
Sergio,

Killing two birds with one stone; as we like to say in the U.S. .
No birds were killed or injured during the making of this project.

...at least until now.
__________________
Sérgio Santos
  Reply With Quote
Old 6th November 2012, 04:54 PM   #566
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
OPA827 has only 0.1mV.
OPA827 is very good for servo, the price is a bit higher.
__________________
Sérgio Santos
  Reply With Quote
Old 6th November 2012, 05:07 PM   #567
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
staccatiss thanks for the list of opamps, and do not worry, the input for the I/V converter for the ESS9018 will be less than 1 ohm,
Just one question regarding the ess9018 . Why use all the 8 output in parallel? one can use only one output per channel and shunt the remaining 6. anyone know the reason?
__________________
Sérgio Santos
  Reply With Quote
Old 6th November 2012, 05:11 PM   #568
MiiB is offline MiiB  Denmark
diyAudio Member
 
Join Date: May 2005
Location: Denmark
The current-mirror, mirrors the impedance you have in your current source, here it's 20 Kohms.. By inserting a Jfet CCS you up that many times. that was the reason for my suggestion.. then you could end with a CCS impedance of app 200k Ohms, or app 10 times better than what you have in your schematic.

Last edited by MiiB; 6th November 2012 at 05:18 PM.
  Reply With Quote
Old 6th November 2012, 05:46 PM   #569
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
MiiB, this is not really a current mirror, it depends the way you look at the circuit.
In this picture there is a simplified schema of the current source, as one can see i have change the 2 bipolar transistors by 2 diodes because they are connected as a diode and they only serves to temperature stabilization.
the voltage at the base of Q7 and Q9 are stabilized by the capacitors C2 and C3 ,normaly you see zeners or led in the place of resistor R8 or R18, but as long as vcc and vdd dont vary with temperature (use a lm317, 337 in power supply ) there is no need as C2 have much less impedance than any zenner or led.

this ccs is also cascoded so the output impedance is very high. Please trust me, this is a very quiet ccs , the noise floor of the I/V converter is realy very low. i will post pictures of that later on.
Attached Images
File Type: png ccs.png (23.6 KB, 268 views)
__________________
Sérgio Santos
  Reply With Quote
Old 6th November 2012, 06:32 PM   #570
smms73 is offline smms73  Portugal
diyAudio Member
 
smms73's Avatar
 
Join Date: Jul 2005
Location: Lisboa, Portugal
This is a test of the ccs , it gives 3.38 Mega ohms of output impedance (1khz).
Attached Images
File Type: png ccs_test.png (70.9 KB, 224 views)
__________________
Sérgio Santos
  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 09:14 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