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 20th June 2008, 01:12 PM   #1
PA0SU is offline PA0SU  Netherlands
diyAudio Member
 
PA0SU's Avatar
 
Join Date: Dec 2006
Location: Eindhoven
Default IV-convertor with op amps....

Hi Folks,
many has been published on the subject in litterature. There are people who 'hate' IV's-with-op-amps as Mr. Hawksford in his article:

http://www.essex.ac.uk/dces/research...0amplifier.pdf

When I measure my IV with three times OPA314 onder the same conditions as Hawksford simulated in fig. 3-9a on page 17, I get the attached picture. Moreover the IV sounds terrific. What should be wrong with (the right) op amps in an IV?

My IV looks like:
http://www.by-rutgers.nl/PDFiles/CD_eind.pdf
on the last page. Sorry that the text is in Dutch....

This IV follows a selected PCM63P__K with 8 times oversampling.
Attached Images
File Type: jpg 19+20khz_dtone_-0,3db.jpg (89.4 KB, 897 views)
__________________
Systems that assume to know too much are more a hindrance than a help.
(Software Tools)
  Reply With Quote
Old 20th June 2008, 01:40 PM   #2
PA0SU is offline PA0SU  Netherlands
diyAudio Member
 
PA0SU's Avatar
 
Join Date: Dec 2006
Location: Eindhoven
Default Input impedance of the first stage

I do not klow how to get more than one picture in a posting so that I do it in this way.

The first stage of the IV looks like:
Attached Images
File Type: jpg iv_with 1pf.jpg (40.8 KB, 948 views)
__________________
Systems that assume to know too much are more a hindrance than a help.
(Software Tools)
  Reply With Quote
Old 20th June 2008, 01:45 PM   #3
PA0SU is offline PA0SU  Netherlands
diyAudio Member
 
PA0SU's Avatar
 
Join Date: Dec 2006
Location: Eindhoven
Default Input impedance of the first stage

When I simulate the input impedance of this circuit, I get:
Attached Images
File Type: jpg z-iv_with_1pf.jpg (68.3 KB, 860 views)
__________________
Systems that assume to know too much are more a hindrance than a help.
(Software Tools)
  Reply With Quote
Old 20th June 2008, 01:48 PM   #4
PA0SU is offline PA0SU  Netherlands
diyAudio Member
 
PA0SU's Avatar
 
Join Date: Dec 2006
Location: Eindhoven
Default Input impedance of the first stage

When I put a 5 nF capacitor over the input of the first stage like:
Attached Images
File Type: jpg iv_with_5nf.jpg (42.2 KB, 844 views)
__________________
Systems that assume to know too much are more a hindrance than a help.
(Software Tools)
  Reply With Quote
Old 20th June 2008, 01:49 PM   #5
oshifis is offline oshifis  Hungary
diyAudio Member
 
Join Date: Mar 2004
Location: Budapest, Hungary
The fast and steep change of the DAC output current from one sample to the next can overload the input of the opamp.

Correct me if I am wrong but you eliminated the output coupling cap by introducing another cap and an opamp in the servo loop. You wrote that this opamp has to be 'mooi klinken' so it surely affects the final quality. Easier were to put a CCS at the output of the DAC so that no DC current flows through the feedback resistor and the output of the first opamp remains at 0V.

I am also questioning the necessity of any low-pass filtering, but this is just a personal (purist) preference, and it can be disputed from engineering point of view. For me it sounds better without.

I tried several opamps in my I/V (1.5k feedback, with output coupling capacitor): SE5534, OPA134, THS4031, now I settled at OPA627. There are bigger difference between them than between no output capacitor and a (reasonable quality) output capacitor. Still, opamp I/V is a compromise for me, until my tube I/V is ready.
  Reply With Quote
Old 20th June 2008, 01:52 PM   #6
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
I think one of the problems is that at high frequencies, the opamp needs to supply a jump in current to charge the feedback cap in short time. Have you tried a first I/V stgae with very little gain, say 100 ohms instead of 2.2k feedback R, followed by a low noise gain stage.
I would expect the input R to be much flatter then.

The input cap doesn't do too much as it is in parallel with the virtual earth.

Jan Didden
__________________
Never explain - your friends don't need it and your enemies won't believe you anyway - E. Hubbart
Check out Linear Audio Vol 7
!
  Reply With Quote
Old 20th June 2008, 02:00 PM   #7
diyAudio Member
 
Join Date: Dec 2004
Location: gran sasso
PA0SU,

I think the valid figure to compare with your [really beautiful] measured output, is not the aforementioned fig 3-9a, with 192KHz Fs, but the fig 3-10a, with Fs 384 [in your case, 352.8 KHz]. And looking at the figures in this way, they are in perfect accordance, the article and your test. You have a noise floor slightly above -120dB, the spurious products in the simulation from the article are all below -120dB!

Ciao, George
  Reply With Quote
Old 20th June 2008, 02:14 PM   #8
diyAudio Member
 
Join Date: Dec 2004
Location: gran sasso
Janneman, Őshifis!

I think the real question that PA0SU is posing here is not "what is wrong with my opamp IV" but rather "explain to me why is it that my opamp IV mesasures so well while it should not, according to the simulations of Hawksford?"

To understand this well, one should read carefully the cited article.
My answer to PA0SU is in the above post.

Ciao, George
  Reply With Quote
Old 20th June 2008, 02:29 PM   #9
PA0SU is offline PA0SU  Netherlands
diyAudio Member
 
PA0SU's Avatar
 
Join Date: Dec 2006
Location: Eindhoven
With this capacitor, the input impedance should look like in the attachement. The input impedance never exceeds 10 ohm any more.
Is this low enough?
Well, the output current of the PCM63 is 2 mA with a slope of 200 ns, so that the voltage over the input never exceeds 20 mV.
The amplification of the OPA314 is 1 at 8 MHz (GB = 1 at 8 MHz) or higher. The only question is: do we get slew rate distortion at 3.528 MHz which is 8 x fs? The total amplification is also 1 there (following my simulation), but it could be that one of the first stages in the op amp would clipp so that slew rate distortion comes in sight.
I gues not because of the excellent results of the measurements. There is no any noticable intermodulation when two signals of -6.4 dB of resp. 19 and 20 kHz has been input....
Could anybody reassure my mind at rest?
Attached Images
File Type: jpg z-iv_with_5nf.jpg (64.7 KB, 777 views)
__________________
Systems that assume to know too much are more a hindrance than a help.
(Software Tools)
  Reply With Quote
Old 20th June 2008, 02:40 PM   #10
PA0SU is offline PA0SU  Netherlands
diyAudio Member
 
PA0SU's Avatar
 
Join Date: Dec 2006
Location: Eindhoven
Quote:
Originally posted by Joseph K
PA0SU,
I think the valid figure to compare with your [really beautiful] measured output, is not the aforementioned fig 3-9a, with 192KHz Fs, but the fig 3-10a, with Fs 384 [in your case, 352.8 KHz].Ciao, George
You are right, absolutely. Thank you!
Why is Hawksford so upset about op amps here? Do you have any idea?
__________________
Systems that assume to know too much are more a hindrance than a help.
(Software Tools)
  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
DC Convertor? zeonrider Power Supplies 2 8th November 2007 02:26 PM
12V to 200V convertor cpaulro Power Supplies 2 27th February 2006 10:51 AM
XLR Convertor Dark Shadow Parts 4 26th October 2003 03:19 AM
DC-DC Convertor Xavier Solid State 2 27th September 2001 05:05 AM


New To Site? Need Help?

All times are GMT. The time now is 11:33 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