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 6th January 2008, 03:54 PM   #1
diyAudio Member
 
Join Date: Jul 2004
Location: us
Default TDA1541 , passive i/v , AD797 for gain ; worth trying??

Time for me to get into the passive i/v game. Finally getting up the courage. Don't wish to do the tube output thing yet. I have on hand a fist full of AD797 ultra low noise op amps that I would like to use for necessary gain. So I have a few questions. First , is it really practical? I would like to know how much gain is required if I use the commonly suggested 30 ohm resistor value at i out. Should the AD797 be used inverting or non inverting , since it is only being used as a gain stage?? Should the i/v resistor be bypassed at all. , or any passive eq used? I have several Philips based players with TDA1541 that I would like to try this on , and use as much of the existing layout as possible. Can this be done on a Brown Dog dual single to dual op amp board , with , say another 797 or 627 as buffer on the other half of this board?
Thank-you for any suggestions.
  Reply With Quote
Old 6th January 2008, 06:47 PM   #2
diyAudio Member
 
Join Date: Jul 2003
Location: Bath, UK
Interesting idea.

My problem with the concept is that the 1541 needs a compliance voltage less than 25mV. SInce it has a peak of -4mA output , that means the I/V impedance has to be 0.025/0.004 = 6ohms or less for undistorted output. (This applies for active I-V input impedance as well, of course...). You might well get away with 10ohms for an I-V resistor.

So then you need roughly 60dB voltage gain (1000x) to get up to line level (well, 1.7V rms) and to do that with minimal SNR degradation I might look at using two 797s providing 30dB each.

However you slice it, careful design will be needed to realise a noise-floor low enough, and very careful execution; this much gain is equivalent to MC phono stage design, but with a much wider bandwidth of spurious noise on the input!
  Reply With Quote
Old 6th January 2008, 08:28 PM   #3
diyAudio Member
 
analog_sa's Avatar
 
Join Date: Aug 2002
Location: Sofia
Quote:
Originally posted by martin clark

So then you need roughly 60dB voltage gain (1000x) to get up to line level (well, 1.7V rms)

Sorry, i don't follow. 60db implies only 1.7mV at input?!

It may not be the best example but Ezdac uses the same concept with a different dac chip and works quite well.
  Reply With Quote
Old 6th January 2008, 11:16 PM   #4
diyAudio Member
 
georgehifi's Avatar
 
Join Date: Sep 2003
Location: Manly Australia (Jewel of the Pacific)
Send a message via MSN to georgehifi
homemade, hi, instead of doing a passive I/V then having very low output and then bringing it back up (with the noise) again with the AD797.
Maybe this should be looked at for the I/V and Gain stage. An AD844 transimpedance opamp, input impedance close to 0ohms just like your passive I/V resistor, yet with gain. A perfect load for the current output TDA1541 to see.

Pedja's design for the use of the AD844 with the TDA1541
http://www.pedjarogic.com/1541a/rev1.htm

Cheers George
Attached Images
File Type: jpg ad844 x.jpg (13.4 KB, 652 views)
__________________
Avatar : Production Lightspeed Attenuator
www.lightspeedattenuator.com
  Reply With Quote
Old 6th January 2008, 11:48 PM   #5
diyAudio Member
 
Join Date: Jul 2004
Location: us
. An AD844 transimpedance opamp, input impedance close to 0ohms just like your passive I/V resistor, yet with gain. A perfect load for the current output TDA1541 to see.

Pedja's design for the use of the AD844 with the TDA1541
http://www.pedjarogic.com/1541a/rev1.htm

Cheers George [/B][/QUOTE]


Thank you all for your replies
I have looked at the AD844 while coming to terms of going this route. It has an input imped of 50 ohms , so I would have thought this to be higher than optimum. Also , I assume this op amp will also need fairly high gain to achieve the 1.7v. The 844 is very fast(2000 slew rate) . Just for fun I tried to use an AD811 in my player. I removed it yesterday and I think it is STILL oscillating.
Regarding the i/v resitor , while I do not dispute the values dicussed , I have read here and elsewhere of around 30 ohms to be acceptable , so perhaps less gain with the 797 might be used.
The 797 just appears to be a little bit easier to use for a beginner . But keep the suggestion coming. I'm ready
  Reply With Quote
Old 7th January 2008, 08:04 AM   #6
diyAudio Member
 
analog_sa's Avatar
 
Join Date: Aug 2002
Location: Sofia
Did you have a look at Ezdac?
  Reply With Quote
Old 7th January 2008, 08:14 AM   #7
diyAudio Member
 
analog_sa's Avatar
 
Join Date: Aug 2002
Location: Sofia
Quote:
Originally posted by homemade
It has an input imped of 50 ohms
This is open loop.
  Reply With Quote
Old 7th January 2008, 08:54 AM   #8
Zoran is offline Zoran  Serbia
diyAudio Member
 
Join Date: Jan 2004
Location: Belgrade
Sorry,
but I think that lower value of 22ohm could damage the DAC chip...
So ...
1. You can start with 100 ohm, for the Rl and lowering the value,
2. in the same time You ned to increase the gain of OP amp 797
3. Put the OP in inverting mode (passive R-IV onverts the signal...)
4. Not to forget to apply current compensation (of app. 2mA) for TDA1541A... Again You can find it on PEDJA site...It is simply done by JFET...
5. And You can try to put the DAC in NON-oversampling mode...
Usualy by overriding the chip before the dac SAA7020 one...
Cheers
__________________
###
  Reply With Quote
Old 7th January 2008, 10:53 AM   #9
diyAudio Member
 
Join Date: Jul 2003
Location: Bath, UK
Quote:
i don't follow. 60db implies only 1.7mV at input?
Good catch - asleep on a sunday evening

2mA peak into 6ohms to 2Vrms requires 47dB of gain...

Zoran -
Quote:
I think that lower value of 22ohm could damage the DAC chip...
Why? the dac has current output, ideally it wants to 'see' a zero ohm load on the output pins.
  Reply With Quote
Old 8th January 2008, 07:03 PM   #10
Zoran is offline Zoran  Serbia
diyAudio Member
 
Join Date: Jan 2004
Location: Belgrade
I dont have personal experiance, but I read somevhere on the net,
there are a lot very god eperiments, measurements about TDA1541A...
Simply try the lowest value but monitor the temperature of dac,..
I think that value between 22 and 47 ohm is quite enough low mind distorsion...
__________________
###
  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
passive biamping any worth? h_a Pass Labs 14 27th July 2007 05:26 PM
passive bandpass dB gain, what is to much? scott beavers Subwoofers 1 26th October 2005 04:18 PM
Passive I/V Trannie for TDA1541? fred76 Digital Source 0 10th February 2005 10:59 AM
TDA1541 with passive filter ashok Tubes / Valves 26 25th February 2003 06:08 AM
Enough gain for passive pre ? protos Pass Labs 1 22nd April 2002 11:36 AM


New To Site? Need Help?

All times are GMT. The time now is 07:55 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