Building the ultimate NOS DAC using TDA1541A - Page 102 - diyAudio
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 4th October 2006, 10:39 AM   #1011
singa is offline singa  Singapore
diyAudio Member
 
Join Date: Apr 2004
Location: near
Send a message via AIM to singa Send a message via MSN to singa Send a message via Yahoo to singa
Quote:
Originally posted by ezkcdude
I don't understand how you can have differential output using one OPA627 per channel. If I understand correctly, you're saying the I/V conversion is passive (using resistor), and then the OPA627 is used for a gain stage, correct? OPA627 only has one output, right? Do you mean you are using OPA627 as a differential amplifier, to convert balanced to single-ended? Could you please explain this, or better yet, post a schematic of the passive I/V section? I want to make sure you understand that I am not being critical here. I just don't understand what you're describing.
As on radio,"If you've just tuned in,welcome".Let's not disturb Ecdesign with trivial details and let the big boy play.(Just kidding)

FYI the I/V stage has 3 X OPA627 per channel.
  Reply With Quote
Old 4th October 2006, 10:55 AM   #1012
Bricolo is offline Bricolo  France
diyAudio Member
 
Bricolo's Avatar
 
Join Date: Nov 2002
Location: Grenoble, FR
Quote:
Originally posted by tubee
Guido Tent has build a new dac. With resisrtor I/V and tube amplification. I have a 6922 waiting to be used for the same task.

If you want to use a Tube, I remember a comment from KYW about using the 2mA DC offset as a bias for the tube.
I think it's worth the try, but I haven't seen anyone here giving some feedback about it.
__________________
Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease
  Reply With Quote
Old 4th October 2006, 11:03 AM   #1013
diyAudio Member
 
-ecdesigns-'s Avatar
 
Join Date: May 2006
Default Passive I/V diagrams

Hi ezkcdude,

Thanks for your reply [post#1007]

The octal D-I DAC has single ended outputs, but uses differential inputs. There are two DAC groups one inverted and one non-inverted, so there is only one OP-amp necessary to combine both differential inputs from the passive I/V stage to the single ended output. I added a schematic diagram. As can be seen, with the D-I system there is no analog filter on the output. The special constuction of the combined zero inductance copper wire I/V resistor is absolutely necessary in order to obtain this level of sound quality.
Attached Images
File Type: jpg passive2.jpg (91.6 KB, 1828 views)
  Reply With Quote
Old 4th October 2006, 12:22 PM   #1014
diyAudio Member
 
Join Date: Dec 2005
Location: Atlanta
Default Re: Passive I/V diagrams

Quote:
Originally posted by -ecdesigns-
Hi ezkcdude,

Thanks for your reply [post#1007]

The octal D-I DAC has single ended outputs, but uses differential inputs. There are two DAC groups one inverted and one non-inverted, so there is only one OP-amp necessary to combine both differential inputs from the passive I/V stage to the single ended output. I added a schematic diagram. As can be seen, with the D-I system there is no analog filter on the output. The special constuction of the combined zero inductance copper wire I/V resistor is absolutely necessary in order to obtain this level of sound quality.

Thanks, that's kind of what I figured. I'm actually using almost exactly the same I/V stage in my DAC, except the capacitor is placed across the two signals. It's nice to hear it sounds good for you. In order for this to work well, the resistor pairs much be matched very closely, but I'm sure you know this already.
Attached Images
File Type: gif eziv.gif (13.0 KB, 1605 views)
  Reply With Quote
Old 4th October 2006, 03:26 PM   #1015
tubee is offline tubee  Netherlands
diyAudio Member
 
tubee's Avatar
 
Join Date: Jul 2004
Location: Netherlands
Hi Erik

I want to use the six to seven 1541A's i have here, and still think about to implement them simple with a passive tubed I/V. For an octal dac will have to get 2 A's extra. PCM1704 (or PCM63) is not a bad choice either!

Nice idea Bricolo, to use the 2mA as tube-bias too. Will search this out.
__________________
Daisy Bell
  Reply With Quote
Old 4th October 2006, 05:16 PM   #1016
Bricolo is offline Bricolo  France
diyAudio Member
 
Bricolo's Avatar
 
Join Date: Nov 2002
Location: Grenoble, FR
Quote:
Originally posted by tubee
Hi Erik

I want to use the six to seven 1541A's i have here, and still think about to implement them simple with a passive tubed I/V. For an octal dac will have to get 2 A's extra. PCM1704 (or PCM63) is not a bad choice either!

Nice idea Bricolo, to use the 2mA as tube-bias too. Will search this out.

it's not my idea but I found the thread I was thinking about

http://www.diyaudio.com/forums/showt...ht=#post409375
http://www.diyaudio.com/forums/showt...ht=#post349480
__________________
Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease
  Reply With Quote
Old 5th October 2006, 07:25 AM   #1017
diyAudio Member
 
Join Date: Jun 2002
Location: Italy
Quote:
Originally posted by -ecdesigns-
Hi stefanobilliani,

Thanks for your reply [post#998]

Are you referring to op-amps like the OPA1632? well I haven't tried them yet, but I will.

Yes OPA1632 ,


Fully differential Audio op amp (pdf)
  Reply With Quote
Old 5th October 2006, 08:37 PM   #1018
tubee is offline tubee  Netherlands
diyAudio Member
 
tubee's Avatar
 
Join Date: Jul 2004
Location: Netherlands
Default Dac's 2 mA biasing 6922

Hi Bricolo/Bob

Found this too:

Yet another output stage for TDA1541

But what happens when there are 8 dacs parallel?

John, i am interested to "turn" some of those twisted self made resistors too, are they relatively easy to make?
__________________
Daisy Bell
  Reply With Quote
Old 6th October 2006, 03:57 PM   #1019
diyAudio Member
 
-ecdesigns-'s Avatar
 
Join Date: May 2006
Default Copper wire resistor construction

Hi Tubee,

Thanks for your reply [post#1018]

When using 8 DAC's in parallel there will be a total of 8*4mA=32mA, this could cause problems with the described circuit. I use differential operation to cancel DC offset voltage, so I can't use that setup. Also keep in mind that a tube faillure could cause all TDA1541A's connected to be permanently damaged.

As I previously noted, the sound quality seems to improve with passive I/V setup, however there is a down side, impulse response time is affected. The very short attack time obtained with the op-amp I/V is missing, so a very smooth laid-back sound is produced. This could be caused by either the limited bandwith of the OPA627 when using higher gains or the relatively high I/V resistor value (18 Ohms instead of the calculated 6.25 Ohms). So I plan to experiment with the OPA1632 (180 MHz gain band width and 0.000022% distortion).

Problem with passive I/V remains the limited allowed voltage drop across the I/V resistor. When using a single TDA1541A DAC it's specified at 25mV, I am still not sure if this voltage can be much higher when using multiple DAC's in parallel as there is nothing mentioned about this in the philips datasheets. Suppose this voltage can be higer proportional to the amount of DAC's in parallel, this would mean that when using 4 DAC's in parallel, maximum voltage could be 100mV. With 4*4=16mA full scale current, the maximum I/V resistor value would be about 6.25 Ohm. If total output voltage of the differential DAC outputs is 2*100mV=200mVpp, then we need a gain of 14.14 in order to obtain 1Vrms output signal. This gain can be achieved without much problems, provided the amplifier circuit used has sufficient bandwidth.

Roll your own copper wire resistors, well it's quite easy, first you need very thin enamelled copper wire (I used wire from a 110V relay). It has about 9 Ohms / meter, in order to determine the resistance of the wire you want to use, cut off a 2m piece and measure the exact resistance, now you can calculate the total length of copper wire needed for a specific resistance. Let's assume you need 78cm for a 6.8 Ohm resistor, next you need to construct a mobius loop to eliminate inductance, all you have to do is fold the wire halfway (at 39 cm in this example). Then twist the 2 wires together (like a twisted pair in a network cable). Next wind the wire on a round piece of plastic or wood, first fixate the end that is folded, then complete the winding. Fixate the winding when completed. The 2 separate wires are the resistor connections, check with a ohm meter if the resistance value is correct. If you make the wire a bit longer, you can exactly calibrate the resistance value after winding, by cutting off small pieces of wire until the exact resistance value is obtained.

Advantages of copper wire resistors are: non magnetic material, very low noise, very low inductance and high resolution.

A good value to start with is around 6.25 Ohms, when using thin copper wire, estimated wire length is about 70cm (35cm when folded).
  Reply With Quote
Old 6th October 2006, 05:32 PM   #1020
Bricolo is offline Bricolo  France
diyAudio Member
 
Bricolo's Avatar
 
Join Date: Nov 2002
Location: Grenoble, FR
with any number of TDA1541A in parallel you should stay with 25mV.

25mV is for optimal operation, but the compliance is a few hundred mV
__________________
Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease
  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



New To Site? Need Help?

All times are GMT. The time now is 03:45 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