AudioSector-chip amp kits, dacs, chassis - Page 203 - diyAudio
Go Back   Home > Forums > Commercial Sector > Vendor Forums > Audio Sector

Audio Sector Kits & PC boards from AudioSector

Reply
 
Thread Tools Search this Thread
Old 19th December 2007, 03:19 PM   #2021
diyAudio Member
 
Join Date: Nov 2007
Quote:
Originally posted by Peter Daniel
Thanks Bob for clearing that up.

The DAC output section of my DAC is very similar to 47Labs DAC, which was measured by Stereophile: http://stereophile.com/digitalproces...00/index5.html

Thanks for the link!

Joshua
  Reply With Quote
Old 19th December 2007, 03:26 PM   #2022
Bobken is offline Bobken  United Kingdom
diyAudio Member
 
Join Date: Dec 2002
Location: UK
Hi Joshua,

I am quite sure that you aren't arguing, and I didn't intend to suggest that you were, which is why I said "with respect etc".

I just wished to make it clear to you that in this particular case there is no question of "likely to be", it is the value of these shunt resistors which *determine* the output impedance of this DAC.

Basically, it is the impedance measured between the 'live' output terminal and ground (this is rarely the same as whatever is in series with the output, and it certainly isn't the same in this circuit), so in a straightforward case like this it is easy to determine simply by checking the schematic.
In truth, it will be a little lower than the exact value of the resistors used here, as there will be some much higher impedances in parallel with these resistors, within the TDA chip, itself, also referenced to ground. However for practical purposes, they will have very little effect on this output impedance in my opinion.

Anyway, I don't wish to hi-jack Peter's thread with this correspondence, but as I just saw your query when I was looking at this thread earlier and I had some time on my hands, I thought I might save Peter some trouble.

Regards,
__________________
Bob
  Reply With Quote
Old 19th December 2007, 03:50 PM   #2023
diyAudio Member
 
Join Date: Nov 2007
Quote:
Originally posted by Bobken
Hi Joshua,

I am quite sure that you aren't arguing, and I didn't intend to suggest that you were, which is why I said "with respect etc".

I just wished to make it clear to you that in this particular case there is no question of "likely to be", it is the value of these shunt resistors which *determine* the output impedance of this DAC.

Basically, it is the impedance measured between the 'live' output terminal and ground (this is rarely the same as whatever is in series with the output, and it certainly isn't the same in this circuit), so in a straightforward case like this it is easy to determine simply by checking the schematic.
In truth, it will be a little lower than the exact value of the resistors used here, as there will be some much higher impedances in parallel with these resistors, within the TDA chip, itself, also referenced to ground. However for practical purposes, they will have very little effect on this output impedance in my opinion.

Anyway, I don't wish to hi-jack Peter's thread with this correspondence, but as I just saw your query when I was looking at this thread earlier and I had some time on my hands, I thought I might save Peter some trouble.

Regards,
Hi Bob,

Thanks for the explanation! Ok, so the output impedance is the impedance seeing from the input of the stage after.
Yes agree, for practical purposes we'll take the lowest impedance which is the external resistor parallel with the DAC output, it should be close....and don't need to plot the output impedance across some range of frequency as well...

No such of hi-jack thing in my opinion, this is the beauty of the forum - learn from different perspective!

Regards,

Joshua
  Reply With Quote
Old 19th December 2007, 03:52 PM   #2024
Bobken is offline Bobken  United Kingdom
diyAudio Member
 
Join Date: Dec 2002
Location: UK
Hi Peter,

I hadn't seen your response when I started my last post (I got interrupted half way through writing it!) and I should apologise for my 'interference' in your thread.

I now see from your reference to Stereophile that their measurement was 2k3 for the 47 Labs version, and if they used this same 2k7 value as yours for the 2 shunt resistors concerned, this is rather lower than I would have guessed.

However, as I just said, any impedance to ground within the TDA chip's circuitry will have some effect on this issue, and needs to be considered in the equation as this will be effectively in parallel with these (output shunt) resistors in this case.

Next time, instead of saying "determined by" or "based on", I should remember to say "will be mainly determined by, or mainly based on".

Regards, and a happy Christmas to you and the family.
__________________
Bob
  Reply With Quote
Old 19th December 2007, 03:55 PM   #2025
diyAudio Member
 
Peter Daniel's Avatar
 
Join Date: Jul 2002
Location: Toronto, Canada
Send a message via AIM to Peter Daniel
The 47Labs DAC is pretty close to this design: http://www.fedde.nu/audio/images/nonoz3.gif and if I'm not mistaken they use 3K resistors for I/V as in attached pic.

Happy Christmas
Attached Images
File Type: jpg shig.jpg (70.7 KB, 1524 views)
__________________
www.audiosector.com
“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC
  Reply With Quote
Old 19th December 2007, 04:13 PM   #2026
Bobken is offline Bobken  United Kingdom
diyAudio Member
 
Join Date: Dec 2002
Location: UK
Hi,

I seem to have dug myself into a bit of a hole here, so it will teach me to mind my own business in future!

The matter is a little more complex (isn't it always?) in that there will be variations at different frequencies (to satisfy Joshua's query), and strictly-speaking the output impedance should be stipulated at a particular frequency. This is why it is called output impedance, and not output resistance.

However for our purposes in a situation like this, it is basically what I have outlined, and few (including Stereophile, it seems) will even indicate at what frequency their measurements have been made, anyway.

This Fedde circuit is rather different as you have several extra caps and resistors in this part of the circuit which are not in Peter's, and these all have paths down to ground and will reduce overall impedances at certain frequencies as a result.

Regards,
__________________
Bob
  Reply With Quote
Old 19th December 2007, 04:36 PM   #2027
diyAudio Member
 
Peter Daniel's Avatar
 
Join Date: Jul 2002
Location: Toronto, Canada
Send a message via AIM to Peter Daniel
Whenever someone asked me about DAC's output impedance I also always specified it as 2k7
__________________
www.audiosector.com
“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC
  Reply With Quote
Old 19th December 2007, 04:38 PM   #2028
Bobken is offline Bobken  United Kingdom
diyAudio Member
 
Join Date: Dec 2002
Location: UK
Default Never give up!

Hi,

One final comment, if that pic of Peter's is a genuine 47 Labs Gaincard interior, then I for one am appalled at the constructional quality and layout.

Compared with anything you ever see of Peter's own work, that looks more as if it has been put together by a real amateur. Also, using parts 'flying in the air' like that is sometimes beneficial to keep certain signal-paths short etc., but this has hardly achieved anything worthwhile in this regard, if at all.

It just looks like a haphazard 'first-attempt' for an initial circuit-trial, like a prototype breadboard design.
__________________
Bob
  Reply With Quote
Old 19th December 2007, 05:21 PM   #2029
diyAudio Member
 
Join Date: Nov 2007
Quote:
Originally posted by Bobken
Hi,

I seem to have dug myself into a bit of a hole here, so it will teach me to mind my own business in future!

The matter is a little more complex (isn't it always?) in that there will be variations at different frequencies (to satisfy Joshua's query), and strictly-speaking the output impedance should be stipulated at a particular frequency. This is why it is called output impedance, and not output resistance.

However for our purposes in a situation like this, it is basically what I have outlined, and few (including Stereophile, it seems) will even indicate at what frequency their measurements have been made, anyway.

This Fedde circuit is rather different as you have several extra caps and resistors in this part of the circuit which are not in Peter's, and these all have paths down to ground and will reduce overall impedances at certain frequencies as a result.

Regards,
Ok, 2k7 ohm is a good approximation of the output impedance

Thanks,

Joshua
  Reply With Quote
Old 19th December 2007, 06:06 PM   #2030
Bobken is offline Bobken  United Kingdom
diyAudio Member
 
Join Date: Dec 2002
Location: UK
Quote:
Originally posted by joshuajoshua


Ok, 2k7 ohm is a good approximation of the output impedance

Thanks,

Joshua
You are very welcome.

Interestingly, if you add the 50k resistors at the output like Fedde's R9 & R10 (and I think Peter's PCB allows for these), this will lower the nominal output impedance from circa 2k7 to not much above 2k5 Ohms, with no other changes.

Regards,

__________________
Bob
  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 10:57 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