Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Digital Line Level DACs, Digital Crossovers, Equalizers, etc.

Improving AK4490 THD(+N) figures
Improving AK4490 THD(+N) figures
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 11th January 2019, 10:37 AM   #11
ojg is offline ojg
diyAudio Member
 
Join Date: May 2003
Location: Norway
Improving AK4490 THD(+N) figures
Quote:
Originally Posted by nihtila View Post
I am using APx555 so there is still some headroom left.
Lucky you! Then please soldier on to find out where the last bits of performance are.
  Reply With Quote
Old 11th January 2019, 10:43 AM   #12
nihtila is offline nihtila  Finland
diyAudio Member
 
Join Date: Jan 2006
Location: Edinburgh, Scotland
Quote:
Originally Posted by ojg View Post
Lucky you! Then please soldier on to find out where the last bits of performance are.
I know. I don't think anyone else in the office considers that as a benefit
__________________
http://nihtila.com - Vegan DIY audio electronics, does not contain snake oil
  Reply With Quote
Old 12th January 2019, 12:11 AM   #13
Joseph K is offline Joseph K  Hungary
diyAudio Member
 
Join Date: Dec 2004
Location: gran sasso
Your values for R4,R5; R8,R9 - seem to be low. (560)

The AK4490's recommended load is about 2kohm.
Did You try to raise these values?
Sure that needs a re-calculation.

Ciao, George
  Reply With Quote
Old 12th January 2019, 12:23 AM   #14
Shinja is offline Shinja  Japan
diyAudio Member
 
Join Date: Jul 2008
ojg, nihtila , I was likely incorrect. If it is the case, freuency-THD+N plot must increase depending on the frequency.
  Reply With Quote
Old 12th January 2019, 08:40 AM   #15
nihtila is offline nihtila  Finland
diyAudio Member
 
Join Date: Jan 2006
Location: Edinburgh, Scotland
Quote:
Originally Posted by Joseph K View Post
Your values for R4,R5; R8,R9 - seem to be low. (560)

The AK4490's recommended load is about 2kohm.
Did You try to raise these values?
Sure that needs a re-calculation.

Ciao, George
See the whole circuit and not just resistor values. Resistors connect the output to the opamp input = extremely high impedance. It is the capacitors that matter, and their value is tens of kohm at 1 kHz, for example, when doing basic THD measurement.
__________________
http://nihtila.com - Vegan DIY audio electronics, does not contain snake oil
  Reply With Quote
Old 12th January 2019, 11:12 AM   #16
mhelin is offline mhelin
diyAudio Member
 
Join Date: Apr 2003
Location: Tampere Finland Europe
Try capacitance multiplier on PSU:
Capacitance Multiplier Power Supply Filter
  Reply With Quote
Old 12th January 2019, 12:22 PM   #17
nihtila is offline nihtila  Finland
diyAudio Member
 
Join Date: Jan 2006
Location: Edinburgh, Scotland
I got some polymer caps yesterday and did a quick measurement after work. No improvement, in this case it seems to be just about the capacitance and not really how good the cap is. This was quick and ugly test though, it's a bit tricky to put caps on my tiny PCB.

I also tried a quick mod by wiring the 5VA reg voltage sensing point from the DAC board, as suggested by ojg. This is quite ugly mod also as the wire is not that short. THD+N at 1kHz remained the same BUT lower frequency distortion got better, see graphs (this is with 680u tantalum polymer I was able to fit on the board, first normally and second with the sense wire mod). This gives some confidence that on-board regulation would improve the lower frequency distortion, and hopefully will push the 1kHz range down as well when all are on solid copper instead of flying wire.

I have decided to make a test board for the AK4490 before re-spinning the W-DAC board. I will try to make and order it during the weekend. I want to try better regulators on that board, along with different cap configurations. This gives better insights on how to modify the W-DAC board. This is something I should have done first, but...

So it will probably take 2-3 weeks before there will be significant updates on this.
Attached Images
File Type: png THDN.png (101.1 KB, 152 views)
File Type: png THDN_mod.png (95.7 KB, 153 views)
__________________
http://nihtila.com - Vegan DIY audio electronics, does not contain snake oil

Last edited by nihtila; 12th January 2019 at 12:25 PM.
  Reply With Quote
Old 13th January 2019, 01:21 AM   #18
sgrossklass is offline sgrossklass  Germany
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
Your issues would seem to reflect LP3878 noise levels, which are quite high below 1-2 kHz in particular according to the datasheet. In hindsight (which proverbially is 20/20, I know), maybe not the best candidate for +5VA. I'd suggest trying a larger capacitor on the BYPASS pin, which would seem to be filtering the voltage reference. The datasheet insists on something low leakage - if you have some 220-470 nF films around, try one of those.

It is unfortunate that the capacitor in parallel to R1 is already used for some pole/zero trickery, but what you could try is another RC combo in parallel to both - an electrolytic in series with maybe the same value as R2 (or half that). This would make the regulator act like a 2 V (or 1.5 V) one for AC, with correspondingly reduced noise gain. I'd try 1 kOhm + 10-22 F (low ESR not needed for obvious reasons... quite the contrary, you want lowish leakage).

With as much capacitance as you have on the VREF pins now, a bit of series R (as mentioned before) is in fact also worth a shot. RC filtering always works much better if R isn't just regulator output impedance, for obvious reasons.

Using two mono DACs in differential output mode avoids a lot of these problems, as the Vref noise subtracts (so it's got inherent PSRR).

Last edited by sgrossklass; 13th January 2019 at 01:42 AM.
  Reply With Quote
Old 14th January 2019, 10:01 AM   #19
ojg is offline ojg
diyAudio Member
 
Join Date: May 2003
Location: Norway
Improving AK4490 THD(+N) figures
Some more armchair-engineering: As much as I like small cute PCBs, it seems in this case it might be limiting the performance you can get. With a larger PCB, perhaps the same size as your PSU board, you would have room for more VREF capacitance and a +5VA regulator.

Why not try a Jung/Didden regulator for +5VA?

It is interesting that the AKM DACS all seem to require lots of VREF capacitance, 1000uF or more, while the similar spec'ed DACS from TI or AD do not have these requirements. What is the inherent difference in DAC design that causes this?
  Reply With Quote
Old 14th January 2019, 10:23 AM   #20
nihtila is offline nihtila  Finland
diyAudio Member
 
Join Date: Jan 2006
Location: Edinburgh, Scotland
Quote:
Originally Posted by ojg View Post
Some more armchair-engineering: As much as I like small cute PCBs, it seems in this case it might be limiting the performance you can get. With a larger PCB, perhaps the same size as your PSU board, you would have room for more VREF capacitance and a +5VA regulator.

Why not try a Jung/Didden regulator for +5VA?

It is interesting that the AKM DACS all seem to require lots of VREF capacitance, 1000uF or more, while the similar spec'ed DACS from TI or AD do not have these requirements. What is the inherent difference in DAC design that causes this?
Capacitance requirement can be quite crazy. In my ugly mods there was a significant change still going from 2200u to 2x2200u. But I hope when I get a good regulator on-board it would not require that much anymore. Odd thing in this is also that why AKM removed the graph from AK4490 datasheet (or EVB manual) showing the impact of VREF cap on low frequency THD? And they don't even use large capacitors on their EVB, nor are the regulators on the same board as the DAC. I really doubt they measure the claimed performance on that board.

I will need changes on all boards but the XLR one so I will re-consider the form factor and interface. At least the interface will change, I will likely have 5VD, 5VA, +15VA, and -15VA, and then use on-board regulators for 3.3V voltages and also for 5-7V for VREFs and such. Also remove two pins from one side for simple keying. Therefore, I can think of making the boards larger but will need to check cost impact as well.

It would be nice to have a bit more information on all the supplies other than VREF, such as AVDD or VDDL/R; how sensitive are those to noise and interference. Will try to do some simple tests on the test board when I have it.
__________________
http://nihtila.com - Vegan DIY audio electronics, does not contain snake oil
  Reply With Quote

Reply


Improving AK4490 THD(+N) figuresHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Lissajous figures rtarbell Everything Else 1 28th October 2005 02:23 AM
Noise Figures northsiderap Solid State 12 7th May 2005 09:48 AM
Xmax figures to actual exurcsions? TRANSDUCEReng Subwoofers 20 21st September 2004 09:19 PM
DC offset figures johnm Chip Amps 7 6th February 2004 02:01 PM
help, need some figures Alex_B Multi-Way 54 9th December 2002 01:36 AM


New To Site? Need Help?

All times are GMT. The time now is 04:19 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki