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hidjedewitje 31st December 2018 11:29 AM

AK4493 DAC
 
Hi,

First of all I'd like to wish you all a happy new year! To start the new year properly I want to build a good DAC.

I want to use the AK4493 DAC. The reference voltage source I have found to be quite high spec'd is the LTC6655 from Linear technology. The problem I have come across is that the AK4493 requires a really high value capacitor (470uF minimum, value dependant on quality of power supply). The LTC6655(5V version) should not be used with capacitive loads larger than 100uF. At 100uF there already is a noise peak, the ideal capacitor should be around 10uF.

My question now is, how do I optimize the VREF capacitor? Should I just go for the 10uF and put the LTC6655 as close as possible?

Thanks in advance!

- Hidde

Thorp 31st December 2018 02:12 PM

Hi,


Happy New Year!


I wouldn't use a VREF cap smaller than 470uF.
Please take a look here: https://www.diyaudio.com/forums/atta...m_avcc_v03-png


Good luck!

phase 31st December 2018 02:46 PM

I would find another source/regulator that gives a spec for esr on the output, then you can find a large capacitor that works.

hidjedewitje 31st December 2018 03:07 PM

Quote:

Originally Posted by Thorp (https://www.diyaudio.com/forums/digital-line-level/331873-ak4493-dac-post5649028.html#post5649028)
Hi,


Happy New Year!


I wouldn't use a VREF cap smaller than 470uF.
Please take a look here: https://www.diyaudio.com/forums/atta...m_avcc_v03-png


Good luck!

Ah, of course! Buffering the voltage reference. Great idea. While it complexes the design a bit, it's a great solution.

Though I'm not quite sure why the negative supply is used here.
Thanks a lot for the tip!

hidjedewitje 5th January 2019 07:33 AM

Does anyone perhaps have recommendations for the output filter?
I can understand that the Q should be 0.707 and THD as low as possible, but what about Fc and order of filter?

I thought the Fc should be 20kHz, but then the phase response becomes really messed up within the audio band.

In the datasheet it states that you can select the oversampling ratio. Since I am using a 11.2896 MHz oscillator. Would that mean that I should use it in normal mode (256x oversampling) and use 11.2896 MHz as Fs?

Also what kind of order is recommended? I've been reading about 4th order, but 4th order messed up the phase response a lot (though I was tinkering with it at 20kHz).


Thanks in advance :D!

nihtila 14th February 2019 11:59 AM

How did you progress with this one?

I have been just experimenting with AK4490 and did a search on AK4493. I want to do similar tests on this one at some point as well although will move forward using AK4490 in the current project.

At least on AK4490 I have noticed that it really depends on the voltage source for VREF if big caps are required or not. When using superb LDO, in my case LT3042, big caps did not bring any benefit. And in fact I have seen small deterioration in performance when trying very large 1000+ uF caps. It may be because big caps are not recommended with LT3042 either.

Probably too late but regarding output filters, you do not need to aim fc at 20 kHz - that is the beauty of oversampling DACs. As you see often in datasheets the corner frequencies are somewhere around 100-200 kHz. Then use a filter type you prefer, for example Butterworth and Bessel are common, and orders anywhere from one to four. Two can be a good compromise.

canvas 14th February 2019 01:06 PM

Quote:

Originally Posted by hidjedewitje (https://www.diyaudio.com/forums/digital-line-level/331873-ak4493-dac-post5648875.html#post5648875)
My question now is, how do I optimize the VREF capacitor? Should I just go for the 10uF and put the LTC6655 as close as possible?

I've tried many Vref caps. IMHO, choosing the right capacitor is more important than the capacitance. I found the classic 47uf/10V OSCON SC is quite good.

Poting

hidjedewitje 14th February 2019 02:57 PM

Quote:

Originally Posted by nihtila (https://www.diyaudio.com/forums/digital-line-level/331873-ak4493-dac-post5697671.html#post5697671)
How did you progress with this one?

At least on AK4490 I have noticed that it really depends on the voltage source for VREF if big caps are required or not. When using superb LDO, in my case LT3042, big caps did not bring any benefit. And in fact I have seen small deterioration in performance when trying very large 1000+ uF caps. It may be because big caps are not recommended with LT3042 either.

Probably too late but regarding output filters, you do not need to aim fc at 20 kHz - that is the beauty of oversampling DACs. As you see often in datasheets the corner frequencies are somewhere around 100-200 kHz. Then use a filter type you prefer, for example Butterworth and Bessel are common, and orders anywhere from one to four. Two can be a good compromise.

My project is currently on hold. I can only work on it in the weekends as I am working on my bachelors graduation project.
I learned about the filters a while ago yes, but thanks for the effort! I am going to look into Bruno Putzeys way of low distortion lowpass filters.

As for Vref caps, I was going to use Nichicon UKZ 1000uF 50V. They seem like nice caps and I only need 2. A local c0g np0 cap will be used near the pin for the high frequencies.

Unfortunately I do not have the luxury of distortion measurement equipment (this project was part of a DIY audio interface for measurements). I was planning on getting it to work and ask a company I used to work for to verify performance once.
I don't think they appreciate me sitting in their lab and use their equipment for a few days for a hobby project. I'd love to do the research though.
Keep me updated on your project though! I'd love to see the performance.

nihtila 14th February 2019 03:09 PM

Quote:

Originally Posted by hidjedewitje (https://www.diyaudio.com/forums/digital-line-level/331873-ak4493-dac-post5697828.html#post5697828)
My project is currently on hold. I can only work on it in the weekends as I am working on my bachelors graduation project.
I learned about the filters a while ago yes, but thanks for the effort! I am going to look into Bruno Putzeys way of low distortion lowpass filters.

As for Vref caps, I was going to use Nichicon UKZ 1000uF 50V. They seem like nice caps and I only need 2. A local c0g np0 cap will be used near the pin for the high frequencies.

Unfortunately I do not have the luxury of distortion measurement equipment (this project was part of a DIY audio interface for measurements). I was planning on getting it to work and ask a company I used to work for to verify performance once.
I don't think they appreciate me sitting in their lab and use their equipment for a few days for a hobby project. I'd love to do the research though.
Keep me updated on your project though! I'd love to see the performance.

I will post something when I try to measure it, and test if it behaves similarly to AK4490. I need to first study the differences of AK4490 and AK4493. If I can use the same boards, I may post something soonish, otherwise some time later. Now mostly focusing on finishing the current project. Also working on evenings and weekends and although I can use quite a lot of free time on that, the progress is still quite slow. Things just take time.

I know not all have access to good measurement kit, then it is more like guessing the technical performance. It will be good if the basics are in order. When you want to get it to the top range and squeeze out the last dBs, then it requires more careful considerations and measurements. Even if I have access to the measurement kit after work, it is so time consuming that I do not want to test too many combinations and spend all nights in the office.

hidjedewitje 14th February 2019 04:07 PM

Quote:

Originally Posted by nihtila (https://www.diyaudio.com/forums/digital-line-level/331873-ak4493-dac-post5697840.html#post5697840)
I will post something when I try to measure it, and test if it behaves similarly to AK4490. I need to first study the differences of AK4490 and AK4493. If I can use the same boards, I may post something soonish, otherwise some time later. Now mostly focusing on finishing the current project. Also working on evenings and weekends and although I can use quite a lot of free time on that, the progress is still quite slow. Things just take time.

I know not all have access to good measurement kit, then it is more like guessing the technical performance. It will be good if the basics are in order. When you want to get it to the top range and squeeze out the last dBs, then it requires more careful considerations and measurements. Even if I have access to the measurement kit after work, it is so time consuming that I do not want to test too many combinations and spend all nights in the office.

Have you considered using a high current (50mA) as a supply for the AK4493?
You can use one high performance voltage reference and a low noise high current opamp.
Something like the AD8599 would be a good choice. Quite the expensive opamp though. I haven't played with discrete buffers yet.


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