The Ultimate DAC Built With Over-sampling AD1865

My best effort when building DAC is when I'm using the TDA1541A in non oversampling, fully independent voltage regulator on every voltage pins, and separate transformer for analog and digital circuit. I called this ultimate DAC built on here and here. I amazed on how great the result from optimized power supply to the sound quality compares to the minimalist approach So I named this built is the first ultimate DAC.

Lately I am in love with the sound of the another ultimate DAC + headphone amplifier using the oversampling AD1865 DAC. It is theoretically better in specification and also great sound quality result as well. It is a smooth sounding DAC, great details, great depth and great stereo separation. You can also check this another ultimate DAC built on my previous post in November 2021.

Since I still have the parts that I need to make another one, so I think I will replacing the DAC board on the first ultimate DAC to this new build oversampling AD1865 DAC. Of course with some improvements.

The first ultimate DAC is built in semi replaceable modules. Inside this thick aluminum chassis, there are 4 boards which consist of:

1. The main analog transformer with small digital transformer and the rectifiers

2. The analog DC regulators

3. The SPDIF receiver, DAC and digital DC regulator

4. The IV converter & LPF board

So I will replace the section number 3 above which is the DAC board with this AD1865. You can check the full schematic of this DAC build by checking on my previous post as well.

The main parts for building this DAC already on my hand. It is YM3623, CXD1144, AD1865, and 74HC04.

For the digital receiver, since SN75157 is already discontinued, so I will using UA9637 instead. Both digital receiver IC has the same function and only pin 1 & 8 that switch between those IC's. Please check both IC datasheet for the details.

Then, I start populating all the parts into the universal board. The layout and schematic is exactly the same with the previous another ultimate DAC built.

I make an additional function in this AD1865 DAC build. The MSB adjust is available on every Analog Device and Burr-brown DAC. It only need few resistors and trim-pot for each channel. The function for this MSB adjust is to reduce the LSB error, yet resulting the improvement in sound quality. I will explain how this MSB adjust impact to the sound quality at the end of this post.

Another additional parts is at the analog output. I am using a pair signal transformer as the buffer output. This 1:1 configuration signal transformer is use one per channel and it is similar to the popular Jensen JT-11P-1.

This transformer are also isolating the output ground with the DAC ground. If you still remember, I also put the digital transformer in the SPDIF input, so all grounds are not connected each others. Cool!

Previously I have checked carefully how this transformers effecting to the sound quality. Especially which is the best way to put this transformer in this DAC circuit. So at the end, I will using this transformers as analog buffer at the output after the LPF circuit. It give the best result by eliminating peaks at high frequency, yet it smoothing all audio frequency band.

Replacing the DAC board to the new one is not consume lot of time. I only spent 2 days working slowly from the first ultimate DAC to become like this. It is quite fast progress in my opinion and now I checking all the connection and voltage before put all the IC's on board.

This new DAC sing from the first time I power it on! Really smooth sounding DAC, very linear, lots of detail, large sound stage, multi layer dimension and warm vocal! This is the DAC with the sound quality that I wish to hear everyday without fatigue.

It is all finished but I add an extra accessories. I am using dual color LED to indicate the signal digital is present or not. I have done this with a simple trick by using the spares inverter at 74HC04 connect to the error flag of the receiver IC. So each of the LED color will get the voltage if the error flag in positive or in negative.

This DAC sound quality is amazing. The sound character from oversampling AD1865 DAC with transformer output is very smooth.

Then I try to adjust the MSB trim on each channel. I don't have sufficient equipment's for this test except my golden ears:)

So the starting point is put both trimer to the center position. Then I dial it at the same time, left and right, with the music play. I hear the sound changing on every single turn, which I stop on few turn clockwise after I heard the very optimal sound especially when the low to mid frequency more pronounce than the first time. I try the opposite turn and find that more error on MSB cause the sound quality thin and less engaging. Then I re-dial again to the optimized setting on each trimer.

That's what I can shares about this revisiting ultimate DAC built, now is using AD1865, it sound really great!

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