Exploring the world of DAC chips

My prefered hobby is design and build audio amplifier and loudspeakers.
Every time I start a new project I am searching to over performance the previous projects.
But is difficult increase the sonic result of an amplifier if the source is not good.
Yes, the source have lot of importance in the final result of the system and in hi-end system this should be without compromise.
The DAC normally included into commercial CD Player (even these of significant price) have a rather low performance because the common usage of high NFB operational amplifiers "destroy" the sound given compression effects and poor soundstage stability.
I have a found a good solution to this problem reading the design concept of Stefano Perugini, for reference he published an article on Glass Audio (Volume 10 - Number 6).
He proposed to use a transformer as output stage of the DAC to clear the signal without active components.
I have started many test about this solution using at the begin the Stefano Perugini transformers and after the Lundahl products.
My idea was to explore the entire world of the DAC chips to find the best.
Build a DAC board is not easy because there are some rules about ground and flow to follows and a wrong design of the board can compromise the final sonic result.
Looking on the Crystal internet web site I have found the evaluation board CDB4390, that is a complete DAC with a optical and coax digital inputs and analog outputs so only changing the output stage with five simple connections you can test the transformer output stage.
The result is incredible especially with the Lundahl transformers LL1527XL connected as show in the fig.1 and over performance the my CDT Musical Fidelity.
In the fig. 2 there is the unique power supply to use with this configuration.

Just a little note about the CDT Musical Fidelity: a good improvement is possible skipping the output tube stage and getting the analog signal from the DAC board.

To test the Burr-Brown chips also I have searched an evaluation board on their web site and the unique found has been the incredible EMV1702, this use four DAC chips per each channel and opto isolators for digital/analog section but this is very expensive.
After many mounths I have found a board produced also as kit by the Italian company Armonia Hi-Hi that have a very interesting low price and use the Burr-Brown the PCM63 considered by many people the best DAC chips.
The PCM63, as the all top line Burr-Brown chips, have an output current instead a voltage so you need an I/V converter connected on output of the chip.
As obvious the simpler I/V conv. is a resistor and I have found that the best
soundstage using the range of 100-125 ohm (here use only first class resistors as the Caddock MK132).
With this low value resistor the output signal voltage is very low so we need to use a transformer with a high turn ratio about 1:20 to increase it.
I have used for this use the Lundahl LL1636 with an amorphous core that don’t store energy (unlike conventional mu-metal core) given better bass frequency.
To use the LL1636 skip any resistor and capacitor on DAC chip output R8,R11,C22,C37 and connect directly the primary of this transformer in the 1:20 configuration and on output connect a 100K MK132 Caddock resistor and a stereo 100K+100K CT1 DACT stepper attenuator (see fig.3 and 4).
In all my new design I use the CT1 or the CT2 DACT stepper attenuators having an  higher sonic detail and lower coloration instead of normal NOBLE or ALPS.

The last chips to test were the Analog Device like the very good AD1860.
In this case I don’t have found an evaluation board or a kit so I have searched a low price product to modify.
I have found the D18 model produced by Monarchy Audio, this is a very interesting company that produce also many class A amplifier without feedback.
The D18 model give a very clear and an incredible near analog signal with the normal output stage.
The DAC chips AD1860 are used with the new Piggy Back technology doubles up the D/A converter chips for even more analog sound and higher output drive.
Also with this DAC I have used the LL1638 Lundahl with a turn ratio 1:20 because 2 AD1860 in the Piggy Back configuration give the same output current of one PCM63 (see fig. 5, 6, 7 and 8).
The Monarchy Audio produce also interesting products with the PCM63-K that is the king version of the PCM63 chip and I think to test it in the future.
The sonic results of all these three DAC modified as described before is very high and the choice is very difficult.

Important references about this article are:

My web site
http://www.geocities.com/ResearchTriangle/8231/

Stefano Perugini web site
http://www.space.tin.it/scienza/stperu/PAE.htm

Crystal chips
http://www.crystal.com/

Armonia Hi-Fi kits
http://www.areacom.it/biz/armonia/

Lundahl transformers
http://www.lundahl.se/

Monarchy Audio
http://www.monarchyaudio.com/

DACT stepper attenuator
http://www.DACT.com