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USB DAC 2b - Hi-Resolution System         
April
 1  st , 2012 - working in progress ......


 
INTRODUCTION

see USB DAC 2

modular design

 


XMOS MODULE

I have tested these USB-I2S interfaces and I have found very big differences in the sound:

After 3 tests in 2 different rooms, 3 different systems, with 3 my friends I can affirm that XMOS module by Lorien is the best. There are many differences in the listening and only the XMOS has an open sound, full of details and never tiring. With the XMOS all musical instruments are easily recognizable in the sound stage.


AD1955 DAC SCHEMATIC

The DAC module use a dedicated power supply with a separated section for the analog stage.
A passive I/V has been created using Hi-end MK132 Caddock resistors.
The pcb use AD1955 mounted on DIP adapter to be simple for DIY.

I have tested 3 different regulators on AD1955 DAC analog power supply.

1) LT1086-5 considered a very good series regulator for audio
2) A very low noise shunt with no op-amp similar to Salas design
3) Tent Labs shunt

The low noise shunt sound a little better than LT1086-5 but the Tent Labs shunt sound much more better.

The sound with Tent Labs shunt is another world, all instruments and voices are distinguishable and separate.

This shunt allows a great increase in the quality of the reproduced sound.
 

AD1955 DAC PCB


CONTROLLER MODULE SCHEMATIC

To use the AD1955 DAC chip with many sampling frequencies is necessary to set the internal register of this chip following this table.

Others DAC chips like the TI PCM1794 and Wolfson WM8740 has a master clock detection circuit that automatically determines the relationship between the master clock frequency and the sampling rate.

In a direct comparison the AD1955 sound much better than PM1794 and WM8740 but you can use the same output module with these DAC chips and in these case you don't need the controller module.

The AD1955 have an on-chip click less analog volume control without lost in resolution and I have implemented this in the PIC firmware so we can save the money necessary to buy a DACT stepper attenuator.

In my first test I have used a little PIC to have only basic functions but in order to display volume level, input sampling frequency, USB link status and to have a remote command as been used a large PIC Microcontroller.

The controller module can be configurated for other functions like a stand-by to power-on manager with a separated transformer. 

CONTROLLER MODULE PCB

CONTROLLER MODULE PHOTOS

video of the test environment

http://www.youtube.com/watch?v=H2LAQonTEXY
 

CONTROLLER MODULE FIRWARE

To program the 16F877A Microchip microcontroller I have used the PICKIT 2 USB Development Programmer/Debugger (cod. PG164120 or DV164121) with a cost about 40-50$

I love the C language because it is very simple if compared to assembler.

The my C source has been compiled with HI-TECH PICC-Lite Compiler V9.82 for PIC10/12/16 MCUs.

HI-TECH Software has provided a  freeware HI-TECH PICC-Lite compiler as a tool for hobbyists and students, but the licence allows its use for commercial purposes as well. It is ideal as a teaching tool for an introduction into the 'C' language and embedded programing on a Microchip device.

 Here you can download the last version of source and hex files.


OUTPUT STAGE SCHEMATIC

The Raleigh Audio Line Stage clone is a Differential parafeed line stage push-pull with a pair of CCS (constant current source)  to isolate  the power supply from the signal.
A 2.2uF parafeed capacitor isolating the output transformer from the power supply current so is possible to use little output transformers with amorphous core.

The CCS are  K&K Audio modules with large heat sink kit, two for each channel.

I have used the normal type CCS but it is possible to use the cascode type.

OUTPUT STAGE POWER SUPPLY SCHEMATIC

Probably the output relay for this circuit is not necessary.

OUTPUT STAGE POWER SUPPLY PCB

For Raleigh Audio Line Stage clone, only the power supply

PHOTOS

...........

 

COMPONENTS

I have used this type of heatsinks for all the mosfets and linear regulator:

     

Raleigh output stage
item description cost in euro
1 pcb 30
2 heatsink 3
2 mosfet 5
1 LM350 3
2 2200uF 25V 6
1 400V 150uF  5
1 2N2905 2
3 220uF 25V 6
1 200ohm multi turn trimmer 2
1 33UF 400V 3
4 tube socket 12
2 5687 40
2 39UF 400V Solen 40
2 2.2UF Z-Cap 36
20 resistors 20
4 CCS  32
2 LL1674 230
1 custom transformer 60
1 chassie 70
2 RCA connectors 5
  total 610