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Tianyun ZERO DAC

After "killing" PHILIPS SBC HP820 I decided to buy something better - AKG K240 MKII. They have low impedance, so it is possible to use them with portable MP3 players or PC soundcard, but without DAC/headamp the sound quality isn't the best. Later I bought electrostatic earspeakers STAX SR-3 with SRD-5 adaptor (adaptor is now replaced by DIY electrostatic amplifier), so I decided to look for something better than PC with cheap sound card.

After reading some feedback I chose Chinese ZERO DAC (with USB module and OPA627 operational amplifiers) over NuForce uDAC2 and Audio-GD Sparrow (thank god for that).
Tianyun ZERO - black version

ZERO DAC parameters (as declared by manufacturer): stereo D/A converter AD1852 max.24bit/192kHz, reciever CS8416, RCA LINE OUT regulated and fixed, headphone autput, inputs: USB+TOSLINK+coaxial, THD 0.0005%, frequency characteristic 20Hz to 30kHz (-0.5dB), SNR 105dB, dynamic range 117dB.

Following table shows some measurements after approx 300hours of operation (headphone amp. without load, THD@1kHz, IMD@60Hz+8kHz, SNR calculated from peak-peak values):
ZERO DAC original 320h
Measurements quite matches with those numbers from manufacturer, except frequency response. (THD 0.002% is distortion of device used to measure THD).
You can also see, that headphone amplifier is much worse than DAC.
I tried to swap operational amplifiers in headamp, but with virtually no improvement - this is because topology of headamp is just bad .

Modifiing ZERO DAC to version "Amarok"

First of all summary of "what can be done":
1) DAC mod: basic (i.e. 6 or 8 new caps), or simplified (2 new caps)
2) New headphone amplifier (with OA of your choice, can be with ALPS - there is room for it)
3) Oversampling switch (×2, ×4 or ×8)
4) I2S input for possible new USB module
5) Connection of PE with metal case of ZERO
6) Isolation of GND from the case connected to PE in order to prevent ground loops
7) Transformer "silencer"

First thing to do is to remove old headphone amplifier and replace it with new high quality headphone amplifier, which is much better than the old one.
It is essential to place cables in same place as they were originally. This picture shows what happens if you place cables near transformer and this picture shows that hum dissapears after moving cables to right position. 
And photo of new headamp before installing into ZERO DAC.

If you look closely on AD1852 output buffer and draw a schematic you must know what is next thing to do:
Output buffer AD1852 schematic
Following graph shows frequency response a phase shift of original ZERO DAC. Frequency response is up to 30kHz, but not -0.5dB (as stated by manufacturer) but -3dB and with huge phase shift.

Original buffer @ AD1852 characteristic
DAC modification is very simple - remove C1 and C2 and replace C2 with 100pF foil capacitor (e.g. WIMA FKP2 100pF/100V)

This photo shows how to modfy DAC (before modification), two 22pF ceramic capacitor are not described here, but you can remove them (they don't have any purpose).
Following graph shows frequency response a phase shift of modified ZERO DAC buffer:
Modified buffer @ AD1852 characteristic
New frequency response and phase shift is much better than the old one resulting in better separation, better soundstage, more details and huge improvement in bass.

Another version of improving DAC is to replace 1nF caps with 470pF, 1.2nF => 150pF and 560pF => 150pF. This results in even better frequency response, I did not hear any differences when comparing to simpler DAC modification, but I reccomend it (because there is no peak in frequency response).

ZERO DAC modified
Modified ZERO DAC - new amplifier with ALPS potentiometer and modified DAC (this is the old ZERO, not 09 version)

This modification allows to switch between 2x, 4x or 8x oversampling, original ZERO is set to x4. Oversampling has almost no efect on sound quality, but you can try it for yourself. Modification is simple - add two switches, one resistor and break one route. Bottom side of PCB - place 10k resistor and wire on pin7, top side of PCB - just solder two wires. Oversampling is switched by two switches - first is switching pin7 to ground and second switches pin10 to ground, following table shows switch combinations (1=ON, 0=OFF):

swiitch "G+7" 1 1 0 0
switch "G+10" 1 0 1 0
oversamling x8 x4 x2  -

I2S input

I2S is used to directly "drive" DA converter and to avoid jittery S/PDIF reciever.
This picture shovs where to connect I2S signal. I2S is connected to AD1852 through resistors 27~270R (the old ones must be disconnected). I used 2-position switch to switch between I2S and S/PDIF input.
My I2S source was USB=>I2S converter based on TE7022 IC, power for converter was taken from nearby 5V PSU in ZERO DAC.
Vhen isung I2S you must deal with one problem: relay shorting LINE OUT when there's no S/PDIF signal. You van either remove this relay, or switch it on in presence of I2S signal, or connect use TE7022 S/PDIF output and connect it to S/PDIF-USB input of ZERO DAC.

ZERO DAC modified
Modified ZERO DAC (as above) with USB => IIS

Modifications that doesn't directly affect sound
Metal case is not connected to protective earth, so first modification is connection protective earth to case (may lead to ground loops!). Second thing is replacing mains connector ("Euro connector") with another that has built-in EMI filter (this is not really necessary).
Last modification is lowering noise level (buzz) of transformer. Solution is quite simple since ZERO consumes only 10W - just add resistor in series with primary winding of toriodal transformer. 150ohm/2W to 390ohm/3W would be sufficient. I used 330ohm/3W and ZERO is virtually silent (hum can be heard only from approx 10cm). Voltage on capacitors in analog section droped from 22V to 21V so there is plenty of voltage drop for voltage regulators.


Folloving table shows measurments of fixed LINE OUT and headphone amplifier of ZERO DAC version "Amarok" (headphoe amplifier measured with AKG K240 MKII as load, measured directly at amp. PCB, THD@1kHz, IMD@60Hz+8kHz, many values of THD and IMD are limited by measuring device - real THD and IMD is smaller, SNR calculated from peak-peak values):
Modified ZERO DAC
And some measurments of headphone output: THD @ 1kHz and max volumenoise spectrum @ max volume and USB inputnoise spectrum @ max volume and coaxial input and @ half volume, THD @ 1kHz and half volume, THD @ 3kHz and half volumeTHD @ 42Hz and half volume, IMD 60+7000Hz @ half volume and @ full volume, IMD 250+8020Hz @ half volume, detail of spectrum of 11kHz with USB input ( => jitter approx. 1ns), THD @ 15kHz and half volume, THD @ 15kHz and full volume, output impedance is smaller than 0.2Ohm through whole audio spectrum.

Fully modified ZERO "Amarok" DAC becomes true high-end DAC with high quality headphone amplifier.

Fully modified ZERO is better than NuForce Icon HD or YuLong D100 or YuLong DAH1 according to owners of modified ZERO.

Author: Luboš "Amarokcz" Støíteský

Photo AKG K240 MKII: www.akg.com

Photo STAX SR-3 and SRD-5: www.kenrockwell.com

Reseller - http://myworld.ebay.com/wsz0304/

Photo Tianyun ZERO 09 taken from manufacturers website