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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).
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):
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:
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.
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:
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).
Modified ZERO DAC - new amplifier with ALPS potentiometer and modified
DAC (this is the old ZERO, not 09 version)
Oversampling
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.
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.
Overview
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):
And some measurments of headphone output: THD @ 1kHz and max volume, noise spectrum @ max volume
and USB input, noise
spectrum @ max volume and coaxial input and @ half volume, THD @ 1kHz and half volume,
THD @ 3kHz and
half volume, THD
@ 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
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