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Glass Audio Article nr. 6/98 |
A 24 Bit DAC
by Stefano Perugini
This
article appeared originally in Glass
Audio nr. 6/1998
The Vacuum Tubes Renaissance appears more
and more as an
intrinsic characteristic of every
sector audio; as well as in realizations
"tout-court" you can see this
fascinating and bright glassy bulbs in
increasing symbiosis with the youngest
products of the solid-state
technology.
The presence of vacuum-tubes into
solid state amplifiers (hybrid
amplifiers) and in the output stage of
the dac converters or in the musical
"processors" of the pro-audio
sector (where in few cm2
of PCB you can
see gathered DSP IC's and electron
tubes) is symptomatic, in my
opinion, of the convicement that
attributes to valves the ability to
compensate the asperities produced by
the solid state devices and allow
therefore the reproduction of a sound
less straining for the human ear.
The musicians for instance seem to
have a preference for the presence of
vacuum-tubes in power amplifiers as well as in the analog signal
processing and pre-amplification
unities, while in the High-End sector
the Audiophile Community reserve a great attention to
dac's with valves in output.
In this article I'm going to introduce
the project of a DAC converter
that uses the recent Crystal's CS4390.
This component that you can see as a
CS4329's up-grade is a complete 24 bit
stereo digital-to analog converter,
that in addition to the traditional D/A function, include a
digital
interpolation filter and a 128X
oversampled delta-sigma modulator.
The sigma modulation [1] has now come also to a technological
maturation and can quietly rivalry both sonically and
technically with the
most traditional multibit
modality.
In this project I have reserved great
importance to the designing of
the power supplies even featuring the
eccentric use, for this context, of a
vacuum-diode rectifer. The output stage, thanks to the versatility
of the
'90, can lend to the most varied designing
interpretations. In this specific
case you will see the implementation
of a passive unity realized with a
signal transformer.
Inside the Blocks.
Fig.1 shows the full block-diagram of the converter. The
signal
coming from the transport unity of
CD-Player is submitted to an initial
conditioning, that consists in an
amplification and slope front
amelioration , Block 2. The Block 2
output is the input of the Interface
Receiver, Block 3. I have used here
another Crystal's device, the
evergreen CS8412. The CS8412 receive
an decode audio and digital data
from a digital audio transmission line
according to the AES/EBU,
IEC958, S/PDIF and EIAJ CP-340
standards. The (low jitters) clocks
generated are:
MCK =
256 X Fs
SCK
= 64 X Fs
Fsync = Fs or 2 X
Fs
where Fs is the sampling
frequency.
SDATA the fourth produced signal, is
correlated directly with the audio
information.This data flow is input to the DAC, Block 4, realized
with
the CS4390. The 90's full Block-Diagram is shown in Fig. 2.
The
oversampling factor permets the
selection of an output filter based on out-
of-band noise attenuation requirement
rather than anti/image filtering [2].
The delta-sigma modulator, Block 2,
convert the interpolation filter
output into 1 bit data @128X Fs. This
data flow is input to the DAC
converter, Block3, where the digital
world bridge the analog world and
the digital word translated into
analog signal. Block 4 perform a low-pass
filtering and allows two analog output
with a phase difference of 180
degrees, Fig 3. Each output produces 1
Vrms for a full scale digital
input. In differential mode, where you
can exalt the cancellation of
common mode errors, noise, distortions
and offsets, you can get up to 2
Vrms.
The output Stage
The noise spectrum in output from the
4390 presented in Fig.4, shows
clearly the uselessness to apply a strong low-pass filtering. In fact for
this
operation the same Crystal recommends
the implementation of a 2nd
order filter, however some experimental tests show
that good results can
already be got with an a simple 1st
order filter. Nevertheless the
4390's good characteristics don't have
to let think that the output filter is
superfluous because some measures,
that have found full confirmation in
listening tests, have put clearly in
evidence the presence of small levels of
distorsions when the signal in output
of the DAC is input to the
rest of the audio-chain directly. In
this project the filtering process has
been realized resorting to the natural
low-pass characteristic that a real
audio transformer exhibits in the
region of the high frequencies. This has
allowed the realization of an output
stage, Block 5 for the L-channel
and Block 6 for the R-Channel in Fig.
1, with excellent sonic characteristics.
The output stage schematic is in Fig. 5.
Since you can see this circuit
as:
a) a differential mode-to single ended mode converter;
b) a low-pass
filter;
it's very simple to profit some
correlated advantages with the architecture
of the output stage internal to the CS4390 .
Unfortunately the simplicity of this
stage is only apparent since a lot of
energies can be dissipated for the
search or the realization of a
transformer that answers to our specifics and for the circuit optimization;
the load seen by the transformer is in fact very complex because you
usually have here a shielded cable
that connects the output of the
conversion unity with the input of the
preamplifier. The knowledge of the
interactions between the parasitic
elements of the transformer,
the
connection cable and the preamplifier input circuit
, it's of primary
importance in order to establish the
position and the entity of all the
resonances, certainly out audio band,
that would be able to move by
intermodulation noise and distortion
into the high region of the
audio frequencies.
I have conducted with profit this
investigation entrusting me, as
usual, to the circuit simulation [3].
The circuit used for the
simulation is shown in Fig. 6. The
components designated with an
asterisk represent the parasitic
elements of the real transformer.
More precisely [4]:
Rti1*, Rti2*, Rti3*, Rti4* are the
windings DC resistances;
L_leak_sc1*, L_leak_pr1*, L_leak_sc2*, L_leak_pr2* are the leakage
inductances;
C_lmpd_p*, C_lmpd_s* are the equivalent lumped capacitances
between windings.
The connection cable (the Signal Cable black-box) has been
schematized as transmission line [5], Fig 7.
I have extracted the
large sample of the High-End and
Consumer production; by measurements
I have extrapolated a real model of worst cable with
(Lp, Cp, Rp) = (3uH, 300pF,
0.1ohm)
that I have used for the simulations.
Fig.8 is a Monte Carlo
analisys related to the frequency response of the circuit in Fig. 6 when
R1
and C_pre have 5% and 100% tolerance
rispectively. Fig.9
show a histogram evaluated with
respect to Bandwidth("node", 1db) Goal
Function [3], extrapolated from Fig.8. You can see
that the
bandwidth @1dB always falls between
21.6kHz and 23.5kHz, therefore
this output stage well "defend" the
correct frequency response from most load
variations.
This last results are not accidental
or causal since with a transformer
of
smaller quality the results won't be
so good. From these simulations
a peculiar behavior of the used
transformer emerges:
the low-pass characteristic that you can observe in
Fig.8, typical of every audio
transformer, has been wanted
acting, during the transformer
building, on the values of leakage
inductances.
Simulations have shown that this
parameter has to lie between 10mH and
20mH. Lower values of 10mH produce an
excessive extension of the
bandwidth worsening the DAC's noise
levels. Greater values of 20mH
produce a premature cut of the
frequency response reducing the
informative content of the audio high
frequencies. Simulations and
measures have shown an optimal
behavior when the leakage inductance
assumes a value of 16mH.
Alternative roads
Naturally a lot of equally valid
variations to the scheme of Fig 5
exist. If in this context you don't
desire to renounce to vacuum tubes
you
could take in consideration the
simplified scheme of Fig. 10; as electron
tubes I recommend the followings:
437A, 3A167M, EC8010, E810F in pseudotriode, 417A,
E55L
in pseudotriode.....
that is small valves with high mu,
high gm and a comforting plate
dissipation in order to make easier
the search or the construction of a
suitable transformer, to get a better
impedance matching and to produce,
in case of necessity, also a
meaningful output power . If you love to listen
to music with headphone this can be the road to follow for
achieving
excellent sonics results.
Moreover since a lot of Sound Card
have a
S/PDIF output, you can use this
circuit to improve the sound reproduced
by the speakers of your Computer dramatically.
By renuncing to the elegant
simplicity of a transformer output
stage (active or passive), you can
entrust yourself to the natural
tendency of an op-amp to the differential-to
single_ended mode conversion.
In Fig. 11 the scheme of a 2-pole
frequency response. This filter has
been designed with a
cross-frequency
of 50kHz, a 40dB/Dec slope and 6dB gain. Obviously
the circuit of Fig.
11 can be implemented with vacuum
tubes technology getting good results
however, Fig. 13.
You can find the grounds of this circuit in GA
1/95
[7]. Nevertheless in comparison to the
original version this schematic use only 6cg7's
and, in order to simplify the
realization I have chosen a SRPP as output stage. Besides is
C10>C3
to limit instability phenomenae in sub-sonic band
that this modifications can produce.
In fact Fig.14 shows what happens in
the frequency response
when, with C3=4uF, C10 is varying
between .2uF and 10uF with .2uF
step . When C10 <C3 overshoots are present in subsonic band.
As
first steps you can for instance
choose C10=3.3uF and C3 = .22uF and
continue with a following tuning on
the real circuit. Unfortunately the
choice of a low value for C3
introduces some disadvantage; Fig.15
shows the simulated frequency response
when Rl varies between
500ohms and 20Kohms with step of
500ohms. A minimal value of
8Kohms is necessary to avoid an
excessive limitation of the low
frequencies. In this context, since
the realization of a vacuum-tubes op-
amp is less critical than that of a wide-band transformer , the
influence of
the parasitic elements can consider
some more negligible and therefore the
electric results more
satisfactory.
Completion
The full schematic of the 24 bit DAC is show in Fig. 16.
The
dimensions and consumptions it features a good ability to supply
current. You don't have to worry about
the elevated capacitive value of C35 and C36
since the repetitive peak current,
even in the most serious condition of
operation is lower of the maximum value, as Fig 17 show.
L1..L4 act in the decoupling of the
high frequencies residuals; nevertheless their
small values doesn't engrave heavily
on the resonances of the power supplies filters,
Fig. 18.
The shunt-type regulator for the
voltage of the ' 90 have been chosen because it effects
a small degrade of the sonic performances in comparison
to the series
regulation;
then whereas audio analog stage are
present , the load is not onerous and,
as in this case, it is not
possible to renounce to the power supply
regulation , I prefere it to the
serie-type regulator.
The hands "in pasta"
I have realized a first prototype of
this converter with a point-to-
point wiring, using small Teflon clippings as support, Photo
1.
Photo1
Although this solution is sonically
effective , lately I have opted for
a PCB realization in order to furnish
in briefer times copies of this
converter to the friends of the
audiophile cenacle remained
fascinated by
his intrinsic sonic
characteristics, Photos 2, 3, 4,
5.
Photo2
Photo3
Photo4
Photo5
Unfortunately this approach determines
an increase in the difficulties. In
fact as every High-Resolution
Mixed-Signal PCB Layout you will have to
respect the followings
constraints:
a) Separate analog and digital
circuits and segment by functionality and speed;
b) Distribute power supplies and
grounds taking care to minimize loop area and return current paths;
c) Isolate noisy return current paths
from more sensitive analog circuits;
d) Minimize interference from
clocks;
e) Decouple ALL IC power supply
pin;
f) Minimize emissions;
g) Reducing the effects of capacitive
and inductive couplings;
h) Minimize return current path
impedances to reduce ground bounce
effects.
For instance with reference to c), the
splitting of the ground into separate
analog and digital grounds is the best
way to guarantee that noisy digital
currents will not flow in the
sensitive analog area.
You can see the adopted solutions in
the Figgs. 19..22 that shows
the
PCB artworks of the DAC.
Conclusions
The ambitious objective to make the
digital source timbre similar to
that of an analogic source, or more
simply to make harshless the resultant
sound, is pursued generally acting
entirely on the output stage side.
Most of the high-end converters
builders realize this circuit using
vacuum
tubes and the results they get are good. Nevertheless, also using an
output
stage with vacuum tubes, I think that
a margin of improvement still exists
if we also re-consider the
constructive philosophy of the power supplies.
In this light you can see the presence
of a vacuum rectifier and a shunt-
regulator in the circuits of this
converter.
Certainly the presence of a vacuum
rectifier into a low-voltage
power supply can appear a bit " freak
", nevertheless I invite you to
experiment a similar solution, not
necessarily in this same context, since,
I am sure, the results will impress
you positively.
This project was conceived initially
two years ago thinking at the
CS4329 (the predecessor of 4390, with the same
pin-out). During the
time continuous improvements have been
effected in order to get both
a minimalist and well sounding object
. A possible evolution of
this object could foresee power
supplies with choke input filter and
vacuum-tubes shunt regulators.
References:
[1] T. Tanaka, T.
Sugimoto, C. Kubomura
18-bit D/A converter with integrated digital and analog filters
91st AES convention, October 1991, New York, Pre-Print
#3113(y-1);
[2] Crystal Semiconductor Corp.
24-Bit, Stereo D/A converter for Digital Audio
DS264PP1, May '97;
[3] Microsim Corp.
Microsim DesignLab Eval. 8, 1997;
[4] Radiotron Designer's Handbook,
Fourth Ed.
pags. 204-206;
[5] Douglas Self
Cable Sonics?
Electronics World, Vol. 103 No. 1738, October 1997;
[6] Crystal Semiconductor Corp.,
Evaluation Board for the CS4329
DS153DB2, Aug. '95;
[7] Fred Forsell
A Vacuum Tube Op Amp
Glass Audio, Vol. 7 No. 1, 1995;
[8] Norman Koren
Improved Vacuum-Tube Models for PSpice Simulations
Glass Audio, Vol. 8 No. 5,
1996;
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