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.

Theinterpolator, Block1, is a 128X
interpolation filter that because the
high

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
thenumerical values of the parasitic elements experimentally
examining a

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-poleanalog filter with differential input
[6] is present . Fig.12 is the

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.

Thevacuum rectifier
selected is the EZ81 since although reduced in

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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