In researching the best options to pursue
for building my own Red Book-optimized digital-to-analog converter (DAC),
I looked over several DACs on diyhifi.org, diyaudio.com and
other forums/sites. I assessed both the topological
(design) aspects as well as users' qualitative comments (i.e., whether
it sounds good).
I have come to the conclusion that Peufeu's
designs are probably the best-executed of the DIY DACs I have analyzed
thus far, especially with respect to price vs. performance.
So much so that in designing my own DAC, I will incorporate several
of his ideas, albeit using a dedicated oversampling (OS) digital filter
and, perhaps, PCM1704s or PCM1794 for the DAC ICs (as opposed to Philips
Peufeu's designs are distributed among two sites: here is the original "Extremist" DAC,
and here is the "What-the-F--k" (WTF)
update. Some basic things that may be learned from Peufeu's notes
are: Simply applying known-good DAC recipes will provide one with a very
good starting base and with much potential (and also, very important: no
Some general tips are as follow:
Pick a well-respected
DAC chip: Philips TDA1541A (double crown, if you can still
find them) or TDA1545A (a decent, budget chip). If you've got some
bucks, go for Burr-Brown/TI's PCM1704 (use it with TI's matching
DF1704 oversampling digital filter).
After picking the DAC, proceed with PCB layout and overall design. Watch
out for these common pitfalls:
use of passive IV without doing distortion tests to check if the
dac's output overloads or not
real crummy printed-circuit-board (PCB) layout (see TentLabs
White Paper on Supply decoupling and optimal DAC
sending high-speed digital signals through flying wires
split ground plane like in Monica design below (split planes can
help, but NOT THIS WAY)
listening to DAC on a full-ranger (loudspeaker) from 1912 treated
with C37 lacquer and only playing Pierre Boulez on it.
Of the various techniques noted on Peufeu's site, master-clock injection
is probably one of the most attractive. I have heard much about this
approach in the past -- notably from "high-end" and professional
manufacturers like dCS --
but never found a clear DIY way to implement the technique ...
until now. Speaking of which…in Peufeu's block
diagram of clock injection, note that before the clock cable (going
to the transport), there is a pulse transformer on the DAC side only.
Since this is sent BNC, one may ask if there any impedance-matching resistors
needed (e.g. 75 Ω)? Yes, and use a 74HC04 buffer in the transport.
Peufeu advises one to use pulse transformers and twisted-pair cable (RJ45),
DAC Power Supply and Regulation
Optimizing snubber values
Snubber should probably not placed around each diode, but rather
across the transformer secondary, which is better. Read this
Hagtech article for more info.
Snubber should be optimized for your specific transformer, diodes,
and supply caps.
Here is one approach:
- Build your power supply. Build it correctly. ie.: you see the T between
rectifiers - capacitor - supply output, don't make that a T, run the
trace through the capacitor pins, for instance.
- Then, place a snubber, with for instance a small capacitor (10-100
nF) + 100 ohm resistor.
- Load the PSU with a current sink, whatever
- Then place a 'scope on the snubber resistor, and observe the diode
switch-off transient. Tune the resistor value until it's clean.
Values for snubber capacitor 10-100 nF and resistor 5-50 Ω...
tune it. Depending on your diodes, you may not need it.
Note: Snubber values depend on stuff that is hard to measure, like
Use a shunt regulator for every important chip
Use ferrite chokes, not inductors
Inductors will LC resonate with your supply caps. A good inductor
has low Ω to dissipate low power. Chokes turn into resistors
at HF, their purpose is to dissipate the HF energy while letting the
DC through with no loss. Pick the cheapest ferrite choke in your supplier's
inventory that has a rather highish resistance at above 1 MHz. Look
at the datasheets curves, and be careful of max current, you don't
want to saturate them, or they will just act like a piece of wire.
Use SMD beads -- 1206 is really quite easy to solder. You need a
fine tip soldering iron, and hair tweezers from your girlfriend's makeup
kit. ($50 professional SMD tweezers are also a possibility...but supermarket
stuff works just fine ;-).
Capacitors for decoupling and bypassing
Put some Starget caps, use your favorite brand of good caps, with
good behaviour at HF (switching power supply caps, for instance). On
the digital chips, OSCON with 100nF ceramic SMD (like 1206 package)
right on the pins is good. Make traces short.
Clocking and Digital Circuits
Which clock frequencies should one choose? If you're injecting
the clock back into the transport you will need to determine the frequency
of the original player/transport clock. See this
list for some examples.
Next, you need to choose the divisor to end up on the frequency you
need, and that depends on the DAC chip; i.e. the sigma-delta types need
higher clock freqs; for the multibit types it depends on the oversampling
ratio, so one should choose a frequency that suits all the chips that
one wants to try. So one will have to select one from the list found
in the DAC chip's datasheet..
Buffering digital circuit (trace) lines
For all resistors on internal digital signal lines, you can use 50Ω SMD
(1206). For exporting the clock, check the ringing on the cable with
a 10x probe and use the highest possible resistor to get a good signal.
You may have seen the values of these digital-signal-line resistors
vary from 50Ω up to 470Ω. Hagtech suggests also adding
a 1uH in series with these resistors. So, what's up? Basically, for
non timing-critical signals, you want to slow down the signal as much
as possible to reduce RF emission and supply current spikes. But if
you slow it down too much, it will not work anymore. So, it depends
on the trace / cable capacitance etc. If you use specified impedance
cable like coax or twisted pair, it makes sense to use the adapted
source termination impedance. Inductors can ring with your cable capacitance
so it is a dangerous game.
Master clock injection and SPDIF
Use a pulse transformer on the clock line, with a ferrite
core from the junk bin, and resistors adjusted for good signal shape...
about 120-ohm. Get a real pulse transformer, or using one from a junked
100BaseT network adapter (then send your clock and SPDIF on a RJ45
cable, these have controlled impedance, much better than RCA jacks,
good signal integrity, cost nothing, and twisted pair is good
for transformer-coupled balanced signals like this). Plus, one connector
means you won't connect the wrong one! It is better to use balanced
signals (less problem with ground), that's why twisted pair is better.
You may ask: RJ45 -- I thought true 75-ohm BNC was best? Also, "twisted
pair"? Coax is good at RF when the grounds of everything are connected
together, since the coax shield is connected to ground. However you
will use transformers to AVOID connecting the noisy CD player ground
to the DAC ground. If you use a coax in balanced mode, ie. connect
it to a transformer without grounding the shield, the shield makes
a *very* good antenna (since it is not grounded and carries half the
signal!) So, for balanced, use twisted pair; for single-ended signals
use coax. If you don't want to join your grounds, this means balanced,
thus twisted pair. For instance if you connect two computers, say 20
m apart with STP cable (shielded twisted pair), the shield connects
both grounds. Since computer cases are already grounded via the mains
ground, you make a huge ground loop. This makes tiny sparks when you
unplug! UTP (unshielded) cable does not connect the grounds : there
are transformers in the ethernet adapters at both ends. So, I have
never tested this, but I think it has potential: maybe RJ45, Cat6,
or even SATA cable! (if it's good at sending 3 GBits/s, it won't care
at 10 MHz...)
Further question: This
diyhifi.org message warned against twisted pair use. But the
diff. between RJ45 twisted pair and that used in the DAC in the diyhifi
post may be part of the misunderstanding. Answer: Yes. This
guy takes a single-ended signal from a coax. He then runs it through
a coaxial RCA connector of unspecified impedance, and through a twisted
pair of unspecified impedance. You can bet sticking a network analyzer
at the end of the cable would reveal a lot of reflected power and
nice resonances from those impedance mismatches! If you use single-ended
signals, then you must use coax all the way, with specified coax
connectors, and proper termination! If you use balanced signals,
then you must use twisted pair all the way, with specified connectors
too, and proper termination! For the twisted pair, if the cable is
shielded you can connect the shield to ground at one side. Not at
both sides: this would make your transformers useless.
Use a 74HC04 buffer in the transport to buffer all signals -- SPDIF
SPDIF vs. I2S
I would advise you to use SPDIF with no data (ie. silence) to send the
DAC master clock to the soundcard or to your transport. Why SPDIF, when
you could just use the clock on a cable? Sending the clock as SPDIF
- you can slave any soundcard to it without modifying the soundcard (for
this you could also use a wordclock but this needs a pro soundcard)
- it is independent of the dac's master clock frequency, so you don't
need to change the oscillator when your transport dies and you get a
However you need a PLL (w/VCXO) in the transport to reconstruct
a clock of the original transport's frequency (since the XTAL is removed).
The key is to put the CS8412/8414 in your DAC in slave mode (double
buffer). It receives the incoming SPDIF, but clocks the bits out with
the master clock you give to it. Obviously this will not work if the
clocks are not at the exact same frequency (it would skip or repeat samples).
But when the transport is slaved, no problem.
Output stage -- current-to-voltage (I/V)
Use a Jocko Homo-type as shown here (circuit
#8a). This one is really simple, you can add a DC servo by controlling
one of the current sources. It costs almost nothing and sounds way
better than op-amps.
A JFET folded cascode may also work. But the above sounds a lot better.
Idle current depends on your DAC... Make the IV a module so you can
Overall DAC design
Don't overdo it! Just remember -- continuous ground plane, sensible
layout, decoupling, short paths...For more info,
document from TentLabs.
Transport / CD Player Issues
The electro-mechanical unit that spins the disc and contains the laser
and optical electronics is known as the transport. If you have
a CD player, the DAC and the transport are in the same chassis. But,
as you probably already know, you can use the player as a just a transport
if you feed its data (via Digital Out) to an outboard DAC. Also, if you
have the skill, you can incorporate a new DAC into the same player's
chassis via I2S connection.
An often-neglected issue with Players and Transports is bit-accuracy.
A possible solution to this is to plug the SPDIF output of the DAC-clock-slaved
transport into a good PC soundcard (e.g. RME). Then feed the DAC from
the soundcard's SPDIF output. So in the end you use the transport
as a SPDIF encoder!
A universal shifting circuit for
interfacing decoder X with digital-to-converter Y.
The circuit shown above allows you to directly
combine any decoder/receiver with any converter without re-sampling,
oversampling or filtering ICs in between. You only have to
figure out how much shifting is required for left and right
channel DATA. Below you'll find some examples for Philips I2S
signals and for the format that's available from Sony decoders
Note: "Philips" DAC TDA1545A uses
EIAJ. So, do not confuse specific "Philips" components exclusively with
I2S or EIAJ.
Details of the circuit and cross-references:
serial DATA coming from decoder/receiver
BCK coming from decoder/receiver
LRCK coming from decoder/receiver
shifted right channel DATA going to converters
shifted right channel inverted DATA going to converters
shifted left channel DATA going to converters (DAC)
shifted left channel inverted DATA going to converters
BCK going to converters (DAC)
LRCK going to converters (DAC)
shifted right channel DATA coming from shifter
shifted left channel DATA coming from shifter
As used in I2S-to-EIAJ conversion for Philips TDA1545A:
DATA: Sig$6 is
pin 12 on 74HC74;
and then, Sig$10 is pin 9
on 74HC74 and
it connects to pin 3 on TDA1545A.
See comments in this
diyhifi.org thread -- they pretty much summarize and conclude many
of the issues discussed below.
After a few months of experimentation (and comparisons with DAC using
other topologies) I've decided to give up on NOS DACs, in general,
Monica 2, in particular. [Please note: while this is generally
a negative review of the Monica 2 DAC, it does not reflect my
purchase experience -- both before- and after-sale -- with the
proprietor(s) of diyparadise.com. That, in and of itself, was good.]
Here are discussion on various forums I've been engaged in on the issue
(these discussions are partly what led me to the decision above):
Counter-argument: They (it) are (is) so inexpensive -- how
can one go wrong at the price?
There are many inexpensive components that also sound great.
Take a late-model Toshiba CD/DVD player.
Add $20-30 worth of tweaking parts and -- bingo! -- you've got a unit
that sounds better than the NOS DAC noted above. [In all honesty, I
personally haven't heard any NOS DAC other than the Monica 2.
But I did some fairly extensive research beforehand -- i.e., read a
lot of reviews and comments -- and, hence, decided to purchase
the DAC. I.e., because it seemed to possibly be (TTBOMK) the best-rated
DAC (for the price). Hence, no reason to purchase others.]
Inexpensive parts and/or simple and easy-to-work with DIP DAC chip.
The old Philips TDA series are "large", uncomplicated designs.
They are easy to work with when compared to recent SOIC or SSOP package
Free technical "How-to" literature/plans on NOS DACs already avail.
on the web. E.g. dddac.de. So original
design/R&D/development costs are minimized.
Counter-argument: What about the generally-favorable reviews
this DAC has received?
My "answer": I've wondered about these reviews and am puzzled?
Couple of possibilities:
If free samples are given to reviewers, it could influence "politics".
Please note that this is mere speculation.
Perhaps the reviewer(s) really hasn't (haven't) heard anything better?
This sounds very unlikely.
The "warm", "tube-like" sound of certain NOS
DACs are simply an in-vogue trend-of-the-moment. People seem to always
be looking for something new, something "interesting." But
not something that's necessarily better or something they really need.
A NOS DAC is something quite different from recent standard designs.
And the sheer novelty of that fact, may to a large extent, drive the
current popularity of this design. Some may recall the brief popularity
single-bit MASH/Bitsream units enjoyed in the early 1990's. Much of
this popularity was the result of Matsushita's and Philips' marketing
BTW: I am still using the Monica 2 DAC
in a portable application (see below (this page) and this
link to a relative Head-Fi.org post). I'm in the process of portablizing
an oversampling DAC, using a modern processor; a breadboarded
version of which has already convinced me that its fidelity is many times
superior to that of the Monica 2.
So what is my main at-home DAC or integrated disc-playback
Below: Tentative cascode class-A JFET output stage for DAC,
such as Monica.
I will add to and update this schematic shortly. Also, to be added is
a volume control option, for pre-amp applications.
Below: Monica2 non-oversampling DAC, from diyparadise.com,
fitted into a Serpac Model
131 enclosure. Note: it appears as if there is no DAC -- a Philips
TDA1545A -- installed in the 8-pin DIP socket. The DAC is there, under the
DIP socket, but not visible in this overhead view. I'm experimenting
with stacking (paralleling) DACs, ala DDDAC.de,
and the shown arrangement allows one to "roll" DACs quickly
and easily for audio-fidelity evaluation. Also note the two-way rocker
switched (to be used for power) is not yet connected.
Below: Monica2, now enhanced (tweaked)
with four Wima 0.1µF bypass capacitors and oscillation-dampening
rope caulk covering the clock.
Below: Monica2 made portable via
Serpac's Model 131 enclosure. A 2.5mm mono jack (rear; not visible in this
view) is used for S/PDIF input.
Acknowledging the need for better I/V for Monica2, I'm not sure if a
vacuum-tube solution is ideal. This NOS DAC, in its native form, sounds "tubey" enough; Grounded
Grid -- diyparadise.com's preferred I/V stage for Monica 2 -- may,
IMO but admittedly w/o having listened to one, color sound too much (again,
based on my specific tastes/experiences with solid-state vs. tube (see
My current home system consists of -- among other components -- self-moded,
all-tube and hybrid pre-amps and amps from Dynaco and avahifi.com (the
FET-Valve line). Most of my listening is via headphones, using DIY (and
mostly portable) solid-state units. After years of listening/experimentation
with tube/solid-state/hybrid gear, I am now leaning towards pure solid-state
for best audio reproduction. It is harder to get rid of all jitter than
to filter it with some tubes "effectively" smearing the digital
grain -- and audio detail/resolution ...i.e. fidelity -- out of the signal.
It is true that digital sounds harsh when there is jitter. There are
several approaches to lowering jitter, but realize that It is extremely
difficult and/or expensive to get rid of all jitter.
As shown above, I have already "portablized" Monica2, using
a Serpac 131 enclosure and am currently experimenting with a decent-sounding,
op-amp-based solution, probably class-A biased. It should fit within
the same enclosure.
BTW: as noted above, I've been experimenting with stacking
TDA1545s, all w/o much success thus far. If anyone has done similar tests,
and has had better luck, please share
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This page was last updated:
Thursday, 2016-12-22 17:41