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Audiophile-sounding DAC for almost no money

0DAC - delivers engaging, immersive sound with a pricetag at least two orders of magnitude from commercial audiophile DACs

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Several years ago, an engineer using the handle 'NwAvGuy' put together a (partly open) DAC design for the masses. He called it 'ODAC' where 'O' meant 'objective' because it was designed according to objective criteria (the usual measurements, in particular THD+N). My design is the complement of that, designed entirely for listening enjoyment with little regard for measurements. The broader aim is to make audiophile sound quality a commodity so everyone gets to get off on the music. Wave a cheery 'goodbye' to fatiguing digital sound! The BOM cost is very low (sub $5) and the circuit board area fairly modest (~25cm^2) - to keep both down it has only I2S input. The output stage is a resistor-loaded current source with around 1VRMS swing.

The name of this design is 0DAC, pronounced 'lingDAC' (zero = ling in Mandarin).

To get a great sounding DAC its important to choose a DAC chip with lots of potential and ignore most of the datasheet specs. 'Having potential' means a simple as possible chip, without the usual bells and whistles which have been added to DAC chips in the past couple of decades. Going back to the 1990s most DAC chips were just that - DACs and had neither on-board digital nor analog filters using opamps. Both of these kinds of filters have the potential to screw up the sound so you do not want them on the chip where its impossible to bypass them.

Philips started a trend with their 'Bitstream' DACs of including on-chip filters and on-chip opamps (SAA7320). But for a while in the 80s and 90s they produced some excellent chips without all that extra fluff - for example TDA1541, TDA1543, TDA1545 and TDA1387. Burr Brown similarly had their PCM-series DACs, culminating in the PCM1704 where by most listeners' ears they'd already started to lose the plot, being seduced by numbers - PCM63 seems to be the pinnacle of their art. Analog Devices have had great designs too, the most recent being AD1865.

I've so far mentioned only audio-targeted DACs but there are others which are pure DAC chips - I've samples of an ancient one from ADI, the AD768. It's an example of a DAC targeted at the communications market, a field which in many ways holds more promise for great sounding D/A than does audio nowadays. TI/BB and Intersil also have offerings in this arena. Of the audio chips I've mentioned, all are out of current production so if you want to have a viable manufactured product you have to look beyond audio to find your chip. Schiit did exactly that in their multibit DAC designs, going to the medical/industrial segment. Metrum too started off with an industrial DAC chip from TI/BB but since have gone over to custom resistor arrays buried inside modules. Hobbyists though aren't constrained by production volumes and so have rarely had it so good with the wide choice of recycled devices on the secondary market.

I must confess I have a real soft spot for Philips (now NXP or even Nexperia) - partly because my first few CD players were Philips (they and Sony together being the innovators of the CD format) but also because their IC designs rock, and not just in DACs. Take in the realm of amps their in-car offerings (TDA8566 for example - I have a design for an amp using it here : Hi end chipamp). TDA1541 has been done to death by DIYers (Thorsten Loesch and Pedja Rogic have commercial product too) - how to choose from its many tweaks? TDA1545 has some DIY designs extant (Peufeu's 'Extremist' is here - TDA1545 DAC) - having listened to my own designs I've abandoned this chip on SQ grounds. TDA1543 has literally dozens of DIY projects and commercial boards/boxes (Lite DAC-AH is my favourite). Strangely TDA1387 has been largely ignored to date - there's a few units on Taobao : L1387 DAC but nothing in the audiophile mainstream. In regard to the tech behind the chips, the TDA1387 takes some beating as it continuously recalibrates itself on the fly so doesn't require the usual extreme resistor matching achieved in other manufacturer's offerings by laser trimming. Its also absurdly cheap as the chips are recycled from old 'Soundblaster' PC cards.

Next up - NOS is my choice here for lowest BOM cost. NOS is a TLA for 'No oversampling' and the craze of NOS began with a Japanese guy by the name of Kusunoki (go here for his web presence, I'm not thereby endorsing his technical justifications - http://www.sakurasystems.com/articles/Non-oversampling-DAC.html) who wanted to bypass the digital filter in a DAC. In the history of Philips' CD players, oversampling has always been used, initially because when CD came out they only had production ready a 14bit DAC (TDA1540) and CDs have 16bits worth on them. So oversampling (4X) was used as a way to gain a couple of bits from the DAC through averaging out 4 faster samples to create one slower one. When the TDA1541 was...

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AIF1fr.jpg

LTspice frequency response plot of AIF1. The rise at the top of the audio band is to counteract the 'NOS droop'.

JPEG Image - 31.11 kB - 10/02/2017 at 07:06

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AIF1.jpg

Anti-imaging and NOS droop correction filter. The inductor values are typicals for SLF7045T-330 and -470 (33uH and 47uH). Capacitors are X7R 50V made up of paralleled 1uF, 470nF and 100nF. Current sources and voltage sources to be expanded in later schematics. DAC input to the right, audio output (1VRMS) from R46.

JPEG Image - 34.14 kB - 10/02/2017 at 07:02

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  • 2 × SLF7045T-470 Inductors, Chokes, Coils and Magnetics / Fixed Inductors, Chokes and Coils
  • 2 × SLF7045T-330 Inductors, Chokes, Coils and Magnetics / Fixed Inductors, Chokes and Coils
  • 14 × C3216X7R2A104K160AA Capacitors / Ceramic TDK 1206 100nF 10%
  • 2 × 220uF 16V Capacitor electrolytic
  • 4 × TDA1387T Data Converters / Digital to Analog Converters (DACs)

View all 11 components

  • Beta prototype assembled

    Richard Dudley11/15/2017 at 05:14 0 comments

    Listening report to follow....

  • Filter I/V boards

    Richard Dudley11/06/2017 at 08:47 0 comments

    Building up the filters takes time due to the painstaking process of ensuring the reactive components (inductors and capacitors) are close enough in tolerance to get to within spitting distance of the target filter response.

    Inductors I measure to 1uH and bin them. The mode for the two values seems so far (with about 40 measured of each to date) 30uH and 43uH, slightly under 10% below the nominal values. Capacitors I trim by adding in paralleled units after measuring them. After soldering its essential to allow enough time for cooling and relaxation of thermal stresses before measuring

    - I allow 10mins minimum for this, but if I see the capacitance still decreasing I'll allow more time. What I call the 'tuning networks' (RC in parallel with inductors) are soldered under the board, using 0805 and 0603s. The main shunt capacitors are generally 1206 and visible in stacks in the above shot. Occasionally the 1206s I'll trim with an 0805 on top, aiming to get within 2% of the value on the schematic.

    At the front of the boards in the picture are the current sources used to bias the I/V transistors (not yet fitted in this pic).


  • Upgrading USB power

    Richard Dudley10/16/2017 at 14:37 0 comments

    I tried quite hard to get this DAC operating on the 'native' USB power from my PC, but it proved not possible to attenuate the low frequency noise on the supply sufficiently. So I've gone for a step up voltage converter to boost the 5V rail to nearly 10V and then regulate it down again.

    An LM2662 performs the step-up task admirably - chosen because its a switched capacitor kind of boost converter, rather than the more normal inductive boost circuit. Inductors tend to be rather leaky and I didn't want to radiate any ultrasonic magnetic field in the tight confines of this DAC's enclosure. Particularly as I have 4 inductors in the anti-imaging filter which are likely to pick up magnetic radiation to some degree.

    The other bits on the board are a TL431 (6.6V) and two NPN pass transistors to provide supplies to the DAC array and I/V.

  • Working on a DAC demonstrator

    Richard Dudley10/06/2017 at 00:58 0 comments

    Since the lingDAC has only I2S input I need a quick and dirty way to provide the rest of the infrastructure to get something to listen to. Taobao comes to the rescue as on there are a few DACs based on TDA1387 which have digital front-ends on them and enough room inside to hide a filter/IV board. Here is the cheapest one : 4 * TDA1387 USB DAC. I have bought my first two of these with cases but the seller also sells just the PCB as a kit. My gf persuaded him to solder on the CM108 as that's a little tricky to do by hand, so we have more on order at the princely sum of 44rmb for just the PCB and kit of parts.

    Firstly its necessary to make some modifications to the board to accept the lingDAC filter/IV stage. Here's a picture of the mods beginning -

    The parts just to the north of the RCA outs are passive IV/filter components which the lingDAC PCB will replace. So they must come out - here I've pulled out caps (2n2 and 220p) and the output coupling caps (CR, CL) which were electrolytics. The 1ks are the I/V resistors, we need to change those to 680ohm.

    The next set of mods are to the power supply as the stock board has inadequate filtering on the 5V USB rail.

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alexandre zanol wrote 09/09/2017 at 10:26 point

Absolutely agree that Philips ICs rock! I have a soft spot for the TDA1543 for all the years of musical enjoyment I had with it. Now playing with the TDA1387 and I am thrilled with the potential of the chip. It is curious indeed that it didn´t find its way into CD players and stand alone dacs - I would say a marketing mistake by Philips, because the little guy is not limited (in performance) by its low power consumption.

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Richard Dudley wrote 09/09/2017 at 10:29 point

I have followed a similar path to you Alex it seems, I did love the TDA1543 for quite a while with its soft, analog style of sound. But after I'd tamed the CMOS chick I couldn't go back....

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alexandre zanol wrote 09/09/2017 at 11:01 point

Richard, the description is spot on. Cassette tape sound.

Onward with the '1387, I can´t wait to get more ambience, detail and some sweet highs with the analog filter :)

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Steven Clark wrote 09/08/2017 at 05:50 point

I can find simple parallel resistor ladder based DACs on digikey if I look.  Do they all have op-amps, or does something else disqualify them?

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Richard Dudley wrote 09/08/2017 at 06:01 point

Have a part number? I'll take a look then and let you know.
 In general ladder based DACs are too glitchy for audio, but some might be able to be made to work.

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Richard Dudley wrote 09/02/2017 at 13:46 point

Yes indeed, they can get the idea of what most systems with speakers are aiming for but mainly fail to achieve as their amps aren't transparent enough. Designing a speaker amp to deliver the goods this beast puts out is the next challenge....

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john.kenny wrote 09/02/2017 at 13:50 point

Right, so it's the project that will just keep giving with further downstream elements when people are ready - excellent!

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