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TNT 1541A - DIY zero-oversampling DAC

The elder brother of our Convertus...

[Italian version]

Product: TNT 1541
Company: not for sale, TNT-Audio free DIY design
Approx. cost: 200$/Euro (just components)
Author: Giorgio Pozzoli - TNT Italy
Published: January, 2003

The TNT Convertus has been one of my favorite designs. It was probably one of the first DAC DIY projects published in Italy, and attracted a lot of attention (well, given the size of the market, obviously...).

A few changes have been made to the original Convertus design, but I never had the time to test another DAC: it would have required a complete overhaul of the project, including a new PSU.

Anyway, rumours about the outstanding quality of another DAC chip have loudly continued - the DAC in question, the TDA1541A, has a slight problem: it is no longer in production.

As I wanted to present an easy to build DAC, this was a condition I was not willing to accept, but after building it, I realized that the design includes a few interesting elements that can be applied to any other dac type. Even though the DAC is now rather difficult to find, especially in the selected versions, it is still available, and in the end the situation is no worse than that of a few well known NOS tubes....

So here is the DAC.

One more note. I will not go back in detail over all the circuitry I have already described in the Convertus, Convertus decima and Convertus Decima Digital articles. Please read them through before reading the present one.

The receiver circuit

For the receiver I used a CS8412. At the time Iím writing this article it is still present in the RS catalogue in a DIL package. The CS8414, the current generation, seems to work a little better, but is not available in DIL format: if you dare working with a surface mount chip, it can be a valid pin to pin substitute.

In order to be able to select any output configuration, the mode pins are controlled by a quadruple microswitch... this lets me select any possible output configuration with ease. Mode 3 is to be used (M0=1, M1=1, M2=0, M3=0), unless you do not wish to implement the digital decimation circuit. In that case, you must select mode 1 (I2S compatible, all signals output).


[MW DAC 1541A receiver schematic]

As you see, there is wide use of local regulation. A large number of TL431s are used on the different boards to decouple all power supplies. This is a precision, programmable, low cost voltage reference that is used (in this application) as shunt voltage regulator. In this configuration it has a dynamic output impedance of 0.1ohm up to 100KHz... Not a very original idea, anyway: I think there are at least two other DAC projects available on the net using the same feature in exactly the same position.

More interesting are the digital inputs. In fact, two inputs are shown on the schematic, but you can add as many as you want, provided that you check the power supply regulation is up to the job.

Each input connection is completely de-coupled using a high frequency transformer. I used a ferrite toroid from RS Components as a core, part number 212-0831, and just wound up both primary and secondary coils with five turns of insulated copper solid core wire (the one used for normal power transformers secondary coils). Unfortunately the original ferrite toroid is no longer available; try finding something similar of small dimensions.

The digital input socket can be either RCA or BNC. I always use BNC, which can specified for a precise 75ohm impedance, but this sometimes creates problems (or requires an adapter to be used...). In either case they must be female sockets, insulated from the panel.

The digital input signal comes out of the secondary of the transformer and is taken to a digital receiver, a DS34C86. This chip contains two differential line receivers and two drivers. The drivers have no use for us, at least for the moment. The receivers work according to the RS-422 electrical standard, which is very similar to S/PDIF electrical standard.

A very interesting aspect of the chip is that the receiver has a three-state output: this means that its output can be disabled, and in this situation presents a very high impedance. The technique is obviously used in buses, where more than one device can get control of the bus depending on the situation.

In our case it allows us to have several different receivers with output in parallel on the same digital line, provided that only one receiver output is enabled at any time. This is achieved by using a simple switch (SW101). If you plan to add more inputs, a rotary switch is recommended.

It is now clear why it is so easy to add other digital inputs: it is just necessary to add one more receiver chip and connect its output to the common output line and its selection input to the input selector.

Note how the receiver outputs are coupled to the 8412 input via a capacitor.

The output signal EN_SPDIF_IN is for future development (guess what for?).

The decimation circuit

I am not going to describe this module in detail, as it has been already described in the article about the Convertus Decima Digital.

If you want to avoid implementing it, you must simply select CS8412 mode 1 through the multiple microwsitch SW101/A-D and just connect FSYNC to LRCK, SCLK to BCK, SDATA to DATA.

As there is switch SW201 that allows to enable or disable decimation, I strongly suggest to test it. If you don't like the results, you can simply bypass the decimation module.

I must admit that while decimation improves the sound of the Convertus in a significant way, the TDA1541A treats high frequencies with such care and detail that decimation can be considered optional.

[MW DAC 1541A decimation schematic]

C204, C205, C206, C207 are bypass capacitors placed as near as possible to U201, U202, U203, U204 respectively, and connected between the power supply and ground pins of each IC.

The DAC circuit

Even the DAC circuit is not anything special. The DAC, configured as in the schematic, directly supports the I2S interface, so connection to the receiver is really straightforward, apart from the decimation circuit.

The DAC chip requires three voltages: +5V, -5V, -15V. As you see there are three separate voltage regulators on board. They should be placed as near as possible to the corresponding DAC chip pin.

In this area all components should be of the highest possible quality: polypropylene capacitors must be used, especially for the 14 capacitors to be connected to the DAC chip and the I/V conversion resistors (R310, R311).

Some explanation is perhaps required for DAC chip quality levels. From old Philips documentation ("Semiconductors for Digital Audio", Designer's Guide, August 1997) it appears that there are four versions of TDA1541A; from the datasheet of February 1991, it appears that the selection was based on the highest linearity attainable, but only 3 selections are defined. I have also included TDA1543 info in the table just as a reference (in referring to Convertus take into account that a quad DAC increases SNR by 6dB).

Type Description Data Format Typ. THD+N at 0dB * Typ. THD+N at
-60dB *
Typ. SNR Typ. Output Current Channel Separation Max diff. linearity error
      dB(%) dB(%) dB mA dB  
TDA1541A high-performance 16- bit DAC I 2 S, up to 8f s -95( 0.0018) -42( 0.79) 112 4.0 98 bit 1-16 EdL < 1 LSB
TDA1541A / R1 high-performance 16- bit DAC I 2 S, up to 8f s -95( 0.0018) -43( 0.7) 112 4.0 98 bit 1-16 EdL < 2 LSB
TDA1541A / S1 single crown 16- bit DAC I 2 S, up to 8f s -95( 0.001) -47( 0.4) 112 4.0 98 bit 1-7 EdL < 0.5 LSB
bit 8-15 EdL < 1 LSB
bit 16 EdL < 0.75 LSB
TDA1541A / S2 double crown 16- bit DAC I 2 S, up to 8f s -97( 0.002) -47( 0.4) 112 4.0 98 N/A
TDA1543(T) economy 16- bit DAC I 2 S, up to 4f -75( 0.018) -33( 2.2) 96 2.3 90 N/A

   * A-weighting

The best version of TDA1541A is obviously the double crown, which was used only on really high-end converters. And this happened not centuries ago: in 1998 it was still the best Philips converter available and in fact when the Marantz CD-7 cd-player was produced, it was chosen for that specific reason, according to Marantz documentation.

I have not been able to find any double crown, and I have used a single crown instead. I also used during first tests a normal version (no crown) just to spare the better ones. There is more refinement and detail in the single crown, but the difference is not so huge as when comparing the quad TDA1543 of Convertus with a single TDA1541A.

Given the huge costs the single crown has reached these days, I do not dare imagine the cost of a double crown, assuming it is possible to find one. Anyway, should you by chance have any spare double crown, please e-mail me...

[MW DAC 1541A DAC schematic]

The C310-323 capacitors must be very high quality polypropylene capacitors (in choosing them, take into account that normally film and metal sheet capacitors are better than metallized film ones) and should be placed as near to the corresponding pin as possible. A package as small (thin) as possible is therefore required.

Also C304,C306,C308 are very high quality polypropylene capacitors, and must be placed as near as possible to the corresponding power supply pin. C301, C302, C303 are OS-CON capacitors placed as near as possible to the previous ones.

For R310-R311, the I/V conversion resistors, I used Holcos. Given the fact there are only two, and their importance in defining the resulting sound is expectedly high, you could experiment with more expensive varieties.

The output circuit

TDA1543, according to Kusunoki, is the only DAC able to drive directly a load up to a normal line output level, and hence the only one not requiring any output stage with gain.

In the case of TDA1541A such a stage is mandatory. I have used a very simple solid state differential stage followed by an emitter follower, in order to achieve the required gain (around 22dB) with a very low output impedance.

There is however a peculiarity: note the position of R421/423. These introduce a small of amount of feedback. In general I agree that the less input/output feedback the better, but here the case is different: this is positive feedback, reducing the stability margin of the stage but increasing its speed. The difference in sound with these resistors in place is really notable: far higher push, impact and apparent speed - a really lively sound.

A few other designs use twin TDA1541As to increase dynamics: I do not really think it is necessary, unless you are a real lover of high dynamics. That said, in our solution, it requires only four more resistors...

The circuit is perfectly stable and has a good deal of local feedback, which helps in giving rather low distortion in spite of the simplicity of the circuit (0.05% at 20kHz at full output level from simulations: direct measurements are not so easy due to characteristics of the zero-oversampled signal...).

Here the highest quality components are again mandatory. In the prototype all resistors are Holcos, all capacitors are polypropylene or OS-CON electrolytic ones. Matching transistors for the same gain and possibly lowest noise cannot do any harm, but I didn't bother and no trouble has arisen.

There is also a low pass filter, composed simply by C407/408 and the collector resistors of the differential couple. This causes only a tiny drop at 20kHz, but also a reduced drop at higher frequencies: in one word, the output is (very) dirty. In compensation, the phase shift due to the output filter is very, very low.

The output is AC coupled via a very high quality polypropylene capacitor. Use the best film capacitor you can think of, and afford...

I definitely suggest avoiding paper in oil capacitors here, as the sound (for my taste) already has enough passion, sparkle and glitter without paper in oil caps!

[MW DAC 1541A DAC schematic]

The circuit has been built on the same board as the DAC, as near as possible to the I/V conversion resistors to reduce the risk of interference.

The power supplies

Given the high number of local regulators, I decided to limit the number of power supplies to two - one for digital components (receiver and decimation circuits), and one for the analogue side. If you wish, you can mount a larger number, dividing their usage between the different modules.

All power supplies are regulated using a simple capacitance multiplier circuit, using either an N- or a P-channel MOSFET depending on the polarity.

Do not underestimate the importance of the pre-regulator: without it, significant mains ripple filtered through to the output, so it is definitely mandatory.

I used a large number of power supply capacitors in parallel in both PSUs. To an extent, the more the better, but don't be excessive.

The diodes are ultrafast rectifiers, with a 25ns recovery time; this reduces spikes and switching disturbances. You could even use the SBYV27 series, which has a recovery time of 15nsec.

Given the distance from the audio circuits, I do not think it is the case of using special capacitors here: standard industrial quality should be enough.

Important note: all the MOSFETs must be mounted on small heat sinks.

[MW DAC 1541A DAC schematic]

[MW DAC 1541A DAC schematic]

The implementation

All the circuits are mounted on single sided printed circuit boards. As usual, the copper side is used as the ground plane and all the components are mounted on this side. Have a look at the assembly instructions for more information.

The internal connections, between the boards and from these to the RCA connectors) are in rather thin, solid-core, silver plated copper wire. When the connections are longer, I used insulated solid-core wire from some UTP Category 5 network cable.

Follow the the order of the schematics exactly in wiring the input pins. As already said the input jacks are isolated from the back panel, and also the output sockets are insulated. All ground leads meet only in one point in each board. Follow the instructions and schematics carefully.

The sound

I am not going to review the unit at all. I am not so arrogant as to pretend to be able to be objective (yes I know, I have some limitations...).

Even so, I understand it is important to give the reader an idea of the sound it produces. Anyway I warn you in advance to consider whatever follows a very biased and personal view...

The sound is lucid and passionate, with highs going up to heaven and further without any hint of hardness. The bass is deep, solid when necessary, controlled and at the same time warm and rounded - you are sometimes surprised to hear a bass line where normally one would only know a bass was playing.

From the imaging point of view, it is precise; the soundstage is naturally deep.

A comparison with Convertus? I used as a reference the latest Convertus version with decimation, and source follower output stage. Tests were made with decimation both active and inactive. All that I can say is the 1541A is in another league. The sound is far, far more lucid, smooth, to such an extent that this seems to be a tonal abberation compared to Convertus, with the highs taking priority. But then the bass comes in - neat, deep, and you feel it is correct this way.

When you come back to Convertus you really miss the smoothness and detail, the softness of 1541A. There is only one area in which Convertus seems to be slightly better, and it is punch: but this is probably down to the laid back and rough high frequencies in Convertus, which as said seems to highlight the bass.

Conclusion

This is not a simple project to build. As said, there are also problems in finding the TDA1541A. You can find the normal version for around 25 EUR, but the cost for the selected version is an issue.

Anyway, the result is so good that I really think you should at least give it a try. If you have a Convertus, you can just mount a board with the DAC and output stage, and give it a test. I am pretty sure that if you don't like the result, you'll be able to re-sell the chip for at least at the same price you bought it...

Kind of investing in chips? Well, let's say... cannot be much worse than stock, these days, no? And remember, guys: stock does not sound at all...

Parts List


Receiver

Code Value Rating Notes
C101 220uF 25V High Quality Electrolytic
C102 0.1uF 35V Polypropylene
C104 220uF 25V High Quality Electrolytic
C105 0.1uF 35V Polypropylene
C107 0.01uF 35V Polypropylene
C108 0.01uF 35V Polypropylene
C111 0.1uF 35V Polypropylene
C112 220uF 25V High Quality Electrolytic
C115 0.047uF 35V Polypropylene
L101 1000uF   RF Choke
L102 1000uF   RF Choke
L103 1000uF   RF Choke
R101 75 1/4W 1% Holco
R102 75 1/4W 1% Holco
R103 100 3.5W Resista
R104 100 3.5W Resista
R105 1k 1/2W 1% Holco
R106 1k 1/2W 1% Holco
R107 1k 1/2W 1% Holco
R108 1k 1/2W 1% Holco
R109 10k 1/4W 1% Holco
R110 10k 1/4W 1% Holco
R111 680 2W Resista
R112 10k 1/4W 1% Holco
R113 10k 1/4W 1% Holco
R114 10k 1/4W 1% Holco
R115 10k 1/4W 1% Holco
R116 1k 1/2W 1% Holco
R117 1k 1/2W 1% Holco
R118 1k 1/2W 1% Holco
SW101     1 way 3 positions
U101 DS34C86C   RS422 Line Driver/Receiver
U102 CS8412   Digital Receiver
U103 TL431   Precision Voltage Reference
U104 TL431   Precision Voltage Reference
U105 TL431   Precision Voltage Reference
  BNC female 75ohm for panel, insulated, qty:2

Decimator

Code Value Rating Notes
C201 220uF 25V High Quality Electrolytic
C202 0.1uF 35V Polypropylene
C204 0.1uF 35V Polypropylene
C205 0.1uF 35V Polypropylene
C206 0.1uF 35V Polypropylene
C207 0.1uF 35V Polypropylene
L201 1000uF   RF Choke
R201 330 1W Resista
R202 1k 1/2W 1% Holco
R203 1k 1/2W 1% Holco
R204 1k 1/2W 1% Holco
R205 1k 1/2W 1% Holco
SW201     1 way 2 positions
U201 74HC161   Binary Counter
U202 74HC161   Binary Counter
U203 74HC74   Dual D-type Flip Flop
U204 74HC00   Quad 2-input NAND gates
U205 TL431   Precision Voltage Reference

DAC

Code Value Rating Notes
C301 220uF 25V OS-CON Electrolytic
C302 220uF 25V OS-CON Electrolytic
C303 220uF 25V OS-CON Electrolytic
C304 0.1uF 35V Polypropylene
C305 0.1uF 35V Polypropylene
C306 0.1uF 35V Polypropylene
C310 0.1uF 35V Polypropylene
C311 0.1uF 35V Polypropylene
C312 0.1uF 35V Polypropylene
C313 0.1uF 35V Polypropylene
C314 0.1uF 35V Polypropylene
C315 0.1uF 35V Polypropylene
C316 0.1uF 35V Polypropylene
C317 0.1uF 35V Polypropylene
C318 0.1uF 35V Polypropylene
C319 0.1uF 35V Polypropylene
C320 0.1uF 35V Polypropylene
C321 0.1uF 35V Polypropylene
C322 0.1uF 35V Polypropylene
C323 0.1uF 35V Polypropylene
C324 470pF 35V Polypropylene
L301 1000uF 100mA RF Choke
L302 1000uF 100mA RF Choke
L303 1000uF 100mA RF Choke
R301 100 3.5W Resista
R302 180 3.5W Resista
R303 180 3.5W Resista
R304 1k 1/2W 1% Holco
R305 5k 1/2W 1% Holco
R306 1k 1/2W 1% Holco
R307 1k 1/2W 1% Holco
R308 1k 1/2W 1% Holco
R309 1k 1/2W 1% Holco
R310 33 1/2W 1% Holco
R311 33 1/2W 1% Holco
U301 TL431   Precision Voltage Reference
U302 TL431   Precision Voltage Reference
U303 TL431   Precision Voltage Reference
U304 TDA1541A   High Performance DAC

Gain Stage

Code Value Rating Notes
C401 220uF 25V Electrolytic OS-CON
C402 0.1uF 35V Polypropylene
C403 220uF 25V Electrolytic OS-CON
C404 0.1uF 35V Polypropylene
C405 10uF 100V Polypropylene very high quality
C406 10uF 100V Polypropylene very high quality
C407 100pF 35V Polypropylene very high quality
C406 100pF 35V Polypropylene very high quality
L401 1000uF   RF Choke
Q401 BC109B   NPN transistor
Q402 BC109B   NPN transistor
Q403 BC109B   NPN transistor
Q404 BC109B   NPN transistor
Q405 BC109B   NPN transistor
Q406 BC109B   NPN transistor
R401 330 2W Resista
R402 330 2W Resista
R403 5k 1/2W 1% Holco
R404 1k 1/2W 1% Holco
R405 5k 1/2W 1% Holco
R406 1k 1/2W 1% Holco
R407 6.8k 1/2W 1% Holco
R408 6.8k 1/2W 1% Holco
R409 270 1/2W 1% Holco
R410 270 1/2W 1% Holco
R411 4.3k 1/2W 1% Holco
R412 6.8k 1/2W 1% Holco
R413 6.8k 1/2W 1% Holco
R414 270 1/2W 1% Holco
R415 270 1/2W 1% Holco
R416 4.3k 1/2W 1% Holco
R417 10k 1/2W 1% Holco
R418 10k 1/2W 1% Holco
R419 220k 1/2W 1% Holco
R420 220k 1/2W 1% Holco
R421 10k 1/2W 1% Holco
R422 150 1/2W 1% Holco
R423 10k 1/2W 1% Holco
R424 150 1/2W 1% Holco
U401 TL431   Precision Voltage Reference
U402 TL431   Precision Voltage Reference
  RCA Pins gold plated for panel, insulated, qty:2

PSU A

Code Value Rating Notes
C501 1nF 35V Ceramic
C502 0.47uF 35V Polypropylene
C503 4700uF 35V Electrolytic
C504 4700uF 35V Electrolytic
C505 4700uF 35V Electrolytic
C506 4700uF 35V Electrolytic
C507 4700uF 25V Electrolytic
C508 4700uF 25V Electrolytic
C509 4700uF 35V Electrolytic
C510 0.47uF 35V Polypropylene
C511 0.47uF 35V Polypropylene
C512 1nF 35V Ceramic
C521 1nF 35V Ceramic
C522 0.47uF 35V Polypropylene
C523 4700uF 35V Electrolytic
C524 4700uF 35V Electrolytic
C525 4700uF 35V Electrolytic
C526 4700uF 35V Electrolytic
C527 4700uF 25V Electrolytic
C528 4700uF 25V Electrolytic
C529 4700uF 35V Electrolytic
C530 0.47uF 35V Polypropylene
C531 0.47uF 35V Polypropylene
C532 1nF 35V Ceramic
D501 BYV27-100   Superfast Diode
D502 BYV27-100   Superfast Diode
D503 BYV27-100   Superfast Diode
D504 BYV27-100   Superfast Diode
M501 IRF630   N-channel MOSFET
M521 IRF9630   P-channel MOSFET
R501 33k 1/2W 1% Holco
R502 4.7k 1/2W 1% Holco
R521 33k 1/2W 1% Holco
R522 4.7k 1/2W 1% Holco
TR501 PSU Transformer 30VA Prim:220V, Sec:25+25V

PSU D

Code Value Rating Notes
C601 1nF 35V Ceramic
C602 0.47uF 35V Polypropylene
C603 4700uF 25V Electrolytic
C604 4700uF 25V Electrolytic
C605 4700uF 25V Electrolytic
C606 4700uF 25V Electrolytic
C607 4700uF 25V Electrolytic
C608 4700uF 25V Electrolytic
C609 4700uF 25V Electrolytic
C610 0.47uF 35V Polypropylene
C611 0.47uF 35V Polypropylene
C612 1nF 35V Ceramic
D601 BYV27-100   Superfast Diode
D602 BYV27-100   Superfast Diode
D603 BYV27-100   Superfast Diode
D604 BYV27-100   Superfast Diode
M601 IRF630   N-channel MOSFET
R601 33k 1/2W 1% Holco
R602 1k 1/2W 1% Holco
R603 10k 1/4W 1% Holco
VR601 2.2k 1/2W Trimmer potentiometer
TR601 PSU Transformer 30VA Prim:220V, Sec:15V
  Mains Filter with IEC Socket with power switch and fuse

© Copyright 2003 Giorgio Pozzoli - http://www.tnt-audio.com

Supervisor: Tom Browne

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