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Crystal DAC For the very best performance from 24-bit/96kHz recordings – uses the Crystal CS4398 DAC and a discrete transistor output stage This new DAC board can be substituted for the original board used in our Hifi Stereo DAC project (Sept-Nov 09) without any major changes, effectively replacing the Burr-Brown DSD1796 DAC IC with the high-end Cirrus Crystal CS4398. Its harmonic and intermodulation distortion figures are significantly lower than before although some people will have difficulty discerning the differences. Try it and find out for yourself. T HE INSPIRATION for this project came from our review of the Marantz CD6003 CD player, which appeared in the June 2011 issue. At the time, we made some measurements using our Audio Precision System One and discovered that it not only had a very low harmonic distortion figure 24  Silicon Chip for a CD player but it was practically flat across the audible frequency band (20Hz-20kHz). We figured that this was partly due to its Crystal (Cirrus Logic) CS4398 DAC (digital-to-analog converter) IC. This is mounted on a large PCB, amongst a forest of discrete and passive compo- nents. So we thought, hmmm . . . could we do something similar for our DAC design? We suspected they were also doing some fancy digital processing using a DSP (digital signal processor) to get that level of performance but that the CS4398 DAC must also be pretty good for such an excellent result. siliconchip.com.au By NICHOLAS VINEN It turns out we were right on both counts. The CS4398 is very good but Marantz seem to be doing some digital interpolation (possibly increasing the sampling rate to 96kHz or 192kHz) to keep the distortion so low. While our new DAC board does not have the benefit of digital interpolation, it is clearly superior to the previous design, especially when processing 24-bit/96kHz program material. If you have already built a Stereo DAC kit and would like to try out this new board, it’s pretty easy. You just build the new PCB and swap it for the old one; it’s the same size and the critical parts are in the same locations. You then reprogram or swap the microcontroller on the input board and Bob’s your uncle. Like the Marantz, we designed the filtering hardware using all discrete components (ie, bipolar transistors and passives). There was some controversy on the internet (unheard of!) over our choice of op amps in the original DAC design (SILICON CHIP, September, October & November 2009). This time we have avoided using those “evil” little black boxes, which should make the extreme audiophile cognoscenti happy (impossible!). The resulting circuit has a lot more components than it would if we had used op amps but they are all cheap siliconchip.com.au and commonly available. The resulting wide bandwidth compared to an op amp means that the output filtering works very well. Performance We tested both the original and new DAC designs extensively, using both our Audio Precision System One and the newer Audio Precision APx525 with digital processing. We also performed numerous listening tests, including blind A/B tests. The first result that became clear from all this testing is that the original design really is very good. Its distortion and noise are low (including intermodulation distortion), its linearity is very good and it generally sounds excellent. However, the new DAC design measures even better, with lower distortion (especially at high frequencies), even lower intermodulation distortion and astounding linearity down to -100dB. Fig.1 shows a comparison of the harmonic distortion between both channels of the original and the new DAC design. These tests were performed on the same unit with just the DAC boards swapped, so they give an apples-to-apples comparison. Note that noise has been digitally filtered out of this measurement completely, for a couple of reasons. First, both DACs have quite a bit of high- frequency switching noise in their output (but a lot less than some DVD and Blu-ray players we’ve tested!) and this can mask the distortion if we set the bandwidth wide enough to capture harmonics of high audio frequencies. Second, the 20Hz-20kHz residual noise of both the original and new boards is similar and this too means that a THD+N comparison would tend to understate the reduction in harmonic distortion obtained with the newer design. As you can see, harmonic distortion with the CS4398 is substantially lower than the original design, both at high frequencies (above 3kHz) and low frequencies (below 100Hz). The differences between channels are due to asymmetries in the PCB layout as well as mismatches between the two channels within the DAC ICs themselves (eg, due to resistor ladder tolerances). Fig.2 shows the channel separation for both units. The lines labelled “left” show how much signal from the right channel couples into the left and the lines labelled “right” show the opposite. In both cases, channel separation is very good and is generally better than -100dB across the audio spectrum. The older design is slightly better in this respect, although the difference is largely academic. Fig.3 compares the linearity of both DACs. This plot shows the deviation February 2012  25 Performance Graphs Harmonic Distortion vs Frequency, 90kHz BW 05/12/11 12:31:22 Crosstalk vs Frequency, 90kHz BW 05/12/11 14:45:09 0 0.01 Left Right Left Right CS4398 DSD1796 0.005 CS4398 DSD1796 -20 Crosstalk (dB) Harmonic Distortion (%) -40 0.002 0.001 -60 -80 0.0005 -100 0.0002 0.0001 20 -120 50 100 200 500 1k Frequency (Hertz) 2k 5k 10k 20k Fig.1: harmonic distortion (ignoring noise) versus freq­ uency for the original (DSD1796-based) and new (Crystal CS4398-based) DACs. The newer design has lower distortion overall but especially above 2kHz. The channels differ slightly due to layout asymmetries and differences in the ICs themselves. The spikes at 1.2kHz and 9kHz are due to aliasing between the test and sampling frequencies. between the expected and actual output level for a sinewave at a range of levels between -60dB and -100dB. Both DACs perform extremely well in this test but the CS4398 is especially good, with a maximum deviation of no more than 0.25dB at -100dB! Its deviation is essentially zero above -84dB while the DSD1796 still shows some deviation up to -70dB. Note that all of the above test results were obtained with the Audio Precision APx525 (which can test in the analog or digital domain) using 24-bit 96kHz signals fed into a TOSLINK input of the Stereo DAC project. Fig.4 shows the FFT frequency spectra for the updated DAC with one channel in magenta and the other in khaki. This was computed with a one million sample window, an equiripple algorithm and 8x averaging. The test signal is at 1kHz and the bandwidth is 90kHz. The harmonics of the test signal are clearly visible at 2kHz, 3kHz, etc. Also visible is some 50Hz and 100Hz mains hum at around -120dB, as well as various intermodulation products of this hum with the fundamental and its harmonics. As we said earlier, both DACs are very good but the updated design generally has better figures. We also ran the SMTPE intermodulation distortion test on both. This involves sending a 26  Silicon Chip -140 20 50 100 200 500 1k Frequency (Hertz) 2k 5k 10k 20k Fig.2: a comparison of channel separation (ie, crosstalk) for the original and new DAC boards. The original is slightly superior but both are very good, with less than -93dB crosstalk at any frequency and separation of at least 100dB up to 1kHz. As is typical, there’s more coupling in one direction (for the new design, left channel to right channel) than the other, again mainly due to asymmetry. 4:1 mix of 7kHz/400Hz sinewaves to the test device. These frequencies are then filtered from its output (400Hz with a high-pass filter and 7kHz with a notch filter) and the remaining harmonics measured. These will generally be the sum and difference frequencies of 6.6kHz and 7.4kHz but possibly other harmonics too. The old design gives an intermodulation distortion level of around 0.0018% (-95dB) while the new design gives 0.0006% (-105dB); a significant improvement. Listening tests The results of our listening tests were somewhat controversial. We used our 20W Stereo Class A Amplifier (May-September 2007) and the M6 Bass Reflex Loudspeakers (November 2006), while the 3-Input Selector presented last month was used to switch between the original and updated Stereo DAC prototypes. The original prototype was set to a volume of -0.5dB and the levels matched almost perfectly, giving seamless switching between the two. The two Stereo DACs themselves were fed with digital audio from a Blu-ray player with separate TOSLINK and S/PDIF outputs. Some staff members could not tell the difference in sound quality between the two DACs while others claimed to be able to hear a distinct difference between the two on certain passages, although the difference was not obvious on other passages. With complex choral music, two of the “guinea pigs” were able to pick the updated DAC as sounding “brighter”. On other types of music, a difference could be discerned but we could not reliably pick which DAC we were listening to. You’ll have to make your own mind up about whether the new design gives an audible improvement. However, we can be certain that this upgraded DAC design gives far superior performance compared to virtually any CD, SACD, DVD or Blu-ray player on the market. And for those people who think that Blu-ray players are generally superior in terms of sound quality, our limited tests demonstrated that this is not necessarily true. Cheap Blu-ray players are just that – cheap! Circuit description Fig.5 shows the circuit diagram for the new board. IC1 is the CS4398 DAC chip and this is wired to 16-pin IDC socket CON1. Its configuration is identical to that of the original DAC board, carrying the 3.3V supply from the control board as well as audio data (pins 4, 6, 8 & 10) and serial control siliconchip.com.au Linearity 05/12/11 14:01:58 Frequency Domain Plot +1.0 +40 Left Right +0.8 CS4398 DSD1796 +20 0 +0.4 -20 +0.2 -40 Level (dBr) Output Deviation (dB) +0.6 0 -0.2 -60 -80 -0.4 -100 -0.6 -120 -0.8 -140 -1.0 -100 -160 -90 -80 Nominal Output Level (dBr) -70 -60 Fig.3: a comparison of the linearity of the original and updated DAC boards. Delta-Sigma DACs typically have good linearity and in fact both are excellent. However, the updated board (with the CS4398) is the best of the two with an astounding deviation of less than one quarter of a decibel at levels down to -100dB! (The dynamic range of CD-quality audio is just 96dB). data (pins 7, 9, 11 & 13). There are also two mute feedback lines (pins 15 & 16), allowing the micro to sense output silence. IC1 has a dual 3.3V and 5V power supply with multiple supply pins for each internal section. Both rails have 100µF bulk bypass capacitors. Each supply pin also has a 100nF bypass capacitor for lower supply impedance at higher frequencies (>100kHz). VLS (pin 27) is supplied 3.3V to suit the audio serial data levels while VLC (pin 14) is at 5V to match the microcontroller’s I/O levels. To avoid switching noise feeding back into the 5V rail, which also powers analog circuitry, a 100Ω stopper resistor is included. VD (pin 7) is the supply pin for the DAC’s digital core (digital filtering and so on). This runs off 3.3V while the internal analog circuitry (op amps, etc) runs off a 5V rail connected to VA (pin 22). This 5V rail is also fed separately to VREF (pin 17) for the DAC reference voltage. Capacitors at FILT+ (pin 15) and VQ (pin 26) smooth IC1’s internal reference voltages. VQ is the quiescent output voltage and generally sits at half supply (ie, 2.5V). We aren’t using the DSD (Direct Stream Digital) input pins on the IC so they are tied to ground. The microcontroller’s serial I/O pins connect to header CON1 via LK1-LK4. siliconchip.com.au .03 .05 .04 .1 .2 .3 .4 .5 1 Frequency (kHz) 2 3 4 5 10 Fig.4: a frequency domain plot (ie, spectrum analysis) of the output of the updated DAC for a 1kHz sinewave. Eight FFTs were averaged to reduce noise. The harmonics are clearly visible at multiples of the fundamental (2kHz, 3kHz, etc) as well as mains hum at 100Hz. You can also see the various intermodulation products of the fundamental and its harmonics with 100Hz. These are closely-spaced pads on the bottom of the PCB which can be bridged with solder. The CS4398 can operate without a microcontroller and to do so, pins 9-12 are connected to either ground or VLC (+5V). This arrangement allows those pins to be connected to configure the DAC correctly, even in the absence of a microcontroller. However, if this is done, many features of this design do not operate properly, such as volume control, automatic input scanning and muting. As a result, we suggest that constructors simply bridge LK1-LK4 and reprogram the micro with the new software. All the features of the original design will then work normally. Analog filtering The DAC IC we used previously (Burr Brown DSD1796) has differential current outputs while the CS4398 has differential voltage outputs. That means we no longer need current-tovoltage converters; they are internal to IC1. However, we still need to filter the outputs to remove the DAC switching noise and convert the differential (balanced) signals to unbalanced, to suit the inputs of a typical amplifier. We have used the recommended filter, a 2-pole Butterworth low-pass arrangement, consisting of six resistors and five capacitors for each channel. These are shown just to the right of IC1. The operation of this filter is quite complicated since the two RC filters for each channel interact with each other. Let’s look at the left channel; the right channel circuit is identical. The noninverted output from IC1 comes from pin 23 (AOUTA+) and the inverted signal from pin 24 (AOUTA-). The waveforms from each pin are (theoretically) identical but opposite in polarity, ie, one swings up when the other swings down and vice versa. Both signals are attenuated, with a gain of around 0.45, by a pair of resistive dividers. While the division ratios are very similar, the actual resistor values differ: 620Ω/510Ω for the noninverted signal and 1.6kΩ/1.3kΩ for the inverted signal. These resistors also form singlepole, low-pass filters in combination with the 18nF (non-inverted signal) and 6.8nF (inverted signal) capacitors. The attenuating resistors are effectively in parallel with each other, for a -3dB point of around 32kHz in both cases. These are then followed by another set of RC low-pass filters – 270Ω/4.7nF for the non-inverted signal and 680Ω/1.8nF for the inverted signal. In isolation, these have corner frequencies of around 130kHz. Note that the bottom ends of the February 2012  27 20 DIGITAL INPUT/OUTPUT +3.3V 1 +5V 3 100 F 100nF 100nF 7 22 VD 27 100 F 620 VA VLS VLC 100 14 510 100nF 100nF 18nF 100 F 4 6 6 4 8 3 10 5 5 13 Vref MCLK SCLK LK1 9 9 LK2 10 7 LK3 11 13 LK4 12 15 25 16 18 2 1 12 2 14 28 1.6k 100nF SDIN 680 100 F LRCLK 6.8nF 1.3k 1.8nF RST IC1 CS4398 100k 11 17 CDIN AOUTA+ AOUTA– 23 +2.5V 24 +2.5V 20 +2.5V 19 +2.5V CCLK CDOUT AOUTB+ AD0/CS AOUTB– AMUTEC BMUTEC FILT+ DSD_B DSD_SCLK VQ DSD_A REF GND 15 26 16 100nF CON1 IDC-16 DGND AGND 21 8 10 F 100 F 10k 620 510 18nF 100 F +15V D5 1N4004 K POWER IN CON2 1 220 +15V 100 F 2 3 SC 2012 100 F 680 6.8nF REG1 78L05 IN 1.6k A +5V OUT GND 1.3k 1.8nF 100 F 0V 10k –15V –15V STEREO CRYSTAL DIGITAL-TO-ANALOG CONVERTER Fig.5: the circuit is based on a Cirrus Logic (Crystal) CS4398 stereo DAC chip (IC1). This has differential outputs (pins 23 & 24 and 20 & 19) and these drive discrete audio output stages based on transistors Q1-Q12 in the left channel and Q15-Q26 in the right channel. Q14, Q28 & dual N-channel Mosfets Q29a-b & Q30a-b mute the outputs when there is no signal from the DAC. Power comes from an external ±15V supply, with REG1 providing a +5V rail for IC1. 28  Silicon Chip siliconchip.com.au 100 K D1 1N4004 220 A Q5 BC559 270 E 47 F 2.2k B 2.2k B B 100 47 F E C C E 47 F Q7 BC559 –15V 100 C B E 2.2k 47 F 10k E 220 C 10k Q2 Q1 BC559 BC559 B E C C 100 4.7nF Q6 BC559 +15V VR1 5k B B C E Q10 BC549 Q11 BC549 TP1 10 TP2 47 F +2.5V 100pF 1nF 10 C Q3 BC549 B B E D2 1N4004 K C E 68 100 Q8 BC549 B C 10nF Q12 BC559 D +5V B C 2.2k Q14 BC559 E Q9 BC549 E G 100pF B 100k 100 ZD1 18V D3 1N4004 220 A Q19 BC559 270 E 47 F 2.2k B 2.2k B B 47 F 10k E C C Q21 BC559 –15V 100 C B E VR2 5k B B C E Q24 BC549 Q25 BC549 TP3 10 TP4 47 F +2.5V 10 C K B E B E 68 100 Q22 BC549 B C 10nF Q26 BC559 D +5V 2.2k 2.2k C Q28 BC559 E Q23 BC549 E G 100pF B S S G Q30b IRF7905 C 100k 100 A 100k D 100 ZD3 18V K –15V BC549, BC559 D1–D5: 1N4004 A siliconchip.com.au Q30a IRF7905 C B 68 100k E B E Q18 BC549 RIGHT OUT CON4 100 100pF 1nF C K ZD2 18V 47 F 2.2k 100 47 F E E K +15V 220 C 10k Q16 Q15 BC559 BC559 B E C C 100 4.7nF Q20 BC559 A A K 100 D4 1N4004 Q29a IRF7905 D 100k –15V Q17 BC549 S S G C 100 A K Q29b IRF7905 C 2.2k 68 100k E B E Q4 BC549 LEFT OUT CON3 100 ZD1–ZD4 A K A K ZD4 18V 78L05 B E A COM C IN OUT February 2012  29 Silicon Chip Binders REAL VALUE AT $14.95 PLUS P & P Features & Specifications Output Level .................................................................................. 1.9V RMS Signal-To-Noise Ratio ........................................................................-112dB Idle Channel Noise ...........................................................................<-124dB Channel Separation ........................................~100dB <at> 10kHz (see Fig.2) Harmonic Distortion (see Fig.1) ... <0.001% <at> 1kHz, <0.002% 20Hz-20kHz THD+N .............................................................................. 0.0014% <at> 1kHz Intermodulation Distortion .................................. <0.001% (400Hz/7kHz 4:1) Frequency Response .........................................-0.25,+0.05dB 20Hz-20kHz Supported Sampling Rates .......... 32kHz, 44.1kHz, 48kHz, 88.2kHz, 96kHz Signature ________________________ 1.3kΩ resistor and 1.8nF capacitor are connected to the output of the following differential amplifier, rather than ground. Because the output is out of phase with the inverted signal from pin 24 of IC1, this acts like a virtual ground. So there is twice the voltage across these compared to the non-inverted signal filter, hence the higher resistance values (keeping the current from each output approximately equal). The overall filter response (determined by simulation) is -3dB at 45kHz, which is above the 30kHz or so you would expect if the filters operated in isolation. This is partly due to their interaction and also partly due to the connection from the differential amplifier’s output to the inverting signal filter. As we said earlier, it’s complicated! The resulting response is -0.1dB at 20kHz. Including the DAC’s internal filtering and the additional filtering at the output, the overall response for the circuit is -0.25dB at 20kHz, which is quite acceptable. The active filter gives around 13dB of attenuation at 100kHz, increasing at around 12dB/decade. This is ultimately limited by the bandwidth of the differential amplifier circuit and so the filter is ineffective at very high frequencies (many MHz). This means that the 1.8nF capacitor in the filter network can couple very high frequencies through to the output but their level is too low to cause problems. Name ____________________________ Discrete op amps These binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold 12 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A14.95 plus $A10.00 p&p per order. Available only in Aust. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or call (02) 9939 3295; or fax (02) 9939 2648 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my  Visa    Mastercard Card No: _________________________________ Card Expiry Date ____/____ Address__________________________ __________________ P/code_______ 30  Silicon Chip As noted above, we have used discrete transistors in this circuit instead of op amp ICs. The design is very similar to that used in the Hifi Stereo Headphone Amplifier (OctoberNovember 2011). Again referring to the left channel only, the base of NPN transistor Q1 is the non-inverting input of the differential amplifier while the base of Q2 is the inverting input. Both transistors have 100Ω emitter degeneration resistors to improve linearity. PNP transistor Q5 acts as a constant current source for the long-tailed pair and this is set to around 3mA by a 220Ω resistor. NPN transistors Q3 and Q4 form a current mirror collector load, with 68Ω emitter resistors to improve current sharing. The current into the base of NPN transistor Q8 is proportional to the difference in voltage between the two inputs (ie, between the bases of Q1 & Q2). Q8 and NPN transistor Q9 act as a beta-enhanced transistor (like a Darlington) and operate as a commonemitter amplifier. PNP transistor Q7 acts as a constant-current collector load at around 3mA. Together, Q8 & Q9 form a trans­ impedance amplifier, converting the current delivered to the base of Q8 into a voltage at Q9’s collector. This voltage controls the output stage which consists of NPN transistor Q11 and PNP transistor Q12 in a push-pull, emitter-follower configuration. NPN transistor Q10 forms a VBE multiplier. This generates an adjustable bias (set by trimpot VR1), so that both Q11 & Q12 are conducting full time, giving Class A operation. The 100pF and 1nF capacitors between Q9’s collector and Q8’s base provide frequency compensation. The two constant current sources (Q5 & Q7) limit their charge and discharge currents and so set an upper limit on slew rate and frequency, reducing gain at siliconchip.com.au very high frequencies below the level required for sustained oscillation. With this 2-pole compensation scheme, the 2.2kΩ resistor to the -15V rail increases the open loop gain available at higher audio frequencies (see “A Look At Amplifier Stability & Compensation”, July 2011). At low frequencies, this resistor shunts much of the current passing through the 100pF capacitor so that it never reaches Q8’s base but at much higher frequencies, the capacitor’s impedance so low that it has no effect. PNP transistor Q6 provides the bias and negative feedback for current sources Q5 and Q7, keeping the voltage across their emitter resistors constant. Its own collector load is a bootstrapped constant-current sink formed from two 10kΩ resistors and a 47µF capacitor. This prevents variations in the supply rail from affecting the current regulation, as this would increase inter-channel crosstalk and reduce supply hum rejection. The signal output appears at the junction of the 10Ω emitter resistors for Q11 & Q12. The output voltage has a 2.5V DC offset which is removed by a 47µF DC-blocking capacitor with a 100kΩ bias resistor. The audio signal then passes through an additional RC low-pass filter (100Ω/10nF) before passing to the output RCA connector CON3 (CON4 in the right channel). Since the output signal swing is about ±2.7V (1.9V RMS), the 100Ω resistor limits the short-circuit output current to 27mA. Otherwise, Q11 or Q12 would quickly burn out with a shorted output. Muting As suggested in the CS4398 data sheet, we have added muting circuitry to the outputs. This consists of a dual Mosfet for each channel, the Mosfets operating as analog switches. These short the output to ground when there is no signal from the DAC. This suppresses any clicks or pops that may occur when the sample rate changes or the DAC selects a different input and so on. It also makes the signal-to-noise ratio appear to be better, by reducing the idle channel noise. But it doesn’t affect the actual signal-to-noise ratio during playback since the muting Mosfets are then switched off. These components are not strictly necessary but don’t add much cost or siliconchip.com.au complexity to the circuit. The example circuit in the CS4398 data sheet uses 2SC2878 NPN transistors rather than Mosfets. These are a special type of bipolar transistor with an unusually high reverse hFE of 150, compared to around 1-2 for a normal NPN transistor. So they can operate normally even with their collector and emitter reversed; in this case, when the collector voltage (ie, signal) swings below ground. 2SC2878 transistors are available but not widely so. By contrast, the dual Mosfets we have used instead can be bought from many different sources. The CS4398 DAC automatically determines the polarity of its AMUTEC and BMUTEC outputs (for the left and right channels, respectively) based on the external biasing arrangement. In this case, they have a resistive path to ground and so the chip drives them low to mute and high otherwise. When the mute output is low, current is sunk from the base of PNP transistor Q14 via the 100kΩ resistor, turning it on. Q14 then pulls the gates of Q29a & Q29b high to 5V via a 100Ω resistor. The 100Ω resistor creates a low-pass filter with the Mosfet gate capacitance, preventing voltage spikes due to stray inductance. The two Mosfets in each pair are connected source-to-source, with one drain connected to the output and the other to ground. As a result, the two parasitic body diodes are connected anode-to-anode so that regardless of the output signal voltage polarity, at least one is reverse-biased. If we had used a single Mosfet instead, the signal would be clipped to within one diode drop to ground when the body diode was forward-biased. These diodes also clamp the sources of both Mosfets to no more than 1V above ground. So when the gates are at +5V, both Mosfets have a gatesource voltage of at least +4V. The on-threshold for the IRF7905 is no more than 2.25V so they are turned on hard in this situation, shorting the output to ground. When the AMUTEC mute output goes high, Q14 turns off and so the gates of Q29a & Q29b are pulled to -15V via a 100kΩ resistor. This is well below the lowest output signal voltage of -2.7V and so both Mosfets switch off and the signal is unaffected. When off, the Mosfets do have some capacitance, due mainly to the Parts List 1 PCB, code 01102121, 94 x 110mm 1 16-pin PCB-mount vertical IDC connector (CON1) 1 3-way mini PCB-mount terminal block, 5.08mm pitch (CON2) 1 white PCB-mount switched RCA socket (CON3) 1 red PCB-mount switched RCA socket (CON4) 2 5kΩ mini sealed horizontal trimpots M3 nuts and flat washers (may be required to adjust new PCB height to suit holes in existing case) Semiconductors 1 CS4398 Stereo DAC IC (IC1) (Element14 1023397) 1 ATMega48 programmed with 0110212A.hex (or reprogram existing micro) 2 IRF7905 dual N-channel SMD Mosfets (Q29,Q30) (Element14 1791580) 1 78L05 5V linear regulator (REG1) 14 BC559 PNP transistors (Q1-Q2, Q5-Q7, Q12, Q14-Q16, Q19-Q21, Q26, Q28) 12 BC549 NPN transistors (Q3-Q4, Q8-Q11, Q17-Q18, Q22-Q25) 5 1N4004 1A diodes (D1-D5) 4 18V zener diodes, 0.4W or 1W (ZD1-ZD4) Capacitors 9 100µF 16V electrolytic 10 47µF 35V/50V electrolytic 1 10µF 16V electrolytic 6 100nF MKT 2 18nF MKT 2 10nF MKT 2 6.8nF MKT 2 4.7nF MKT 2 1.8nF MKT 2 1nF MKT 4 100pF NP0/C0G Resistors (0.25W, 1%) 7 100kΩ 2 510Ω 6 10kΩ 2 270Ω 10 2.2kΩ 5 220Ω 2 1.6kΩ 17 100Ω 2 1.3kΩ 4 68Ω 2 680Ω 4 10Ω 2 620Ω February 2012  31 + 10k 10k 2.2k Q1 Q2 680 1.3k Q5 3 x 100F CON1 16 2 1 220 +15V 0V -15V 15 DIGITAL I/O REG1 4004 100nF 100F 2.2k 220 2.2k 2.2k 100F 47F 18nF D5 100k 100nF + + 100 + (UNDER) Q6 1.8nF CAD latsyrC CS4398 100nF 100nF + 100nF 4004 100 100 510 620 270 1.6k 6.8nF 10F Q7 1nF Q8 + 100nF D2 D1 4004 + 620 1.6k 6.8nF 100F 100pF + 510 Q14 47F Q9 47F ' 2012 + 01102121 18nF 68 68 100pF 12120110 + Crystal DAC 1.8nF 100k 4.7nF 2.2k 2.2k VR2: 5k 100 100 2.2k D4 D3 4004 4004 100k 100F Q15 100 100 680 270 1.3k 4.7nF + 100F 100k 10k Q28 Q19 100F Q10 VR1: 5k Q3 1nF Q18 Q17 Q16 100k 10k 100pF 47F 47F 18V 100 100pF Q22 220 68 68 10k 2.2k 10k 18V 100 + Q24 Q12 + 220 2.2k 100k 10nF 2 x IRF7905 10nF Q4 18V 18V (UNDER) ZD3,4 ZD1,2 + Q20 47F Q11 TP2 TP1 100k 100 47F Q23 Q21 100 + + 47F 100 100 100 2.2k 220 + 100 CON4 100 Q26 CON3 + TP3 10 10 + Q25 47F R OUT 100 L + 47F 10 10 TP4 RIGHT (RED) LEFT (WHITE) TOP SIDE OF BOARD CON2 Fig.6: follow this layout diagram to install the through-hole parts on the PCB. Take particular care with the transistors. There are two different types (BC549 & BC559) – don’t get them mixed up. Left: this is the fully-assembled PCB. Note the orientation of the IDC socket. 32  Silicon Chip drain-source capacitance which is at a maximum of about 350pF when the drain-source voltage is zero. However, most of the time, the two capacitances are in series and so there is effectively no more than 200pF additional capacitance at each output. This is swamped by the parallel 10nF capacitors and so has no effect on distortion. A pair of back-to-back 18V zener diodes between the gates and sources of each Mosfet protect them from damage in the case of a voltage spike or static discharge. Due to the low currents normally involved, the zeners will conduct below 18V, clamping the gate-source voltages below the 20V maximum rating. The 100pF capacitor between the emitter and collector of Q12 helps keep it on when power is first applied, preventing start-up clicks or pops. Q12 is then held on by the resistors between its base and ground until the DAC IC begins actively driving the mute outputs. Power supply The ±15V supply for the amplifier circuitry is provided by an external power supply board (as used in the original Stereo DAC), wired to CON2. This powers the output stages directly, while the rails feeding the input stages are applied via RC filters. These filters each comprise a 100Ω resistor in series with each rail plus a 47µF capacitor between the two rails. This improves the channel separation by preventing supply voltage variations to the input stages due to current demands from the output stages. Diodes D1 & D2 in the left channel and D3 & D4 in the right channel prevent the 47µF capacitors from pulling either supply rail to the wrong side of ground during power-up or power-down. The +5V supply is derived from the +15V rail using REG1. D5 prevents REG1 from being damaged if the +15V rail collapses faster than the +5V rail. The associated input and output capacitors ensure regulator stability and reduce output noise, while the 220Ω resistor reduces dissipation in REG1 and helps filter any ripple from its input supply. Building it All the parts are mounted on a double-sided PCB coded 01102121 and measuring 94 x 110mm. Fig.6 shows the parts layout. siliconchip.com.au siliconchip.com.au UNDERSIDE OF BOARD R OUT IRF7905 L IRF7905 01102121 2012 IC1 CS4398 LK1 LK2 Crystal DAC LK3 LK4 The DAC IC (IC1) should be fitted first. This device is in a 28-pin TSSOP (thin shrink small outline package) with a 0.65mm lead pitch and is installed on the underside of the PCB – see Fig.7. That’s done by first placing the PCB copper-side up, with IC1’s pads to the left and right (ie, with the board rotated 90°). That done, apply a very small amount of solder to the upper-right pad with a clean soldering iron (use a medium to small conical tip). Next, pick up the IC with tweezers and position it near the pads with the correct orientation (ie, with its pin 1 dot positioned as shown on Fig.7). That done, heat the tinned pad, slide the IC into place and remove the heat. Now check its alignment carefully, using a magnifying glass if necessary. It should be straight, with all the pins over their respective pads and an equal amount of exposed pad on either side. If not, reheat the solder joint and gently nudge the chip in the right direction until its position is perfect. The diagonally opposite pin should now be soldered, after which you can solder the remaining leads. Don’t worry about solder bridges; they are virtually inevitable and can easily be fixed. The most important job right now is to ensure that the solder flows onto all leads and pads. Once the soldering is complete, apply a thin smear of no-clean flux paste along the leads, then remove the excess solder using solder wick. Once the flux is heated to boiling point, this should happen quickly. Be sure to trim the end off the wick if it gets solder-logged. You should now make a final inspection to ensure that there are no remaining solder bridges and that the solder has not “balled” onto a lead without flowing onto its pad. If there are still bridges, clean them up with more flux and solder wick. For further information on soldering SMD packages, refer to these two articles: (1) “Soldering SMDs – It’s Becoming Unavoidable”, December 2010; and (2) “How To Hand-Solder Very Small SMD ICs”, October 2009. Mosfets Q29 & Q30 go in next. These are also SMDs but come in SOIC-8 (small outline integrated circuit) packages with much wider leads and greater pin spacing than the DAC chip. The leads can be soldered individually although it’s a good idea to add a small amount of flux paste and use solder –15V INPUT DIGITAL I/O Fig.7: this diagram shows how the SMD parts are installed on the bottom of the PCB. Note that you also have to install solder bridges for links LK1-LK4 but temporarily leave these out if you want to test the completed board without reprogramming the microcontroller – see text & panel. wick to remove excess solder when you have finished. This also helps to reflow the solder, ensuring good joints. Again, be careful with the orientation. The Mosfets may not have a dot to indicate pin 1. Instead, SOIC packages normally have one bevelled edge and pin 1 is located on that side. Links LK1-LK4 The next step is to bridge the solder pads for LK1-LK4 (see Fig.7). This connects pins 9-12 of IC1 to CON1 and it’s simply a matter of soldering across the four pairs of closely spaced pads. However, be careful not to bridge adjacent links or to bridge to the 0V and 5V pads on either side of the four links. Note: if you want to test the board without reprogramming the microcontroller, leave these links open and connect pins 9-12 to either 0V or +5V, as detailed in the accompanying panel. Through-hole parts The larger through-hole parts can now be installed, starting with the resistors, diodes D1-D5 and zener diodes ZD1-ZD4. Table 1 shows the resistor colour codes but you should also check each resistor with a DMM before installing it, as some colours can be difficult to read. It’s also a bit of a hassle to remove an incorrectlyplaced part from a PCB with plated through-holes. If you do need to remove a resistor or diode, first cut the lead off one side, near the body. That done, heat the pad on the opposite side and gently pull the body until it comes away. Finally, grab the remaining lead with pliers, heat its pad and again pull it out. Once the part is out, you can then clear the holes with a solder sucker. Other parts can be removed in similar fashion, ie, by cutting away the body and then removing the leads one at a time. Check that each diode (and zener diode) is orientated correctly before soldering its leads. The 78L05 regulator (REG1) can then go in. Orientate it as shown and bend its leads with February 2012  33 Table 1: Resistor Colour Codes o o o o o o o o o o o o o o No.   7   6   10   2   2   2   2   2   2   5   17   4   4 Value 100kΩ 10kΩ 2.2kΩ 1.6kΩ 1.3kΩ 680Ω 620Ω 510Ω 270Ω 220Ω 100Ω 68Ω 10Ω pliers to match the holes on the PCB. Now for the transistors. There are two different types, BC549 (NPN) and BC559 (PNP), so don’t get them mixed up. Crank their leads so that they mate with the pads, then push them down onto the PCB as far they will comfortably go before soldering their leads. Follow with the two horizontal trimpots, then mount the ceramic and MKT capacitors. That done, solder the electrolytic capacitors in place. These are all polarised so be sure to orientate them correctly. That just leaves the four connectors (CON1-CON4). Make sure that the IDC socket is installed with its notch towards the edge of the PCB and that it is pushed down fully before soldering its pins. It’s best to solder two diagonally opposite pins first and check that it’s sitting flat before soldering the rest. Similarly, terminal block CON2 must go with its wire entry holes towards the edge of the PCB and must be flush against the board. Be sure also to push the RCA sockets down as far as they will go before soldering their pins. The red socket is mounted on the righthand side as shown on Fig.6, while the white (or black) socket goes to the left. Chassis mounting Once the assembly is complete, the PCB can be mounted in the chassis. Assuming you built you Stereo DAC from an Altronics kit, it’s just a matter of removing the old DAC board and mounting the new board in its place (the mounting holes are in the same locations). Note, however, that you may need to 34  Silicon Chip 4-Band Code (1%) brown black yellow brown brown black orange brown red red red brown brown blue red brown brown orange red brown blue grey brown brown blue red brown brown green brown brown brown red violet brown brown red red brown brown brown black brown brown blue grey black brown brown black black brown install some washers under the spacers to get the RCA sockets at the correct height. If so, install these between the spacers and the bottom of the case. If you put the washers under the PCB, they could short some of the component leads to earth. The connectors are also in essentially the same locations, so the new PCB should slot straight in to any case that’s already in use for the original Stereo DAC. Reprogramming the micro You will now need to either reprogram the Atmel microcontroller on the Input PCB or replace it with a micro that has the new software. The hex file (0110212A.hex) is available for download form the SILICON CHIP website. If you don’t have an Atmel programmer, you can either purchase a programmed micro from SILICON CHIP or send yours in to have it reprogrammed for a fee (contact SILICON CHIP for details). Input board modifications There are other changes we suggest you make to the Input Board. First, the original design had 33pF capacitors between each TOSLINK receiver’s output and ground. These were recommended in the data sheet for the Jaycar ZL3003 16Mbps TOSLINK receivers we used originally. However, we subsequently found that these capacitors caused some TOSLINK receivers to oscillate under no-signal conditions and published an errata in June 2010 which recommended increasing the capacitor values to 100pF. The problem with this is that with 5-Band Code (1%) brown black black orange brown brown black black red brown red red black brown brown brown blue black brown brown brown orange black brown brown blue grey black black brown blue red black black brown green brown black black brown red violet black black brown red red black black brown brown black black black brown blue grey black gold brown brown black black gold brown Table 2: Capacitor Codes Value 100nF 18nF 10nF 6.8nF 4.7nF 1.8nF 1nF 100pF µF Value 0.1µF 0.018µF 0.01µF .0068µF .0047µF .0018µF .001µF   NA IEC Code EIA Code 100n 104   18n 183   10n 103   6n8 682   4n7 472   1n8 182    1n 102 100p 101 the 100pF capacitors, the TOSLINK inputs can no longer reliably receive data with a 96kHz sample rate. As a result, we removed these capacitors altogether from our unit (there were no ill effects) and were then able to test it at 96kHz. So if you want to use the DAC with 96kHz data, first check that you have TOSLINK receivers capable of 16Mbps. The aforementioned Jaycar ZL3003 are suitable and Altronics now stock a similar part (Cat. Z1604). If you do swap them over, be sure to check that the link selecting 3.3V/5V operation is in the correct location. You must then remove the 33pF (or 100pF) capacitors at the outputs of the TOSLINK receivers. While you are at it, be sure to change the 300Ω resistor across the S/PDIF input socket (CON1) to 82Ω (see Notes & Errata, December 2011). Setting up & testing The new DAC Board can now be tested but first a warning: never apply power to the unit without both CON1 and CON2 (on the DAC board) wired up. If you do, you could damage IC1. siliconchip.com.au The new DAC Board (top, right) is a drop-in replacement for the older board. Be sure to connect both the I/O cable and the supply leads befor applying power, otherwise you could damage the DAC chip. Check also that the power supply polarity to CON2 is correct before applying power. Before switching on, turn trimpots VR1 and VR2 fully anti-clockwise, then back clockwise about a quarter of a turn. That done, apply power and check the voltage between TP1 & TP2 using a DMM. You don’t need PC pins; just push the probe tips into the test point holes. The reading should be below 10mV. If it’s higher, switch off and check for faults. Also, check the voltage between TP3 & TP4; it should also be less than 10mV. Assuming these readings are OK, monitor the voltage between TP1 & TP2 and slowly turn VR1 clockwise until you get a reading of about 20mV. That done, repeat this procedure by monitoring TP3 & TP4 and adjusting VR2. This sets the quiescent current through the output transistors in each channel to around 2mA. That’s sufficient for them to operate in class A mode for any load of 1.3kΩ or more. For lower load impedances or highly capacitive loads, the circuit will automatically switch into class B mode. siliconchip.com.au Testing The PCB Without Reprogramming Communications between the DAC (IC1) and the microcontroller on the other board (via CON1) go via LK1-LK4 which are closely spaced pairs of pads on the underside of the PCB. These are normally shorted with solder. We could have used permanent tracks instead but this way, it’s possible to test the DAC board without having to reprogram the microcontroller. This is because the CS4398 has multiple different configuration modes and the simplest involves tying pins 9-12 either high to +5V (VLC) or tying them low. These are the same pins used for serial communications and they are connected to LK1-LK4. Most constructors should just short the four links as shown on the overlay diagram, then reprogram the microcontroller. However, if you want to test the new board out first, you can instead connect pins 9-11 of IC1 to the small, nearby 0V pad and pin 12 to the adjacent 5V pad. In this mode, many DAC features do not work properly (eg, the volume control, input scanning and muting) but you can at least verify that the new board is functioning and use it in a limited manner. If for some reason you want to drive a 600Ω load in class A mode, increase the quiescent current to 6mA by adjusting VR1 & VR2 for 60mV between the associated test points. There’s no thermal feedback between the VBE multipliers and output stages but at these current levels, transistor self-heating is low and thermal runaway should not occur. Changes in ambient temperature will be compensated for though, as it will affect all transistors more or less equally. Finally, connect a signal source and check that the sound is undistorted. It’s also a good idea to check that the volume control, scanning, muting and so on are all working correctly. This will confirm that the microcontroller can communicate with the DAC IC (IC1). Once it’s up and running, its operation is identical to the original Stereo DAC – see the November & December SC 2009 issues for further details. February 2012  35