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w o L a r t Ul & e s i o N n o i t r o t Dis By JIM ROWE A 2-Channel Balanced Inp For Audio Analysers & Dig This project is designed to extend the measurement capabilities of low-cost USB test instruments like the QA400 Stereo Audio Analyser or the USB DSOs we reviewed recently. It provides balanced/ differential inputs for each channel in addition to unbalanced inputs, combined with three attenuation/measurement ranges: 1:1, 10:1 and 100:1. B ACK IN MARCH, we reviewed the QuantAsylum QA400 low-cost USB Stereo Audio Analyser and we were most impressed with its performance capabilities. Yet at the same time we were disappointed with two limitations, which restricted its practi64  Silicon Chip cal applications quite severely. One limitation was a maximum input level of only 1.41VRMS/4.00V peak-peak for both input channels. That makes it fairly useless for a lot measurements; you would have to use external input attenuators if the QA400 were to be used for making useful measurements on hifi, guitar and PA amplifiers. The QA400 also only provided unbalanced inputs, whereas you need balanced inputs in order to efficiently test professional audio equipment. Balsiliconchip.com.au 1 µF +2.5V 250V 22pF 1M 90.0k 0.1% /1 /10 2 33Ω +IN A 9.0k 0.1% /100 D2 LOW NOISE & DISTORTION DIFFERENTIAL AMPLIFIER –IN R1 +IN R1 1.0k 0.1% 1 µF 250V 22pF 1M 90.0k 0.1% –2.5V D3 /1 /100 S1b A D4 A 1.0k 0.1% R1 K K 9.0k 0.1% OUTPUT +2.5V 68Ω /10 –2.5V Fig.1: the basic configuration used for each channel of the 2-Channel Balanced Input Attenuator. The balanced inputs (+IN and -IN) feed a matched pair of attenuator/ dividers with ganged switching, followed by a differential amplifier to subtract the two signals and provide the unbalanced output. Left: the 2-Channel Balanced Input Attenuator is built into a case that’s almost exactly the same size as QuantAsylum’s QA400 Stereo Audio Analyser, so that the two can be stacked together. put Attenuator gital Scopes anced or differential inputs also allow instruments like the QA400 to be used to make accurate measurements on signals at the output of bridge-mode analog amplifiers or class-D digital amplifiers where neither side of the outputs is earthed. As a result, we realised that the applications of instruments like the QA400 could be greatly expanded by designing an “outboard” 2-channel input attenuator to allow measurements at significantly higher audio power levels, combined with balanced/differential inputs for each channel in addition to unbalanced inputs. Such a project is not restricted to siliconchip.com.au R1 A RANGE SWITCHING 33Ω 3 D1 S1a K BALANCED INPUT –IN 1 K 68Ω enhancing audio analysers like the QA400, either. Many, if not most, lowcost USB DSOs have similar limitations, and would therefore benefit in the same way. And we should also mention the Digital Audio Millivoltmeter described in the March 2009 issue of SILICON CHIP, which had similar limitations. Finally, we should also mention that this project would make a useful addition to any oscilloscope when you need differential inputs, albeit its bandwidth does limit its use to signals with harmonics no higher than 750kHz (eg, square-wave signals to about 75kHz) – see specifications panel So you can see the design concept is quite straightforward but producing a design which was “good enough” turned out to be a real challenge. This was largely because of the need to introduce as little additional noise and distortion as possible, because this would detract from the excellent performance of the QA400. Basic configuration Fig.1 shows the basic configuration for one channel: balanced inputs (+IN and -IN) feeding a matched pair of attenuator/dividers with ganged switching, followed by a differential amplifier to subtract the two signals and provide the unbalanced output. Don’t worry about the circuitry shown inside the differential amplifier at present – we’ll move onto that shortly. Just note that the purpose of Schottky diode pairs D1/D2 and D3/ D4 at each input of the differential amplifier are to limit the signal levels to within windows between ±2.7V, to protect both the differential amplifier and the input of a following instrument, such as the QA400. The 68Ω resistors in series with the “/1” position of switches S1a & S1b are there to limit the current in these diodes, together with the 33Ω resistors in series with each input. Ideally we’d like to make these series resistors somewhat larger than May 2015  65 The front panel carries two 3-pin XLR sockets for the balanced inputs, two BNC sockets for unbalanced inputs and the range selector switch. 101Ω (= 33Ω + 68Ω), because the diodes we’re using for D1-D4 have a fairly low maximum current rating. However, we are forced to compromise at the values shown because these resistors are directly in series with the inputs when S1 is switched to the 1:1 range. This means that their thermal (Johnson) noise is added directly to the input signals, thus degrading the attenuator’s noise performance. As set out later in an accompanying panel, the thermal noise generated in a resistor is directly proportional to the square root of its resistance multiplied by the absolute temperature and the bandwidth being used. This means that if we were to increase the value of the input series resistors to, say, 1kΩ, the RMS thermal noise voltage at each input of the differential amplifier would rise from 186.79nV (-134.6dBV) to 587.6nV (-124.6dBV), measured at 25°C and over the band from 20Hz to 21.0kHz. In other words, the noise level at –IN 820Ω each input would be degraded by some 10dB. Note that since the two sources of thermal noise are not correlated, the output noise level of the differential amplifier would be degraded by a further 6dB even if the amplifier itself was totally noiseless. So with the resistor values shown in Fig.1, the RMS output noise level will always be above 373nV (-128.6dBV), while if the input resistors were increased to 1kΩ it would always be above 1.175µV (-118.6dBV). Those 33Ω resistors in series with each input are mainly to form low-pass filters in conjunction with the 22pF shunt capacitors, to improve the RFI/ EMI rejection of the overall circuit. You’ll find that in the final circuit we have also fitted small inductors in series with the 33Ω resistors, to further improve EMI rejection. The 1µF coupling capacitors on each input reject any DC that may be present, while having minimal effect on the low frequency response. And 820Ω OUTPUT +IN 820Ω 820Ω 66  Silicon Chip Fig.2: to achieve better performance in terms of noise and distortion, this is the configuration used for the output differential amplifier. In practice, an array of four of these are used in parallel. the 1MΩ shunt resistors on the input side of the capacitors are to bleed away any charge remaining on those capacitors when the inputs are disconnected from a source of DC. Finding the right amplifier(s) Let us now consider the crucial aspect of the project’s design: how to achieve the best noise and distortion performance from the output differential amplifier section (shown inside the dashed rectangle of Fig.1). In other words, which is the best op amp to use and what is the best configuration to use it in? We began by searching through all the data we could find on low noise, low distortion op amps. Initially, this led us to the Analog Devices AD797, a device with particularly impressive noise and distortion specs: 1.2nV/√(Hz) maximum input voltage noise density between about 80Hz and beyond 10MHz, coupled with a typical THD figure of -120dB at 20kHz. However when we looked closely at the performance of the AD797 when used as a differential amplifier, we found that its noise performance wasn’t as good: the output voltage noise spectral density jumped up to around 9nV/√(Hz), giving an RMS noise output of close to 1.3µV (-117.5dBV) over the 20Hz – 21kHz audio bandwidth. Even to achieve this level of performance, the resistor values shown as R1 in Fig.1 had to be lowered to 1kΩ, making it very difficult to achieve a total input resistance of more than 2kΩ siliconchip.com.au on the 1:1 range of S1. This obviously wasn’t high enough, suggesting that voltage follower/buffers were going to be needed ahead of the differential amplifier. There was one more drawback regarding the AD797 – its price, which in Australia turns out to be $14.51 plus GST. Since at least two of these were going to be needed (one per channel), this meant that the op amps alone would account for just on $32 of the project’s cost. So we looked for an alternative approach. And ultimately we found such an approach in the book Small Signal Audio Design (Focal Press/Elsevier, Second Edition 2015; ISBN 978-0-41570973-6), by renowned audio engineer Douglas Self. In chapter 18 of this book, starting on page 483, Douglas Self gives a great deal of useful information on the design of low-noise balanced input stages. He explains why the standard differential amplifier configuration like that shown in Fig.1 cannot achieve an output noise level as low as an unbalanced input stage using the same op amp, unless the resistor values are reduced to a level that gives an unacceptably low input resistance – regardless of the actual op amp being used. He then explains that the best approach is to use the configuration shown in Fig.2, where the differential amplifier is preceded by a pair of op amps connected as voltage follower buffers. This allows the four resistors around the differential amplifier to be reduced to a value giving an acceptable noise level, while the voltage followers provide a unity-gain impedance step-up for the two inputs. At the same time, the input buffers don’t degrade the CMRR (commonmode rejection ratio), because this is still defined by the tolerance of the 820Ω resistors around the differential amplifier and also by its bandwidth. Douglas Self then goes on to analyse the performance of this configuration and explain why the resistor values can’t really be reduced below 820Ω, without degrading the distortion performance. (This is mainly because of the current drive capability of the input buffers and also of the differential amplifier itself.) He explains that by using 820Ω resistors with the well-known (and much lower cost) 5532 low-noise op amps in all three positions, the noise output of siliconchip.com.au Main Features & Specifications Description: a 2-channel balanced input attenuator with low noise and distortion suitable for extending the measurement range of audio analysers and digital oscilloscopes (both self-contained and USB linked). It provides a choice of either balanced/differential or unbalanced inputs for either or both channels, plus the ability to quickly select one of three measurement ranges. Input resistance (DC): 1MΩ Input impedance (AC): 100kΩ shunted by approximately 25pF Maximum input voltage: 10V to ground, 20V peak-to-peak/7V RMS differential on 0dB range; 100V to ground, 200V peak-to-peak/70V RMS differential on other ranges Output clipping level: approximately 4V peak-to-peak (1.4V RMS sinewave) Attenuation/measurement ranges: 1:1 (0dB); 10:1 (-20dB); 100:1 (-40dB) Frequency response (both channels): 0dB range: ±0.1dB from 11Hz – 35kHz, -3dB at 750kHz -20dB range: ±0.1dB from 11Hz – 20kHz, -0.5dB at 35kHz, -3dB at 1.5MHz -40dB range: ±0.1dB from 11Hz – 10kHz, -0.3dB at 20kHz, -3dB at 4.25MHz (Note: these figures apply for both balanced and unbalanced inputs) Gain/attenuation accuracy: ±2% (±0.2dB) Signal-to-noise Ratio (20Hz-80kHz measurement bandwidth): 0dB range: 114dB with respect to 1.4V RMS input/output -20dB range: 108dB with respect to 14V RMS Input/1.4V RMS output -40dB range: 98dB with respect to 26.6V RMS input/266mV RMS output Output noise level: 0dB range: -136dBV (158nV) 400Hz-40kHz; -113dBV (2.24µV) at 15Hz -20dB range: -138dBV (126nV) 400Hz-40kHz; -116dBV (1.6µV) at 15Hz -40dB range: -138dBV (126nV) 200Hz-40kHz; -116dBV (1.6µV) at 15Hz Total Harmonic Distortion (20Hz-80kHz measurement bandwidth): 0dB range: <0.0005%, 20Hz-20kHz -20dB range: <0.0005% 20Hz-2kHz, <0.0025% 2kHz-20kHz -40dB range: <0.002% 20-25Hz, <0.0015% 25Hz-2kHz Channel separation with a 1V RMS signal (QA400 Analyser alone: 100dB): 0dB range: >100dB, 20Hz-20kHz -20dB range: >80dB, 20Hz-1kHz; >60dB, 1kHz-20kHz -40dB range: >95dB, 20Hz-1kHz; >70dB, 1kHz-20kHz Common mode rejection ratio: 0dB range: >60dB, 20Hz-20kHz; typically >80dB -20dB range: >50dB, 20Hz-20kHz; typically >65dB at 1kHz -40dB range: >40dB, 20Hz-20kHz Power supply: runs from an external ±15V DC supply, with a current drain of approximately 200mA the Fig.2 configuration can be lowered to -112.4dBV. This is about 7.3dB above the level that could be achieved with a single AD797 differential amp, so it’s still not good enough. Multiple op amps & noise cancellation As Douglas Self moves on to explain, there is a fairly easy way to improve noise performance quite significantly: by using an array of identical differential amplifiers driven by an array of input buffers. So that’s what we are using in this project, with four differential amplifiers connected in parallel, driven by four pairs of unity-gain input buffers. The thinking behind this is that connecting two identical amplifiers in parallel causes the noise generated in each to mostly cancel, because they are not correlated. This happens each time the number of amplifiers is doubled, so that by using four identical differential amplifiers in parallel, we can achieve a 6dB drop in the overall noise output. Similarly, we can achieve a further 3dB drop in noise output by using a separate pair of input buffers for each differential amplifier, to achieve better buffer noise cancellation. The end result of moving to this eight-buffers-driving-four-differentialamplifiers configuration gives a total improvement in noise level of about May 2015  67 9dB – so even if we use 5532 op amps throughout, the noise output level drops to -119.2dBV. This is a couple of dB better than we could achieve with a single AD797, even if it were preceded by a couple of AD797s as input buffers. But what about the price to be paid for this increase in circuit complexity, in order to achieve that low noise level? The good news is that the 5532 device is a dual op amp, whereas the AD797 is only a single op amp. So we only need six 5532 devices at a current price of around $2.00. So the total op amp price tag for one channel is only about $12 – less than the price of a single AD797. The end result is that by using Douglas Self’s “array” technique, we are able to achieve an impressive output noise level of -119.2dBV in our two channels. We do have to allow for a more complex PCB but we believe that the end result is worth it. Circuit description Now have a look at Fig.3 which shows the circuit of the left channel (the right channel is identical). Notice that we have added an unbalanced input, using CON2, to provide the option of connecting the attenuator channels to unbalanced signal sources. As previously mentioned, induct­ors RFC1 & RFC2 have been included in series with the 33Ω suppressor resistors at the three inputs, to provide additional rejection of RFI/EMI signals. RFC1 is bifilar wound to provide improved rejection. Note that we provided for discrete high-frequency compensation capacitors across the upper arms of the attenuator dividers, marked C1 and C2. We thought that these would be needed to compensate for the capacitance of the input voltage limiting diodes D1D4 and the input capacitance of the array of voltage followers (IC1, IC3, IC4 & IC6). However, during prototype testing, we discovered that discrete compensation capacitors were not necessary – partly due to the very low capacitance of D1-D4 and partly to incidental capacitance between the short lengths of wire connecting the lugs of range switch S1 to the PCB. As can be seen in the specification panel, the resulting frequency response is quite acceptable. Note that the outputs of the four 68  Silicon Chip differential amplifiers (IC2a/b and IC5a/b) are combined using 10Ω (1%) resistors. This ensures that the final output at CON3 is an average of the four differential amplifier outputs and they won’t “fight” each other. As a result, there is no drop in signal gain but there is a welcome drop in noise output due to cancellation. Before leaving the circuit of Fig.3 we should perhaps draw attention to the notes panel. Part numbers for the right channel circuit are listed here and also shown on the circuit in grey. Power supply details Now let’s move on to consider the Attenuator’s power supply. Natsiliconchip.com.au Fig.3: the complete circuit for the left channel of the 2-Channel Balanced Input Attenuator (the right channel is identical). It’s based on six NE5532D dual lownoise op amps (IC1a-IC6b, plus six more for the right channel. urally both the ±15V supplies for the op amps and the ±2.5V rails for the input clipping diodes need to be as quiet as possible, if the full low noise performance of the attenuator itself is to be realised. The first approach we tried was a fairly standard configuration with an external 17VAC plugpack feeding two siliconchip.com.au half-wave rectifiers, each of which was then driving a 15V regulator followed by a 2.5V regulator. Apart from the external AC plugpack, everything was on the same PCB as the rest of the attenuator’s circuitry and therefore inside the shielding metal box. While this did work, it proved to be virtually impossible to prevent 50Hz hum components and their harmonics from finding their way into the signal circuitry – possibly via radiation from the tracks on the PCB carrying current between the rectifier diodes and the input filter capacitors. The only practical way to solve this problem was to remove the rectifiers, input capacitors and ±15V regulators from both the PCB and the box, and modify the design so that the unit is operated from a well-filtered and regulated external ±15V DC supply. As it happens, we were also developing an enhanced version of the March 2011 Universal Regulator module, so the logical approach was to arrange for one configuration of this new Universal Regulator Mk2 to be used for the Attenuator’s external ±15V supply. You’ll find the Universal Regulator Mk2 described elsewhere in this issue. Redesigning the attenuator in this way allowed us to simplify its internal power supply circuit to that shown in Fig.4. It has the two incoming 15V supply lines passing directly through to the attenuator’s op amps and a pair of low-power TO-92 adjustable regulators (REG3 and REG4) used to provide the ±2.5V rails for the clipping diodes. A 3mm green LED (LED1) is connected between the two 2.5V rails via a 330Ω series resistor to provide power indication. Because the 17V AC plugpack we’re using with the Universal Regulator Mk2 has an untapped secondary winding, we are forced to use a half-wave rectifier configuration. However, at the same time, this plugpack does provide a mains earth output lead and to make use of this we decided to pass this mains earth through the new Universal Regulator Mk2 PCB and thus make it available for load equipment like our Balanced Input Attenuator. By connecting the attenuator to the regulator module using a four conductor shielded cable as shown at the bottom of Fig.4, we were able to bring the mains earth right through to pin 2 of the attenuator’s power input connector (CON7). As a result, the attenuator’s metal shielding box can be permanently connected to mains earth for shielding. However, the earth/0V side of the attenuator’s circuitry should not be connected permanently to this mains earth, because in some measurement situations this would have the potenMay 2015  69 +15V REG3 LM317L +15V 1 0V 4 IN 2 MAINS CON7 ADJ EARTH LIFT S2 +2.5V OUT 100nF +2.5V 120Ω 470 µF 10 µF 16V 330Ω 16V EARTH 120Ω 5 3 120Ω 0V BOX 100nF A 10 µF 470 µF 120Ω ADJ –15V IN OUT POWER λ LED1 16V 16V K –2.5V –2.5V REG4 LM337L –15V LM317L LED LM337L OUT OUT K IN A ADJ IN ADJ (SHIELDING BRAID) TO CON2 ON UNIVERSAL REGULATOR Mk2 Ver.C * –15V E SC 1 2 0V 5 4-CONDUCTOR SHIELDED CABLE INTERCONNECTING POWER CABLE 20 1 5 4 +15V 2-CHANNEL BALANCED INPUT ATTENUATOR 3 5-PIN DIN PLUG (MATES WITH CON7 OF ATTENUATOR) * DESCRIBED SEPARATELY IN THIS ISSUE INTERNAL POWER SUPPLY CIRCUITRY Fig.4: the power supply circuitry built into the Balanced Input Attenuator, plus the wiring of the power cable used to run the unit from the Universal Regulator Mk2 module described elsewhere in this issue. tial to create an “earth loop” and hence inject 50Hz hum into the attenuator’s signal circuitry. That’s why we have fitted EARTH LIFT switch S2, so that the connection between the attenuator’s earth and mains earth can be broken, to see which setting gives the better results. Note that the cable used to connect the attenuator to the regulator module should be shielded, as shown at the bottom of Fig.4. This is to ensure that hum and EMI are not picked up and fed into the attenuator via the ±15V power lines. It is the shield braid that also connects the mains earth to the attenuator, via pin 2 of CON7. Construction Building it is straightforward, with all parts (except for range selector switch S1) mounted on a double-sided PCB coded 04105151 and measuring 160 x 80mm. This board is housed in a small extruded aluminium case measuring 170 x 85 x 54mm (W x D x H). It’s similar in size to the case used for the QA400 Audio Analyser, making it easy to stack the two together. 70  Silicon Chip Figs.5 & 6 shows the parts layout on the PCB. As shown most of the parts are fitted to the top of the PCB. The only parts mounted on the bottom are output buffers IC5 & IC12 and their associated components. These are all fitted in the two areas indicated on the underside overlay (Fig.6). All of the parts used in the input sections of the Balanced Input Attenuator (ie, ahead of range switch sections S1a-S1d) are conventional “leaded” components. This was done to give maximum ruggedness and reliability, and to make the assembly easier. The power supply circuitry along the rear of the PCB also uses leaded components. However, SMD parts are used in the signal circuitry between S1 and output connectors CON3 & CON6. PCB assembly Here is our suggested order of assembly, to make this task as easy as possible: Step 1:  fit the SMD resistors and capacitors to the top of the PCB. Step 2:  fit SMD diodes D1-D8. These go on the top side near the front cen- tre of the PCB (behind where S1 will be after final assembly). Be sure to fit each diode with the orientation shown in Fig.5. Step 3: install the NE5532D dual op amp ICs to the top side of the PCB (IC1-IC4 & IC6-IC11). These come in an SOIC 8-lead SMD package. Make sure that you fit each IC with the correct orientation. Don’t worry if you get solder bridges between the pins when soldering these ICs in; they can be easily removed afterwards using solder wick and a hot iron. Step 4:  repeat step 1-3 for the parts on the underside of the PCB – see Fig.6. Step 5:  once all the SMD components are in place, install the resistors followed by the non-polarised capacitors and the polarised capacitors. Regulators REG3 & REG4 and LED1 can then go in. The latter must be fitted with its longer anode lead towards the centre rear of the board and with its body 18mm above the PCB (use a cardboard spacer between the leads). The LED is later bent down through 90° so that it protrudes through a siliconchip.com.au (TOP OF PCB) CON1 LEFT IN BAL E LEFT IN UNBAL D4 100nF 100nF D8 S1d IC9 5532 68Ω C1 820Ω IC8 5532 22pF 100nF IC7 5532 10 µF 1 100nF 1 µFC 250V 2015PP 15150140 33Ω RFC4 10 µF 820Ω 10 µF 1 1 µF 04105151 250V PP S1c 100nF 820Ω 100nF 5102 C 100nF 100nF 1 820Ω 820Ω 10 µF IC10 5532 1 1 820Ω 100nF 820Ω 22pF 820Ω 10 µF IC11 5532 820Ω 22pF 10Ω 22pF 100nF D5 100nF D7 RANGE 820Ω 100nF 22pF 33Ω 33Ω 22p 100nF 100nF 100nF 820Ω D6 D1 D3 S1 100nF 10Ω 330Ω 1k 3.0k 3.0k 3.0k 30k 30k 30k 68Ω D2 120Ω 120Ω LM337L 1k CON5 RIGHT IN UNBAL RFC3 2 1 3 1M 1M CON2 C1 -40dB 1k C2 -40dB C1 S1b 0dB C2 RFC2 C2 100nF S1a 470 µF -2.5V 30k 30k 30k 3.0k 3.0k 3.0k 1k V 5 1- 100nF 820Ω 100nF 68Ω 33Ω 22pF 10 µF 68Ω 30k 30k 30k 3.0k 3.0k 3.0k 22p 1 100nF 100nF 100nF 820Ω 1206 33Ω 33Ω 3 4 CON6 RIGHT OUT REG4 + 100nF V0 C1 1 1 µF 250V PP 1M 1M 2 IC6 5532 1 1 µF 250V PP RFC1 820Ω 10 µF IC4 5532 IC3 5532 1 100nF 100nF 1 820Ω 10 µF 10 µF 820Ω 820Ω 820Ω 10 µF IC1 5532 100nF 22pF 820Ω 820Ω 22pF IC2 5532 820Ω 100nF 22pF 5 +2.5V 820Ω V 531- 2 C2 LM317L V 511 + 100nF K 3.0k 3.0k 3.0k 30k 30k 30k 120Ω 120Ω 100nF 100nF 10Ω 10Ω 10 µF REG3 POWER A CON7 + + 22pF 470 µF LED1 HTRAE S NIA M 1 10 µF 820Ω S2 + CON3 LEFT OUT + 15V DC INPUT – BOX GND 820Ω EARTH LIFT CON4 E RIGHT IN BAL Fig.5: follow this layout diagram to install the parts on the top of the PCB. A mixture of leaded (through-hole) and SMD components is used, with some SMD parts also fitted to the underside of the board as shown on Fig.6. The only component not mounted on the PCB is range selector switch S1, which mounts on the front panel. The photo below shows the completed PCB. siliconchip.com.au May 2015  71 (UNDERSIDE OF PCB) MAINS EARTH 7 NO C 9 CI 2355 0 1 CI 2355 1 1 CI 2355 100nF 10Ω 10Ω 22pF 820Ω 820Ω IC5 5532 22pF 820Ω 22pF NOTE: ALL COMPONENTS FITTED ON THE UNDERSIDE OF THE PCB ARE IN THESE TWO AREAS ONLY 820Ω 820Ω 7 CI 2355 V 5. 2 + -15V 10 µF 100nF 22pF 0V 22pF 22pF 3 GER +15V V 5. 2- 22pF 10Ω 100nF 10Ω 820Ω IC12 5532 820Ω -15V 3 NO C 6021 22pF 820Ω 4 GER 1 2S 1 6 NO C 10 µF 100nF 6 CI 2355 4 CI 2355 3 CI 2355 1 CI 2355 C 2015 PP5V100522 CFµ 1 04105151 1P5P1V5005124 0Fµ 1 PP V 0 5 2 Fµ 1 PP V 0 5 2 Fµ 1 3 CFR 1 CFR 4 CFR 1 3 2 CFR 1 2 1S E 4 NO C 5 NO C 3 2 E 2 NO C 1 NO C Fig.6: here’s how to install the SMD parts on the underside of the PCB. As shown, these parts are fitted to two areas at the top left and top right of the diagram. matching hole in the rear panel when the unit is assembled into the case. Step 6:  wind the four EMI suppression inductors (chokes). Each inductor is wound on a 5mm-long, 4mm-OD ferrite bead, using 0.25mm enamelled copper wire. All four inductors have only two full turns but the winding details vary. RFC2 & RFC4 have only a single 2-turn winding. By contrast, RFC1 & RFC3 have two turns wound in bifilar fashion, ie, two short lengths of wire are threaded through the bead together. The ends of these wires are then cut short (about 7mm long at each end) and tinned, ready to be soldered to the pads of the PCB. Take care not to transpose the end connections of the two wires passing through RFC1 & RFC3, or you’ll get a mysterious phase reversal! The four inductors can now be fitted to the PCB (just behind the positions for CON1, CON2, CON4 & CON5). Step 7:  fit connectors CON1-CON7 to the top of the PCB. Be sure to push each one all the way down so that it sits flush against the PCB before soldering its leads. Step 8:  fit earth lift switch S2 to the rear of the PCB. This is a very small slider switch but it’s no harder to solder in place than the SMD components. Step 9:  fit a single PCB terminal pin at the rear of the board, in the posi- Table 1: Resistor Colour Codes   o o o o o o o o o    No.     4   12   12     4     1     4     4    6 72  Silicon Chip Value 1MΩ 30kΩ 3.0kΩ 1kΩ 330Ω 120Ω 68Ω 33Ω 4-Band Code (1%) brown black green brown orange black orange brown orange black red brown brown black red brown orange orange brown brown brown red brown brown blue grey black brown orange orange black brown tion labelled BOX GND in Fig.5 (just between S2 and CON7). Step 10: complete the PCB assembly by fitting four 4-pin SIL headers in the positions indicated in the front centre of the PCB, grouped around diodes D1-D8 and their bypass capacitors. These headers will be used to make the connections to the four sections of range selector switch S1. Preparing switch S1 The PCB assembly can now be put   Table 2: Capacitor Codes Value µF Value IEC Code EIA Code 1µF   1µF   1u0   105 22pF  NA  22p   22 5-Band Code (1%) brown black black yellow brown orange black black red brown orange black black brown brown brown black black brown brown orange orange black black brown brown red black black brown blue grey black gold brown orange orange black gold brown siliconchip.com.au Fig.6: the underside of the PCB carries op amps IC5 & IC12 and their associated SMD parts. Be sure to orientate the op amps correctly and use solder wick to clean up any solder bridges between their pins. aside while you prepare switch S1, as follows: Step 1:  cut its control spindle to about 12mm long, then smooth off any burrs using a small file. Step 2:  cut a piece of 4-wire rainbow ribbon cable into four 35mm lengths and strip 5mm of insulation from both ends of all four wires. Carefully tin the ends of all wires, using a minimum of heat and solder. Step 3: solder one end of each wire in each 4-wire cable to one section of switch S1. The first wire is soldered to the inner rotor lug, while the other three wires are soldered to the outer contact lugs as shown in the accompanying photo. Note that in each group the second wire connects to the “most clockwise” contact lug (looking from the front), the third wire to the centre contact lug and the fourth wire to the “most anticlockwise” contact lug. Step 4: solder the other ends of the ribbon cable wires to the connection lugs of four 4-way SIL sockets (again as shown in the photo). Note that in each case, the wire from the switch siliconchip.com.au This close-up of the rear of range switch S1 shows how the four short ribbon cables are attached to its connection lugs and also to the four small SIL female header sections used to connect to the PCB. May 2015  73 Above: switch S1 is mounted on the front panel, while the four SIL sockets at the ends of its ribbon cables are plugged into matching pin headers on the PCB (see text for details on socket orientation). Note: this photo shows the original metal front panel supplied with the case, whereas the final version uses a PCB front panel and a PCB rear panel. Both the front and rear panel PCBs are available from the SILICON CHIP Online Shop. rotor connects to one end lug of the SIL socket, with the other three wires soldered to the remaining lugs of the socket in the same order as before. This should be clear if you look closely at the photo. Alternatively, if you can obtain 4-way cables with “DuPont” connectors already fitted, you can save yourself some effort. Just cut them to length and solder them to the rotary switch. Your range selector switch assembly is now complete. PCB front & rear panels No case preparation is necessary since pre-drilled PCBs with screened lettering are used for the front and rear panels. These take the place of the panels supplied with the case. The front-panel PCB is coded 04105152, while the rear panel PCB is coded 0410515. Both boards measure 170 x 64mm and can be purchased from the SILICON CHIP Online Shop. Once you have the panels, the next 74  Silicon Chip step is to fit the front panel PCB to the main PCB. That’s done by first bringing it down at an angle so that the notches at the top of the XLR socket holes slip down behind the PUSH levers on the two sockets. At the same time, the two 13mm-diameter holes must be slipped over the BNC sockets, after which the panel is straightened and pushed all the way up to the PCB, so that it fits close to the four input sockets. It’s then just a matter of securing the panel in pace by fitting the nuts that come with the BNC sockets and by installing pairs of 6G x 6mm selftapping screws through the 3mm holes adjacent to each XLR socket. Range selector switch S1 can now be attached to the front panel PCB. That’s done by first removing its mounting nut and checking to make sure that its locating spigot is set correctly to give three positions. The switch is then fed through its mounting hole and secured by doing up its mounting nut to hold it firmly in position. S1’s knob can then be fitted to its spindle and its grub screw tightened firmly. Once the switch is in place, connect the four SIL sockets to their matching pin headers on the PCB. The “rotor wire” end of each socket goes to the header end labelled S1a, S1b, S1c or S1d. As shown on Fig.5, these labels are at the rearmost ends of the headers for S1b & S1c, while they are at the far left and far right of the headers for S1a & S1d. It’s important to get these socket/ header connections correct, otherwise you’ll get some very strange results. Final assembly Now for the final assembly – fitting the front-panel/PCB assembly into the case. There are no mounting screws or pillars, because the extruded case has a series of horizontal PCB mounting slots running along each inside end. The main PCB simply slips snugly into the lowest slot at each end, until siliconchip.com.au The left and right channel BNC output sockets, the earth lift switch and the green power LED protrude through matching holes in the rear panel. Access is also provided through the rear panel to the 5-pin DIN power supply socket. the front panel PCB meets the case. The back of the main PCB will then be only about 1mm in from the rear of the case, so that the power socket is accessible when the rear panel PCB is later fitted in place. Once the PCB assembly has been slid into place, secure it using five of the supplied M3 x 12mm socket-head screws (these go through the holes in the front panel). However, before fitting the screw into the lower frontcentre hole, it’s a good idea to fit a thin M3 star lockwasher between the panel and the case. This is to make sure that there’s a good electrical connection between the case and the front panel PCB earth pattern when the screw is tightened up. The rear panel PCB is attached to the rear of the case using the five remaining M3 x 12mm screws but before doing this, there are two small jobs to do. The first is to fasten a small solder lug to the inside of this PCB, using an M3 x 6mm machine screw, M3 nut and star lockwasher. This screw passes through the 3mm hole in the rear panel PCB just to the right of the 15mm diameter power input hole in the centre (and just above the rectangular hole for the earth lift switch actuator). Fit the star lockwasher over the screw before fitting the solder lug and the nut. This will ensure a good electrical connection between the solder lug and the rear panel PCB earth pattern when the assembly is tightened up. That done, cut a short length (say 50mm) of insulated hook-up wire, strip siliconchip.com.au Resistors & Thermal Noise Back in 1926, John Johnson of Bell Labs in the USA discovered that electrical noise was generated in all electrical conductors at temperatures above absolute zero (0K = -273°C), due to thermal agitation of the charge carriers (eg, the electrons). This happens regardless of whether the conductor concerned has any voltage applied to it or is conducting any current. It is basically determined by the resistance of the conductor and the temperature, although the bandwidth of measurement also plays a role in terms of the actual noise voltage. Johnson’s Bell Labs colleague Harry Nyquist worked out how this noise is generated and came up with a number of expressions which allow its power density and/or RMS voltage level over a given bandwidth to be calculated. The most useful of these expressions is the one to calculate RMS noise voltage for a given measurement bandwidth: Vn = √(4.kB.T.R.∆f) where kB is Boltzmann’s constant in Joules per Kelvin (1.38 x 10-23), T is the temperature in Kelvins (°C + 273), R is the resistance in ohms and ∆f is the measurement bandwidth in Hertz. For example, a 1kΩ resistor at 25°C (= 298K) will generate an RMS thermal noise voltage of 0.5876µV (ie, 587.6nV or -124.618dBV), when measured over a bandwidth of 20,980Hz (20Hz – 21.0kHz). Note that thermal or Johnson (or Johnson/Nyquist) noise is quite different from Shot noise, which is the additional noise generated in a conductor when a voltage is applied and a current begins to flow through it. Thermal noise also has nothing to do with the actual conducting material inside a resistor or other component – it’s purely to do with the resistance and the temperature. So if you have two 1kΩ resistors, one with a metal film element and the other with a carbon composition element, they will both generate the same thermal noise at 298K when measured over the same bandwidth. about 5mm of insulation from each end and tin the wires. One end of this wire is then soldered to the solder lug on the inside of the rear panel, while the other end is soldered to the PCB terminal pin at the rear of the PCB (between CON7 and earth lift switch S2). The second small job is to bend LED1’s lead down by 90° (so that it faces outwards) at a point about 10mm up from the PCB. This will ensure that the LED’s body will line up with its matching hole in the rear panel PCB and protrude slightly through it when May 2015  75 Parts List 1 aluminium instrument case, 170 x 85 x 54mm (W x D x H) (Box Enclosures B4-080SI, element14 code 930-7443) 1 double-sided plated-through PCB, code 04105151, 160 x 80mm 1 front panel PCB, code 04105152, 170 x 64mm 1 rear panel PCB, code 04105153, 170 x 64mm 1 ±15V DC power supply assembly plus 17VAC earthed plugpack (Jaycar MP3022) (see text) 4 ferrite beads, 4mm OD x 5mm long 1 200mm length of 0.25mm enamelled copper wire (for winding RFC1-RFC4) 1 4-pole 3-position rotary switch (S1) 1 instrument knob, 20mm diameter with grub-screw 1 subminiature SPDT slide switch, PCB mounting with side actuator (S2) (element14 code 120-1431) 2 3-pin XLR compact female sockets, 90° PCB-mount (CON1, CON4) (Altronics P0875) the rear panel is fitted. Once that’s been done, position the earth lead so that it won’t get damaged, then fit the real panel. Make sure that LED1 and S2 pass through their matching holes in the panel, then fit the mounting nuts to CON3 and CON6 and the five remaining case assembly screws. Another lockwasher As with the front panel, it’s a good idea to fit a thin M3 star lockwasher between the rear panel and the lower 4 BNC sockets, 90° PCB-mount (CON2,CON3,CON5 & CON6) 1 5-pin DIN socket, 90° PCBmount (CON7) 1 5-pin DIN line plug 1 1m length 4-core shielded cable 4 4-pin SIL header strips 4 4-pin SIL female headers 4 35mm lengths of 4-wire ribbon cable or 2 x 4-way cables with DuPont header plugs at each end (these also replace the SIL female headers) 4 6G x 6mm self-tapping screws 1 M3 x 6mm machine screw 1 solder lug 1 M3 star lockwasher 2 thin M3 star lockwashers 1 M3 nut 1 PCB terminal pin, 1mm diameter 1 50mm length of insulated hookup wire 4 adhesive rubber feet Semiconductors 12 NE5532D dual low-noise op amps, SOIC-8 SMD package (IC1-IC12) (element14 code 958-9856) centre of the case, before you fit the lower centre screw. This is again to ensure that there will be a good electrical connection, this time between the rear panel and the case once that screw is tightened. It also means that, the case (and both the front and rear panels) will be reliably connected to mains earth for shielding when the Balanced Input Attenuator is connected to the Universal Regulator Mk2. Your Balanced Input Attenuator is now assembled and ready for use. However, it’s a good idea to fit four 1 LM317L adjustable regulator, TO-92 (REG3) 1 LM337L adjustable regulator, TO-92 (REG4) 1 3mm green LED (LED1) 8 1N5711W-7-F Schottky diode, SOD-123 SMD package (D1-D8) (element14 code 185-8640) Capacitors 2 470µF 16V RB electrolytic 2 10µF 16V RB electrolytic 12 10µF 35V MLCC, SMD 1210, X7R dielectric 4 1µF 250V polypropylene 5% 32 100nF 50V MLCC, SMD 1206, X7R dielectric 2 100nF multilayer ceramic 4 22pF 100V disc ceramic, NP0 16 22pF 50V ceramic, SMD 1206, C0G/NP0 dielectric Resistors (1% tolerance) 4 1MΩ 0.5W metal film 12 30kΩ 0.5W metal film (0.1%) 12 3.0kΩ 0.5W metal film (0.1%) 4 1kΩ 0.5W metal film (0.1%) 32 820Ω 1/8W, SMD 1206 (0.1%) 1 330Ω 0.5W metal film 4 120Ω 0.5W metal film 4 68Ω 0.5W metal film 6 33Ω 0.5W metal film 8 10Ω 1/8W, SMD 1206 adhesive rubber feet to the underside of the case, so that it can be placed on top of the QA400 Analyser or another instrument without scratching it. All that remains is to wire up the power cable, using the diagram at the bottom of Fig.4 as a guide. This will allow you to connect the unit to the Universal Regulator Mk2 (Version C). Once you’ve done this, plug the 17VAC plugpack into a power outlet and check that LED1 on the rear of the attenuator lights, to show that it has SC powered up correctly. Issues Getting Dog-Eared? Keep your copies of SILICON CHIP safe with these Buy five and get handy binders them postage free! REAL VALUE AT $14.95 PLUS P & P Available Aust. only. Price: $A14.95 plus $10.00 p&p per order (includes GST). Just fill in and mail the handy order form in this issue; or fax (02) 9939 2648; or call (02) 9939 3295 and quote your credit card number. 76  Silicon Chip siliconchip.com.au