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For those times when near enough isn’t good enough! PRECISION “AUDIO” PRECISION SIGNAL AMPLIFIER There’s a law in electronics which says you can never have too much test equipment. Even if it is pretty specialised; even if you only need it once every blue moon, there will come a time when you do need it! This one fits the bill perfectly: you’re not going to need it every day . . . but when you do, you’ll thank your good sense that you do have one on hand! S o what are we talking about? It’s a Precision “Audio” Signal Amplifier. It’s used when you need to know – exactly – what “audio” signal you’re dealing with. “Audio” is in quotes because it will actually handle signals way below the normal audio range (down to just 6Hz) and it can go all the way up to more than 230kHz . . . or even higher. Talk about the proverbial “DC to Daylight” amplifier . . . this one is not far off! It can deliver a particularly impressive 30V peak-to-peak (10.6V RMS) up siliconchip.com.au to around 230kHz. It has two switched gain settings of either 1.00 times (0dB) or 10.000 times (+20dB), and it’s powered from a standard 12V AC plugpack. OK, so why would you want one? Let’s say you’ve built some audio gear and want to check it out properly. Or maybe, you need to accurately calibrate other test gear. Or perhaps (and we imagine this will be the biggest market) you’re in By Jim Rowe Australia’s electronics magazine the service game and need to troubleshoot a misbehaving unit. You may already have a low-cost waveform generator (there are many on the market these days) and they are increasingly built into DSOs. They can usually generate sine, square, triangle and often ‘arbitrary’ waveforms with varying frequencies and amplitudes. But the maximum amplitude is usually limited to about 5V peak-to-peak and often, that simply isn’t sufficient. This project overcomes that limitation. It can be connected to the output October 2019  91 of the waveform generator to provide exactly ten times gain, boosting the signal level to just over 10V RMS. And because the gain applied is very precise, you don’t need to check the output level. You just set the generator to produce a waveform with 1/10th the needed amplitude and the signal amplifier does the rest. I came up against this problem when calibrating our new Digital Audio Millivoltmeter, described starting on page 42 of this issue. I have a few signal generators, but none of them could produce a sinewave with sufficient amplitude to calibrate its “HIGH” range. So I decided to design and build this precision amplifier, to generate accurate signals of a high enough amplitude for me to calibrate it. And I realised just how useful this would be for other audio projects! It provides a choice of two accurately known gain ranges (1:1/0dB or 1:10/+20dB), over a relatively wide range of frequencies, from 20Hz up to beyond 200kHz. Circuit details The Signal Amplifier circuit is shown in Fig.2. As you can see, the amplifier itself (lower section) is quite straightforward. That’s because we are using a rather special op amp, the Analog Devices ADA4625-1 (IC1). It offers very high input resistance, thanks to the use of JFET input transistors. It has a typical gain-bandwidth product of 18MHz, very low noise, fast settling time (to within 0.01% in 700ns), a rail-to-rail output swing and the ability to operate from a ‘single Features & specifications Input impedance: .............................. 100kΩ//9pF Output impedance: ........................... 51Ω (each output) Gain: .......................................................... A=1 (0dB) or A=10 (+20dB) Frequency range: (see Fig.1)........ For A=1: 6Hz to >1MHz (+0,-0.3dB) ........................................................................ For A=10: 20Hz to 230kHz (+0,-0.3dB) Maximum input signal level: ..... For A=1: 10.6V RMS (+20.5dBV) ........................................................................ For A=10: 1.06V RMS (+0.5dBV) Maximum output signal level: ... 10.6V RMS (+20.5dBV) THD+N: .................................................... For A=1: 0.0007% (-103dB) ........................................................................ For A=10: 0.007% (-83dB) Power supply: ...................................... 12V AC at <100mA supply’ of up to 36V. Other features include a low output resistance in closed-loop mode (typically 2Ω when gain=1 or 18Ω when gain=10) and the ability to drive load capacitances up to 1nF in closed-loop unity-gain operation. We are using IC1 in a standard noninverting configuration, with the input signal from CON1 coupled to its noninverting input (pin 3) via a 1µF metallised polyester capacitor. The output from IC1 is then fed to output connectors CON2 and CON3 via another 1µF metallised polyester coupling cap, with a 51Ω protective (and impedance-matching) resistor in series with each connector. Switch S1 is used to alter the feedback around IC1, to provide either unity gain or a gain of 10. In the A=10 position, the 100kΩ 0.1% resistor forms the top arm of the feedback divider, while the lower arm is formed by the series combina- 20.5 SIGNAL AMPLIFIER GAIN in dB 20.0 x10 RANGE 19.5 19.0 0.5 0.0 x1 RANGE –0.5 1Hz 10Hz 100Hz 10kHz 1kHz 100kHz 1MHz FREQUENCY Fig.1: this shows a frequency response plot for the Signal Amplifier at both gain settings. The response in both modes is entirely flat from 100Hz to 50kHz, so ideally, calibration and measurements should be made within that range. But it gives acceptable performance (within 0.3dB) from 20Hz to 230kHz, which more than covers the audio range. 92 Silicon Chip Australia’s electronics magazine tion of the 10kΩ and 820Ω fixed resistors together with VR1, a 15-turn 500Ω trimpot. The trimpot allows us to set the amplifier’s gain to exactly 10.000, by compensating for within-tolerance variations in the value of the 10kΩ and 820Ω resistors (both 1% tolerance) as well as the 100kΩ 0.1% tolerance resistor. Although it’s easy to calculate the nominal lower-arm resistance for a gain of 10.000 (it’s 11.111kΩ), this would need to be made up from at least two more 0.1% tolerance resistors (11.0kΩ and 110Ω), to give a gain of 10.0009 with a tolerance of +0.018% and -0.0162%. By using two 1% tolerance resistors and a 15-turn trimpot, we can achieve even better potential accuracy at a significantly lower cost. But how do you set the gain to exactly 10.000? You just need a relatively accurate DMM. You measure the value of the 100kΩ 0.1% resistor (which should be between 99.9kΩ and 100.1kΩ), then divide that by nine, and adjust VR1 so that the total lower-arm resistance matches the calculated value (which should be close to 11.111kΩ). The upper part of the circuit exists primarily to generate a 32V DC supply voltage from the 12V AC plugpack, so that IC1 can deliver output signal amplitudes as high as 30V peak-to-peak or 10.6V RMS. This is achieved in two stages. First, diodes D1 and D2 and the two 470µF capacitors form a simple ‘voltage doubler’ rectifier configuration, which derives about 38V DC from the incoming 12V AC. This is followed by voltage regulator REG1, an SMD version of the familiar siliconchip.com.au D3 1N5819 K A REG1 LM317M +32V OUT K 10k 3.3k D4 1N5819 100 +16V INSULATED SINGLE HOLE MOUNTING BNC SOCKET 220nF  LED1 K 100k K A 12V AC INPUT 470 F CON4 25V A 35V LOW ESR K D2 1N5819 5.6k 220nF A 470 F 50V 10k 100nF 220nF ADJ 240 D1 1N5819 47 F 50V A IN 25V 330 INPUT CON1 1 F 7 3 IC1: ADA4625 6 100V 2 4 1 F CON2 51 OUTPUT 1 100V SELECT GAIN S1 A = 10 SET x10 GAIN SC 20 1 9 820 VR1 500  15T 10k 10 F CON3 51 OUTPUT 2 A=1 LED1 100k 0.1% ADA4625 8 4 K 25V 1.5pF 1 A 1N5819 precision audio signal AMPLIFIER A K LM317M (SOT-223-3) ADJ OUT IN TAB (OUT) Fig.2: the Signal Amplifier circuit is based around precision JFET-input op amp IC1 and uses a precision resistor and trimpot to provide a very accurate 10 times gain (+20dB), to boost the level of signals from devices such as arbitrary waveform generators. The 12V AC supply is boosted and regulated to 32V DC using a full-wave voltage doubler configuration (D1 & D2), followed by a low-ripple adjustable linear regulator (REG1). The SILICON CHIP Inductance - Reactance - Capacitance - Frequency READY RECKONER For ANYONE in ELECTRONICS: HU 420x59G4Em on heavy photo pa m per You’ll find this wall chart as handy as your multimeter – and just as ESSENTIAL! Whether you’re a raw beginner or a PhD rocket scientist . . . if you’re building, repairing, checking or designing electronics circuits, this is what you’ve been waiting for! Why try to remember formulas when this chart will give you the answers you seek in seconds . . . easily! Read the feature in the January 2016 issue of SILICON CHIP (you can view it online) to see just how much simpler it will make your life! All you do is follow the lines for the known values . . . and read the unknown value off the intersecting axis. It really is that easy – and quick (much quicker than reaching for your calculator! Printed on heavy (200gsm) photo paper Mailed rolled in tube for protection Limited quantity available Mailed Rolled in Tube: Just $20.00 ORDER NOW AT siliconchip.com.au inc P&P & GST www.siliconchip.com.au/shop Australia’s electronics magazine October 2019  93 Parts list – Precision Signal Amplifier 1 double-sided PCB, code 04107191, 92 x 51mm 1 diecast aluminium box, 111 x 60 x 54mm [Jaycar HB5063] 1 12V AC plugpack (100mA or higher) with 2.1 or 2.5mm plug 1 SPST mini toggle switch (S1) 1 insulated BNC socket, single hole panel mounting (CON1) 2 BNC sockets, single hole panel mounting (CON2,CON3) 1 PCB-mount concentric DC socket, 2.1mm or 2.5mm inner diameter (to suit plugpack) (CON4) 4 25mm long M3 tapped spacers 8 M3 x 6mm panhead machine screws 2 1mm PCB stakes (optional) 9 30mm lengths of hookup wire (to connect S1 & CON1-3 to the PCB) Semiconductors 1 ADA4625-1ARDZ low-noise JFET input op amp, SOIC-8 SMD package (IC1) 1 LM317M adjustable voltage regulator, SOT-223-3 SMD package (REG1) 4 1N5819 40V 1A schottky diodes (D1-D4) 1 3mm green LED (LED1) Capacitors 2 470µF 25V RB electrolytic 1 47µF 35V RB low-ESR electrolytic 1 10µF 25V multi-layer ceramic (X5R 3216/1206 SMD) 2 1µF 100V polyester (radial leaded) 3 220nF 50V multi-layer ceramic (X5R 3216/1206 SMD) 1 100nF 50V multi-layer ceramic (X5R 3216/1206 SMD) 1 1.5pF 100V multi-layer ceramic (C0G 1206 or 0603 SMD) Resistors (1% all SMD 3216/1206 SMD unless otherwise stated) 1 100kΩ 0.1% 0.25W axial leaded 1 100kΩ 3 10kΩ 1 5.6kΩ 1 3.3kΩ 1 820Ω 1 330Ω 1 240Ω 1 100Ω 2 51Ω 1 500Ω 15-turn horizontal trimpot (VR1) ration, each filter capacitor only recharges at 50Hz. The 32V rail is also used to provide a ‘half supply voltage’ bias of 16V for the non-inverting input of IC1, via a 10kΩ/10kΩ resistive divider with a 220nF ripple filter capacitor. LED1 is a power-on indicator, con- 50V D4 CON2 TP 32V 100 1 1210 100k 1 F 51 51 CON3 100nF A S1 (ABOVE) 10 F D3 50V 220nF OUTPUTS 3.3k GAIN 10k IC1 10k SET x10 820 LED1 0.1% 1.5pF TP GND Fig.3: all of the components mount on this PCB, except for CON1-CON3 and switch S1. The design uses a mix of through-hole and surface-mounting parts. Fit them where shown here, being careful to ensure that IC1, LED1, diodes D1-D4 and the electrolytic capacitors are mounted with the correct polarity. 94 Silicon Chip Almost all of the circuitry and components are mounted on a single PCB which fits inside a diecast aluminium box, for shielding. The PCB measures 92 x 51mm and is coded 04107191. Refer now to the overlay diagram, Fig.3, along with the matching photo. The only components not mounted on the PCB are input and output connectors CON1-CON3 and range selection switch S1. These all mount on the box lid/front panel, with short lengths of hookup wire linking them to the PCB. It’s easiest to fit the SMD components to the PCB first, starting with the passives (resistors and capacitors) and then REG1 and IC1. Make sure IC1’s pin 1 dot/divot (or bevelled edge) is orientated as shown in Fig.3. Then fit the leaded parts, starting with the 100kΩ 0.1% resistor and diodes D1-D4 (with the orientations as shown), trimpot VR1, the two 1µF capacitors, the two 470µF and 47µF electrolytic capacitors (longer lead towards + sign) and then the power input connector, CON4. The final step is to fit LED1, which is mounted vertically just below the centre of the PCB. First solder a 2-pin SIL header to the PCB, then solder the LED’s leads to the header pins, with the LED anode towards the front. The underside of the LED body should be about 24mm above the top 5819 47 F 35V LOW ESR POWER VR1 500  15T 220nF 25V D1 100k 4625 10k 1 F Construction 240 REG1 LM317M 470 F 5819 + CON1 5819 330 5.6k 220nF INPUT 470 F25V 12V AC IN CON4 04107191 C 2019 RevC + 19170140 04107191 02 C C9 12019 D2 CRevC v eR 5819 LM317 adjustable regulator. Here it’s configured to provide a regulated output of 32V which is fed to IC1 via a 100Ω resistor. The 220nF capacitor from the ADJ (Adjust) pin to ground improves its ripple rejection, which is helpful here as with the voltage doubler configu- nected to the +32V line via a 3.3kΩ series resistor. And here’s the almost-complete PCB immediately before final assembly. Naturally, S1 and the connectors are not yet fitted because these mount on the front panel and connect to the PCB via short wire links. The PCB “hangs” off the front panel via 25mm M3 tapped spacers, which are screwed to the four holes in the PCB corners. Australia’s electronics magazine siliconchip.com.au of the PCB. This will allow it to protrude through the box lid/front panel when the unit is assembled. Your Signal Amplifier PCB is then virtually complete. The next step is to set the gain of its 10x/20dB range. Use a DMM with the best resistance accuracy possible. Monitor the resistance between the junction of the 10kΩ resistor and 10µF capacitor near VR1, and the PCB’s ground. Then adjust trimpot VR1 until this resistance is as close as possible to one-ninth the resistance of the 100kΩ 0.1% resistor. If you’re not confident of your DMM’s accuracy, it may be easier to simply adjust the lower arm’s resistance to measure 11,111Ω (11.111kΩ, or 100kΩ÷9). But if you can measure both values on the same range, any proportional inaccuracy in the DMM itself should be cancelled out as it applies to both measurements. It’s now time to test the completed PCB by connecting a source of 12V AC, such as an AC plugpack. LED1 should light up. Measure the voltage between TP 32V and TP GND. You should get a reading close to 32V. If so, you can disconnect the power lead and put the PCB aside while you work on the box. Preparing the box along with switch S1, and then turn the panel over and solder short lengths of insulated hookup wire to the rear connection lugs of the connectors and S1. Next, attach the four 25mm-long M3 tapped spacers to the corners of the front panel, using four 12mm long M3 screws. Then you can cut the hookup wires soldered to CON1-CON3 and S1 to a length which will enable them to A just pass through the PCB holes when the board is attached to the rear of the spacers. Remove about 6mm of insulation from all of the wire ends, so that they can be easily soldered to the matching PCB pads. After bending these wires so their ends are positioned to meet with the holes in the PCB, offer up the PCB 42 42 A C 21.5 C 41 9.5 A 1 15 CL 41 9.5 21.5 B C A 42 A 42 CL HOLES A: 3mm DIAMETER HOLE B: 6.5mm DIAMETER HOLES C: 9.0mm DIAMETER ALL DIMENSIONS IN MILLIMETRES (FRONT OF BOX) 3.5mm DIAMETER 18.5 24.5 This is fairly straightforward. It involves drilling a total of nine holes in the box lid/front panel, another hole in the front of the box itself and then a larger hole (12mm diameter) in the box rear. The locations and sizes of all these holes are shown in the cutting diagram, Fig.4. After all of the holes have been drilled, cut and de-burred, you can attach a dress front panel to the lid, to give the Signal Amplifier a neat and user-friendly look. You can copy the front panel artwork shown in Fig.6, or download is a PDF file from the SILICON CHIP website. Then you can print it out and laminate it in a protective pouch to protect it from getting soiled. It can then be attached to the box lid using double-sided adhesive tape. The final step is to use a sharp hobby knife to cut the holes in the dress panel, to match those in the lid underneath. Now fit the three input and output BNC connectors to the front panel, Fig.4: here are the locations and sizes of the holes that need to be drilled in the diecast aluminium enclosure. For the larger holes, it’s best to start with a smaller pilot hole (eg, 3mm) and then enlarge it to size using either a stepped drill bit, a series of larger drills or a tapered reamer. That ensures accurate positioning and a clean, round hole. You can copy this diagram and attach it to the box using tape to use it as a template siliconchip.com.au Australia’s electronics magazine CL (REAR OF BOX) 17 12mm DIAMETER October 2019  95 And here’s an end-on view from the input end. x10 gain is calibrated via the multi-turn pot (blue component) in the foreground. Above are two views of the assembled unit from the front (top) and the rear (bottom). assembly to the spacers on the rear of the panel. With a bit of jiggling, you should be able to get all of the wires to pass through their matching holes. You can then attach the PCB to the spacers using four more 6mm long M3 screws, up-end the assembly and solder each of the wires to its PCB pad. Your Signal Amplifier is now complete, and should look like the one shown in our photos. All that remains is to lower the lidand-PCB assembly into the box and fasten them together using the four M4 countersunk-head screws supplied with it. Checkout & use At this stage, your Signal Amplifier should be ready for use. Remember that it can deliver a maximum output voltage of 30V peak to peak or 10.6V RMS, assuming that it is feeding a high-impedance load, of 50kΩ or more. If the load impedance is much lower, the maximum output amplitude will be slightly reduced. Note that you can check and even adjust its calibration even after it has been sealed in its box. To do this, you will need an audio oscillator or function generator and a DMM with trueRMS AC voltage range with reasonable accuracy and resolution. Set your oscillator or function gen96 Silicon Chip erator to produce a sinewave at 1kHz and around 1V RMS, then connect its output to the input of the Signal Amplifier. Then connect your DMM’s input to one of the Signal Amplifier outputs. With the Signal Amplifier’s gain set to unity (A=1), power it up and your DMM should indicate an AC voltage very close to 1.00V. If not, you may need to tweak the output of your oscillator/ generator until this reading is achieved. Then all you have to do is select the Signal Amplifier’s x10 range, whereupon the DMM reading should jump to 10.000V, or very close to it. Adjust trimpot VR1 with a small screwdriver or alignment tool (through the small hole in the front of the box), until the DMM is reading 10.000V. SC Fig.5: this scope grab of the unit’s output with a full-swing (32V peakto-peak) 20kHz square wave demonstrates the fast slew rate and quick settling time of the ADA4625 op amp. You can see that there is minimal rounding and overshoot after each transition and it settles close to the target value in well under 1µs. 12V AC INPUT www.siliconchip.com.au INPUT OUTPUT PRECISION AUDIO SIGNAL AMPLIFIER Rout = 51 POWER OUTPUT Rin = 100k (3Vp–p MAX) A = 1.00 A = 10.00 Rout = 51 SET x10 GAIN Fig.6: this 1:1 front panel artwork can be copied and fixed to the lid or can be downloaded from the SILICON CHIP website, printed and then applied. Australia’s electronics magazine siliconchip.com.au