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BASS CONTROL TREBLE CONTROL TONE DEFEAT MOTORISED VOLUME CONTROL INFRARED REMOTE CONTROL Ultra-low-distortion Preamplifier with Tone Controls Part 1 We have published many fine low-distortion stereo preamplifiers. But often without tone controls – and we are regularly asked, ‘how do I add tone controls?’ Well, this design not only has tone controls but also has infrared remote volume control, input switching and muting. Meet the 2020 Ultra-low-distortion Preamplifier! Features RELAY INPUT SELECTION  Very low noise and distortion  Remote-controlled input selection and volume control with muting  Manual volume control plus bass and treble cut/boost controls  Tone control defeat switch bypasses bass and treble controls  Minimal interaction between tone controls  Can be used with just about any power amplifier  Designed to be mounted in the front of a stereo amplifier chassis, but is also suitable for standalone use  Three status LEDs  Runs from ±15V DC 14 INBUILT LED INDICATORS MANUAL INPUT SELECTORS by John Clarke Constructors – please read note in the Parts List before purchasing components. Practical Electronics | April | 2020 W e present a high-quality, low-distortion and low-noise stereo preamplifier that can be used with just about any power amplifier modules to form a stereo amplifier. It can also be used as a standalone preamplifier. A low-cost infrared remote control is used to switch between three separate inputs, adjust the volume or temporarily mute the output. It also includes manual volume, bass and treble controls and pushbuttons to select between the three stereo inputs. LED indicators in the pushbuttons show which input is active. It also has power, acknowledge and mute status LEDs. All in all, it offers considerable advantages over previous models. You could use it with the easy-tobuild, low-cost SC200 amplifier modules (January-March 2018; Altronics kit Cat K5157). Or build it in a case and use it with an existing power amplifier. It’s up to you. It has a motorised potentiometer for volume control, so you can adjust the volume directly with a knob if you don’t want to use the remote. It has an effectively infinite number of possible volume settings, unlike most digital volume controls, which can have quite large steps. This Preamplifier provides widerange bass and treble adjustment knobs to allow you to overcome deficiencies in your loudspeakers, compensate for the room response or just adjust the sound to be the way you like it. While the performance is excellent when the tone controls are active, we have provided the option to bypass them using a push-on, push-off switch. Its integrated LED indicator shows when the tone controls are switched in or out. This switch has three benefits. One, it’s difficult to centre the tone controls precisely when you want the response to be flat, so the switch provides an easy way to achieve that. Two, it provides slightly better performance with the tone controls switched out. And three, it gives you an easy way to hear exactly what effect the tone controls are having, by toggling them on and off. A PIC microcontroller is used to provide the remote control, muting and input selection functions. Input selection is by way of a separate PCB interconnected to the main preamplifier using 10-way ribbon cable. If you don’t need the input selector, you can build the project without it. The micro remembers the last input selection, so it will go back to the same set of inputs even if it’s switched off and on again. Practical Electronics | April | 2020 Specifications (2.2V RMS in/out, 20kHz bandwidth unless otherwise stated): Frequency response: .......... flat from 20Hz to 20kHz (see Fig.3), -1.25dB <at> 100kHz Bass adjustment range: ...... ±15dB at 20Hz; ±13dB at 75Hz Treble adjustment range: .... ±15dB at 20kHz; ±14dB at 10kHz Input impedance: ................ 22k Output impedance: .............. 100 THD+N: ................................ <0.001%, 80kHz bandwidth; ............................................. typically <0.0003%, 20kHz bandwidth (see Fig.1) Signal-to-noise ratio: .......... -121dB with tone controls out; -114dB with tone controls in Channel separation: ............ >80dB <at> 1kHz; >67dB <at> 10kHz (see Fig.4) Input separation: ................. >98dB <at> 1kHz; >80dB <at> 10kHz Maximum gain: ................... two times (6dB) Signal handling: .................. up to 4V RMS input, 8V RMS output Performance This preamplifier has excellent performance. It uses low-distortion, low-noise op amps throughout, plus we have taken great care to specify very linear types of capacitor and to keep resistor values low, where their Johnson (thermal) noise contribution is likely to affect the signal. Inevitably, the tone control circuitry adds some noise when it is switched in. But performance is still very good with the tone controls in, giving a THD+N figure of just 0.00054% at 1kHz and 0.0007% at 10kHz. By comparison, with the tone controls out, those figures become 0.00044% and 0.00048% respectively – see Fig.1. Those measurements were made with a bandwidth of 20Hz-80kHz, which is necessary to measure distortion at higher frequencies accurately. But such a measurement includes a significant amount of ultrasonic noise (ie, in the 20-80kHz range). And Fig.1 shows that the distortion performance is dominated by noise. So we also made measurements with a 20Hz-22kHz bandwidth, shown in blue on Fig.1, and this reveals that the true audible distortion and noise level is closer to 0.00025% – an astonishingly low figure. Fig.3 shows the frequency response with the tone control at either extreme, and switched out (the blue curve). This demonstrates that when you’re not using the tone controls, the frequency response is very flat. You can barely see the deviation on this plot; zooming in, we can see that the response is down only 0.2dB at 20Hz and less than 0.1dB at 20kHz. Fig.4 shows the coupling between channels, which is typically less than −80dB, and the coupling between adjacent inputs, typically around −100dB. So isolation between channels and inputs is very good. The signal-to-noise ratio figure is especially good; over 120dB with a 2.2V RMS input signal (typical for CD/DVD/Blu-ray players), the tone controls switched out and the volume pot at unity gain. In summary, you can be confident when using this Preamplifier that it will not negatively affect the audio signals passing through it, regardless of whether you are using the tone controls. Capacitor and potentiometer selection We mentioned earlier that we’re using linear capacitor types where that’s important, and also keeping resistance values low to minimise thermal noise. For capacitors between 10nF and 100nF, we have specified MKT polyester (plastic dielectric) types. While polyester is not quite as linear as polypropylene or polystyrene dielectrics, none of those capacitors are critical enough to cause a measurable increase in distortion, as demonstrated by our performance graphs. But there are some capacitors with values below 1nF where the dielectric is important and this presents us with some difficulty, since MKT capacitors with values below 1nF are not particularly easy to get. However, we’ve found them (see parts list) and that is what we have used in our prototype, with good result. If you can get MKP (polypropylene) capacitors instead, those will certainly work well and we would encourage that. But we have also mentioned the possibility of using NP0 ceramics. We have tested these in the past and found that they are just as good as the best plastic dielectrics in situations where linearity is critical. However, do be careful because many ceramic capacitors are not NP0 (also known as C0G) types, especially values above 100pF. Fig.5 shows a distortion plot for a simple low-pass filter comparing two capacitors of the same value, one polypropylene and one ceramic (not NP0/C0G). As you can see, the ceramic capacitor produces a lot more distortion. So make sure you use one of the types specified. 15 0.01 Preamplifier THD vs Freq., 2.2V in/out 01/13/19 10:27:03 0.01 Total Harmonic Distortion (%) Total Harmonic Distortion (%) Tone in, 80kHz BW Tone out, 80kHz BW Tone out, 22kHz BW 0.002 0.001 0.0005 0.002 Tone in, 22kHz BW Tone out, 22kHz BW 0.001 0.0005 0.0002 0.0002 20 50 100 200 500 1k 2k Frequency (Hz) 5k 10k 20k Fig.1: distortion across the entire range of audible frequencies is extremely low, whether the tone controls are active or not. There is a slight rise in distortion above 10kHz, but below that, the distortion is below the noise floor. Regarding resistance, you may find it a bit strange that we have specified a 5kΩ volume control potentiometer as values in the range of 10kΩ-100kΩ are more commonly used. But we have chosen 5kΩ because the thermal noise contribution of the volume control pot can be a major limiting factor in the performance of a low-distortion preamplifier and suitable motorised pots are available. Op amps IC1a and IC2a buffer the signal from the source so that it does not have to drive the 5kΩ impedance; these op amps are more than capable of driving such a load without increased distortion. If you can’t get the 5kΩ motorised pot (available from Altronics; see parts list), you can use a 20kΩ pot instead; also a pretty standard value. In that case, we have made provision for two 4.7kΩ shunt resistors to lower the impedance seen by the following stage, giving you most of the performance benefits of a 5kΩ pot. These have minimal effect on the pot curve, so it still works well as a volume control. Fig.6 shows the difference in distortion with and without these shunts (the signal level is lower here than in the other figures, hence the higher base level). The performance with the proper 5kΩ pot is slightly better again. Remote control Pressing the Volume Up or Volume Down buttons on the infrared remote causes the motorised pot to rotate clockwise or anticlockwise. It takes about nine seconds for the pot to travel from one end to the other using these controls. For finer adjustment, the Channel Up and Channel Down buttons on 16 01/13/19 10:32:39 0.005 0.005 0.0001 Preamplifier THD vs Level, 1kHz, gain=1 0.0001 0.05 0.1 0.2 0.5 1 Level in/out (V RMS) 2 5 Fig.2: this shows the effect of noise; as you reduce the volume and thus the output signal level, the fixed circuit noise becomes larger in proportion and so total harmonic distortion goes up. However, even at very low volume levels, it’s below 0.01% so it won’t be noticeable. the remote can be used instead. These cause the pot shaft to rotate about one degree each time one of these buttons is briefly pressed. Holding one of these buttons down rotates the pot from one end to the other in about 28 seconds. If any of these buttons is held down when the pot reaches an end stop, a clutch in the motor’s gearbox begins to slip so that no damage is done to the motor. The code also provides a convenient automatic muting feature. Press the Mute button on the remote and the volume control pot automatically rotates to its minimum position and the motor stops. Hit the button again and it returns to its original position. If you don’t want the pot to return all the way to its original setting, you can simply increase the volume to your desired new level instead. So how does the unit remember its original setting during muting? The microcontroller monitors the time it takes for the pot to reach its minimum setting and the minimum pot setting is detected when the load on the motor increases at the potentiometer end stop, as the clutch begins to slip. When the Mute button is pressed again, power is applied to the motor drive for the same amount of time, rotating it back to the original position. The orange ‘Ack’ LED flashes whenever an infrared signal is being received from the remote, while the yellow Mute LED flashes while the muting operation is in progress and then remains on when the pot reaches its minimum setting. Circuit description Fig.7 shows the main preamplifier circuit – but for clarity, only the left channel components are included. The right channel is identical and the matching part designators are provided, in brackets. The following description refers to the left-channel part names. The audio signal from the Input Switching board is AC-coupled to the input of the first op amp (IC1a) via a 22µF non-polarised (NP) electrolytic capacitor and 100Ω resistor. A 22kΩ resistor to ground provides input DC biasing and sets the input impedance to around 22kΩ. The 100Ω resistor, ferrite bead and 470pF capacitor form a low-pass filter to attenuate radio frequency (RF) signals ahead of the op amp input. IC1a operates as a voltage amplifier with a gain of two, due to the two 2.2kΩ feedback resistors. The 470pF capacitor combines with the feedback resistors to roll off the top-end frequency response, with a −3dB point at about 150kHz. This gives a flat response over the audio spectrum while eliminating the possibility of high-frequency instability or RF demodulation. IC1a’s pin 1 output is fed to the top of volume control potentiometer VR1a (5kΩ log) via a 22µF nonpolarised capacitor. The signal on its wiper is then AC-coupled to the pin 5 non-inverting input of IC1b via a 4.7µF non-polarised capacitor. This coupling arrangement prevents direct current from flowing through any part of the volume control potentiometer, VR1. Even a small direct current can cause noise when the volume is adjusted. As mentioned earlier, the circuit was designed for a 5kΩ motorised volume control pot, as this results in good noise performance. However, if you Practical Electronics | April | 2020 +20 Preamplifier Frequency Response +15 -0 +5 +0 -5 -10 -30 -40 -50 -60 -70 -80 -90 -100 -15 -110 20 50 100 200 500 1k 2k Frequency (Hz) 5k -120 10k 20k 20 can’t get one you can use a more common 20kΩ potentiometer and fit resistors R1 and R2, so that the circuitry has a similar impedance, resulting in the same overall frequency response. lC1b operates as a unity-gain buffer and provides a low-impedance output regardless of the volume control setting. Its pin 7 output is fed to the tone control section and also to switch S4a. When S4a is set to the ‘tone out’ position, the output from IC1b is coupled via the 22µF capacitor to output socket CON3, via a 100Ω resistor. Therefore, the tone controls are effectively out of circuit. The 100Ω resistor isolates the op amp output from any capacitive loads that might be connected to ensure stability. 50 100 200 500 1k 2k Frequency (Hz) 5k 10k 20k Fig.4: the crosstalk and separation figures are good. Crosstalk is how much of the left channel signal feeds into the right channel or vice versa. Channel separation is how much signal from input #1 couples into input #2 or vice versa. Fig.3: the blue line shows the Preamplifier’s frequency response with the tone controls switched out; you can see that it’s very flat, varying by only 0.2dB across the entire audible frequency range. The red and green curves demonstrate the range possible with bass and treble adjustments. This resistor and ferrite bead in series with the output also attenuate any RF noise which may have been picked up by the board. Tone controls When S4a is in the ‘tone in’ position, output CON3 is instead driven from the tone control circuitry, so potentiometers VR2a and VR3a adjust the amount of bass and treble in the signal. Op amp IC3a forms the active tone control in conjunction with VR2a and VR3a and associated resistors and capacitors. The bass and treble tone circuitry is a traditional Baxandall-style design. This is an inverting circuit, so it’s inverted again by unity-gain buffer IC3b to restore the original signal phase. When the wipers of potentiometers VR2a and VR3a are centred, the impedance between output pin 1 of IC3a and each wiper is equal to the impedance between the wiper and output pin 7 of IC1b. So in this condition, IC3a operates as a unity-gain inverting amplifier for all audio frequencies. Therefore, in this case, the tone controls have little effect on the signal – they just add a little noise. Bass adjustment The bass control (VR2a) provides cut (anti-clockwise) or boost (clockwise) to low frequencies. The impedance of each of the two 100nF capacitors for high-frequency signals is low and so they can bypass VR2a entirely. THD+N vs Frequency, 20Hz-80kHz BW, 1.5V in/out THD+N vs Frequency, 20Hz-80kHz BW, 1V in, 2V out 09/15/11 11:41:02 09/15/11 11:41:02 0.01 0.01 With 4.7k shunt resistor Without 4.7k shunt resistor 470pF Ceramic (X7R) 470pF MKT Polyester 0.005 Total Harmonic Distortion + Noise (%) 0.005 Total Harmonic Distortion + Noise (%) 01/13/19 10:30:25 Crosstalk right-to-left Crosstalk left-to-right Channel separation left Channel separation right -20 +10 -20 Preamplifier Channel/Input Separation -10 Relative Amplitude (dBr) Relative Amplitude (dBr) 01/13/19 09:55:36 Tone controls full boost Tone controls full cut Tone controls bypassed 0.002 0.001 0.0005 0.0002 0.002 0.001 0.0005 0.0002 SC SC 20 1 9 0.0001 20 50 100 200 500 1k Frequency (Hertz) 2k 5k 10k Fig.5: distortion versus frequency of a simple low-pass filter using either a 470pF MKT capacitor or a 470pF ceramic (non-NP0/C0G) capacitor. As you can see, distortion rises dramatically at higher frequencies with the ceramic capacitor due to its non-linearity and its lower impedance at higher frequencies, which causes it to shunt more of the signal and thus have a stronger effect. Practical Electronics | April | 2020 20k 20 1 9 0.0001 20 50 100 200 500 1k Frequency (Hertz) 2k 5k 10k 20k Fig.6: if you must use a 20k motorised potentiometer to build this Preamplifier, fitting the two extra 4.7k resistors (R1 and R2) will keep high-frequency distortion low, by lowering the input impedance seen by the following buffer stage. This allows it to perform optimally and also lowers thermal noise. 17 +15V LEFT IN (CON2) CON1 22F NP 100 2 470pF 22k IC1 – IC 4 : NE5532 OR LM833 FERRITE BEAD FB1 (FB2) 3 IC1a (IC2a) 1 22 F NP VR1a (VR1b) 5k LOG 2.2k LOW-PASS FILTER 35V 10 VOLUME 4.7F NP 100 R1 (R2) 4.7k 470pF 2.2k 22 F NP 100 F 100nF 100k 5 8 6 IC1b (IC2b) 7 4 AMPLIFIER GAIN = 2 BUFFER FIT R1 & R2 ONLY IF DUAL 20k POTENTIOMETER IS USED FOR VR1 (NOTE: ONLY LEFT CHANNEL SHOWN; LABELS IN BRACKETS REFER TO RIGHT CHANNEL) –15V 100 +5V +5V 100 F 16V 2.7k 100nF A 10k 14 10k LK3 OUT: MUTE RETURN LK3 IN: NO MUTE RETURN IRD1 3 LK3 3 1  6 MCLR RB3 RA4 RB4 RB0 RA0 1k 9 Q1 BC327 B INPUT1 CON7 1 2 3 4 5 6 7 8 9 10 12 INPUT2 13 INPUT3 11 RB1 RB6 RB7 RB2 RB5 +5V 15 X1 4MHz 22pF 22pF 16 AN3 OSC2 RA1 RA2 OSC1 Vss 5 B C 1k 10 Q3 BC327 E E C 100nF CON6 17 MOTOR – + IC5 PIC16F88-I/P 2 TO INPUT BOARD POWER  LED1 K Vdd 4 100 F 16V 1k 7 1k 8 Q2 BC337 2 18 330 1 B 330 ACK LED2 A A  MUTE  LED3 K K 18k C E ENDSTOP ADJUST VR4 1k 10nF B Q4 BC337 C E CURRENT MONITOR 10 100nF SC LOW NOISE Low-noise Preamplifier volumeCONTROL control PREAMP with WITH tone TONE controls CONTROLSand & Rremote EMOTE VOLUME 20 1 9 Any change in the position of VR2a’s wiper will thus have little effect on high frequencies. For example, at 1kHz, the 100nF capacitors have an impedance of 1.6kΩ each. That is considerably lower than the 5kΩ value of the half of the potentiometer track that they are connected across when VR2a is centred and therefore the capacitors shunt much of the signal around VR2a. But at 20Hz, the 100nF capacitors have an impedance of 80kΩ and so minimal current passes through them; almost all of it goes through VR2a. Therefore, VR2a has a significant effect on the amplitude of a 20Hz signal and so it provides much more boost or cut at lower frequencies. When VR2a is rotated clockwise, the resistance from output pin 1 of IC3a to its wiper increases, while the resistance 18 from the wiper to the input signal decreases, providing increased amplification. And when rotated anti-clockwise, the opposite occurs, decreasing amplification. Because the capacitors shunt a different amount of signal around the pot at different frequencies, this gain is also frequency-dependent. The 1.8kΩ resistors set the maximum boost and cut range. They have been chosen to allow up to ±15dB adjustments at around 20Hz, dropping to around ±1dB at 1kHz. The measured frequency response with the controls at minimum, centred and at maximum is shown in Fig.3. Treble adjustment Treble control VR3a operates differently to VR2a. It is configured to have more effect on higher frequency signals. This is achieved by connecting capacitors in series with the pot channel, rather than across it. At low frequencies, the 15nF capacitors have a high impedance, eg, 106kΩ at 100Hz. This is very high compared to the 10kΩ channel resistance and so most of the feedback signal at this frequency will flow through the bass network, which has a DC resistance of 13.6kΩ and therefore a much lower impedance. So VR3a will have little effect on the gain at low frequencies. At high frequencies, the 15nF capacitors have a lower impedance, eg, around 1kΩ at 10kHz and so the treble controls are effectively brought into circuit, providing adjustable gain similarly to the circuitry surrounding VR2a. The 1kΩ resistors at each end of VR3a set the maximum boost or cut for Practical Electronics | April | 2020 +15V +15V 47pF 100 F 15nF 100nF BASS VR2a (VR2b) 10k LIN 1k BOOST 12k 1k BOOST CUT CUT 100nF 100nF 1M 1.8k TONE CONTROLS 2.2k 1k 1.8k 2 8 3 IC3a (IC4a) TONE OUT SWITCH 5 1 22 F NP GND 22 + 15 V CON5 +15V IN 1 0 0 F 16V 100k INVERTER –15V OUT FB3 (FB4 ) 4 –15V REG 1 7805 LEFT OUT FERRITE BEAD IN 7 IC3b (IC4b) (CON4) CON3 100 S4a (S4b) 6 15nF 100 F 100 F 16 V 2.2k –15V TREBLE VR3a (VR3b) 10k LIN 100k 10k OUT OUT S4c +15V LEFT CHANNEL ONLY 100 F 35VW LK4 IN A IN LED (IN S4) 470 F 10 16V LEFT G ROUND 0V 10 470 F RIGHT G ROUND 16V  –15V K K B E 1 C 2 3 NE5532/LM833 7805 IRD1 BC327, BC337 LEDS A –15V GND IN GND OUT 4 8 1 Fig.7: here’s the circuit diagram for the main Preamplifier PCB, incorporating the volume and tone controls and tone switching (at the top) and the infrared remote volume control and input switching circuitry (at bottom). The analog signal path is built around dual low-noise op amps IC1-IC4 and motorised potentiometer VR1. The volume control and input selection circuity is based on microcontroller IC5, motor driver transistors Q1-Q4 and infrared receiver IRD1. high frequencies, up to around ±15dB, similar to the bass control. You can see this in Fig.3. The 12kΩ and 1kΩ resistors between the bass and treble potentiometer wipers minimise the inevitable interaction between the two controls. Note that while the treble potentiometer is isolated from direct current flow due to the 15nF capacitors in series, the bass potentiometer requires two extra 100µF capacitors. These do not affect the action of the bass control; they are just there to block direct current flow through VR2a. This is for the same reason that DC is blocked for VR1; to prevent noise during adjustments. The 1MΩ feedback resistor between pins 1 and 2 of IC3a provides DC bias for the pin 2 input, while the 47pF capacitor prevents high-frequency oscillation of the op amp by reducing the gain at ultrasonic frequencies. When S4a is set to the ‘tone in’ setting, the output from IC3b (reinverting IC3a’s signal inversion) is then fed to the CON3 output as mentioned above. Another pole of the switch (S4c) controls the indicator LED that is contained within the switch. It is powered from the ±15V supplies via a 10kΩ resistor and therefore receives about 3mA. Practical Electronics | April | 2020 Jumper link LK4 can be removed to prevent this LED from lighting, or moved into one position or the other to invert its function. In other words, LK4 selects whether the LED lights when the tone is in or out. Note that the ‘tone out’ position of S4 is when the switch is pressed in. In other words, it acts like a defeat switch. Remote control circuitry The Remote Control circuitry is also shown in Fig.7. Signals from the handheld remote are picked up by infrared receiver IRD1. This is a complete infrared detector and processor. It picks up the 38kHz pulsed infrared signal from the remote and amplifies it to a constant level. This is then fed to a 38kHz bandpass filter, after which it is demodulated to produce a serial data burst at its pin 1 output. The resulting digital data then goes to the RB0 digital input (pin 6) of PIC16F88-I/P microcontroller IC5 for decoding. Depending on the button pressed on the remote, IC5 either drives the volume control motor (via an external transistor circuit) to change the volume, or sends one of its RB6, RB7 or RB5 output low to select a new input. The input routing is controlled by the Input Selector board which is connected via CON7. IC5 is programmed for a remote control which sends Philips RC5 codes. It supports three different sets of RC5 codes, normally referred to as TV, SAT1 or SAT2. You must also program the universal remote control with the correct number for one of these sets of code. We will explain how to do that next month. You also need to set IC5 to expect the correct set of codes; we will also describe that next month. Driving the pot motor IC5’s RB1-RB4 outputs (pins 7-10) drive the bases of transistors Q1-Q4 via 1kΩ resistors. These transistors are arranged in an H-Bridge configuration and control the motor. The motor is off when the RB1-RB4 outputs are all high. In that state, RB3 and RB4 turn PNP transistors Q1 and Q3 off, while RB1 and RB2 turn NPN transistors Q2 and Q4 on. As a result, both terminals of the motor are pulled low and so no current flows through it and it won’t rotate. The emitters of Q2 and Q4 both connect to ground via a common 10Ω resistor, which is used for motor-current 19 sensing. The transistors operate in pairs so that the motor/potentiomenter can be driven in either direction, to increase or decrease the volume. To drive the motor clockwise, RB2 goes low and turns off transistor Q2, while RB3 goes low and turns on Q1. When that happens, the left-hand terminal of the motor is pulled to +5V via Q1, while the right-hand terminal is pulled low via Q4. As a result, current flows through Q1, through the motor and then via Q4 and the 10Ω resistor to ground. Conversely, to turn the motor in the other direction, Q1 and Q4 are switched off and Q2 and Q3 are switched on. As a result, the righthand motor terminal is now pulled to +5V via Q3, while the left-hand terminal is pulled low via Q2. Regardless of the direction of rotation, current flows through the 10Ω shared emitter resistor and so the voltage across it varies with the current drawn. Typically, the motor draws about 40mA when driving the potentiometer but this rises to over 50mA when the clutch is slipping. As a result, there is about 0.4-0.5V drop across the 10Ω resistor. This is ideal because the motor is rated at 4.5V and the result of subtracting the resistor voltage from the 5V supply is that it provides the correct motor voltage. Current sensing and muting Once the potentiometer has reached full travel in either direction, a clutch in the motor’s gearbox begins to slip. This prevents the motor from stalling and possibly overheating if the button on the remote continues to be held down. The clutch mechanism also allows the user to rotate the pot shaft manually. As mentioned earlier, when you press the mute button on the remote control, the volume control is rotated fully anti-clockwise. Microcontroller IC5 detects when the wiper reaches its end stop by detecting the increase in the motor current when the limit is reached and the clutch slips. That’s done by taking a sample portion of the voltage across the 10Ω resistor using trimpot VR4. The voltage at VR4’s wiper is filtered using an 18kΩ resistor and a 100nF capacitor to remove the motor commutator hash and is applied to lC5’s analogue AN3 input (pin 2). IC3 then measures the voltage on AN3 to a resolution of 10 bits, or about 5mV (5V ÷ 210). Provided this input is below 200mV, the PIC microcontroller allows the motor to run. However, as soon as 20 the voltage rises above this 200mV limit, the motor is stopped. When the motor is running normally, the current through it is about 40mA, which produces 0.4V across the 10Ω resistor. VR4 attenuates this voltage and is adjusted so that the voltage at AN3 is slightly below the 200mV limit. Note that the AN3 input is monitored only during the muting operation. At other times, when the volume is being set by the Up or Down buttons on the remote, the clutch in the motor’s gearbox assembly slips when the potentiometer reaches its clockwise or anticlockwise limits. As described above, pressing Mute on the remote again after muting returns the volume control to its original setting, by driving it clockwise for the same amount of time that it was driven anti-clockwise to reach its end stop. This mute return feature in the software is enabled by leaving shorting link LK3 open. This allows the RA4 input (pin 3) to be pulled to 5V by a 10kΩ resistor. Installing the jumper shunt at LK3 will pull RA4 to ground, disabling the mute return feature. Status LEDs LEDs1-3 indicate the status of the circuit. The blue Power LED (LED1) lights whenever power is applied to the circuit. The other two LEDs, Acknowledge (LED2) and Mute (LED3) light when their respective RA2 and RA1 outputs are driven high (ie, to +5V). LED2 indicates that an infrared command was received and LED3 lights when the mute function is active. Pins 15 and 16 of IC5 connect to the oscillator which drive 4MHz crystal X1, providing the microcontroller system clock. This oscillator runs when the circuit is first powered up for about 1.5 seconds. It also runs whenever an infrared signal is received at RB0 or when a button on the front panel switch board is pressed and then for a further 1.5 seconds after the signal ceases. The oscillator then shuts down and the processor goes into sleep mode, as long as a muting operation is not in process. This ensures that no noise is radiated into the sensitive audio circuitry when the remote control circuit is not being used. A 10nF capacitor connected directly across the motor terminals also prevents commutator hash from being transmitted along the supply leads, while further filtering is provided by a 100nF capacitor located at the motor output terminals on the PCB. This reduces the amount of noise that gets into the preamplifier signals when the volume pot motor is being driven. Input selection Digital outputs RB6, RB7 and RB5 of IC5 (pins 11-13) control the relays on the Input Selector board. These outputs go low when the 1, 2 or 3 buttons on the remote are pressed respectively; they are high-impedance (set as inputs) the rest of the time. As shown, RB6, RB7 and RB5 are connected to pins 1-6 of 10way header socket CON7; each output is connected to two pins in parallel. Pins 7 and 8 of CON7 are wired to the +5V rail while pins 9 and 10 go to ground. CON7 is connected to a matching header socket on the Input Selector board via an IDC cable. This provides both the control signals and the supply rails to power this module. The Input Selector circuit is shown in Fig.8. It uses three 5V DPDT relays (RLY1- RLY3) to select one of three stereo inputs: Input 1, Input 2 or Input 3. The relays are driven by PNP transistors Q5-Q7, depending on the signals from the IC5 microcontroller in the Remote Control circuit (and fed through from CON7 to CON8). One relay is used per stereo input so that the audio signal only has to pass through one relay. As shown, the incoming stereo line-level inputs are connected to the NO (normally open) contacts of each relay. When a relay turns on, its common (C) contacts connect to its NO contacts and the stereo signals are fed through to the left and right outputs via 100Ω resistors and ferrite beads. The resistors isolate the outputs from the audio cable capacitance, while the beads and their associated 470pF capacitors filter any RF signals that may be present. When button 1 is pressed on the remote, pins 1 and 2 on CON8 are pulled low (by output RB6 of IC5 in the Remote Control circuit). This pulls the base of transistor Q5 low via a 2.2kΩ resistor and so Q5 turns on and switches on RLY1 to select input 1 (CON11). Similarly, RLY2 and RLY3 are switched on via Q6 and Q7 respectively when buttons 2 and 3 are pressed on the remote. Only one relay can be on at any time. Pressing an input button (either on the remote or the switch board) switches the currently activated relay off before the newly selected relay turns on. If the input button corresponds to the currently selected input, then no change takes place. The last input selected is restored at power up. Fig.9 shows the circuitry for the separate front panel Pushbutton Switch Board. This consists of three momentary contact pushbuttons with integral blue LEDs (LEDs1-3) plus a 14-way header socket (CON10) which is connected to CON9 via an IDC cable. Practical Electronics | April | 2020 A variety of infrared remote controls can be used to control the Preamplifier: this one came from Altronics. One side of each switch is connected to ground, while the other connections to S1-S3 are respectively connected back to the RB6, RB7 and RB5 digital I/Os of IC5 in the Remote Control circuit. When a switch is pressed, it pulls its corresponding pin low and this wakes the microcontroller up, which then turns on the corresponding relay and promptly goes back to sleep again. The anodes of LEDs1-3 are connected to +5V, while their cathodes are respectively connected to the RB6, RB7 and RB5 I/Os of IC5 (pins 11-13) via 2.2kΩ current-limiting resistors. As a result, when one of these pins goes low to select a new input, it lights the corresponding switch LED as well. This occurs whether the input was selected using the remote control or pressing a switch button. The cathodes of the other LEDs are held high via 2.2kΩ pull-up resistors to the +5V rail and are off. Note that the pins which are used to sense when buttons are pressed and drive the switch LEDs are the same pins which are used to drive the transistors which drive the relay coils. So if you press the button corresponding to the input which is already selected, that line is configured as an output but it’s already low (at ground potential), so pressing the button has no effect. If you press one of the other buttons, as mentioned earlier, that pin on IC5 has been configured as an input and there are 2.2kΩ pull-up resistors on the Input Selector board. So pulling that line to ground will bring that line low, signalling to the microcontroller that you wish to switch inputs, which will then switch off the relay selecting the currently active input. Preventing switch conflicts Comparator IC4 and NPN transistor Q8 prevent more than one relay from switching on if two or more input switches are pressed simultaneously. This circuit also ensures that the currently activated relay is switched off if a different input button is pressed, before the newly selected relay is switched on. Practical Electronics | April | 2020 IC4 is an LM393 which is wired so that its non-inverting input (pin 3) monitors the three switch lines via 100kΩ resistors. These resistors function as a simple DAC (digital-to-analogue converter). If one switch line is low, the voltage on pin 3 of IC1 is 3.3V; if two are low (eg, if two switches are pressed simultaneously), pin 3 is at 1.67V; and if all three lines are low, pin 3 is at 0V. This pin 3 voltage is compared to a 2.5V reference on IC1’s inverting input (pin 2), formed by a resistive divider across the 5V supply. So its pin 1 output is high only when one switch line is low and this turns on Q8 which connects the bottom of the relay coils to ground. This allows the selected relay to turn on. However, if two or more switch lines are low, lC4’s output will be low and so Q8 and all the relays turn off. Similarly, if one switch line is already low and another input is selected (pulling its line low), IC4’s output will briefly go low to switch off all the relays before going high again (ie, when the micro changes the state of its RB5-RB7 outputs) to allow the new relay to turn on. Power supply The Preamplifier is powered from ±15V rails. These are typically derived either from two separate 15V windings on the main power transformer, or a small secondary 15-0-15 transformer and rectifier. These 15V rails are bypassed on the preamp board by 470µF capacitors. There are other capacitors connected across the supply rails at various points of the circuit which provide local bypassing for the op amps on the PCB. We use both 100nF capacitors and 100µF capacitors to ensure low impedance at a range of frequencies. The capacitors connected across the full 30V supply are rated at 35V or more. The 5V supply for microcontroller IC5 is derived from the +15V rail via a 22Ω dropping resistor and 5V linear regulator REG1. The 22Ω resistor reduces the dissipation in REG1 and provides some additional filtering, in combination with REG1’s 100µF input capacitor. The power LED (LED1) lights up when 5V is present and its current is set by a 2.7kΩ series resistor. We published a suitable regulated supply design in the May 2016 issue: the 4-Output Universal Voltage Regulator. It has adjustable outputs which can be set for ±15V, plus 5V and 3.3V outputs that could be used to power other circuitry in your preamplifier/ amplifier. Its PCB (coded 18105151) is available from the PE PCB Service. Construction Fig.10 shows the assembly details for the main Preamplifier module. It is built on a PCB coded 01111119, which measures 216 × 66mm. Begin by installing the resistors (use your DMM to check the values), followed by the four ferrite beads. Each bead is installed by feeding a resistor lead off-cut through it and then bending the leads to fit through their holes in the PCB. Push each bead all the way down so that it sits flush against the PCB before soldering its leads. Following this, install the IC sockets for the five ICs. Make sure that each socket is seated flush against the PCB and that it is oriented correctly, as shown in Fig.10. Note that IC5 faces in the opposite direction to the op amp ICs (IC1-IC4). It’s best to solder two diagonally opposite pins of a socket first and then check that it sits flush with the board before soldering the remaining pins. The MKT and ceramic capacitors can now go in, followed by the electrolytic capacitors (regular and nonpolarised). The electrolytic capacitors must be oriented with the correct polarity; ie, with the longer lead through the pad marked with a ‘+’ symbol. The 100µF capacitors that are marked on the overlay and PCB with 35V must be rated at 35V or higher. If you use ceramic 470pF or 47pF capacitors, make sure they are the specified NP0 (or the equivalent C0G) type. Using other types of ceramic capacitors in these positions will degrade the distortion performance. The next step is to install the four transistors (Q1-Q4) in the remote control section. You need to ensure you use the correct type at each location. Q1 and Q3 are both BC327s, while Q2 and Q4 are BC337s. The PC stake (near VR3), 2-way SIL pin header for LK3 and 3-way SIL header for LK4 can now be installed, followed by polarised pin header CON6 and box header CON7. Crystal X1, trimpot VR4, the 3-way screw terminal block (CON5) and the four vertical RCA sockets (CON1-CON4) can then be fitted. Ensure the terminal block wire entry holes face the nearest edge of the PCB. Use white RCA sockets for the left channel input and output positions and red ones for the right channel positions. Switch S4 can be mounted now. Take care that all the pins are straight before attempting to insert them into the PCB. Press the switch fully down onto the PCB before soldering each pin. Also fit REG1, taking care to orient this correctly. 21 CON 1 1 FERRITE BEAD 100Ω CON14 L OUT L1 IN 470pF 100Ω R1 IN CON 1 2 FERRITE BEAD 100Ω RLY1 CON15 R OUT L2 IN 470pF 100Ω R2 IN 100Ω RLY2 CON 1 3 L3 IN 100Ω R3 IN RLY3 E B C K C Q7 BC327 10 µF K D2 D3 A A A 2.2k 2.2k 2.2k 4 E B RLY2 D1 3 Q6 BC327 K RLY1 TO CON 10 ON FRONT PANEL SWITCH BOARD 2 Q5 BC327 C 3x 2.2k 1 E RLY3 B 2.2k 7 8 1 2.2k 9 3 5 10 6 7 2.2k 11 9 12 14 10k 3 CON9 BC327, BC337 D1–D3: 1N4004 K E A 2 2.2k 6 10k 8 IC4 5 100nF 100nF B 8 10 CON8 3x 100k 13 2 4 TO CON7 ON PREAMP 5 6 1 2.2k 4 B C Q8 BC337 10 µF E IC 4 : LM393 C SC ultra LOW NOISE Ultra-low-noise Preamplifier InputINPUT Selector PRE AMPLIFIER SELECTOR 20 1 9 Fig.8: the circuitry of the optional module used for input switching. One of DPDT relays RLY1-RLY3 is energised at any given time, feeding one of the input pairs (CON11-CON13) through to CON14/CON15, which are wired to inputs CON1 and CON3 on the main Preamplifier board. IC4 and Q8 ensure that only one relay can be energised at a time, so the signal sources are not shorted to each other. 1 K  K  3 4 A LED2 LED1  A LED3 A K 5 6 7 8 9 10 11 12 13 S1 S2 S3 TO CON 9 ON INPUT SELECTOR BOARD FRONT PANEL SWITCH BOARD 2 14 CON10 Fig.9: the circuitry on the front panel pushbutton switch board. LEDs 1-3 are actually inside the pushbutton switches and light when the corresponding input is selected 22 Mounting the pots Before mounting the potentiometers, the shafts should be cut to length. The length depends upon the knobs and the type of box that the preamplifier is to be mounted into. The thickness of the front panel will have an impact on the required shaft length. Make sure the motorised pot (VR1) is seated correctly against the PCB before soldering its leads. Once the pot fits correctly, solder two diagonally opposite pot terminals and check that everything is correct before soldering the rest. The two gearbox cover lugs can then be soldered. That done, connect the figure-8 wire to the motor terminals along with the 10nF capacitor that also connects to these terminals. These leads pass through a hole in the board immediately behind the motor. They are then secured to the underside of the PCB using cable ties and then brought up to the top side of the PCB just behind CON6. Strip the wire ends and crimp them to the header pins. The wire from the positive motor terminal (marked with a red dot) should connect to the CON6 pin that is closer to IC5. Insert the pins into the 2-way shell and plug it into the CON6 header. Before fitting VR2 and VR3, scrape off some of the coating on the top of the pot body using a file so that they can be soldered to. Don’t breathe in the resulting dust. VR2 and VR3 must be seated correctly before being soldered to the board. They are then earthed using 0.7mmPractical Electronics | April | 2020 LK3 Mute Return 100 F IRD1 + REG1 7805 100 4.7 F NP 22 F NP 100k 4.7 F NP 22 F NP VR1 2x 5k LOG GEARBOX * OPTIONAL – ONLY REQUIRED IF 20k POT IS USED FOR VR1 (SEE TEXT) 100 100k R1 * VOLUME R2 * S4 100k LK4 1M 12k 47pF 2.2k 4x 15nF 1k 1k 1k 1k 1k 1k 10k GND 1.8k 1.8k 2.2k 1.8k 12k + 1M 4x 100nF 100 F 1.8k VR3 10k Lin 100 TREBLE VR2 10k Lin L A K R 22 F NP 100 IC4 5532 FB3 100nF 470pF MOTOR 91111110 OERETS ESI O N W OL REIFILP MAERP 22 F NP To Chassis FB4 100nF 2.2k 2.2k 22 F NP 01111119 C 2019 REV.B 100k LOW NOISE CON1 STEREO PREAMP Right out 470pF IC3 5532 100 F + 22pF FB1 2.2k 47pF 100 2.2k 35V 2.2k + 100nF 100 330 1 2 100 F 100 + Left in IC1 5532 22k CON4 CON3 9 10 2.2k 22 F NP * 10 470pF + 22 F NP * see text * 10 MUTE 100 F FB2 470pF 100 F LED3 A 35V 22k 100k 100 F 35V Left out IC2 5532 100 F 100k CON5 + 100nF 22 F NP + 22 CON2 Right in –15V 0V +15V + 100 F 1k 22pF 2 x BC327 LED2 Fig.10: use this PCB overlay diagram as a guide when building the main Preamplifier board. Don’t forget to cut the pot shafts to length before soldering them. You will also need to remove some of the passivation layer from the top of VR2 and VR3 to allow you to solder the GND wire to earth the pot bodies. Bend the leads of LED1LED3 and IRD1 to suit your case, so that the LEDs protrude through the front of the case. You can make a hole for infrared light to reach IRD1 at the same level and cover it with a small piece of perspex to prevent dust ingress. See the parts list for details on the red capacitors. BASS + CON6 470 F 4MHz X1 CON7 Q3 Q1 + ACK. A 100nF IC5 PIC16F88-I/P 100nF 470 F 330 10k Q2 1k 100nF 1k Q4 + 1k 1k 2 x BC337 100 F POWER 18k VR4 + 10 LED1 A 2.7k 10k 100 F diameter tinned copper wire soldered to the GND PCB stake and the top metal shield on both pots. Make sure that you apply sufficient heat for the solder to form a good joint. Mounting the LEDs and IRD1 We mounted the infrared receiver lRD1 with its lens about 18mm above the PCB. Similarly, the LEDs were mounted with the base of the LED body 18mm above the PCB. This will allow sufficient length Practical Electronics | April | 2020 for the LED leads to be bent forward, to line up with the potentiometer shafts, and then poke forward through the front panel of the amplifier. When bending the LED leads, the longer (anode) leads must go into the PCB pads marked ‘A’. IRD1 should be fitted with its hemispherical lens facing towards the front of the board. The assembly can now be completed by installing the spade connector to the left of the motorised pot, secured with an M4 screw, shake-proof washer and nut. Leave the ICs out of their sockets for now. They are installed later, and only after the power supply checks have been completed. Conclusion Next month, we’ll describe the Input Selector module and Switch Board assemblies and detail the test procedure. We’ll have more details on the power supply arrangement and describe setting up the remote control. 23 Parts list – 2020 Ultra-low-distortion Preamplifier with Tone Controls Main module 1 double-sided PCB, code 01111119, 216 x 66mm 1 universal remote control [Altronics A1012 or similar] 1 dual-gang 5kΩ log motorised potentiometer (VR1) [Altronics R1998] (a 20kΩ log pot can be substituted) 2 dual-gang 10kΩ linear 16mm potentiometers (VR2,VR3) [Altronics R2296] 1 1kΩ mini horizontal trimpot (VR4) 3 knobs to suit VR1-VR3 1 4PDT push-on, push-off switch (S4) [Altronics S1451] 4 8-pin DIL IC sockets (for IC1-IC4) 1 18-pin DIL IC socket (for IC5) 4 ferrite beads (FB1-FB4) [Altronics L5250A, Jaycar LF-1250] 1 4MHz crystal (X1) 2 vertical PCB-mount RCA sockets, white (CON1,CON3) [Altronics P0131] 2 vertical PCB-mount RCA sockets, red (CON2,CON4) [Altronics P0132] 1 3-way PCB-mount terminal block, 5.08mm pitch (CON5) 1 2-way vertical polarised header, 2.54mm pitch (CON6) [Altronics P5492, Jaycar HM-3412] 1 2-way polarised header plug (for CON6) [Jaycar HM-3402, Altronics P5472 and P5470A] 1 10-pin PCB-mount IDC vertical box header (CON7) [Altronics P5010, Jaycar PP-1100] 1 2-way SIL pin header (LK3) 1 3-way SIL pin header (LK4) 2 jumper shunts (LK3,LK4) 1 6.35mm chassis-mount single spade connector 4 12mm long M3 tapped nylon spacers 1 M4 x 10mm panhead machine screw 1 M4 hex nut 1 M4 star washer 4 M3 x 6mm panhead machine screws 2 100mm cable ties 1 150mm length of light-duty figure-8 hookup wire 1 50mm length of 0.7mm diameter tinned copper wire 1 PC stake Semiconductors 4 NE5532AP or LM833P dual op amps (IC1-IC4) 1 PIC16F88-I/P microcontroller programmed with 0111111A. hex (lC5) 1 infrared receiver module (IRD1) [Altronics Z1611A, Jaycar ZD1952] 1 7805CV 5V regulator (REG1) 2 BC327 PNP transistors (Q1,Q3) 2 BC337 NPN transistors (Q2,Q4) 1 3mm blue LED (LED1) 1 3mm orange/amber LED (LED2) 1 3mm yellow LED (LED3) Interconnecting cables 1 350mm length of 14-way IDC cable 1 250mm length of 10-way IDC cable 2 10-pin IDC line sockets [Altronics P5310] 2 14-pin IDC line sockets [Altronics P5314] This is a rewarding but complicated project with several options. It is spread over three issues and constructors are strongly urged to read the panel opposite before purchasing components. 24 Capacitors 2 470µF 16V PC electrolytic 3 100µF 35V PC electrolytic 8 100µF 16V PC electrolytic 8 22µF small non-polarised electrolytic 2 4.7µF small non-polarised electrolytic 11 100nF MKT polyester 4 15nF MKT polyester 1 10nF MKT polyester 4 470pF MKT polyester, MKP polypropylene or NP0 ceramic [eg, element14 1005988] 2 47pF MKT polyester, MKP polypropylene or NP0 ceramic [eg, element14 1519289] 2 22pF ceramic Resistors (all 0.25W, 1% metal film) 2 1MΩ 6 100kΩ 2 22kΩ 1 18kΩ 2 12kΩ 3 10kΩ 1 2.7kΩ 8 2.2kΩ 4 1.8kΩ 10 1kΩ 2 330Ω 7 100Ω 1 22Ω 3 10Ω Input Switching module 1 PCB, code 01111112, 109.5 x94.5mm 3 DPDT 5V relays, PCB-mount (RLY1-RLY3) [Altronics S4147] 3 PCB-mount vertical stacked dual RCA sockets (CON11-CON13) [Altronics P0212] 1 vertical PCB-mount RCA socket, white (CON14) [Altronics P0131] 1 vertical PCB-mount RCA socket, red (CON15) [Altronics P0132] 1 10-pin PCB-mount IDC vertical box header (CON8) [Altronics P5010, Jaycar PP1100] 1 14-pin PCB-mount IDC vertical box header (CON9) [Altronics P5014] 2 ferrite beads [Altronics L5250A, Jaycar LF1250] 4 12mm long M3 tapped nylon spacers 4 M3 x 6mm panhead machine screws Semiconductors 1 LM393P comparator (IC4) 3 BC327 PNP transistors (Q5-Q7) 1 BC337 NPN transistor (Q8) 3 1N4004 diodes (D1-D3) Capacitors 2 10µF 16V electrolytic 2 100nF MKT polyester 2 470pF MKT polyester, MKP polypropylene or NP0 ceramic [eg, element14 1005988] Resistors (all 0.25W, 1% metal film) 3 100kΩ 2 10kΩ 11 2.2kΩ 6 100Ω Front Panel Pushbutton module 1 PCB, code 01111113, 66 x 24.5m 1 14-pin PCB-mount IDC vertical box header (CON10) [Altronics P5014 3 PCB-mount pushbutton switches with blue LEDs (S1-S3) [Altronics S1173, Jaycar SP0622] 4 6.3mm long M3 tapped nylon spacer 4 M3 x 6mm panhead machine screws Reproduced by arrangement with SILICON CHIP magazine 2020. www.siliconchip.com.au Practical Electronics | April | 2020