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AUDIO OUT AUDIO OUT L R By Jake Rothman Low-noise Theremin Power Supply – Part 1 (CE, known cynically as ‘Chinese Export’) don’t care about interference emitted below 1MHz. This is one reason there are so many disillusioned audio, and low-frequency radio engineers. Low-noise design Fig.1. The completed Theremin power supply board, delivering +9V and +15V. O h no, not another power the pitch. It’s even worse if the PSU is a switch-mode design. Many SMPSs have 4.7nF Y capacitors connected from the mains input to 0V, guaranteed to make Theremins have an absolute fit. For most electronic equipment – such as PCs, TVs or printers – PSU RF (radiofrequency) emissions don’t matter. The European regulations supply unit!, I hear you say. ‘You can buy them online for a fiver and I’ve got 20 in the kitchen drawer’. True, if it’s noisy, un-earthed black plastic ‘wall-warts’ destined for landfill that you want. Plug such a PSU into a Theremin, AM radio or Hi-Fi preamplifier and their performance will be degraded. Theremins generate a horrid 50Hz warble sound as the mains-related electromagnetic interference (EMI) emissions from cheap PSUs modulate There is one way out of this modulation misery and that is to build your own power supplies. In the quiet of the present lock-down, the uniquely lownoise of this design shown in Fig.1 will be even more noticeable. The simplest way to reduce EMI-affecting circuits is distance, because fields obey the inverse-square law (double the distance, the intensity goes down four times (ie, two squared)). Although I dislike external PSUs, there is no substitute for distance to reduce noise. If you are building expensive audio/music gear, where an internal PSU is expected, then the use of a screened toroidal transformer is almost mandatory. For a Theremin housed in a big or long box, it’s often possible to position the power supply out of the way. Down to earth To deliver maximum playing range, a Theremin should be earthed to complete Mains transformer L * snubber capacitors typically 10nF to 100nF Mains input * * – + * 2200µF * 25V Bridge rectifier V+ + N Smoothing capacitor 0V Fig.2. Snubber capacitors connected across the diodes in a bridge rectifier suppress switching spikes. 48 Fig.3. Example of snubber capacitors from an old Robert’s radio – these are essential for hum-free reception on AM (long/medium wave). Practical Electronics | August | 2020 n T o scope Scope probe 0V earth D iode under test L 23 0V A C 15V A C + Ω 10W load resistor 3 3 00µF 25V N Fig.4. Simple half-wave power supply used to compare rectifier diode switching spikes. Scope 1nF T o diode anode 1kΩ 0V earth Fig.5. High-pass filter on scope probe blocks 50Hz AC (‘mains hum’). the capacitive circuit between the hand and the antenna. Most wall-warts have no earth at all; often having a plastic pin in the earth position on the plug, which easily breaks off. In the UK, this prevents it being plugged into the mains socket resulting in more e-waste. My power supply is – of course! – earthed via a proper three-pin socket and mains plug. Rectifiers In the old days of AM radios, the bridge rectifier diodes in the power supply A lmost ve rtical spike w ith lots of harmonics Very narrow lots of R F – always had snubber 17 V capacitors of around P ow er supply load 10nF to 100nF conL nected across them + (Fig.2) to stop what Mains was known as ‘mod+ input ulation hum’. These are shown in the 1978 N Robert’s RM30 table 11V 1nF radio supply in Fig.3. These were needed to Scope Spare probe suppress RF bursts w inding produced by a sharp 1kΩ spike when the diode turned off. I’ve always added these T o scope capacitors as a matter of habit in all equipment. For this article, Fig.7. An isolation transformer is normally needed to view I decided to do a bit of switching spikes on bridge circuits. However, if the transformer has investigation. These an extra secondary winding this can provide an isolated output. spikes are produced I had to use an isolation transformby the stored charge from the recombier on the ‘scope input to avoid earth nation of holes and electrons at the diode currents with bridge rectifiers to look junction. This effect is especially proat the noise. It’s easy if the transformer nounced with old slow silicon rectifiers, has dual secondary windings, since the such as the good-old 1N4001. scope can then be connected to a floating This problem is especially bad with unused winding. In this way the mains half-wave rectification, since the transtransformer becomes its own isolation former has no load on half cycles where transformer, as shown in Fig.7. it is free to ring undamped. To examine Newer fast rectifiers, such as the the spikes, initially I used the half-wave UF4001 (the UF stands for ultra-fast), rectifier circuit in Fig.4 since it only needs soft recovery, and Schottky diodes give a one rectifier. For experimentation, it is four-times smaller spike and consequentsimpler to change one diode than the ly less RF noise. Snubbing capacitance four of a full-wave rectifier. Also, with is still needed, albeit a reduced amount, a full-wave rectifier the switch-off diode to reduce emissions further. is damped the moment the other diode When it comes to suppressing these turns on, so the effect lasts a shorter time. spikes, surprisingly cheap ‘n’ nasty It’s a good idea when looking at this sort capacitors can be very effective beof thing to put a high-pass filter consistcause they have high losses. So those ing of a 10nF capacitor and 1kΩ resistor much-derided barrier-layer ceramic disc on the scope probe to block 50Hz, as capacitors from the Far East work very shown in Fig.5. The narrow well. X7R and Y5V multi-layer ceramic spike with its low repetition types are also good. Plastic-film capacfrequency defied photography itors may need a series 2.2Ω to 100Ω on an analogue oscilloscope, resistor to provide a defined loss. The so it is drawn in Fig.6. 50H z repetition freq uency 2V 8 0% more spike w ith a 1N4 001 Schottky or ultra- fast diode 1V 0. 7 V 0V off Fig.6. Illustration of rectifier switching spike. Practical Electronics | August | 2020 Fig.8. High-pass filtered waveform from a snubbed bridge rectifier circuit. Notice the resonant bursts and spikes. 400mVpk-pk (100mV/div) 100Hz repetition frequency . 49 Scope probe 1nF This Zobel circuit is optional, since it is not strictly needed with the Theremin, but is worthwhile for studios where multiple power supplies are in use. A plastic film type is best, but a non-polarised capacitor can be made from two back-to-back electrolytic capacitors – Ca and Cb on Fig.11. Note the series connection reduces their value to half. Solid-aluminium types are the best choice here, since the high ripple current won’t dry them out over time. A third capacitor C3, with six times the value of the non-polarised capacitor acts, as a capacitive potential divider. This develops a few volts across it to drive the LED. It also maintains the high frequency bypassing effect. A Zener diode stabilises the voltage at 2.7V to drive the LED via a current-limiting resistor. On negative cycles the Zener acts as a normal diode, clamping the reverse voltage to 0.7V. This means a normal polarised capacitor can be used. Since most of the current bypasses it, a tantalum type is usable. R1 sets the damping and limits any surge currents at switch-on. T o scope 1kΩ 0V Z obel netw ork L 23 0V input 15V, 1A N 1W Ω Load should be floating otherw ise an isolation 4 × 1N4 001 transformer is needed 4 × 100nF ceramic disc on the scope lead – 3 3 00µF 25V + Load + 4 . 7 µF Ω 10W Fig.9. Adding a Zobel network across the transformer secondary. Fig.10. The effect of adding the Zobel network – the resonant bursts are damped down to 80mVpk-pk and the spikes are reduced. capacitors should be wired across the diodes to minimise inductance and radiation loop area. New buzz on the block Diode switching also excites the resonance produced by the transformer’s leakage inductance and winding capacitance (Fig.8). This is at a much lower frequency, typically 10kHz to 80kHz and can easily be suppressed by a Zobel (series RC) network across the secondary, shown in Fig.9. This does not affect radios and Theremins, but can affect audio if multiple power supplies are being used, causing a buzz due to beating between their respective frequencies. Typical values would be 1Ω to 100Ω and 1µF to 10µF. large. A non-polarised electrolytic can be used, such as those used in loudspeaker crossover networks. Since the current through this capacitor is quite large at 38mA, you might think this is a waste of energy. However, because the current is 90° out-of-phase with the voltage, the real power loss is quite small. The current can be put to good use to drive an LED, avoiding the heat produced from the normal dropper resistor, by using the circuit in Fig.11. Someone told me recently that this was a typical mad Jake circuit! I always like to include one in every column, otherwise you might as well be reading a textbook. Adding spike-snubbing capacitors without damping resistors boosts this transformer resonance. The effective damping produced by adding a Zobel network can be seen in Fig.10. On one transformer, I was pleased to find adding the Zobel also reduced the high frequency content of the mechanical hum. Using the transformer specified here, the Zobel was effective for both rails when put on one winding only. This is because the windings are magnetically closely coupled. A network of 2.2Ω and 10µF was found to be most effective when wired across the lower voltage 11V secondary. The capacitor has to be non-polarised and is therefore physically 50 W ire across 11V secondary Whacky circuit E q uiva lent to Ω Ω 0. 25W Special offer transformer! If there’s one thing I detest it’s the scrapping of perfectly sound components. I bought a whole load of mains transformers for scrap value that were being dumped because they had no built-in thermal cutout. I throw more transformers in the bin due to random thermal cut-out failures (where they are often embedded in the windings) than any other cause. I always insist on an external cut-out, where it can be replaced. The ‘Right to Repair’ movement is gaining ground and hopefully practices like embedded cut-outs will be banned. There will be no shortage of these transformers, because PCB designer Mike Grindle and I have over 200 in stock. Even if we do run out, Mike will quickly edit the PCB for a new transformer. The transformer used has two secondaries of different voltages, as shown in Fig.12. Thus, the board has provision for two separate power supply circuits. Alternatively, the two secondaries can be wired in series to give a higher + C a 15µF* 16 V C b + 15µF* 16 V 4 7 µF + 6 V T ant P rimary L Ω 0. 25W Secondary 1 11V I Max = 254 mA ( 2. 51W at 9 . 9 V) Voltage measured off- load 23 0- 24 0V 2. 7 V 4 00mW R ed N 18 V I Max = 23 5mA ( 3 . 7 5W at 16 V) Secondary 2 Fig.11. A whacky circuit? Incorporating an LED into the Zobel network. This avoids a heat-dissipating resistor. Fig.12. The transformer specified has two secondaries. Practical Electronics | August | 2020 Fig.13. These excellent transformers were going to be dumped – nothing wrong with them and they are ideal for our Theremin power supply. www.poscope.com/epe voltage single supply. The transformer is shown in Fig.13. It is possible to connect the two positive supplies to give a dual-rail plus-andminus supply with a centre ground, as shown in Fig.14. This is not as good as a proper dual-rail supply with a negative voltage regulator. The 0V reference can bounce around if too much current goes from the ‘negative rail’ into ground. This is because the output impedance of the regulator is in series with the ground line. This was how early op amp circuitry was originally powered; we just designed it so current was never dumped into ground. It was always arranged to go from one rail to the other. This is still good practice today, unless your ground is a superconductor. Transformer specification When using surplus components with no written specification (such as this one), it is important to take some measurements. The first aspect to consider is the transformer’s physical size, which will give some indication of the power rating. This is usually specified in terms of VA for transformers rather than watts because the current is pulsed when feeding a capacitor-smoothed rectifier. Most small mains transformers are built up from standard lamination sizes, so by looking at the Danbury and Vigortronix catalogues I determined it was about 8VA. Small transformers like this generally have a regulation figure (how much the voltage drops on full load current) of 22%, so the current rating of the secondaries can be determined by loading with big wire-wound resistors until the expected voltage drop is reached. Finally, the transformer should be left on for a long time at estimated full load for a while to make sure it does not get too hot. Above around 70°C is too hot; if a smell of burning polyurethane varnish fills the air, it is definitely time to turn it off. Transformer measured specification Size (mm) 47 × 36 × 40 (w × l × h) Weight (g) 250 Off-load voltages 18V and 11V. On-load voltages (at max current): 16V <at> 235mA, load 68Ω 9.9V 254mA, load 39Ω. With the secondaries in series, 26V was obtained with a load of 240mA, temp rise was 45°C above ambient. - PWM - Encoders - LCD - Analog inputs - Compact PLC Next month That wraps it up for this month. As you can see, even simple power supplies can be complicated, or rather they need care and consideration at the design stage. Next month, we’ll build the circuit. + I nput +12V regulator + L - USB - Ethernet - Web server - Modbus - CNC (Mach3/4) - IO O utput + - up to 256 - up to 32 microsteps microsteps - 50 V / 6 A - 30 V / 2.5 A - USB configuration - Isolated PoScope Mega1+ PoScope Mega50 +12V 12V – 23 0- 24 0V 0V N + I nput + +12V regulator O utput + 12V – E – 12V I f using the ‘ special offer’ transformer then use the 18 V secondary for the positive rail because current draw is uaually higher for the positive rail in most synthesisers. Fig.14. Connecting two positive regulators to make a plus/minus power supply. Practical Electronics | August | 2020 - up to 50MS/s - resolution up to 12bit - Lowest power consumption - Smallest and lightest - 7 in 1: Oscilloscope, FFT, X/Y, Recorder, Logic Analyzer, Protocol decoder, Signal generator 51