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AUDIO OUT AUDIO OUT L R By Jake Rothman Switching in audio equipment – Part 1 S witching is the most basic function in electronics, beginning with the old-fashioned knife switch (Fig.1) from which the circuit symbol is derived today. However, audio systems require specialised switching of very small signals, with no added noise, clicks or distortion. There are also psychological aspects to audio switching, such as how the mechanical aspect of the switch feels to the user. Mechanical noise is also very important; a switch that emitted a loud mechanical clunk, for example, would not be acceptable in a recording environment. There is also the whole area of ergonomics or the science of controlsurface design to consider. Welldesigned switch positioning is vital to avoid mistakes in operation, such as the output of a radio station inadvertently being turned off. In addition to this, music technology and Hi-Fi equipment also has to satisfy the artistic needs of creative people. (In my experience these people seem to have what I call a wider ‘dynamic arm envelope’ – they are are much more likely to accidentality knock switches! Usually, at the worst possible time.) Power switching. Even mains power switching has an aspect that is almost unique to audio. The switch position is often down for ‘on’, Fig.2. In audio it’s considered ‘much worse’ if a switch gets knocked to off, so I prefer a switch orientation such that down is always on. Fig.3. This old cassette deck has the mains switch actuated by a mechanical rod to keep it away from the sensitive playback head. as shown in Fig.2. This is in contrast to most other electronic equipment (such as a bench power supply) which you wouldn’t want to turn on accidentally; it is ‘up’ for on. It doesn’t stop there; audio mains wiring from the input power socket to the front panel switch mustn’t emit hum into the sensitive circuitry. Some Hi-Fi designers obviate this by putting the mains wiring and switch at the back and use a mechanical rod to couple the front panel button (Fig.3). This is difficult for home and small batch construction since custom mouldings and specialist mechanical items are needed. There is also the travesty of rear-mounted mains switches, great (not!) when the unit is mounted in a rack against a wall. I always front mount the mains switch and have always used it to switch both the live and the neutral – which nowadays is a mandated safety requirement. This needs a double pole (DP) switch, such as the rocker switch shown in Fig.4. The two pairs of wires going to and from the switch need to be twisted tightly and moved away from sensitive circuit areas to reduce magnetic emissions. These wires should also be run along the corner of the metal box for partial electrostatic screening. In Orfoton moving-coil pickup pre-amplifiers, the mains wiring is enclosed in a copper tube for screening. I’ve used this idea, employing short lengths of micro-bore central heating pipe. In low-cost equipment, hum (and DPDT switch Fig.1. It’s horror movie time. The old-fashioned knife switch would today be called a ‘wiping contact single-pole double-throw (SPDT) switch’ and would be considered irredeemably ‘non-compliant’ by any health and safety inspector; and they’d be right! 58 Fig.4. Mains switches need to be carefully wired to prevent hum emissions to the rest of the circuit. Note how wires to the black DPDT switch on the left are twisted and placed into the top corner of the enclosure. Practical Electronics | June | 2024 Snubber network 10nF, 250V (Y capacitor) 100Ω, 0.5W Live Fuse Mains input Neutral S1a DPST switch Transformer VDR S1b Snubber network 10nF, 250V (Y capacitor) 100Ω, 0.5W Fig.5. (left)To prevent loud cracks when turning off mains power, snubber networks can be used across the switch contacts. Fig.7. (right)There must be no DC on signal lines to be switched or clicks will result. Input channel 1 C Input op amp R = Pull-down resistors 1MΩ film types 100kΩ tantalum 22kΩ electrolytic Electrical vs electronic mains switching These days, a solid-state power switch, such as a zero-crossing triac circuit can be used for mains switching. These can give click-free performance and allow cheap (low-current, momentary, push) switches to be used rather than a clunky latching mechanical type. The problem is that the UK Health and Safety Executive (and others) do not consider silicon to offer proper safety isolation because semiconductors fail short circuit, so an additional mechanical switch is still needed. (Electronic signal switching, as opposed to mains switching, using FETs is a whole subject in itself, which we will come to later.) Clicks When switching audio there must be no DC present on the signal or a click will be generated. Even a few millivolts of DC offset or capacitor charge is going to cause an equal voltage pulse when switching occurs. It will also have very fast rise and fall times creating a lot of audio bandwidth high frequency signals, which are clearly audible as a characteristic ‘click’. This is the bane of audio signal switching and audio Fig.6. Close-up of switch contacts showing hysteresis-curved spring mechanism to ensure snappy action. Ideally, the contacts should be gold plated to reduce oxide build up. Practical Electronics | June | 2024 R 0V C Switch pops Some power switches are designed to switch quickly regardless of how slowly they are moved. They have a spring mechanism which defines the switching time. This is important for some power applications, but I don’t think it matters that much for audio. The slow ones tend to make fewer clicks and less Output R Input channel 2 mechanical noise due to the less abrupt transition. A ‘snappy’ sprung pair of contacts is shown in Fig.6. Quick-make, quick-break Output op amp C Input op amp safety compliance legal problems) are side stepped by buying in external DC ‘wall-wart’ power supplies. Power switches need to be well suppressed to avoid generating audible pops and clicks. This is caused by arcing when switching inductive loads, such as transformers, generating electromagnetic interference. RC snubber networks across each contact are usually necessary to reduce arcing and subsequent noise. In-line mains EMI (electromagnetic interference, not the British record company) filters and voltage-dependent resistors (VDRs) normally complete the job, as shown in Fig.5. Switches employing built-in mains neon indicators can also generate strange EMI noises from flickering as they age. To avoid the complexity of a mainspowered power-on indicator LED, it’s always simplest to add the LED to the DC side of the power supply. Channel select switch: changeover or SPDT 0V R 0V C = DC blocking capacitor 100nF to 22 µF depending on input impedance of output op amp circuit designers go to great lengths to isolate DC by using coupling capacitors and pull-down resistors on every input and output, as shown in Fig.7. Electronic and relay switching has the additional problem of the control signal breaking through, giving the possibility of more clicks and thumps. Latching vs momentary Audio engineers like to be able to see what position a switch is in, and they like it to stay there, even after power loss, which means they have a strong preference for latching-type switches. Momentary push switches should only be used for doorbells! For audio, switches ought to have an accompanying LED and mechanical ‘memory’. Unfortunately, latching mechanisms add mechanical complexity, cost and lower reliability, meaning they fail sooner. A lifetime of 25,000 operations is typical for a good quality switch. Consumer latching types last about 10,000 cycles, making the switch the shortest-life component in most audio systems. The most cost-effective switch in the world is the ‘tact’ switch used in car key fobs. I use one in my Stylophone Theremin for the sound trigger with a big moulded springy plastic button on top. (Fig.8) After testing lots of switches with differing torque ratings/actuation pressures we found Fig.8. Momentary tact switch; these are used for the trigger button on the Stylophone Theremin. They are very cheap and available with different push-to-operate force options. 59 Input S1a DPDT Input Processor eg, fuzz In S1b DPDT Output Out In Output Out Bypass line Dotted line denotes switch sections mechanically linked Fig.9. The classic ‘hard-bypass’ circuit for switching in and out a signal processor or effect. a combination of switch and moulding that gave a ‘musical feel’ at minimal cost. Contact material For low-level audio signals (<100mA, down to µV levels), gold-plated contacts ensure low distortion since gold does not oxidise. It has to be reasonably Fig.12. The 1,000,000-cycle EAO pushbuttons are Swiss made with a thick (5µm) or it may wear off when switching high price to match, but they give your front panels that Star Trek console look. currents, which will ‘burn’ the gold off the contacts. These industrial-type switches are made by several companies. Most are Silver-plated contacts are the next best and can be used illuminated, often by odd filament bulbs, such as T5.5 types. for high level (>1V) audio signals. In the presence of atmospheric pollution, such as from wood burners, diesel engines and cigarette smoke, black sulphide may form shown in Fig.12, have an NO and NC pair of contacts in each on silver contacts giving rise to crackles and distortion. These section. These can be joined together to make a changeover contacts need to be cleaned with a contact cleaner such as switch, as shown in Fig.13. Kontakt 60, then lubricated with Servisol or Kontakt 10 Super. Some switches have self-wiping contacts which maintain good Make before break (MBM), break before conductivity so long as they are moved regularly. Alps-style make (BBM) pushbuttons (the manufacturer, not the mountain range), slider MBM and BBM are not fancy food fads, but defined forms of and rotary types have this feature. Toggle switches, rockers and switching action. Most switches break from one contact before pushbuttons generally don’t and thus have shorter lives. they move onto the next, thereby maintaining isolation – a switching action called ‘break before make’ or ‘BBM’, as shown in Fig.14. Occasionally, on some switches with sliding contacts Poles, ways and odd acronyms it can still be joined to the previous contact as it moves to the If a switch has two switch positions then it is often described next one. This is achieved by making the wiper wider than the as a ‘changeover’ (C/O or CO) switch or ‘double throw’ (DT). spacing between the contacts. This is called ‘make before break’ I suspect this is a throwback to the old knife switch design (MBB), or shorting contact. MBB can be a useful property in audio (Fig.1). Two-section changeover switches are often called switching, providing a seamless transition from one signal source ‘double-pole, double-throw’ or DPDT, these are the most to another, often with less chance of generating a click. MBB is common types in audio systems. A classic circuit is the effect also useful if you want to avoid big steps, intermittently down to in/out switch used on guitar pedals shown in Fig.9. It’s also zero between switch positions with multiple resistive steps, such useful for the motor direction reversing, channel-flip or phaseas switched attenuators. The Fender Stratocaster guitar pick-up change circuit shown in Fig.10. It can be used as a left/right switch was originally a three position MBB switch. Guitarists channel flip. It’s easy to wire up, as shown in Fig.11. Lighting soon realised they could move the switch to get unusual sounding designers and electricians know them as an ‘intermediate phase cancellations in the ‘intermediate’ positions (Fig.15). Fender switch’, often used on stair landings. responded by making the switch have five positions. Normally closed (NC), normally open (NO) No, we’re not talking about provincial British tea rooms; normally closed (NC) and normally open (NO) are standard switch nomenclature referring to the default switching position. A lot of pushbuttons, such as the Eao types Input 1 Normal 1 NC Lamp Input 2 A 3 B NO 2 NC A 4 B Lamp, typically rated at 50mA, 24V Output 1 1 NC 2 3 4 = NO Configuration to create a changeover switch Output 2 Normal Fig.10. (above) The phase flip circuit which switches over two lines. It could be used to flip the phase of a loudspeaker. 60 2 3 NO 4 NO Flip Flip NC 1 Fig.11. (right) Rear of a DPDT toggle switch wired for ‘phase flip’ action. Note the characteristic ‘cross over’ wiring. The two wires coming in go to the cross wires and the two wires coming out are taken from the centre tags. Electricians use an ‘X’ symbol to denote an intermediate switch pattern. Fig.13. EAO pushbuttons (see Fig.12) and similar switches often have two sets of contacts per section. One is normally closed (NC) and one is normally open (NO). The NC and NO contacts change over when the button is pressed. Practical Electronics | June | 2024 Fig.14. Make before break (MBB) contact. Fig.17. I use these Arrow TS6 toggles in top-quality studio gear. They are now made by Eaton/Hart. Centre-off You can get toggle switches with three positions, the middle one is centre off, described as ON-OFF-ON. These are useful for selecting, say channel one for up, middle for mute and down for channel two. C&K Fig.15. MBB moving contact in transition joining two switch positions in a Fender-style lever switch. Toggles Miniature toggle switches are the most popular switching device for smallproduction audio equipment because they are the simplest to use. One thing to watch out for is that the position of the closed contacts is usually the opposite to lever (or dolly as it is correctly called), as shown in Fig.16. This is not normally a problem since the switch can be turned round if it’s hard-wired. Since the tags form the internal contacts it can be assumed if they are gold plated the contacts are gold. One of my favourite toggle designs is the Arrow Hart Eaton TS6 shown in Fig.17. I’ve had some in continuous operation for 30 years. C&K are a US company which makes some of the best toggles switches. They do a special type of DPDT switch (such as their 7211 shown in Fig.18) that looks like a centre off type and has a very versatile (but confusing) contact arrangement, as shown in Fig.19. This is called an ON-ON-ON switch. These can be used to give an interesting set of series parallel variations of pick-ups in electric guitars. Practical Electronics | June | 2024 Centre NC (linked) Rear view of switch tags NC (linked) Right Left Biased operation Toggle switches can also come with a momentary option called ‘biased’, which means it can be moved to an on position but springs back when released. It is possible for the switch to be biased in one position and latching in the other. This would be called an (ON)-OFF-ON switch which would be centre-off with the brackets denoting the momentary or biased position. I used one of these on a music sequencer where the biased position is used to increment the next step for tuning the note; centre is off, and ‘on’ cycles through the steps once the notes ae all tuned. I’ve also employed them as quick trigger-off-continuous drone switches for synths. Slide Fig.16. On most toggle switches, when the dolly is up, it’s the bottom contact that is closed, and vice versa. Make sure you get this right for PCBmounted switches. Fig.18. C&K were the original manufacturers of some of the best miniature toggle switches. You can spot them from their distinctive dark red mouldings that don’t melt. This is a 7211 PCB-mount right-angle toggle switch, many of which I installed in RTS intercom belt-packs used in the TV industry. Note the unthreaded bush – no nut is needed, just big platedthrough PCB holes. These are the cheapest switches for production audio equipment. They came to popularity as wave-change switches in cheap radios. They are often found in three-position versions, used with the LW/MWFM selection in mind. It’s easy to see what position they are in, but they are prone to dirt ingress, so it’s worth moulding a protective top to stop this and at the same time Fig.19. Contact positions for C&K style DPDT ON-ON-ON switches. add a larger actuator. Sliders need tidy punched rectangular holes, so they are only suitable for mass production. I used them in my recent Stylophone theremin, as shown in Fig.20. Next month In Part 2 next month we have a little more mechanical switching to cover before we move on to the important subject of audio ‘silicon switching’. Fig.20. Slider switches can do a lot for low cost. I used these on my pricebarrier-smashing Stylophone Theremin. (Product of the year at NAMM show!) 61