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AUDIO OUT AUDIO OUT L R By Jake Rothman Switching in audio equipment – Part 2 T his month, we carry on from where we left off in June and finish our look at the many varied types of mechanical audio switches. Fig.21. Rotary wafer switches have been around since the swinging 1920s. They have very long life, are easy to service but are labour intensive to wire. Fig.22. Lorlin/Alpha standard pattern rotary switch, 6-way 2-pole. These are available in both BBM and MBB versions and are well sealed. 44 The Rotary Club Phono socket Rotary switches, sometimes Aux called wafer switches (Fig.21), are the most versatile, but often require a lot of costly hand-wiring; although this is FM tuner not really a problem for home constructors and prototypes. The old Paxolin wafer types CD player have an open construction Volume which means they easily get Left dirty but are also very easy 10kΩ output log to clean. It is even possible Cassette deck to power 0V amplifier to buy kits, such as the RS Maka-Switch to assemble your own specialised multiwafer switches. (These are popular among older engineers who played with 0V Meccano as kids!) Most PCB-mounting types 10nF are expensive and have Aux unique overlays and pinouts. I use the Lorlin pattern types 1kΩ 10nF (Fig.22) which are also dual FM tuner sourced by Alpha and have a standard pin-out. These 1kΩ have a washer under the nut Right 10nF output CD player to set the number of ways. to power amplifier The military and aviation 1kΩ sectors seem to favour rotary 10kΩ Volume 10nF switches for their ruggedness log Cassette deck 0V (>100,000 cycles) compared to lightweight slide switches 1kΩ for example. Those made by Plessey, Painton, Diamond Fig.24. Circuit for rotary Hi-Fi selector switch. On some H, Elma and Greyhill are more complex designs, the earth connections are also the best and come in up to switched as well to minimise earth loops. 29-way and eight-wafer (or ‘bank’) options. They are often used as of poles. A typical amplifier selector switched attenuators in audio and can switch circuit will have four ways and cost between £20 and £100 each. A fine two poles (one section for each left and example is shown in Fig.23. right channel) as shown in Fig.24. A The selector switch on Hi-Fi amplifiers useful dedicated (standalone) audio is traditionally a multi-position rotary source selector box is shown in Fig.25. switch. In switch parlance the number of positions is the number of ‘ways’. Crosstalk The number of sections is the number A common problem with selector switches is crosstalk. This is where a signal from an unwanted source bleeds Fig.23. Some rotary switches are beautiful into the selected source. An example pieces of mechanical engineering. Such as of this happening is when listening this Plessey 72 series military switch. Practical Electronics | July | 2024 Fig.25. A dedicated three-position source selector switch with earth switching using an expensive 02 series Elma rotary switch. Fig.27. An extension spindle enables a switch to be placed in an electrically quiet area. In this case a flexible coupling has been used to decouple vibrations. a high frequency, say 20kHz, and can be in the order of −70dB, improving to −120dB in the best cases. By screening the cables and using buffers with a low output impedance the crosstalk can be greatly reduced, as shown in Fig.26. Another popular dodge is to put a rotary switch on the back panel next to the input sockets. This switch is then mechanically coupled to the knob at the front using an extension spindle and brackets – see Fig. 27. This approach also minimises hum and noise and is popular in expensive integrated (combined pre- and power) amplifiers. It requires care with mounting to the front panel bushing to prevent play and a floppy feel. I use a viscous damping grease such as Kilopoise and a felt washer under the knob. Shaft extensions were used in to a quiet radio signal (say, classical music on BBC Radio 3), and having inadvertently left the CD player going, one can hear a faint frequency from the CD player in the background. (Hi hat cymbals seem to bleed through especially well.) This inadvertent coupling is due to the capacitance between the contacts and cables. It gets worse if the source impedance of the selected source is high – in the case of some FM tuners and valve pre-amps this may be up to 10kΩ. The capacitance effectively makes a high-pass filter with this resistance, so if it is high the coupling frequency is lower hence the hi hat being especially noticeable. Crosstalk is specified in the same way as signal-to-noise ratio, in decibels (−dB) below the signal. This figure is given at Zero output impedance amplifier Input 2 *Reduces distortion from output capacitors 0V 10kΩ* 220kΩ Zero output impedance minimises crosstalk 47pF 220nF + 68Ω NE5534 220kΩ 22µF Selector switch Output 22µF + – + Input 3 2x back-to-back 3.3kΩ tantalum capacitors creates a bipolar device Input 4 0V Parasitic capacitance Zero output impedance amplifier 0V Screened cable shield connected at only one end Fig.26. Feeding switches from low impedance sources, such as zero-output impedance buffers, reduces crosstalk between switching elements and cables. Practical Electronics | July | 2024 T Giesberts Mini-preamplifier (Elektor, February 1994). To prevent crosstalk there are several other effective techniques. Rotary switches sometimes include a metal screen between the two sections, as shown in Fig.28. On PCBs it is always worth putting earthed guard tracks between the input lines going to the switch, along with a ground plane to mimic screened cables. Another trick is to ground the unused sources. Care has to be taken to ensure this does not cause any damage to the source though. Normally a series resistor of around 600Ω prevents excessive current. However, there can still be the problem of preventing current going to ground which can cause resistive crosstalk. This can be alleviated by connecting the ground to a low impedance point, such as the power supply. In the days of the Maka-Switch one could buy a ‘rotating open-circuit’ wafer, as shown in Fig.29. By linking across to an adjacent normal wafer, all the unused sources were automatically grounded. It’s also possible to do the Fig.28. An earthed metal screening plate can be placed between two switch sections to reduce coupling. Useful for preventing crosstalk between left and right channels. 45 Fig.29. A ‘rotating open-circuit’ wafer. This can be used to ground all contacts except the position selected. same thing with interlocking push buttons, as in Douglas Self’s preamplifier (Wireless World, September 1996). Multiple push buttons An alternative to the rotary switch is an interlocking push button, where only one button can be on (in) while the others are always off (sticking out) as illustrated in Fig.30. These are often easier to operate and offer the possibility of clearer labelling. They were popularised by German industrial designers such as Dieter Rams of Braun with his 1959 SK4 radiogram. Indeed, they are replicated on computers screens today in graphical user interface (GUI) software as ‘radio buttons’. Mechanical interlocking has a sliding bar interlinking all the switches (see Fig.31). Such assemblies are prone to go wrong and it’s often impossible to get a replacement. There was one design of button that incorporated a black ‘eyelid’ which opened to reveal a florescent orange disc when pressed – see Fig.32. Most designers today would go for an electronic interlocking system with LEDs which, unlike the ‘eyelid’, requires power. Fig.32. An interesting push button by Schadow that uses internal ‘eyelids’ to reveal a fluorescent ‘on’ indicator when the actuator is pressed. 46 Fig.30. Mechanically interlocked push buttons. Notice the spring. Post Office/BBC switches I mention these because they often turn up in old BBC equipment, and are sometimes called ‘lever switches’. They are very reliable, achieving a million operations. They have an open construction with easy-to-clean multicontacts, as shown in Fig.33. They also make no mechanical noise, but on the downside they are expensive, and mounting them is awkward. They need a rectangular hole and two screw holes with precise relative dimensions. Fig.31. Interlocking is accomplished by a spring-tensioned sliding bar with latching tabs underneath all switches. medium-scale production I outsource the front panels and then the rectangular switches just clip in. (Getting them out again for replacement/maintenance though is another matter!) Soldering meltdown Some cheap switches use thermoplastic cases rather than the old Bakelite thermosetting material. Take care with these when soldering. If the wire is pulled while it is still hot, the tag/contact Funny shapes, odd holes For small-scale constructors, switches that are fastened with a nut in a round hole are the most convenient, such as toggle and rotary types. They also allow the front panel to be completely sealed, unlike rows of interlocking push buttons. In general, I try to avoid switches that require rectangular holes which either need an expensive cutout punch or fiddly, time-consuming sawing and filing. This precludes a lot of rocker and some push switches – the kind shown in Fig.34. I have even had 3D-printed adaptor plates made for some push buttons – see Fig.35. Of course, for Fig.34. These lovely LED illuminated Japanese push buttons require a difficultto-cut hole. Fig.33. Old-fashioned Post Office switches used by the BBC were very reliable and you could get them in almost any combination. Fig.35. A square switch in a round hole. An adaptor plate for the switch type shown in Fig.34. A 3D print designed by my friend, Jeremy Paine. Practical Electronics | July | 2024 Fig.36. Once you get the front panel rectangular hole right, the switch just clips in with the plastic tabs. This is a typical rocker mains switch. A pig to get out though, since all the tabs have to be squeezed in together. Note these switches melt and can fail when soldered. They are designed for push-on insulated spade connectors. Fig.37. Black sulphide tarnish on switch tags can be impossible to solder. However, in this situation the switch surprisingly usually still works. switch tags, the solvent can wash the flux into the switch contacts where it dries like a varnish causing an open circuit. The black silver tarnish that often occurs with switches (Fig.37) can often make the tags impossible to solder, even though the switch often works fine with enough current. The solution is to use a glass fibre brush to clean them – the type shown in Fig.38. PCB mounting I used to avoid PCBmounted switches since the high Fig.39. Here’s some multi-gang PCB mount rotary switches mechanical forces on an equaliser module. Last time I bought one of these imparted on the board Greyhill switches it cost £35. gave rise to dry joints. This is much less of a problem today because of plated-through holes. For maintenance work there is still the problem of finding switches to fit a PCB layout since an exact physical replacement is always required. Fig.39 shows an API-inspired audio equaliser with rotary Greyhill switches connected to resistor ladders giving precise dB control steps. It is possible to get a complex multi-pushbutton assembly made up (see Fig.40) which is soldered into the PCB as one unit. The goal is to eliminate a stack of wiring and possible Fig.40. A custom-made push-button errors – assuming you get the PCB layout assembly ready to solder into a PCB. right! These set-ups are common in Hi-Fi This has two high-current switches for pre-amps and audio test sets such as the speakers and a small-signal DPDT switch. Wayne Kerr AMS1 audio analyser shown in Fig.41 and 42. Switch odyssey One of the switches failed in my Wayne Kerr AMS1, by shooting out of the front panel across the room. It’s contacts and other small parts were scattered in the process, most of which were never found. Unfortunately, it was the most important button on the front panel; in this case, the 1kHz test signal switch. It took a year for me to find a replacement for these white switches – they turned up quite by chance at a radio rally. Then it took half a day of microsurgery to fix it. All that hassle because of a plastic-tab failure on the end of the internal plastic slider (Fig.43). I inserted a cropped Veropin in a 1mm hole drilled where the missing Fig.38. A glass fibre brush is the best way to clean contacts. can move as the plastic softens, resulting in an open circuit. This also happens with mains rocker switches (Fig.36). They are designed for push-on tags, not soldering. Don’t clean solder flux off Practical Electronics | July | 2024 Fig.41. A complex PCB-mounted interlocked push button assembly on the AMS1 audio analyser front panel. Switch problems in systems built like this are a nightmare to fix. 47 Fig.43. The bane of my life, the plastic tab failure. This little bit of plastic breaking off the (upper) internal switch slider resulted in half a day’s work. Fig.42. Interior of my AMS1. The Centralab switches are white and mounted on the lower PCB. The latching spring and contacts went missing, and they weren’t in the Hoover bag. Fig.44. The plastic tab was replaced by inserting a truncated Veropin in its place. Fig.45 (left) The small parts of the AMS1 switch ejected. Note the spring and specially bent contact strips. The originals was never found. These are the internals of a replacement; (right) switch with the top prised off to show the sliding contacts of the DPDT switch. This is in the on (pushed in) position. tab was (Fig.44). Even then I couldn’t get the slider back in until I realised I had to raise the Vero pin for clearance. It then had to be pushed down when the slider was inserted with new springy contacts from a sacrificed switch (Fig.45) and precariously held in place. I had to drill a 1.2mm hole to do this (Fig.46). It was a real pain to troubleshoot and fix, but definitely better than desoldering the whole switch with the possibility of damaging the PCB, which really would have been difficult to fix. Multi-function mayhem Audio switches are often combined with potentiometers to give dual functions with one knob. Switching can be at either Fig.46. I had to drill another hole at the back (switch sixth from left) to access the Veropin to push it down after inserting the slider into the switch casing. No way was I going to unsolder 78 pins to get this switch assembly out. 48 end of a pot’s rotation – clockwise or anticlockwise – or the shaft can be pulled/ pushed to actuate the switch. Old audio gear often combined the mains power switch with the volume control: a classic case of design trumping function and a sure way to induce hum and bangs. It’s fine for signal and control switching though, and adds to the designer’s ergonomic arsenal. For example, the AMS1 is full of switched pots, as shown in Fig.47. Allan Bradley series 70 ModPots were among the most popular types in good quality audio equipment, offering a lifetime of 100,000 cycles. However, they were very expensive. Finally, it’s possible to add a mains switch to a rotary switch – see Fig.48. Fig.47. The Allan Bradley (now Clarostat/Honeywell) Mod Pot series enabled complex potentiometer and switch combinations to be made up for custom designs. (These are the black square devices). Practical Electronics | July | 2024 noise pick up that may result in a pop when the effect is engaged, as shown in Fig.51. I now use this method in all my Colorsound guitar effects pedals. Next month My overview of audio mechanical switches went on longer than expected, but next month we really will move on to the topic of audio electronic switching. Fig.50. A Chinese Daier foot switch used on a Colorsound Powerboost preamplifier pedal. Guitar pedal builders call these ‘stomp’ switches. (Do not adjust your monitor, I’ve always built 1970s-style electronics in orange cases. Matt black is only for heatsinks). Fig.48. Here’s a 5-way rotary switch with an integral mains switch on the first click. It’s possible to get almost anything made up. This one was from Blore Edwards. Output FX 0V Input Output In Input Unused contacts In Output FX output There are often spare sections on multipole switches. Don’t waste them, wire them in parallel as shown in Fig.49 to improve reliability and reduce contact resistance. The switch here is a 3PDT guitar pedal bypass switch from Daier Electron in China – see Fig.50. This switch also has an improvement to the classic bypass circuit shown originally in Fig.9 last month. Here the input circuitry can be a single switch contact. If a changeover contact is used, the unused contact can be grounded to stop Fig.49. The spare section on this 3PDT hard bypass footswitch on this Macaris Colorsound wah-wah pedal is wired in parallel to improve reliability. Bypass Input Gnd option Input FX input Fig.51. Superior hard bypass. Avoids one extra contact in bypass mode compared to a traditional circuit. If a DPDT switch is used, the unused contact can be used to earth the processors input. 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