Fullscreen HTML5Med Res (??MB)HTML4

AUDIO OUT AUDIO OUT L R By Jake Rothman Switching in audio equipment – Part 3 L input + 100µF + I finished off last month promising to move on to electronic switching in this month’s Audio Out, and we will make a small start, but first there are a few odds and ends to polish off in the mechanical audio switching section. L output – 100kΩ Mono ‘Soft’ bypass Mono switch 22kΩ Out Fig.53. Traditional mono switch circuit. it cancels out across the left and right channels. In more sophisticated units, a little bit of gain is introduced via a proper summing amplifier (Fig.54) to counteract the attenuation. A typical ‘fudge value’ being +4.5dB. ‘Juggler’ switch It is sometimes necessary to switch the order of two processing blocks. A common dilemma facing music engineers is ‘do we have the EQ followed by the compression or the other way round?’ It sounds very different depending on which way you + 100µF 47Ω 22kΩ Mono S1a L output 0V + 100kΩ 100µF 47Ω – 5kΩ 22kΩ Mono gain 0V 100µF 0V Mono 100µF S1b 47Ω + R input Stereo – + In R output 0V + In EQ 620Ω 100kΩ + Output 100µF – 1.5kΩ Out + + This is an essential test switch for music producers to listen for phase problems in stereo mixes, accomplished by mixing the left and right channels. This must be done using resistors, since it can be dangerous if the left and right channels are simply joined together. Imagine if there were two out-of-phase 10Vrms signals from two op amps (not impossible in audio) and their outputs were joined. The ‘electronic smell of death’ from burning epoxy would soon fill the air. The simple resistive summing circuit shown in Fig.53 leads to a halving of the voltage level (−6dB) if only one channel has a signal on L input it. It remains the same if it’s the same signal on both channels, say a vocal in the middle of the mix. This means material in the (stereo) middle 22kΩ is louder relative to sources panned left or right when ‘mono-ed’. This is fine for checking mixes, but if one really does want to listen to stereo mixes in mono it’s not ideal, and compensating for this is complex. However Hi-Fi buffs with old record collections also like a mono button for playing mono records with a stereo pickup. This reduces the surface noise because some of R input Stereo 0V + A ‘soft’ bypass, as opposed to a hard bypass shown in Fig.51 last month, is an arrangement that works well for internal bypassing in systems where the input is driven from a low impedance. However, it does not work well for self-contained units such as guitar pedals where the high-impedance guitar signal is loaded by the input stage at all times, pulling the impedance down. A typical place where this is used is in Hi-Fi preamplifiers to bypass the tone control – see Fig.52. Input from lowimpedance drive 620Ω – R output Stereo 22kΩ Bass Treble 0V Fig.52. Alternative bypass circuit where the processor’s input is always connected, such as in this Hi-Fi preamplifier configuration. 48 Fig.54. Mono switch circuit with a bit of gain tweaking to equalise the volume difference between mono and stereo. Practical Electronics | August | 2024 Fig.56. Interior of an Arcam 65 Plus amplifier. It used a microcontroller employing a rotary encoder to accomplish full remote control. Too high tech for me, I couldn’t fix it. (Lovely Douglas Self power amplifier though!) The thick white wires are a bypass ‘bodge’ from the Aux input sockets to the tone control inputs. ended up bypassing the whole selector section, reducing it to a single-input computer amplifier. A real benefit of electronic switching can be had by removing easily corrupted small-signal wiring Electronic switching connected to mechanical switches at That’s i t f or me c h a n i c a l a u d io the front panel and confining it to the switching; now we can move on to PCB at the rear. The DC control signals the important topic of electronic audio can then go anywhere around the front switching, which offers the possibility panel for the most ergonomic layout. of remote control that has become This means there is no need to worry an essential marketing tool today. I about hum when placing a switch next don’t rate it that highly as I’m always to the mains switch. losing ‘remotes’ and I don’t think a The simplest and highest quality sedentary lifestyle is so wonderful. If way of implementing DC control is I’m going to select a new source, I see to use relays, but it is debatable if no problem in getting up to operate a this is truly ‘electronic’ – more like switch. A modern amplifier following ‘electromechanical’. On the other this tradition is the Arcam 65 Plus hand, I tend to view relay control as shown in Fig.56. This design uses a electronic because it involves electrical rotary encoder and a microcontroller control signals. Again, the relays can to control the electronic switching. be located at the back and only the Unfortunately, the micro died in mine DC control wires need to go to the and could not be replaced thanks to front panel switch. Relays are most proprietary programming/silicon. I commonly available with dual change-over contacts and have to be used in the special circuit shown in FX A S1b In Out Fig.57 to achieve a fourA then B position input selector. A S1a possible problem with this Input Output approach is that the signal has to go through more than B then A FX B In Out one contact. If the relays S1c are well-sealed (better still hermetic) with gold-alloy contacts then this is not an issue. Relay control is Fig.55. A ‘juggler’ switch swaps over the order of two still the choice in most circuit blocks. S1 is a 3PDT device. An alternative top tier Hi-Fi systems and approach could use a 4053 logic IC or latching relay. do it. One option is quite a convoluted circuit called a ‘juggler’ using three ganged change-over switches (3PDT) as shown in Fig.55. Practical Electronics | August | 2024 Input 1 RL1a Input 2 RL2a Input 3 RL1b Input 4 1 2 V+ coil voltage 0V 1 0V NC 2 Input select 3 4 Fig.57. Four input selector switch using DPDT relays. Duplicate for stereo use. Suitable relay is Relpol R8M822N, Rapid 61-6018. V+ S1 Off Push to break NO NC RL1 S2 On Push to make 0V Fig.58. Latching relay circuit. 49 Debouncer section monostable S1 S1 Push to 100nF make 1MΩ – + To switch sections CD4013a CD4001a 10nF D CD4001b 100kΩ 741 V– 100nF 1.5MΩ V+ V+ Push to make 0V 10nF 1MΩ +5 to 15V R S Q Output (inverted) Q Output 100kΩ Output +10V until S1 pressed then –10V 220kΩ 10kΩ 10kΩ 0V Fig.60. Op amp latching circuit. Avoids the use of unreliable CMOS chips and give higher output voltage as well. 100kΩ 4.7kΩ Low-current LED, on when S1 pushed 0V Fig.59. CMOS switch debouncer and flip-flop. Ideal for controlling 4066 chips. Shown using single-rail supply, but could use a dual-rail supply. test gear. My Audio Precision analyser takes the number of relays to extremes, and many can be heard clicking away as it runs through various test routines. This mechanical noise is annoying and strangely reassuring at the same time. Relay coils consume relatively high levels of power at 100mW or more, and can generate high back voltages when turned off. It is vital to use clamping diodes, an independent power rail and an independent earth to avoid clicks and bangs. Control logic One of the great advantages of using electronic control is that latching/maintained switches can be replaced by cheaper momentary types. Gold-plated contacts are not needed either since we are dealing with switching DC rather than low-level signal currents, sometimes called ‘dry switching’. However, we need to generate a latching action to hold the selected position. It is possible to use a spare contact on a relay to do this job (Fig.58), but it is more convenient to use a latching logic element, such as a flip-flop/bi-stable to do this. This can be done with standard logic ICs such as the 4013 dual D-type flip-flop. A common problem affecting counting logic when controlled by mechanical switches is switch bounce. This can cause erratic operation, such as jumping a few steps randomly. A monostable made with a couple of NOR gates makes a good de-bouncer to feed the flip-flop, as shown in Fig.59. It’s also possible to make a latch with an op amp using a circuit that appeared in Elektor Summer Circuits (July 1983) as illustrated in Fig.60. I have found this arrangement useful since it can use a standard op amp power supply. It is also a good use for old spare op amps that are too noisy for audio, such as 741s. A drawback is the problem of an uncertain state upon switch-on. One way round this is to have some kind of capacitive delay on one side of the bi-stable. Fig.61 shows a discrete bi-stable used in Boss effects pedals. Next month That was a very brief start to electronic audio switching. Next month, we will dive properly into this important and fascinating topic. ESR Electronic Components Ltd All of our stock is RoHS compliant and CE approved. Visit our well stocked shop for all of your requirements or order on-line. We can help and advise with your enquiry, from design to construction. 3D Printing • Cable • CCTV • Connectors • Components • Enclosures • Fans • Fuses • Hardware • Lamps • LED’s • Leads • Loudspeakers • Panel Meters • PCB Production • Power Supplies • Relays • Resistors • Semiconductors • Soldering Irons • Switches • Test Equipment • Transformers and so much more… Monday to Friday 08:30 - 17.00, Saturday 08:30 - 15:30 +9V 1MΩ 56kΩ 470pF 470pF 100kΩ 100kΩ 56kΩ Output Output BC549B 1kΩ 470pF Foot switch push to make 10nF 56kΩ 56kΩ BC549B 470pF 0V Fig.61. Good old discrete flip-flop; still used in Boss guitar pedals. 50 Station Road Cullercoats North Shields Tyne & Wear NE30 4PQ Tel: 0191 2514363 sales<at>esr.co.uk www.esr.co.uk Practical Electronics | August | 2024