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V+ AUDIO OUT Control Controlled current sources AUDIO OUT 10kΩ 10kΩ – L Series switch 2 + – R + By Jake Rothman Input Output Series switch 1 –1 Audio switching Part 5 – more on electronic switching Shunt switch 0V W e pick up here where we left off last month, describing various practical audio switching circuits implemented using solid-state devices like JFETs or ICs, rather than mechanical relays and such. They have various advantages, such as silent operation, reduced cost and lower power consumption. Capacitor coupling When the FET is switched off with a negative control voltage, a DC path to ground is necessary for the bootstrap resistor, which can be the low impedance of an op amp output. Without this path, it won’t switch off. If a coupling capacitor is used to block the DC offset of the stage feeding the JFET to prevent clicks, this path is blocked. Douglas Self connects the bootstrap resistor before the capacitor to give the required path, as shown in Fig.83. Solid-state logic (SSL) Back in January 1989, I took the SSL maintenance course in Begbroke, Oxfordshire. This was for the massive £250,000 G-series mixing consoles that every top studio then had to have. One of the lecturers was SSL’s principal analog design team leader, Andy Millar, who explained the subtleties of FET switching. His team’s work led to the development of the SSM2402 audio switch at Analog Devices/Precision Monolithics Inc. In this circuit (Fig.84[a]), the bootstrapping Input + – 220nF J112 Output 22kΩ 100kΩ (Load) DC path 1N4148 Capacitor input +Ve 10V to turn off –Ve 10V to turn on 1MΩ 0V Fig.83: the AC-coupling arrangement for a JFET with bootstrapped DC path. 36 was done by an op amp, which ensured that VGS was zero while allowing the gate control signal to pass through. The control input to the op amp was via a current source; otherwise, the follower would become an amplifier boosting the bootstrap signal. I built one section of this circuit (Fig.84[b]) and found that the op amp clipped on the negative cycle first, but this was only in mute when the negative control voltage was added on. So it was not a problem. Making a discrete version of this discontinued IC warrants further research, since the distortion was 0.0015% at 1VRMS (0.01% at 4VRMS) into a high impedance load. However, it was a bit worse than the diode bootstrapping method in my initial experiments. Interestingly, the JFETs in the 2402 are P-channel types. This results from a similar ion implantation process to that used for Texas Instrument’s Bi-Fet TL0xx series op amps. They also have P-channel JFETs, which is the outcome when made in association with the standard NPN bipolar transistor IC process. Audio Input Inverter 1µF 100kΩ 0V J112 1mA current regulator diode Audio Output +18V 7 3 + 4 100kΩ 10kΩ 6 2 – Control input +10V: mute 0V: On 0V 0V 8 5 22pF Audio and inverted control signal –18V 10kΩ Fig.84(b): I found this discrete version of the SSM2402 concept to work OK. This system is really the minimum required for a decent audio switch. It avoids the need for bootstrapping because the signal voltage across switch TR1 is very low when it is conducting, so VGS is also very low. A shunt JFET switch (TR2) attenuates the input in conjunction with R1 when the switch is off, preventing breakover. Noise is slightly higher with the inverting op amp than with the voltagefollower op amp system (a unity-gain inverting op amp has a noise gain of two). A good compromise between noise, input impedance and distortion is to use 4.7kΩ for R1 and R2. Switch block My circuit uses blocking capacitors A switch block I developed based on C1 and C2 because FET input op amps, virtual-earth topology is shown in Fig.85. especially the low-noise ones, V+ Control have become expensive. The Controlled current sources two capacitors are connected back-to-back in the series chain, effectively making a composite 10kΩ 10kΩ non-polarised capacitor, which reduces their distortion, especially with tantalum types. – – I used a P-channel J175 for Series switch 2 TR2 with a slight reduction in + + maximum attenuation. An inverter stage could be added to allow Input Output two N-channel J112 FETs to be Series switch 1 employed (as shown in Fig.84[b]). –1 C5 is a phase-lead compen­sation Shunt switch capacitor to compensate for the Inverter 0V 0V capacitance of TR2 and layout capacitance around pin 2. The Fig.84(a):1µF the SSM2402 chipJ112 uses interesting measured distortion was 0.001% Audio Audio op amp bootstrapping with three FETs in a Output Input at 10V RMS from 20Hz to 20kHz. bidirectional series/shunt/series network. 100kΩ 0V +18V 7 3 + 2 – 100kΩ 10kΩ 6 8 0V Practical Electronics | October | 2024 +V TR1 R6 R5 R8 100kΩ TR2 (J112 n-chan) D/S* G S/D* BC546B Control input J112 TR2 p-chan J175 TR1 p-chan Inverter R9 10kΩ C5 22pF R2 4.7kΩ –V + + +18V 2 3 TR2 J175 D1 1N4148 R3 10kΩ R5 6.8kΩ C3 220nF R7 680kΩ 0V C2 47µF 15V Tant TR1 J112 R1 4.7kΩ C1 47µF 15V Tant By combining two JFET switch circuit blocks (Fig.85) with a shared virtual earth op amp circuit, a changeover switch can be made, as shown in Fig.87. This can be driven by the flip-flop switch circuit from Fig.59. Since its control outputs are complementary -V and +V, all the JFETs can be N-channel J112s or even J111s because the negative voltage can go up to -16V. *Interchangeable FET switch circuit R6 6.8kΩ R4 330kΩ D2 1N4148 C4 220nF IC1 7 NE5534 – + 4 R8 680kΩ R9 47Ω 6 8 5 C6 22pF Output C7 100nF –18V 0V Fig.85: the final JFET mute switch, suitable for professional audio gear. Attenuation was about -90dB at 10kHz with the J175. Ultimately, an electronic switch needs around 16 components just to approach the qualities of a simple mechanical switch. Ramping Each gate has two RC networks, R5/ C3 and R6/C4, to ramp the JFET gate voltages, giving almost a fade rather than an abrupt transition. It is a distorted fade when switching a high-amplitude sinewave, but it is less noticeable than a click when music is switched. This is done on the SSM2402 IC by special on-chip silicon nitride capacitors, ensuring smooth breakbefore-make action. There was a 2412 chip variant with faster switching for broadcast use. More circuit tricks I have seen some tricks applied on the Soundcraft 6000 series desks that are worth looking at. It is possible to have a DC-blocking capacitor in series with the lower JFET switch, as shown in Fig.86. This capacitor is thus out of the audio path when the whole switch circuit is passing audio. Control –15V: Off +15V: On Resistor R3 is used to equalise the DC offset levels, preventing clicks. This technique needs FET-input op amps, such as the expensive OPA1641, since the bias currents from bipolar devices like my favourite, the NE5534, will cause clicks. Minimising RON Although N-channel JFETs are on at 0V gate bias, a bit of extra positive bias (200-300mV) can minimise RON and consequently distortion. For a typical J112, at 0V VGS, RON = 38Ω, but with a VGS of 300mV, it drops to 28Ω. This can be achieved by the potential divider action of a 10MΩ resistor across the diode and a 680kΩ resistor to ground from the gate with a +7.5V control signal. I only got 300mV while there was no signal passing through, since the ‘diode action’ of the gate generated a small negative voltage, so I did not think this technique was worthwhile. It’s also a problem at high temperatures due to the gate’s diode leakage being temperature-dependent. Input 1 Control 0V Output 0V 4.7kΩ Keeps capacitor at same 47µF offset voltage of op amp 25V (Only in circuit path in Off mode) 47µF 10V 3.3kΩ Flip-flop switch (Fig.59) FET-input op amp R3 22kΩ + 47pF V+ (V–) + Control (inverted) Virtual earth block FET switch block R2 4.7kΩ – The Quad selection circuit is the simplest electronic channel selector I’ve seen and makes the circuit given in Fig.59, with its debouncing, seem over the top. This circuit (Fig.88) is ideal for use with CD4066 devices. Contact bounce does not seem to be a serious problem in practice with Hi-Fi input selection. The occasional ‘jump’ is not as catastrophic in audio as it would be in an industrial control system. Sufficient debouncing can usually be achieved with capacitors across switches, especially with slow 4000-series logic. The Quad circuit has a capacitor wired across S2 to ensure the circuit always defaults to this position when powered. It can be moved to whatever position is required at power-on. 40xx series logic gates alone often had insufficient output current for high brightness with older technology LEDs, which is why the Quad circuit had driver transistors. The latest highefficiency LEDs are bright enough at 1mA, such as the red TruOpto (Rapid 55-2190) or Kingbright (72-8989). So, if one of them is used, the driver Off (On) 33pF R1 Input 4.7kΩ Interlocking switches I first saw a similar topology in the Studer A779 mixing desk. + Input A changeover switch Virtual earth amplifier D/S* S/D* G 330kΩ V – (V+) – + 47Ω Output Input 2 FET switch block On (Off) Fig.86: you can put the DC blocking capacitor in series with the lower JFET instead of the op amp input. Fig.87: a changeover switch can be made by feeding two mute circuits into one op amp. The op amp flip-flop (Figs.59 & 80) can control this. All FETs can be J112s. Practical Electronics | October | 2024 37 IC1b 4001 6 8 4 5 9 10 10kΩ V+ 14 IC1c 4001 13 Output 2 1 IC1d 4001 V– –7.5V Output 1 S1 Unused NOR gate 6 5 A 4 10kΩ S2 S3 Radio +VE Output 1 Output 3 Disc 1MΩ 10kΩ 10kΩ S2 Aux 4.7nF Delay capacitor +7.5V +8.6V 8 9 B 10 10kΩ 1MΩ BC182 BC182 1MΩ Start-up 4.7nF capacitor +VE Output 2 1MΩ +VE 7.5V S1 2 11 7 IC1a 4001 S1 BC182 –7.5V 12 13 C 11 10kΩ 1MΩ –9.4V 1kΩ 1N4148 Fig.88: the Quad 34 3-channel selector switch circuit. The power rails for the ICs are +7.5V (pin 14) and -7.5V (pin 7), so the audio signal can be DC-biased at 0V. positions, but the package has a spare NOR gate. I had it redesigned by Grant Stevens to make it a four-position switch with the addition of some diodes, as shown in Fig.90. This ties up nicely with the 4066 chip that has four switches. The startup position capacitor has been increased in value R9 to allow for the slow 1MΩ ramp-up of some power + supplies. 0V 0 out 3 Note that when feeding 1 IC1a a high-impedance CMOS 1 4001 gate with diodes, it is R10 1MΩ often necessary to follow the diode with a 100kΩ to + 0V 0 out 4 1MΩ pull-down resistor 2 (R9 to R12 in Fig.90) due IC1b 2 4001 to the leakage current. R11 Grant also designed a 1MΩ decimal to binary-coded + 0V decimal (BCD) diode 0 out 10 3 logic circuit to drive IC1c 3 4001 a 4052 audio switch, described below. R12 transistors can be dispensed with, as shown in Fig.89. The circuit only consumes 0.9mA at 9V and 1.9mA at 15V. The Quad circuit had only three +VE 4 3 2 1 (Reset to 1 ) 4.7nF Switch selected 1 3 2 2 3 1 2 4 1 In R5 10kΩ VE+ 0V 4 4 3 Select 6 5 2 R6 10kΩ 1 4 1 8 9 3 R7 10kΩ 2 1MΩ 2 1 13 12 4 3 0V 11 IC1d 4001 R8 10kΩ R1-4 1MΩ 0V Convert to BCD (for 4052) 1 NC 4 The Playmaster Series 200 + out One of the first DIY amplifiers using a similar switching system was the Playmaster Series 200, R13 6.8kΩ published in Electronics Australia in March 1985 (page 38). It used 4052s 1 2 3 4 (two four-way switches B 0 0 1 1 in a package) and 4011 4 A 0 1 0 1 2 A B Pin 5 6 3 5 6 4 8 9 10 12 13 11 3 1 On 2 On 0 0 1 1 0 0 1 0 0 0 1 0 4 3 On 4 On 1 0 0 0 1 0 0 0 1 1 0 0 1 0 0 0 0 1 0 1 0 0 1 0 0 1 0 0 1 0 1 0 0 0 0 1 4x 1N4148 1MΩ Outputs A and B to 4052 control inputs (pins 9 and 10) 38 0 1MΩ 0V Fig.90: why waste a gate in a quad package? Here is the Quad circuit expanded to four buttons. The inset diagram is a diode logic driver for use with a 4052. –VE –7.5V Output 3 Outputs Pin number 5 6 4 8 9 10 12 13 11 1 2 3 Input switches A On 0 0 1 0 1 0 1 0 0 1 0 0 B On 1 0 0 0 0 1 0 1 0 0 1 0 C On 0 1 0 1 0 0 0 0 1 0 0 1 Fig.89: a simplified Quad circuit with logic analysis by Grant Stevens. NAND gate flip-flops, as shown in Fig.91. Some may prefer this circuit because the switches go to ground. Using a couple of diodes, a backup voltage source could be added to the power pins of the CMOS chips to memorise the switch positions. The current consumption of these devices is below 10µA; I used a 100µF 10V lowleakage tantalum capacitor, which gave a backup time of about half an hour. Of course, the LEDs have to be connected to the anode side of diode D9. The 4052 is run from split power rails to accommodate the bipolar audio. The control signal can be normal logic levels of 0V and +5V because there is internal-level shifting circuitry in the 4052. Breakover occurs at 16V peak-topeak and clipping at 13V peak-to-peak. The original circuit used a BCD decoder made from another 4011 to drive the LEDs shown in Fig.92. A single 4052 was originally used, switching both left and right channels. The original op amp was a TL071, but this has a phase inversion at negative clipping when used as a follower, so it was changed to an NE5534. Attenuation was only -40dB at 10kHz on the messy breadboard shown in Fig.93, but this soon cleared up when the unused inputs were grounded. If the inputs are all buffered, which they should be to protect the delicate CMOS gates, it is around -90dB. The distortion was surprisingly low and flat with frequency: 0.0006% at 1V RMS, rising to 0.004% at 4V RMS. Practical Electronics | October | 2024 LED1–4 12 +7.2V +7.5V (Power rail to D9 anode) 16 R5 RLED 1kΩ 14 13 15 6 11 –7.5V or 0V Phono (from preamp) 1 CD 2 Aux 3 Tuner 4 Should be buffered 1 0 0 Pin 1 2 0 1 Pin 5 3 1 0 Pin 2 4 1 1 Pin 4 Possible battery back-up PP3 0V – +15V 0V 5 3 2 4 C2 100nF 2 – 4 A B 7 10 9 BCD output –7.5V A: Q B: Q R7 100Ω 8 Output 5 From 4011 flip-flop –15V 10 11 +7.5V D3 IC2 4011c 14 8 10 9 D1 D7 12 13 1 2 D6 D8 R4 100kΩ CD Tuner S2 S3 D1-D8 1N4148 2 4 Q Q 11 IC2 4011a 5 6 3 4 Q Q 7 IC2 4011b D4 Aux Push-to-make momentary switches 0V Discrete button pushing There is a mismatch between the high negative switching voltages of low-RON JFETs and standard logic chip families. There is also the hassle of providing dedicated logic power rails of different 12 A4 Q2 4 11 Q4 A2 5 10 Q3 B2 6 9 B3 VSS– 7 8 A3 +8.6V Q +7.5V Q 14 9 8 11 4011 1 2 Q Q 470Ω 10 3 6 5 4 Fig.92: you can use this BCD decoder to drive the LEDs if needed. If the input is binary 11, LED 4 illuminates. When feeding in 00, LED 1 illuminates. S4 Phono Q1 3 0V IC2 4011d R3 100kΩ D2 13 B4 7 R2 100kΩ D5 B1 2 12 13 R1 100kΩ 0V 14 VDD+ To right channel D9 BAT46 7.2V (to pin 16, IC1) A1 1 4011 Quad two-input NAND C3 22pF 0V C1 100µF + 10V Tant low leak S1 IC3 NE5534 6 7 3 + R6 220kΩ D10 1N4001 + 8 IC1 4052 Switch B A Output 1 Fig.91: a simplified version of the Playmaster 200 input selector; the spare four-way switch drives the LEDs. Using two separate 4052s simplifies PCB routing and reduces crosstalk. 0V voltages. Solutions to these problems include using level-shifting circuits or designing one’s own logic using discrete transistors. Discrete logic is old-fashioned, but the requirements for audio switching are very simple. Many young engineers reach for an Arduino with scanned switches; contact bounce is important and can be dealt with in software. There are not many discrete logic circuits about now, but I remember as a teenager (in the mid-1970s), there was a big thing about touch-switch channel selectors on TVs. I think some 1974 GEC C211X series sets used discrete transistors with little neon bulbs! As usual, I found a suitable circuit by perusing old Wireless World magazines. In WW October 1974’s Circuit Ideas section (page 380), I found an inspiring circuit by P. G. Hinch. I got it to work after a bit of semiconductor polarity flipping and resistor value changing; the final circuit is shown in Fig.94. Power Powerbus bus +18V +18V R1 R1 4.7kΩ 4.7kΩ TR2 TR2 BC556B BC556B Alternative Alternativebulb bulbwiring wiring +17.4V +17.4VLED LEDon on –15.5V –15.5VLED LEDoff off Output Output Repeat Repeatcircuit circuit C1 C1 2.2nF 2.2nF S1 S1 R4 R4 3.3kΩ 3.3kΩ 6mA 6mA Reset Resetbus bus TR1 TR1 BC546 BC546 D2 D2 1N4148 1N4148 D1 D1 1N4148 1N4148 R2 R2 6.8kΩ 6.8kΩ Power Powerbus bus Fig.93: a Playmaster-style channel selector on a breadboard. Practical Electronics | October | 2024 R3 R3 100kΩ 100kΩ TR2 TR2 Output Outputtoto FET FETswitch switch 3.3kΩ 3.3kΩ Bypass Bypass 28V 28V 40mA 40mA LED1 LED1 D1 D1 R5 R5 1.2kΩ 1.2kΩ R5 R5 220Ω 220Ω 0.5W 0.5W –18V –18V –18V –18V Fig.94: the final circuit (per switch) for the discrete interlocking circuit. The polarity is inverted from Hinch’s design. 39 Fig.98: the rotary switch display PCB. I described this in the Modern Amateur Electronics Manual years ago (Supplement 24). Fig.95: a discrete transistor interlocking switch circuit with filament light bulbs on a breadboard. The blue breadboard is the JFET audio switch from Fig.85. 22 swg tinned copper wire Reset bus LED LED LED Switch Switch Switch V+ V– Power Fig.99: the TSL audio monitor, a highly-regarded broadcast unit, excellent for driving LS3/5A speakers and monitoring the stereo compatibility of mixes. The current consumption for the bulb circuit is 45mA from both rails. In this circuit, the bulbs are under-run at 23V, so they last a long time. I had to prove the concept on a breadboard; see Fig.95. I’ve kept a whole bag of these momentary switches in stock for years – now I have a use for them! The great thing about this discrete circuit is that it can be divided into a separate section for each switch. You can have a string of buttons as long as you like, as shown in Fig.96. allow the LED current to be increased for higher Switch 2 Switch 3 Switch 1 brightness and can easily Fig.96: a suggested system for one PCB per switch, extendable as long as you like with busbars. drive the 28V 40mA (eg, CML CM334) filament bulbs in EAO switches by changing R5 to The capacitor across the switch can 220Ω ½W. Unlike LEDs, bulbs do not be used to set which switch comes on require a series resistor (R4), so the bulb when the circuit is powered. If you is wired from the collector of TR2 to R5. want S1 to be the default, increase C1 If the bulb blows (which they do every to 220nF. 4000 hours), the interlocking ceases This system is ideal for controlling to function, so each bulb must have the switch block in Fig.85, which needs a 3.3kΩ resistor wired in parallel to +15V for on and -15V for off using ensure enough current flows to trigger standard op amp power rails. the circuit. Discrete transistor circuits also Discrete diode circuits Com +5V 3, 14 1 2 Com Rotary switch gang 0 3 1 4 5 6 7 D9 13 D6 8 D1 8 D3 D11 D12 D2 10 D10 D5 D4 D7 D8 7 2 11 A Common anode B a f C D E F b g e c d G R1-R7 150Ω Kingbright SA04-11HWA SA03-11GWA 0V D13 Select R1-R7 to suit rail voltage and required LED current. Fig.97: a diode-based seven-segment display driver for a rotary switch. You need an extra gang on the switch but avoid complex front-panel artwork. 40 Some switching logic is so simple it can be done with diodes, such as the rotary switch LED readout circuit shown in Fig.97. There was an excellent story about this in the Radio Constructor series, In Your Workshop, May 1978. The assembled PCB is shown in Fig.98. Doing it properly The Television Systems Limited (TSL) analog audio broadcast monitor shown in Fig.99 exemplifies electronic switching and general construction. The whole Practical Electronics | October | 2024 audio signal path is dealt with in a self-contained plug-in Euro card at the back of the unit near the audio XLR connectors. The interior is shown in Fig.100, the Euro card is shown in Fig.101 and the bilateral electronic switches used are Vishay’s DG509. The DG509 is a device that can operate from supply voltages of up to 44V maximum and it contains two four-way switches; a posh CD4052, in effect. The binary input to switch it is also the same. It costs around £4 from Mouser. The front panel switches are all mechanically latching C&K paddle types, just controlling DC, which is fed to the Euro card by a grey ribbon connector. Fig.100: the interior of the TSL audio monitor, a great example of audio gear built properly. The audio processor card is in its own screened box. New chips on the block There are some new audio switch chips available. They are all surfacemounting low-voltage (5V) types, so they are difficult to incorporate in conventional ±15V powered op amp audio circuitry. The Diodes Incorporated PI5A3157 has a 6Ω RON but it comes in a hardto-solder SOT-363 package. The Texas Instruments TS5A3159­ DBVR in the SOT-23 package is even lower at 1Ω and claims to have a THD of 0.005% at 4.9V peak-to-peak into a load of 600Ω. I reckon these chips could work well in low-voltage, low-impedance portable audio circuitry. Samples are coming; it will be worth checking how they perform in the virtual earth circuit PE in Fig.85. Fig.101: the TSL analog audio processor card. The DG509 switches are still made; sadly, the Analog Devices SSM2141 balanced line receivers and SSM2142 balanced line drivers are obsolete. TSL certainly splashed out on parts! Transform Your Passion into Expertise with Our Embedded Software Mentoring Programme! Join Us Today – Perfect for Beginners, Enthusiasts, Future Professionals! 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