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AUDIO OUT AUDIO OUT L R By Jake Rothman Universal single op amp board (optimised for audio electronics) – Part 1 audio work. The best way to test this is to put the offending op amp into a highgain amplifier and have a listen if you have good ears, or check with a ‘scope if – like me – your hearing is no longer top notch. In the end, I decided that to test op amps the best I could to do was to design a universal circuit board where most of the standard op amp configurations could be easily built. Getting board RS used to sell a generic op amp board, as shown in Fig.1. Maplin also did one. I’ve used lots of the now discontinued RS boards over the years, such as in the EPE Test Amplifier and the Upwards Compressor shown in Fig.2. Since such a useful board is no longer available, it’s obviously time to produce a PE replacement, and while we’re at it, Fig.1. The discontinued RS op amp PCB. Now we have a better replacement, optimised for audio, but perfectly useable for general op amp applications. O ften the subject of this column is initiated by a reader’s request. In this case, Mr Martin van Doorn from Holland needed an op amp tester. To create a tester that covered every op amp parameter would be very complex – more a job for a team at say Peak Electronic Design than my own ‘sole-trader’ approach. So, having dodged that job, I did come up with a viable alternative idea. Fig.2. Here’s a typical studio application for the RS boards, adding balanced XLR inputs to an upwards compressor (low-level ambience booster). Mission creep In audio equipment, op amps are surprisingly reliable, with a failure rate almost as good as single small-signal transistors. Their most common failure mode is the inputs getting zapped by abuse – for example, connecting large, charged capacitors or 48V phantom power with no protection. This usually results in the output permanently stuck at one of the power rails or a short between the power pins. In such cases it’s a straightforward go/no-go test with a DVM/multimeter or simply sniffing for smoke. A more subtle failure is input transistor degradation, resulting in a high noise level, a major problem for 48 Fig.3. A problem with the RS board was a lack of provision for the large capacitors required for audio work. Practical Electronics | December | 2022 R14* C4 + R7 Bias C2 R1 V+ R2 R9 R11* C13 3 +input pad R6 Vb bias C5 R8 *Feedback components V+ 2 Vb + R10* R5 + Input + 0V R4 + + Input – C3 C8* –input pad R3 C1 C7* 7 – IC1 + 5 8 C6 0V R12 6 4 C10 100nF Vb Vb R6 R3 0V C11 C9 Output + Sum input V+ V– Op amp inputs R13 Power on 3-pin connection V– C12 Link pads 0V Fig.4. The general circuit diagram of the universal op amp board. Not all the components are used at once, only a selected few for a specific circuit. Fig.7. There are links provided for bias and grounding. Note the PE boards have an extra grounding pad. There are also connection points going to input pins 2 and 3 on the op amp near C5. These are useful when pots have to be connected for gain control. There is also a link to R3 for biasing bipolar capacitors made up of C1 and C3. Finally, there are three ground pads – see Fig.5. amps, the NE5534, sometimes requires a compensation capacitor across pins 5 and 8. Fig.5. The overlay of the general op amp board. Chip multipacks Some of the best audio op amps, such as the LM4562, are only available as dual devices. Generally dual devices are cheaper than singles, so we’ll be needing a dual-channel board as well, a design which will be coming later. Quad op amps, such as the TL074, are rarely used Fig.6. The board has provision for experimentation in the form of four ‘blob points’. Essentially, five pads connected in strips like a bread board. upgrade the late-1970s single-sided unplated-through-hole implementation. The RS boards were designed for DC instrumentation use, reflecting RS’s mainly industrial clientele. When I used them for audio applications there were often large capacitors hanging off precariously, as shown in Fig.3. Practical Electronics | December | 2022 So, accommodation for these larger components was necessary in my new design. On the other hand, some instrumentation requirements, such as the large off-set adjust preset, could be omitted. Also needed was a half-rail bias network for single-rail operation. Plus, one of the most popular audio op Fig.8. The output resistor outline is extralarge to allow a capacitive load isolator inductor network to be added. 49 for audio since it’s difficult to get large audio capacitors to fit around them on the PCB. Apart from the LM837, there are very few low-noise quad devices for audio anyway, so we won’t be bothering with quads. I do think a plug-in dualto-quad adapter board is a good idea though. I would love to replace some noisy quads with a couple of NE5532s in some of my vintage gear. Has anyone designed one? If there is enough demand, C4 1µF Sum input I’ll do a 2x 8-pin dual to 14-pin quad converter PCB. Well-padded board design The overall schematic for the board is shown in Fig.4. It is optimised for audio applications, but is certainly not limited to audio. For any given circuit only some of the components will be inserted. The overlay with all the possible components is shown in Fig.5. R14 10kΩ + R7 10kΩ Basic circuit C7 47pF C8 Rin R3 Input – C3 R4 + + C1 C2 0V R1 R6 V+ R2 R9 Vb bias Vb + C13 R8 + –input pad R5 + Input + IC1 R11 2 – 3 + +input pad C5 7 IC1 5 8 V+ 0V 0V C9 10µF 25V C11 100nF 6 4 C10 100nF C6 22pF R13 100kΩ Power on 3-pin connection V– C12 100nF 0V Link pads Link Used Not used R7 C 6 R13 C 10 0V Input 0V Output IC1 C9 + C11 C12 Normally, for a board like this, the components would be annotated according to their function. On the RS board for example, the negative feedback resistor was called Rb. To keep things simple for the PCB designer and constructor I’ve decided to stick with conventional numbering. H e r e ’s a l i s t o f t h e components and their possible functions. Things will become clearer when specific circuits are shown where many of the board’s component positions are left empty. R1, R2 and R13 R 14 C4 Output R12 47Ω Fig.9. Inverting amplifier, a standard op amp configuration. R 12 Output Gain = – Rf/Rin + R10 Component functions Rf – Input Bias All the big capacitor positions have extra pads for different outlines. These allow radial or axial components to be used, and capacitors wired in parallel. There are also four interconnected pad groups or ‘blob-points’ for adding extra components for experiments on the lefthand side of the board. (Note how the blob points have been used to make it easy to add extra capacitors to make a parallel C8, as shown in Fig.6.) 0V V+ V– Fig.10. Component placement for the inverting amplifier. These resistors are an important audio addition. They are pulldown resistors which prevent clicks when input and outputs are connected. They hold the outputs of the coupling capacitors to 0V. Typical values would be a few tens of kΩ when using electrolytic coupling capacitors and around a few hundred kΩ for film and tantalum types (because of their lower leakage). C1, C2 and C3 These are coupling capacitors on the op amp inputs. C1 and C3 are for the inverting input. They can be wired as a non-polarised capacitor for low distortion. The capacitor distortion can be reduced still further by biasing the mid-point positive terminals of C1 and C3 at 5V using R3 and Vb. C1 and C3 can also assume the role of the lower-arm feedback capacitor for non-inverting configuration when grounded by putting a shorting link in R1s position. C2 is connected to the non-inverting input. R4 Fig.11. The assembled board – the op amp is a TDA1034, the original NE5534, designed by Philips. The date code is 1977 and this one was used in Pink Floyd’s touring mixing desk made by Midas for Britannia Row. It’s still working fine, having been installed many times. This is the input resistor for an inverting amplifier configuration, commonly designated ‘Rin’. It can also be the negative-phase (‘cold’ in audio parlance) input resistor on a differential amplifier. R6 is usually linked to ground in inverting amplifiers via a link. A low value, such as 560Ω, is often used for audio for low noise while providing a degree of 50 Practical Electronics | December | 2022 R7 Input – R1 V+ R2 Input2 C13 R8 – 10kΩ IC1 2 – 3 + 7 IC1 5 8 0V R12 6 4 C10 100nF C6 Output + 0V C11 C5 Vb 10kΩ V+ +input pad R6 Vb bias R9 + R11* R5 + This can be the positive phase input resistor on a differential amplifier. In the non-inverting configuration, it becomes the input RF filter in conjunction with C5. R10* –input pad R4 C3 22µF 10kΩ C2 0V C8* + + R3 C1 22µF Input + C4 and R7 Feedback components C7* Input1 10kΩ C9 Output + C4 Bias R5 Basic circuit R14* Sum input + input current limiting in the event of faults such as power supply misconnection. In instrumentation, it is usually the same value as the feedback network giving lowest DC offset. For single-supply rail use, it is connected to the halfrail bias network consisting of R8 and R9 via another link. The link area is shown in Fig.7. R13 Power on 3-pin connection V– C12 0V Link pads Fig.12. Summing or mixing amplifier with an extra input – the composite bipolar capacitor (C1 and These provide a second input C3) is optimised for good LF response. Note: red components shown are in addition to those in Fig.9. to the inverting amplifier’s input when used as a virtual earth 0V C1 summer or mixer. –input R 12 R14 This is the all-important negative feedback resistor. There is provision for more complex feedback networks, such as RIAA equalisation comprising R10, R11, C7 and R8. Finally some stability components. If a phase-lead capacitor is needed across feedback resistor R14, it can be C7 with C8 linked out. Typical values are 33 to 100pF. R12 is needed to isolate the op amp from capacitive loads, such as screened cables. An inductor can also be used instead for lower AF output impedance. This can consist of 40 turns of wire wrapped around a 1W 39Ω resistor or a 10µH inductor and resistor wired in parallel, as shown in Fig.8. There’s the usual decoupling capacitors C10, C11 and C13. Capacitor C6 is the compensation capacitor for NE5534 op amps for gains below 5. + R 14 R7 R4 C4 C 6 IC1 C 10 + R13 0V Output C3 C11 C9 C12 + 0V Input 0V V+ V– Fig.13. Overlay for summing amplifier. Component list (and function) Semiconductors IC1 Single op amp, such as NE5534. Can be 8-pin DIP through-hole or SOIC surface-mount part. Resistors All standard 0.25W case size, usually metal-film, use the 1% 0.6W MRS25 series for audio. R1, R2, R3 input grounding/pull-down resistors 22kΩ to 100kΩ R3 bipolar capacitor bias resistor or link to ground for using big lower-arm feedback capacitor in C3 position. R4, R5 o p amp input resistors,  typically 1kΩ to 100kΩ Practical Electronics | December | 2022 Fig.14. The completed summing amplifier. R6 n o n - i n v e r t i n g i n p u t grounding / bias resistor, typically 1kΩ to 100kΩ ( m u c h h i g h e r, 1 M Ω t o 4.7MΩ, for FET op amps) R7 summing input resistor R8, R9 half-rail bias resistors equal value, 10kΩ to 100kΩ R10, R11 extra feedback resistors for filters R14 f eedback resistor zero to  220kΩ (much higher, 1MΩ to 4.7MΩ, for FET op amps) R13  utput capacitance isolation o resistor 39Ω to 600Ω Capacitors C1, C2 input coupling capacitors,  typically 1µF to 22µF C3 part of bipolar capacitor with C1 or big lower arm feedback capacitor C4 extra input coupling  capacitor for summing input or small lower arm feedback capacitor 51 C4 100µF 16V Sum input R14 10kΩ R7 1.1kΩ C7 47pF R1 100µF 1.1kΩ C8 other applications–do let us know your ideas. Rf 10kΩ Basic circuit Inverting amplifier – + Apart from a voltage follower, this is the simplest Feedback Blocking amplifier circuit that can be R10 R11 components Gain = 1 + (Rf/R1) capacitor Bias built on the board. There V+ are two ways of feeding –input R3 Bias 1/2 supplyV+ pad C11 C9 the input. The simplest is 100nF C3 C1 R4 100µF Output 2 7 to come in via the two-pin 25V Input – – 0V 6 R5 IC1 input Molex connector 1kΩ 3 4 Input + + R12 shown in the circuit in 8 C10 47nF 5 C2 100nF Fig.9 and the overlay in R6 220nF +input 0V V+ 100kΩ Power on 3-pin R13 pad R1 Fig.10. An alternative is to 100kΩ connection C6 Vb bias R9 V– R2 22kΩ use the inverting input on C5 1MΩ 220pF Link to connect the three-pin connector via C12 Vb negative rail to 0V C1. This enables a bipolar 0V + R8 input coupling capacitor Link pads C13 22kΩ Link Used Not used 10µF composed of C1 and C3 to be used. This network Fig.15. Non-inverting amplifier with gain of 10x and single-rail biasing. can be biased to give the lowest distortion. This is useful for low-impedance applications R C7 where the input resistor is very small. 14 0V –input A completed inverting board is shown R +input C2 R2 6 R in Fig.11. R5 IC1 + Output + + + Input 12 R7 C5 C 10 R 9 R 8 Summing amplifier R13 C4 IC1 + C13 + 0V Output C9 C11 + 0V V+ Fig.16. Overlay for non-inverting amplifier – note bias links. This is just an inverting amplifier with an extra input resistor. Both inputs on J1 and J3 are used. The circuit is shown in Fig.12. The bipolar capacitor set up was used for an equaliser where a low-pass filter output of a state-variable filter was mixed with the high-pass creating a notch filter. The high value used (11µF) minimised the LF response droop. The overlay is shown in Fig.13 and Fig.14 shows the construction. Non-inverting amplifier Fig.17. Fully stuffed non-inverting amplifier. C5 C6  F filter, 47pF to 470pF R NE5534 compensation; not  used for gains over 5: 22pF for unity gain, 4.7pF for RIAA stages, 2.5mm pitch C7, C8 feedback, C8 is for extra-large polystyrene filter capacitors C10 power rail decoupling 0.1µF ceramic 5mm C11, C12 0.1µF ceramic 5mm or up to 10µF electrolytic 52 Connectors Molex 0.1-inch pitch PCB connectors Molex equivalent 2 off each JYK P2500-02 two-pole straight header and three-pole P2500-03 Rapid order codes 22-0950 and 22-0955 Classic configurations Here’s a few of the possible circuits that can be built on the board; I’m sure readers will adapt the board for many This is probably the most common op amp circuit, typically giving any gain from 1 to 1000 (0dB to 60dB) along with a high input impedance. A circuit giving a gain of 10x is shown in Fig.15. This circuit is more complicated because it is shown designed for single-rail (rather than the normal dual-rail powering). This is achieved by feeding the non-inverting input with a half-rail bias voltage. The overlay is given in Fig.16. A useful application of this configuration is a microphone preamplifier. This consists of a noninverting amplifier with variable gain and a step-up input transformer. Fig.17 shows the completed board. To run it off the dual-rail supply, leave off the half-rail bias and link R6 to ground. Next month In Part 2, we will finish describing this design with a differential and RIAA phono amplifier, plus a useful and unusual low-frequency compensated op amp amplifier. Practical Electronics | December | 2022