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AUDIO OUT AUDIO OUT L R By Jake Rothman Universal single op amp board (optimised for audio electronics) – Part 2 C4 R14 10kΩ R7 C7 100pF Feedback components Bias Balanced input C1 10µF Cold R3 C3 C2 10µF Gnd R1 100kΩ R2 100kΩ V+ R6 10kΩ + L C13 R8 3 Link pads ast month, we introduced our Universal Single Op Amp Board, a really useful op ampbased amplifier PCB that is perfect for both design/development and finished products. It takes a through-hole or SMD op amp, around which you have lots of options for adding passive components so that you can build a whole host of common amplifier configurations. I’m an audio engineer, so it’s not surprising that I’ve designed the board with audio in mind, but there is absolutely no reason why it can’t be used for instrumentation or any other high-quality op amp application that requires a quick and easy PCB solution. We’ve already covered the op amp non-inverting, inverting and summing amplifier configurations, and this month we will look at three more handy designs. 2 C5 100pF Vb IC1 + R 14 5 8 4 C 10 R3 10kΩ Gain = (Vin1–Vin2)(Rf/R3) Output R12 47Ω R13 C10 100nF 100kΩ Power on 3-pin connection V– C12 100nF Link Used C1 0V Not used R1 C2 R2 0V –input +input R4 C5 C 6 All resistor equal C9 22µF 0V 6 C6 22pF R5 R 6 C11 100nF 7 – C7 R 12 R2 10kΩ R11 Output + V+ +input pad Vb bias R9 – IC1 Vin2 R10 R5 10kΩ + Hot Vin1 C8* –input pad R4 10kΩ R1 10kΩ + Sum input + Remember that for each amplifier shown, only the red components and green links are used. Rf 10kΩ Basic circuit + Fig.18. Differential or balanced input amplifier, possibly the most common addon op amp circuit for converting consumer audio equipment to professional operation. R13 0V Output IC1 C11 0V V+ V– C9 + C12 Power Fig.19. (right middle) Component placement for differential amplifier. Fig.20. (right) The finished differential amplifier board. 46 Practical Electronics | January | 2023 Audio Precision A-A FAST RMS FREQUENCY RESPONSE +30 +30 +25 +25 +20 +20 +15 +15 +10 +10 d B r +5 +5 +0 +0 d B r A –5 –5 B –10 –10 –15 –15 –20 –20 –25 –25 50 100 200 500 Hz 1k 2k 5k 10k RIAA network *Can be scaled (see text) C4 + + Gnd R5 + Input R1 C2 100µF 10V R9 R2 180kΩ Vb + C13 V+ + 220µF R11 63.4kΩ* R6 68kΩ – 3 + IC1 5 +input pad Vb bias 7 8 IC1 6 Output + R3 10kΩ 0V C11 10µF 25V (Solid aluminium) 0V 4 Output C12 10µF + 25V (Solid aluminium) R8 Link pads Link 0V Used Differential amplifier This amplifier is useful for adding a balanced input to consumer audio equipment. The input is connected via the three-pin Molex. C3 is normally replaced with a wire link (short). As with the non-inverting amplifier, R6 can be linked to the bias point for single-rail operation. The circuit is shown in Fig.18, the overlay in Fig.19 and the final stuffed board in Fig.20. Now we’ve covered the basic amplifiers, here’s a couple of more specialised circuits that can be built on the board. Fig.22. Phono amplifier circuit – if you are using only one op amp (as here) to accomplish the RIAA equalisation, then odd RC values are needed for the negative feedback network. R12 C9 47Ω 100µF C10 25V 100nF Power on 3-pin R13 connection 100kΩ V– C6 4.7pF C5 100pF 50.15nF – V+ 2 14.32nF Input 0V –input pad C3 R4 220µF 220Ω 63.4kΩ 220Ω C8 50.15nF* R10 5.23kΩ* + R3 C1 Feedback components 0V Bias R14* C7 14.32nF* R7 5.23kΩ + Sum input Basic circuit –30 20k Not used Extra cap C7 E x t r a c a p C8 R 12 C2 R 6 C5 C 6 C 10 R2 + IC1 0V –input +input R4 R R 10 11 R13 C3 + –30 20 Fig.21.Measured RIAA curve of the phono amplifier circuit in Fig.22. Mid-band gain at 1kHz is +30dB. Max gain is +50dB at 20Hz giving 150mV for a typical cartridge input of 5mV at 1kHz. This requires a lot of open-loop gain for a single op amp; however, this approach generally gives the best noise and overload capability if all the gain and equalisation is done in one first stage. 0V Output C11 + + C9 + C12 0V V+ V– Power Fig.23. Component placement for phono amplifier. Note extra capacitors wired in parallel. RIAA phono amplifier This is probably the most complex design to be built on this board. The equalisation to implement the RIAA curve, shown in Fig.21, requires nonPractical Electronics | January | 2023 standard-value components with a tolerance of 2% or better. These values are difficult to calculate, but Douglas Self has got them all worked out in his book Electronics for Vinyl. Getting the E96 resistors is not too difficult now, and Farnell’s pricing is reasonable with their Vishay 1% MRS25 series 47 compensation capacitor (C6) is 4.7pF, which works in practice, even though the gain of the circuit falls to unity at high frequencies. The theoretical value should be 22pF, but the lowest high-frequency distortion is achieved with lower values. The coupling capacitors can all be tantalum. There is no distortion problem since the signal levels are all below 1Vpk-pk. The polarity of the capacitors is unimportant for dual rail, although fusspots may wish to align them with the offsets. For bipolar op amps with high input bias currents feeding NPN input transistors, such as the NE5534, the output offset is usually negative. Therefore, the negative terminal of the electrolytic capacitors should be connected to the op amp. The circuit is shown in Fig.22 and the overlay in Fig.23, along with the final construction in Fig.24a and b. The NE5534 is still one of the best op amps for moving magnet cartridges and the NE5534A version has guaranteed noise specs. I’ve only got significantly better than this by using ±25V discrete circuits with an expensive J74 JFET on the front end. Cherry low-frequency compensation Fig.24. (above) Phono amplifier construction – a bit untidy, but it can at least be accommodated on the board; (below) Alternative RIAA amplifier construction. Single capacitors are used for C7 and C8, but half the value. Resistors R10, R11 and R4 are scaled up to 10.5kΩ, 127kΩ and 442Ω respectively. if you buy a lifetime’s supply of 100! Mouser are cheaper, supplying the 1% Yeago MFR-25FBF52 series. (Often, E96 resistors are 0.1% and can cost a fortune, so these 1% options are well worth considering). Getting accurate capacitors is more difficult and ±1% or 2.5% tolerance devices are considered top-notch, but here in Wales, home of the UK’s ‘capacitor cluster’, LCR (who absorbed Suflex) still make excellent polystyrene capacitors, available from the author – see contact details in the AO Shop ad on p.50. To aid constructors I’ve made available some low-cost kits of these odd-value resistors and capacitors (also via the AO Shop). The 50.15nF capacitor (C8) can be made up using two surplus Suflex 1% 24.76nF capacitors in parallel (giving 49.52nF) using the extra pads as shown earlier in Fig.6. You can add an extra 680pF as well for more precision. The other capacitor (C7) needs to be 14.32nF and I made mine out of a parallel 13nF and 48 1.3nF. I rescued these caps out of a skip at the end of a production run of filters and they are all still spot-on. Of course, five 10nF capacitors, or any other combination could be used for C8. These were the original Doug Self values. For a simpler version (Fig.24, lower), I decided to double the resistor values and halve the capacitors, so only two capacitors, one of the 24.76nF and an LCR 7.15nF were needed. The noise increases by about 1dB by doing this because of the higher impedance. To provide the final part of the RIAA curve, −20dB at 20kHz relative to 1kHz, a further low-pass filter is required. This is due to the op amp’s response flattening off to unity, since the noninverting configuration has a minimum gain of one. This is achieved with a 4.7nF capacitor inserted in R13’s position in conjunction with a higher-than-normal value for R12 (510Ω). This network should be omitted if a roll-off at 66kHz is provided further on, as is usually the case, such as with stabilising capacitors or an output balancing transformer. The I put this capability on the board because it’s a current interest of mine and I think the technique needs to be more widely known. DC coupled circuits give a perfect square-wave response at low frequencies, but can suffer from dangerous DC offsets. Placing a capacitor in the lower arm of the feedback network of a non-inverting amplifier offers protection by reducing DC gain to unity, but a low frequency square wave, such as 20Hz, will suffer from a pronounced ‘tilt’ at the top and bottom of each cycle. Professor Ed Cherry devised a scheme to compensate for this by adding an extra RC network (see ETI magazine, 60W NDFL Amplifier, May 1983). This technique can also be used to minimise the value of the capacitor, which can often be hundreds of microfarads in a power amplifier, down to say 47µF. This then allows longer-life tantalum capacitors to be used. For those worried about distortion, the capacitor can then be configured as a bipolar part by putting two capacitors back-to-back and applying a 5V bias voltage to the centre connection. A demonstrator circuit is given in Fig.25, the overlay in Fig.26 and the finished board in Fig.27. Positive feedback The vast majority of op amp circuits for audio, as well as instrumentation and other signal-processing designs, use Practical Electronics | January | 2023 R14 Sum input C4 + R7 C7 Feedback components + + + R3 R5 C2 10µF V+ 10V R9 1MΩ R2 100kΩ Vb Gnd + R10 15kΩ C13 R11 33kΩ R6 100kΩ R8 470kΩ R1 750Ω V+ 2 – 3 + 7 IC1 5 +input pad 8 + C11 4 C6 Output 0V 6 C5 Vb bias +5V Basic circuit C8 2.2µF Polyester –input pad C3 C1 100µF Bias 100µF R4 20V 20V 750Ω Input Low-frequency compensation boost R12 47Ω C10 100nF Power on 3-pin connection V– 47µF Used 33kΩ 2.2µF – IC1 + Output Gain = 1 + (Rf/R1) 0V R13 Output 0V Link 0V C9 C12 Link pads Input Rf 15kΩ Not used With compensation 0V Time Without compensation at 20Hz Fig.25. Circuit for demonstrating Cherry low-frequency compensation. Feed with a 20Hz square wave and short out the compensation network C8 and R11 to see the effect. The ‘scope must be set to DC input coupling. Gain is 21x or 26dB. R 6 + C5 C 10 R2 C2 C3 IC1 0V Output C11 + R 9 0V –input +input R4 R R 10 11 + R 12 C1 + C8 C7 R 8 + C12 0V V+ V– Power Fig.26. Component placement for Cherry low-frequency compensated amplifier. Fig.28. There is provision for SMD op amps, such as this MOSFET CA3140. outline in parallel with the DIP socket pins. This is shown in Fig.28. Allowing for a few diagonal links, it should be possible to accommodate almost any audio op amp circuits, such as pre-emphasis/de-emphasis filters, integrator/servos, gyrators… and I’ve thought of about 20 more circuits. I’m sure readers will adapt the boards for themselves in ways limited only by their ESP (electronic spatial perception). Fig.27. The final Cherry compensated op amp board. The big red capacitor (C8) provides a compensating low-frequency boost to compensate for the roll-off induced by the lower arm feedback capacitor (C1, C3) and associated resistor R4. negative feedback. Positive feedback is generally only used for oscillators, and comparators with snap-action hysteresis. This board does not really cater for these circuits, but I’m sure a way will be found. Practical Electronics | January | 2023 Soldering on There may come a time when some audio op amps are only available in surfacemount packs (heaven forbid), so the board includes a standard SOIC 8-pin PCB for Universal single op amp board The PCB described in this twopart series was designed by Mike Grindle and is available from the January 2023 section of the PE PCB Service, part no. AO1-JAN23. 49