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AUDIO OUT AUDIO OUT L R By Jake Rothman Universal op amp board (optimised for audio electronics) – Part 3 (dual op amp version) R14L* Sum input L C4L R7L + Bias L R3L 0V C3L Input – L R5L + C1L + Input + L C2L V+ R2L R1L R9 R6L Vb bias C8L* R10L* R11L* + R8 C13 –input pad L 2 – 3 + + R7R R12L 6 IC1a C9L 0V Link pads L C7R* C8R* *Feedback components R Power input 0V R10R* Fearful symmetry Output L R13L +input pad L R14R* C4R *Feedback components L C5L Vb Sum input R C7L* R4L + and January 2023 issues of PE we described a universal board for single op amps. This month we’ll introduce one for dual op amps which are more common and offer better value. The dual NE5532 for example, is half the price of its single counterpart, the NE5534, so you get four times as many op amps for the money. Also, one of the few op amps that’s noticeably better for audio than the NE5532/4, the LM4652, only comes in a dual package. At the same time, PCB designer Mike Grindle and I decided to do a whole host of adapter boards for various op amp packages, which we’ll come to next month. + I n the December 2022 RS2 0.5W R11R* 0V + supply + + + Bias R V+ The general circuit of the + dual board is the same as the – supply –input R3R D2 C11 0V pad R single PCB (Fig.4 in Part 1) C1R C3R R5R 0V Output R but duplicated, as shown in 6 8 Input – R C9R – 7 Fig.29. The dual chip (IC1a IC1B R4L 5 Input + R + R12R and IC1b) is shared between 4 C10 C2R the left and right channels. R13R 100nF +input R6R RS1 This gave Mike an excuse pad R R2R 0.5W V– to create a symmetrical R1R C5R mirror-imaged board. The D1 C12 overlay is shown in Fig.30. 0V Most of the part numbering Link pads R is the same – except for R4 and R5 which are swapped. The component numbers common to both channels Fig.29. Circuit diagram for the dual universal audio op amp PCB. This is virtually the same as the are prefixed L and R. The single circuit, bar a single bias network and some extra power supply zener regulators. half-rail bias components are renumbered R8, R9 and C13, since only is to be supplied from say a power Don’t be deferential, one network is needed, shared between amplifier, where the rails are typically go differential! both channels. Finally, a dual-rail Zener ±22V to ±60V. This situation commonly A suitable dual-channel differential regulator circuit is added to enable the crops up where an amplifier needs the amplifier to create balanced inputs is board to be used with supply voltages addition of balanced inputs, such as shown in Fig.31. The main differential higher than the op amps’ maximum. when converting a Hi-Fi amplifier for amplifier resistors R4, R5, R6 and This facility is useful if the board professional use. R14 are all 3.3kΩ. This gives a total + + 52 Practical Electronics | April | 2023 R2L + R7L R11L + R12L C8L* R4L D1 R R 9 8 C5R 0V Output L RS1 C12 C10 IC1 C4R C9L + C5L 0V V+ V– C13 + C11 D2 RS2 C9R R4R C8R* + R12R R13R R6R R5R + + C3R R11R C1R R3R 0V Bias C2R 0V –input R** +input R** **Ignore the error on the PCB’s silkscreen, the Right input Molex +/– pin positions are as shown above. R2R Non-inverting input IC1b Access these pads for linking signals to bias or 0V R1R 0V Sum input R Inverting input IC1b + R10R 0V Output Vbias R3R R6R Sum input L The vinyl countdown For those of us who go round charity shops buying scratchy old sound effects records, it seemed sensible to make a *Note for C8L/R there is space for two devices so that capacitors can be in parallel, or an axial device. R14L 3.3kΩ 1% C4L R7L + C7L C3L 22µF 16V R3L 0V C1L Input – L C2L R4L 22µF V+ 3.3kΩ 16V 1% R2L 100kΩ Vb bias L R9L R1L 100kΩ VbL + 2 C4R – IC1a 3 LM4562 1 + R6L 3.3kΩ 1% 0V C7R R11R Bias R C1R Input – R C3R 22µF 16V R5R 3.3kΩ 1% R1R 100kΩ C2R 22µF 16V R2R 100kΩ R4R 3.3kΩ 1% R6R 3.3kΩ 1% Link pads R Fig.31. Dual-channel differential/ balanced input amplifier. Practical Electronics | April | 2023 Link Used Not used C8R 330pF 5% 0V Power input 6 8 – IC1b 5 LM4562 + C11 47µF 25V 0V 7 4 +input pad R C5R 330pF 5% + V– D1 BZY88C16 16V 0V RS2* 0.5W D2 BZY88C16 16V V+ –input pad R + Input + R Output L C5L 330pF 5% R14R 3.3kΩ 1% R7R R3R C9L 100µF 10V R13L 100kΩ +input pad L R10R 0V R12L 47Ω R8L Link pads L C13L Sum input R C8L 330pF 5% –input pad L R5L 3.3kΩ 1% + Input + L R11L R10L Bias L + bridging impedance of 9.9kΩ across the input pins on the XLR. 10kΩ is the standard accepted minimum, so we are near enough. If the equipment driving the amplifier can accept a lower load impedance, such as 600Ω, reducing the resistance will reduce the Johnson noise. The overlay is shown in Fig.32 and the final assembled board in Fig.33. The photo in Fig.34 shows the board installed in an old Citronic SA 200 power amplifier, banishing its unbalanced jack inputs for a hum-free heaven. The power rails were ±43V and the Zener resistors (RS1 and RS2) were chosen to be 2kΩ 0.5W, dropping 27V. The current is split with the 16V Zener diodes (D1 and D2) passing 5mA, and the op amp 8mA. (Note that if you want just normal op amp supply voltages (±15V) then RS1/2 should both be 22Ω and the Zener diodes can be omitted). Note that since the board mounting holes are connected to signal ground, 0V + C7R + + R14R R7R an earth loop could develop with the mains safety earth if metal bolts are used. It’s best to use plastic standoffs in this situation. A subtle point: adding the balanced input board made the amplifier have nasty turnon/off thumps, somewhat negating its advantages. Of course, this problem only became apparent in the field, not on the test bench. I’ve found the 5534 series op amps to be especially bad in this regard. Replacing it with a LM4562 stopped the turn-off thump and gave slightly lower hiss. The switch-on thump was greatly reduced by putting a 470µF 35V electrolytic capacitor (C10) across the op amp’s positive/ negative power pins. Inverting input IC1a 0V Sum input L R6L R13L C7L R14L C1L + + C4L 0V –input L +input L C2L R1L R5L R10L 0V + R3L C3L Vbias 0V Fig.30. Overlay for the dual op amp board. It’s a shame to conceal its beautiful symmetry in a metal box, but hum pick-up is a nightmare with RIAA pre-amps because of the bass boost. Access these pads for linking signals to bias or 0V + supply – supply C9R 100µF 10V Output R + R6L R12R 47Ω C10 100nF RS1* 0.5W + R3L Bias Non-inverting input IC1a R13R 100kΩ C12 47µF 25V 0V *RS1/2: 2kΩ for ±43V supply, 22Ω for normal op amp supply (±15V) 53 Access these pads for linking signals to bias or 0V Bias R3L Fig.32. Overlay for dual balanced input board. R6L 0V Vbias 0V R2L C3L 0V –input L +input L + C2L R1L R5L + R6L R13L R14L R12L C8L* R4L + C10 C5L C9L C12 D1 IC1 C5R RS1 + C11 C8R* R12R + D2 RS2 C9R R4R 0V Output L 0V V+ V– 0V Output R14R R6R R5R R1R + C3R + R13R C2R 0V –input R +input R R2R 0V Access these pads for linking signals to bias or 0V Bias R3R lower-cost stereo RIAA preamplifier using one board, useful for installing inside rescued cheap decks. It can be possible to derive the preamplifier power from the motor transformer using voltage-doubler networks. In Vbias *Note for C8L/R there is space for two devices so that capacitors 0V can be in parallel, or an axial device. R6R the old days, when mains-powered synchronous motors were used, an overwind was sometimes placed on the coil, effectively making a transformer. Going from two single NE5534s to a dual NE5532 incurs a small 1dB decrease in the signal to noise ratio (−77dB to −76dB referenced to 5mV input). The impedance of the feedback network was also doubled, as shown in Fig.24 (lower) in Part 2. This increases noise a little, but in the context of a needle ploughing a dust-filled groove, it is practically insignificant. However, the lower loading (442Ω at 20kHz) of the feedback network improves the overload performance on scratches. The circuit is shown in Fig.35, the overlay in Fig.36 and the assembled board in Fig.37. If you want to use a single-rail power supply for this design, the half-rail bias network is deployed and the extra components are inserted. These comprise R9, R8 (both 22kΩ) and C13 22µF 25V. R6L and R6R must be connected to the bias pads via links. Bidirectional caps Note that the polarisation of electrolytic coupling capacitors does not matter with op amps on dual-rail supplies. It makes no real difference since aluminium electrolytic and solidtantalum capacitors can tolerate a continuous reverse polarity of up to a few hundred millivolts. With the 5534 series of op amps, where high source impedances are presented to the input pins (as in the RIAA amplifier) input bias current flowing through the resistance always generates a small negative offset. These ICs have NPN input transistors run at a relatively high collector current, giving rise to significant input currents. Components The special value 1% RIAA equaliser components C7 (24.76nF), C8 (7.15nF), R5 (442Ω), R10 (127kΩ) and R11 (10.5kΩ) are available in a bargain pack from PE PCB Service or the author (see page 35 for contact details). Having built several of these boards and plotted their frequency responses on an Audio Precision analyser I was pleased to see all the plots matched looking like a single plot when overlaid. This shows the benefits of 1% tolerance devices and the near cancellation of the −200ppm/°C Fig.33. A view of the stuffed dual balanced input board. For the eagleeyed among you, note that there was a small error on this prototype board – the op amp’s pin 5/6 connections were reversed. This has been corrected for production boards. (They will also have a smart red silk screen!) 54 Practical Electronics | April | 2023 negative temperature coefficient of polystyrene capacitors with the +100ppm/°C positive coefficient of metal-film resistors. Distorted view + + + + I thought it would be interesting to measure the distortion of the RIAA amplifier with regard to the high gain and complex filtering. Interestingly, the only way I could get meaningful results was to precede the RIAA amplifier with a passive inverse RIAA filter – shown in Fig.38 – to get an overall (theoretical) flat frequency response. This attenuation followed by gain, gives rather high noise and hum levels which are added to the distortion residual. The end result was a total harmonic distortion and noise (THD+N) figure of 0.02%, as shown in Fig.39. This is ten-times higher than a normal non-inverting 5532 amplifier with moderate gain. I thought this was bad, but other amplifiers I measured were much worse. Again, in the context Fig.34. Another view of the assembled balancing board installed in a power-amplifier. of vinyl playback distortion, typically in the order of a few R14L percent, it is little to worry Sum input L about. However, distortion C4L C7L* C8L* R7L 24.76nF, 1% 7.15nF, 1% plots do reveal much about amplifier behaviour during R10L* R11L* *Special RIAA design, even if its effect 127kΩ, 1% 10.5kΩ, 1% components Bias L may never be heard. A R3L –input strange surprise was that the C3L R5L* 0V pad L 100µF 442Ω C9L THD+N was lower with the 10V C1L 1% 100µF R12L Output L 2 10V single-rail version. I suspect Input – L – 47Ω 1 IC1a R4L additional power supply 100Ω 3 NE5532 Input + L + noise was to blame. C2L + A useful device 33µF V+ R1L R2L 10V 75kΩ R9 R6L 130kΩ Vb bias R13L 220kΩ +input pad L + + + + These general-purpose op C5L amp boards were designed Vb 220pF with experimentation in 0V + R8 Link pads L mind and of course that’s C13 what happened while messing about with the RIAA R14R board. I was thinking about Sum input R the power supply for a singleC4R R7R C7L* C8L* Power 24.76nF, 1% 7.15nF, 1% rail RIAA preamplifier, and it 0V input suddenly struck me: why not RS2 0V R10L* R11L* *Special RIAA 22Ω 127kΩ, 1% 10.5kΩ, 1% components make it phantom powered? 0.5W +18V This would be very useful Bias R because most mixing desks V+ + C11 –18V –input R3R C3R R5R* 10µF and USB audio interfaces D2 0V pad R 100µF 442Ω 25V C9R have microphone inputs C1R 1% 10V 0V 100µF Output R 6 8 Input – R – 10V (with phantom power) not 7 IC1b R4R RIAA inputs for record 100Ω 5 NE5532 Input + R + R12R 4 decks. A quick conflab with 47Ω C10 R13R C2R R6R RS1 100nF 220kΩ +input 22µF Mike Grindle revealed he was 130kΩ 22Ω pad R 16V R1R 1% 0.5W wondering how to connect a R2R record deck to his mixer for a V– 75kΩ C12 C5R D1 gig and my son had a similar 10µF 220pF 25V 0V problem with a Focusrite Link pads R computer interface. The Link Used Not used classic and expensive way of phantom powering a circuit is to use output transformers Fig.35. Stereo RIAA pre-amplifier circuit. The special RIAA equalisation components marked * are and harness the power from available as a kit from the PE PCB Service. + + Practical Electronics | April | 2023 55 Bias R3L Access these pads for linking signals to bias or 0V R6L 0V Vbias 0V R2L C3L 0V –input L +input L + C2L R5L + R6L R10L R13L C7L R11L R12L C8L C9L R4L + RS1 C12 C10 C5L IC1 C5R RS2 C11 C8R R4R R12R C7R R13R R6R + C1R C2R 0V –input R +input R + R2R 0V Access these pads for linking signals to bias or 0V Bias 0V Output + C3R 0V V+ V– R11R R10R R5R 0V Output L R3R Vbias *Note for C8L/R there is space for two devices so that capacitors 0V can be in parallel, or an axial device. R6R Fig.36. Overlay for the stereo RIAA amplifier. a centre tap, as shown in Fig.40. The physical ‘lash-up’ is shown in Fig.41. A few measurements of the current capability of some representative phantom-powered input XLRs showed a typical maximum current delivery of 6.6mA. This concurs with the standard dual 6.8kΩ feed resistors used. Most commercial units tested gave less than the official +48V. A £29 Behringer mixer gave +44V and the Focusrite gave +46.5V. However, it was clear that it was possible to power a 5532 op amp, that typically needs a 30V single rail drawing 8mA. The setup was tested and worked perfectly. However, the transformers were expensive (at least £20 a pair) and I was worried about the stress on the single op amp chip, already doing 50dB gain at 20Hz, along with turntable rumble, possibly saturating the transformer cores. My usual approach is to prove a concept works with nocompromise expensive parts – and then find a way of doing it for less. The transformers’ unique advantage, a floating output, was not realisable here anyway, because of the earth return required for the phantom power. Quasi-balancing Since the output of the RIAA amplifier is around 150 to 600mVpk-pk with a typical moving magnet cartridge, a truly balanced output is not necessary for low noise. It is perfectly satisfactory to drive just one leg of the balanced line. The other leg is held at the same impedance to ground to obtain common-mode interference rejection of the balanced input. Of course, both lines can be used to provide phantom power. This is blocked off by capacitors and steered to the op amps’ power rail via resistors. Unlike the centretapped transformer supply where cancellation occurs, there is audio riding on top of the +48V DC, so a big supply decoupling capacitor (C11) is necessary. The circuit is shown in Fig.42. Connecting the 820Ω phantom feed resistors must be done partly offboard, as shown in Fig.43a. Lighting-up time It’s a good idea to put in a low-current LED to show phantom power is present (there could be one for each XLR). I use Kingbright L-7104SEC orange LEDs because they are bright enough with 1mA. They are ‘costly’ (for LEDs) at 25p from Rapid (part No. 72-8972), Fig.37. Assembled stereo RIAA PCB. 56 Practical Electronics | April | 2023 Output Oscillator Control link RIAA amplifier 30dB gain at 1kHz Output Input Input Passive inverse RIAA filter Loss approx 40dB at 1kHz Input Distortion analyser dB Fig.38. (left) Plotting the distortion of RIAA amplifiers is tricky. In this set-up, the overall frequency response is rendered flat by inserting an inverse RIAA filter on the input. Fig.39. (above) Distortion plot for RIAA amplifier using an NE5532. Most of the distortion was noise due to the massive attenuation (-40dB) from the inverse filter. f Opposite curves in series give flat response Single-rail stereo RIAA board V+ 28V to 34V D2 BZX61C36 36V, 1.3W 8mA C11 + 47µF 35V 1N4001 470Ω 0.5W Polyfuse 100mA PTC 2x Schottky diodes eg, BAT49 80V, 500mA 5mA Currents 6.6mA each 0V IC1a NE5532 0V T1 VTX-101-007 + Input Optional external power C9L 100µF 25V Equipment with phantom power +48V 13nF Internal 6.8kΩ resistors 3 (–) Adapting the board 1 1.1kΩ 2 (+) XLR output 0V 0V IC1a NE5532 T2 VTX-101-007 + Input C9R 100µF 25V +48V 13nF Internal 6.8kΩ resistors 3 (–) 1 1.1kΩ 2 (+) XLR output but worth it. Every milliamp counts in this application. To power the LED, I used four 150kΩ resistors connected to each XLR live pins, as shown in Fig.43b. The maximum voltage available on a 5532 op amp with this phantom power circuit is 27V. However, this increases to 32V using an LM833 (an early National Semiconductors audio op amp) which works just as well in this application. 0V 0V Fig.40. Feeding a circuit from phantom power using output transformers. Universal boards can often end up rather messy – as shown in the overlay in Fig.44 – illustrating the adaptation of a single-rail stereo RIAA board. Note the spare input on the balanced input connector is used to feed the ‘fake’ balanced output pin to its grounded capacitor (C1). All boxed up Since we now have no big transformers to worry about, the PCB, two phono connectors and two male XLR connectors can all be fitted in a neat Hammond diecast box, as shown in Fig.45. The final unit of symmetrical design, shown in Fig.46, can be placed alongside the turntable, where its low impedance output will drive very long cables. It is effectively a phantom-powered RIAA direct-inject (DI) box. Fig.41. The dual op amp board can be phantom-powered using a pair of Vigotronix transformers wired centretapped to obtain a 48V output (yellow wire), as shown here using the PE transformer mounting boards. Practical Electronics | April | 2023 57 C11 + 330µF 50V 820Ω D2 BZY88 36V 2 (+) Phantom power 1 input 0V LM833 (or similar) 820Ω 820Ω 0V + 3 (–) 0V + + C1R 47µF 50V 47Ω** C9R 47µF 50V R12R 47Ω – * 3 (–) 0V + + 820Ω 47Ω** C9L 47µF 50V R12L 47Ω +34V – C1L 47µF 50V + 0V 2 (+) 820Ω supply resistors are off board *Middle pin on input connector **Links can be replaced with 47Ω resistors for * better balance. Phantom power 1 input 0V Fig.42. Circuit for phantom powered, quasi-balanced outputs. This trick is often used in ‘transformerless’ microphones, such as the Neumann TLM170. Mis-cued tracks An eagle-eyed reader has noticed that the links near R6 on the single channel RIAA board in Fig.23 Part 2 have to be dropped down by one hole. The links are shown correctly in the photo of Fig.24 (upper) Part 2. Fig.43. a) (top) The phantom power is collected together via four 820Ω resistors and fed into the board; b) (below) using four 150kΩ resistors to powering an LED to show phantom power is on. PCB / RIAA parts The Dual Op amp PCB (AO1-APR23) and the stereo RIAA set of precision capacitors and resistors (AO2-APR23) are available from the PE PCB SERVICE. Fig.44. (below) Overlay for the phantom powering components on the PCB. 0V Vbias R6L 0V Phono input R2L C3L + 3 (–) + R7L C7L R6L R13L 2 (+) R11L R12L C8L R4L C10 R R 9 8 IC1 C5R C11 D2 Power in C9R R12R C7R R5R 820Ω 820Ω 820Ω XLR output R11R R13R R6R C1R + Fig.45. The assembled phantom-powered RIAA preamplifier interior. 0V R4R 1 + R7R 820Ω C13 + C8R + C3R 0V C9L + C5L R10R XLR 1 output + R10L C2L C1L R5L C2R 2 (+) 3 (–) R2R 0V 0V Vbias R6R Fig.46. Finished phantom-powered RIAA pre-amp. 58 Practical Electronics | April | 2023