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AUDIO OUT AUDIO OUT L R By Jake Rothman Three discrete class-A buffer modules O ne of my long-term goals with Audio Out is to make a complete range of audio modules, so that any analog audio system can be made, whether it’s a massive synthesiser or minimalist Hi-Fi amplifier. Once a great modular system comes together, it can be combined and made into a viable product. I used to make modular systems from the late 1970s to the 1980s, in the ‘golden age’ of audio modules, before I could design at component level. If you can get access to the April 1977 issue of Practical Electronics, have a look at the advert on the inside front cover for Stirling Sound/ Bi-Pre-Pak. It’s where I got the SS125 modules, which I resurrected in the July 2025 issue. There were at least nine audio module companies in that issue. My soldering iron just dripped with nostalgic lust. By the way, regarding that July issue, I’ve found a substitute for the ZTX694B VAS transistor (TR3): the 2SC2364. It is available from Profusion and AOShop at just 13p each in 100s. Just to be awkward, its collector is the centre pin, like most Japanese transistors. Boards for old buffers Back in the February and May 2024 issues, I described the circuits for some FET-based class-A discrete buffer circuits. The good news is I’ve now had Grindle, my partner in crime design, create some PCB designs for them; three altogether. A circuit without a PCB today is almost of academic interest only, especially if it includes surfacemounting parts. My son questioned the point of making discrete circuits with SMT parts at all, since it detracts from long life and repairability, which is the main point of discrete circuits. For the circuits here, it’s simply a matter of cost; SMT matched dual transistors cost around 30-50p each, while their hermetic leaded metal-can equivalents cost £5-12 each. Besides, SMT is widely used now, and rework gear is available at low cost. It is not overly difficult to operate once you have a little practice. And there’s no fundamental reason that an SMD should have a shorter life than a throughhole equivalent (they’re just packaging differences, after all). In fact, part of the reason that SMD was developed in the first place was the desire for reliability by organisations like NASA (see page 2). I will probably design a dual-outline board for those who have access to such gems as LM394 transistors and 2N5564 JFETs; I could always do a dual outline on the board. Grindle and I do our bit to make our SMT boards repairable; he designs our PCBs with the values and part numbers clearly visible alongside each component. If that unmarked ceramic capacitor dies, the person repairing it at least stands a chance. Plug-in 8-pin DIL module This is designed to replace dual op amp chips for those desiring a minimalist discrete solution. It has less noise and high-frequency distortion than the ubiquitous JFET-input TL072 often employed as buffers in the Sallen-Key filters used in active loudspeakers. Fig.1 shows the distortion of a good original Texas TL072. Normally, if one is to improve on the TL072, a singlesourced and therefore expensive audio JFET input op amp has to be used, which is overkill for a buffer. My circuit, shown in Fig.2, is based on the White follower and uses a favourite device of mine, the Toshiba 2SK2145 dual JFET. Fig.2: the 8-pin plug-in dual JFET buffer. Using matched JFETs minimises the distortion. Pin 3 +VIN Pin 2 (NC) –VIN Total Harmonic Distortion (%) TR1 2SK2145-BL 3 4 2 5 VO Pin 1 1 R2 390kΩ 0.2 0.1 0.05 R4 82Ω 0.34V –V Pin 4 +V rail VO TR1 –VIN 0.02 0.01 .005 VO +VIN –V rail .002 .001 .0005 600Ω lload 600Ω oad –VIN TR2 50 100 200 500 1k Frequency (Hz) 10kΩ 1 0kΩ lload oad 2k 5k C4 100nF +VIN Pin 5 +VIN Pin 6 (NC) –VIN 20 R8 330Ω TR2 2SK2145-BL 3 10k 20k Fig.1: the distortion of a TI TL072 JFET-input op amp wired as a buffer at 1V RMS. 14 R3 330Ω C2 100nF +0.33V 0.5 .0002 .0001 +V Pin 8 4 2 5 1 R7 390kΩ R9 82Ω Practical Electronics | October | 2025 VO Pin 7 Semiconductors 2 2SK2145-BL dual N-channel JFETs (TR1, TR2) [Mouser 757-2SK2145BLTE85LF] I made a little adaptor board for this device for experimenters, which was originally shown in the November 2023 issue of Practical Electronics on page 63 (Fig.56). The basic 2SK2145 specifications are: • Vgs maximum: 50V • Transconductance/gain (YFS): typically 15mA/V • Input capacitance: 13pF • Noise factor (NF): 1dB <at> 1kHz, source impedance 10kΩ • IDSS: 1.2-3mA (Y suffix), 2.66.5mA (GL suffix) or 6-14mA (BL suffix). The markings are X.L, XG and XL, respectively. • Power dissipation: 300mW maximum (total for both FETs). The BL grade is best for driving low impedances and has the lowest noise, while the Y is best for low-current battery applications, such as guitar pedals. Using the GL grade in the buffer circuit uses the same current as a TL072, in the order of 4mA (note the voltages across the resistors are halved). The specified BL grade draws 7.5mA, nearly that of the NE5532. It has been in production since 1993, and like exceptional components by Toshiba valued by audio designers, it may be deleted soon, so I bought in a 2 s1/2 3 d2 R9 C4 R8 R3 Q2 TR2 R7 Q1 1 d1 C2 TR1 O/PL I/PL NC –V Capacitors 2 100nF ±10% 63/100V polyester film, 5mm pitch (C2, C4) Resistors (all SMD M3216/1206 ⅛W ±1% thin film) 2 390kΩ (R2, R7) 2 330Ω (R3, R8) g2 4 R4 1 double-sided 19 × 17mm PCB coded AO-OCT25-01 2 4-way 0.1-inch (2.54mm) pitch gold-plated round-pin headers (square pins won’t fit into IC sockets) [Tayda type X19525, SKU A-4662] g1 5 R2 Parts List – Dual JFET buffer +V O/PR I/PR NC Fig.4: the dual JFET buffer component overlay. You can’t get anything the wrong way around! 2 82Ω (R4, R9) few hundred. Currently, they are 31p each in 100s from Mouser. This design uses SMD resistors. Standard SMD resistors are mostly thick-film types that are non-linear and will introduce unwanted distortion, so make sure you stick to thin film types, which are available but a bit more expensive. They perform just as well as through-hole (axial) thin-film resistors. Having said that, standard thick-film types from Tayda work fine. Specifications The distortion curves for buffers with BL and GL JFETs are shown in Fig.3. Note that there is no rise at HF, a characteristic of op amps when used as buffers. This is because high levels of HF compensation are not required for stability at unity gain with the discrete buffers. Since the JFET is rated at 50V, the buffer can be used at up to ±23V, but one has to be careful with loading to make sure the dissipation limit is not exceeded. The output impedance is higher than an op amp, at 100Ω. Construction Fig.4 shows the overlay, and the completed buffer is shown in Photo 1. The SMD dual JFETs are considerably smaller than through-hole equivalents, so if your vision is far from perfect, you will want good light and a decent magnifier (ie, with an anti-reflection coating). A syringe of flux paste makes soldering the small leads much easier by allowing the solder to flow more readily. Start by mounting the PCB so it doesn’t move about. One way is to first put the IC socket pin strips into a breadboard and push the board on top to solder it. This way, it aligns the pins and provides a firm base at the same time (see Photos 2 & 3 overleaf) so you can solder them. Next, solder the JFETs in place. Luckily, they have asymmetrical packages, so you can’t get them the wrong way around. Add the resistors around the edge next, then add the radial plasticfilm modulation capacitors, C2 and C4. Don’t use ceramic X7R types as they will cause bad low-frequency distortion. I made a version with AVX 100nF 35V SMT tantalum chip types, which gave low distortion. They had to be positioned with correct polarity, with the positive end facing the drain of the upper JFET. However, they cost ten times as much as the film types. Total Harmonic Distortion (%) 0.5 0.2 0.1 0.05 GL, G L, 600Ω 600Ω lload oad BL, 600Ω BL, 600Ω lload oad 0.02 0.01 .005 B BL, L, 1 10kΩ 0kΩ lload oad .002 .001 .0005 .0002 .0001 GL, L, 1 10kΩ 0kΩ lload oad G 20 50 100 200 500 1k Frequency (Hz) 2k 5k 10k 20k Fig.3: the distortion curve for the Fig.2 circuit using the BL and GL versions of the 2SK2145. The BL version is delivering 4V RMS into both 600Ω and 10kΩ, while the lower-current GL version is delivering 1V RMS into 600Ω & 4V RMS into 10kΩ. Practical Electronics | October | 2025 Photo 1: the discrete plug-in dual buffer. I used slightly smaller resistors than specified as I already had them. 15 C1 (C3) R11 (R12) 22nF 2.2kΩ 1/2 of DIP-8 pack assembly Pin 8 +V R3 (R8) 330Ω C2 (C4) 100nF C9 100nF 3 TR1 (TR2) Pin 3 (5) C5 (C6) R5 (R10) 47µF 25V 22Ω 4 VIN Pin 1 (7) 2 C7 100nF + Fig.5: the plug-in buffer should be connected like a standard dual op amp to function as a complete buffer. VO 5 R1 (R6) 1MΩ 0V 1 R2 (R7) 390kΩ R4 (R9) 82Ω Pin 4 –V C8 100nF 0V Soldering Every engineer has their own take on SMT hand-soldering. I always like to coat the pads with a flux pen, then let them dry a bit to help stick the device in the correct position. Solder one pin quickly, before it has a chance to move. It helps if you use Multicore 26 SWG (0.46mm diameter) low melting point (LMP) solder. This is 36% lead, 62% tin and 2% silver, with 3% activated rosin flux. If you use that awful low-residue 96% tin, 4% copper, lead-free stuff, expect trouble. Tin/lead solder is still used in military and avionic work. If you put too much solder on and bridge the pins, a bit of 3mm desolder braid usually helps soak it up (adding a little extra flux paste makes it work even better). You can get rid of the flux residue with a good scrub with a stiff bristle artist's brush and isopropyl alcohol. That’s good practice, even if you’re R2 220Ω 0.5V 0.5V 2mA 1 TR4/5 HN1A01FGR Dual C8 100nF VIN C4 100nF MKT R1 1MΩ 0V TR5 TR4 6 2,5 1 D TR1 Testing Recall that this unit is designed to replace a dual op amp used as a buffer, so it needs to be surrounded by normal buffer supporting circuitry to check that it works. A suitable test circuit is shown in Fig.5. There are pull-down resistors on the gates (R1, R6) along with gate resistors (R11, R12) to prevent oscillation. The same applies to the output resistors. R3 220Ω R7 10kΩ 2mA 5mA 4 3 C1 150pF using no-clean flux, as it can prevent you from seeing problems in the solder joints. It’s worth checking not only for possible bridges between adjacent pins on the JFETs but also to make sure that the solder fillet is covering both the pin/lead and the PCB pad. It’s possible to get solder to stick to the lead but not the pad, which may not be obvious unless you look closely under magnification (with good light). 46V C6 100nF 0V + R8 180Ω S 2 S TR1/2 2SK2145-BL Dual 4mA TR3 BC546 R4 240Ω R5 2.2kΩ C2 100nF LED1 Red R6 2.2kΩ TR7 BC546 High-spec Mosfet buffer This relatively complex circuit, shown in Fig.6, will beat most expensive JFET audio op amps. Its operation was fully explained in the May 2024 issue. It’s basically a JFET long-tailed pair, TR1/TR2, loaded by a current mirror (TR4/TR5) based on a PNP dual transistor, the HN1A01F. Fig.7 shows the distortion plot. Each PNP transistor in the package is rated at 50V and 150mA, so it’s similar to two BC556s, and it’s cheap R10 47Ω C5 22µF 35V Bipoplar VO +60mV G 4 Capacitors C7, C8 & C9 are required for rail decoupling, with C5 & C6 for output DC offset blocking. Note that ‘pins’ 2 & 6 are not connected (NC). In practice, these may be linked directly to the output pins on most PCBs. To test it, power it up and check that the supply current and output offset voltages are correct. If so, it should pass audio with symmetrical clipping (if the level is high enough). If there is a problem, it is normally due to incorrect resistor values having been used, or solder bridges or dry joints on the JFET pins. Photo 3: it’s essential to get the header strips straight so it fits in an IC socket. TR6 BS170 D D 3 TR2 G 5 1.13V C3 22µF 50V +25V 17.5mA<at>±15V 20mA<at>±25V 1W dissipation Photo 2: a breadboard can provide support to align the pins. The TL072 IC shows the relative size. 10mA 0V 1.7V C7 R9 100nF 100Ω 1.12V –25V Fig.6: the Mosfet buffer provides very high performance from a more complex circuit. 16 Practical Electronics | October | 2025 Parts List – High-spec Mosfet buffer 1 double-sided 55 × 30mm PCB coded AO-OCT25-02 2 2-pin 0.1-inch (2.54mm) pitch polarised headers 1 3-pin 0.1-inch (2.54mm) pitch polarised header Photo 4: the single-channel Mosfet buffer board. Semiconductors 1 2SK2145-BL dual N-channel JFET (TR1/TR2) [Mouser 757-2SK2145BLTE85LF] 2 BC546 NPN small-signal transistors (TR3, TR7) 1 HN1A01F dual 50V 150mA PNP bipolar transistor, SM6 (TR4/TR5) [Mouser 757-HN1A01F-GRTE85LF] 1 BS170 60V 500mA N-channel Mosfet (TR6) [Mouser 512-BS170] 1 standard 5mm red LED (LED1) Capacitors 1 22µF 50V radial electrolytic, 2.5mm pitch (C3) 1 22µF 35V bipolar electrolytic (C5) 4 100nF 50V X7R ceramic, 5mm pitch (C2, C6-C8) 1 100nF 63/100V ±10% polyester film (C4) 1 150pF ±5% C0G/NP0 ceramic, 5mm pitch (C1) Resistors (all ¼W ±1% metal film axial) 1 1MΩ (R1) 2 220Ω (R2,3) 1 10kΩ (R7) 1 180Ω (R8) 2 2.2kΩ (R5, R6) 1 100Ω (R9) 1 240Ω (R4) 1 47Ω (R10) c1 g2 c2 6 out 5 the4 the values of R3 and R4. I left preset that was in the original design, Q2 assuming AC coupling would Q1 be used in most cases. 1 e1 Construction 2 g1 2 g1 C8 C6 C7 R 5 R 7 TR3 3 e2 0V R 3 R 2 C3 + C1 TR4/5 R 8 e b c TR1/2 R R R 9 –V 0V LED1 d g s R 0V Power ±25V Input +V R e b TR7 c + C4 C2 C5 The assembled board shown 1 4 6 10 Input in Photo 4 is relatively simple, given the circuit, and the Fig.8: the high-spec Mosfet buffer PCB overlay. corresponding overlay is shown down. It will work either way because in Fig.8. The SMT transistors are the the two transistors are connected such first components to solder; otherwise, that the pinout is symmetrical (as protruding components will get in the shown in the diagram above). way of your iron. Then fit the rest of the leaded Bipolar transistor TR4/TR5 is unique components in the usual way, starting in that the package can be mounted in with the lowest-profile devices first. two possible positions, with pin 1 up or The board should be tested with a bench power supply with current limiting set around 300mA. Current consumption on both rails should be around 20mA, and the DC offset (measured on R10) should be less than ±100mV. If oscillation occurs, it can usually be fixed by inserting a gate-stopper resistor of 1kΩ in series with the input. 0.2 0.1 0.05 16V RMS 16V RMS 4V 4 V RMS RMS .002 .001 .0005 8V 8 V RMS RMS 1V V RMS RMS 1 A stereo buffer 20 50 100 200 500 1k Frequency (Hz) 2k 5k 10k 20k Fig.7: the distortion plots for the Mosfet buffer. The load is 600Ω for all curves; 16V RMS is just on the verge of clipping (45.4V peak-to-peak, delivering 430mW). Practical Electronics | October | 2025 R 7 R 3 3 e2 Testing 0.02 0.01 .005 R 5 TR3 1 e1 0.5 Total Harmonic Distortion (%) c2 4 Q2 Q1 + at 10p each in 100s. The long-tailed pair’s non-inverting output then drives a Mosfet source-follower output stage (TR3) loaded by a modulated current sink (TR7). This is reasonably shortcircuit-proof, but only on the negative cycle. The output impedance is less than 1Ω, but a resistor is in series with the output to help with stability with long screened leads. If a low output impedance is needed to drive output transformers, the 47Ω output resistor (R10) can be bypassed with a small inductor of around 4µH. If the unit is to be DC-coupled (as is also essential for audio transformer driving), output capacitor C5 should be omitted. The DC offset will then have to be trimmed out by tweaking .0002 .0001 g2 5 c1 6 I’ve also had a stereo board designed based on a minimised version of the JFET input circuit in the April 2024 issue (see Fig.27 on page 63 of that issue), which is easier to build and use than two separate boards. The 17 Output 0V C Parts List – Dual Mosfet buffer Photo 5: the stereo Mosfet buffer board takes the place of two original mono boards described in the April 2024 issue. 1 double-sided 65 × 52mm PCB coded AO-OCT25-03 2 2-pin 0.1-inch (2.54mm) pitch polarised headers 2 3-pin 0.1-inch (2.54mm) pitch polarised headers 1 4-pin 0.1-inch (2.54mm) pitch polarised header Semiconductors 2 2SJ113 N-channel JFETs (TR1, TR4) [AliExpress] 2 BC556 PNP small-signal transistors (TR2, TR5) 2 BC546 NPN small-signal transistors (TR3, TR6) 2 standard 5mm red LEDs (LED1, LED2) Capacitors 2 100µF 35V non-polar radial electrolytic, 5mm pitch (C4, C13) 2 100µF 25V radial electrolytic, 5mm pitch (C8, C7) 2 22µF 35V radial electrolytic or tantalum bead, 2.5mm pitch (C3, C12) 2 10µF 6.3V tantalum bead, 5mm pitch (C5, C14) 1 220nF X7R ceramic, 5mm pitch (C6) 2 100nF ±10% polyester film, 5mm pitch (C1, C10) 2 270pF ±5% NP0/C0G ceramic, 2.5mm pitch (C9, C15) 2 100pF ±5% NP0/C0G ceramic, 2.5mm pitch (C2, C11) Resistors (all ¼W ±1% metal film axial) 2 1MΩ (R2, R13) 2 100kΩ (R11, R20) 2 6.8kΩ (R8, R19) 4 2.2kΩ (R3, R7, R14, R18) 2 620Ω (R1, R12) 2 300Ω (R5, R16) 2 220Ω (R9, R10) 2 100Ω (R4, R15) 2 33Ω (R6, R17) circuit is repeated in Fig.9 because the component numbering is different. I thought it could be simplified further by employing one voltage reference for the current sinks. This caused problems in that if one side clipped, it transferred a glitch to the other side. I’ve noticed this problem on a few dual op amp chips too. The overlay is given in Fig.10, while Photo 5 shows the completed board. There is nothing untoward about the construction, and it should appeal to those who love symmetrical PCBs. and studio monitoring setups is simply a stereo volume control in a box, sometimes called a passive preamp. The potentiometer often used is the Alps Blue Velvet (shown in Photo 6) on account of its smooth audio taper and channel matching at low levels. There is a catch, however, in that it is usually only available in 50kΩ. This results in an output impedance varying from 25kΩ to 4.5kΩ as it is rotated. This can interact with long cables and the input impedance of the power amp or active speaker being driven, causing high-frequency loss, pot-law deviation and distortion. These Buffer board applications A popular item in minimalist Hi-Fi R9 220Ω are common problems with simple passive preamps. The solution, of course, is to buffer the output of the pot. This now makes it an active preamp, which has the problem of needing a power supply. C’est la vie. Sourcing the parts All three PCBs and the parts that mount on them are available from the AOShop (see page 13). These PCBs are also available at the same price from our PCB Service, in case you want to order them along with some for other projects. Coming up next month Enough of clean, low-distortion 32mA +25V C8 100µF 25V 1.5mA + C9 270pF 3-5V 0V C6 220nF R1 620Ω C1 100nF R3 2.2kΩ R5 300Ω TR2 BC556B 9.53mA C15 270pF 34V 3.44V 5mA C3 + 22µF 35V TR1 J113 1.5mA R8 6.8kΩ 2.86V 31.4V C4 100µF 35V R6 33Ω VIN, LEFT VO R12 620Ω C10 100nF C2 100pF 0V C13 100µF 35V R17 33Ω VO R11 100kΩ R13 1MΩ R7 2.2kΩ C11 100pF TR6 BC546B 0V R20 100kΩ 11mA R18 2.2kΩ 0V R4 100Ω 1.1V C5 + 10µF 6.3V LED1 1.8V Red 1.8V R15 100Ω 1.1V C14 + 10µF 6.3V LED2 1.8V Red –25V 32mA + C7 100µF 25V 0V 18 34V 5mA C12 + 22µF 35V VIN, RIGHT 0V R10 220Ω R19 6.8kΩ 2.79V +1.0V 11mA TR3 BC546B 9.3mA TR4 J113 +1.0V R2 1MΩ R14 2.2kΩ R16 300Ω TR5 BC556B Typical output offset is +1.0 to 1.2V Fig.9: the circuit for the stereo buffer. This is the same as the April 2024 design but with a shared decoupling network. Practical Electronics | October | 2025 1.8V The Wireless for the Warrior books are references for the history and development of radio communication equipment used by the British Army from the very early days of wireless up to the 1960s. Volume 1 covers early transmitters and transceivers used between 1932 & 1948. Volume 3 covers army receivers from 1932 to the late 1960s. The book not only describes receivers specifically designed for the British Army, but also the Royal Navy and RAF. Photo 6: an Alps volume pot installed in a NAD 3030 amplifier. It needs a lowimpedance drive and high-impedance load to minimise the effect of track resistance tolerance. These pots’ rotational characteristics are optimised when they are fully buffered. Volumes 1 & 3 are still available, but stocks are running out, so they won’t last long. Order a printed copy now from: https://pemag.au/link/ac20 living for now. In the following article, we’ll ‘get dirty’ with a transformer-coupled germanium transistor fuzz box! Fuzz boxes are used by musicians, especially in rock and jazz music, to alter the sound of instruments (usually an electric guitar or bass) to add harmonics. These give the sound more of a ‘bite’. It is especially helpful for a bass guitar because the higher-frequency harmonics can be more audible in the mix, while the lower frequencies are often more felt than heard. Germanium fuzz boxes are generally thought to PE give the best sound. 0V Output 0V Output C4 R 7 R 4 TR3 C2 C1 R 6 Input 0V left C5 R 11 c e b R b 3 e c g d s R R 1 2 LED1 LED2 + TR1 + C8 R 5 + C14 + R 8 R 19 TR2 C3 + C9 R 16 R 17 e c b R b 14 c e C15 C12 g d s + C6 +V R 20 TR5 + R 9 C13 0V R 10 R 15 R 18 TR6 C11 C10 TR4 C7 + –V R R 13 12 Input 0V right Fig.10: the PCB overlay for the stereo version of the buffer. Practical Electronics | October | 2025 19