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AUDIO OUT AUDIO OUT L R By Jake Rothman Op amp adaptor boards: surface mount and through-hole versions Fig.1. The original inspiration for these adaptor boards: an SMD-to-DIL op amp adaptor board from HobbyTronics. 60 such as HobbyTronics Ltd – Fig.1, https:// bit.ly/pe-may23-dil – and I will produce one of these too. Replacements and upgrading Audio engineers don’t like to waste money, and like grumpy farmers, they expect all equipment to last at least 50 years. Unfortunately, some op amp designs become obsolete and can be hard to find, but on the up side, new versions may well have much better noise and distortion figures. There’s no point scouring eBay for replacement NOS (new-oldstock) quad-pack 741-style devices – it’s much better to replace expired op amps with some NE5532s. Pin 1 for 14-pin socket under board Pin 1 IC1 IC1 we’ve described some generalpurpose audio op amp boards designed for single and dual op amps. While I was designing these, it also seemed like a good idea to make some general-purpose adaptor boards to allow different package op amps to be adapted to any socket. For example, for the dual op amp board covered last month I wanted to compare a pair of single op amps with a dual device without changing anything else. To achieve this, I designed a special adaptor board to allow two 8-pin single op amps to be plugged into a socket with the pinout for one 8-pin dual op amp. I liked the idea and decided to take this concept further, producing a board that enables two 8-pin dual op amps to be plugged into a 14-pin quad op amp socket. Once I had sorted that out for DIL (dual-in-line) ICs I realised that it would be just as useful to have SMD (surface-mount device) versions of these adaptors. This would not only make the assemblies more compact, but also allow the use of some new op amps that are only available in SMD packaging. One important point to note here: there are quite a few varieties of SMD packaging, I have opted to use the most-common 8-pin SOIC. The result is four adaptor boards. (Note that 8-pin-op-amp-SMD-to-DIL adaptor boards are also available from companies Pin 1 IC2 IC2 O ver the last few issues, Pin 9 for 14-pin socket under board Fig.2. Overlay for the two-duals-to-quad op amp adaptor board. J-FET replacements A quick word or two of advice here. If you’re working on old equipment that uses J-FET input op amps, such as the 1980s Texas Instruments TL0xx Bi-Fet series, then it’s essential to use J-FET replacements because bipolar input op amps have much higher input bias currents. If a bipolar NE5532 is used to replace a TL072 in a mixer channel or preamplifier then then the result is likely to suffer from switch clicks and pot scratching. While the bias currents could be blocked by adding extra coupling capacitors and DC path resistors, it is a lot of work, probably involving track cutting. Good audio J-FET op amps tend to be very expensive, such as the Burr Brown designs. There is only one relatively low-cost replacement for the TL072, the SMT-only Nisshinbo NJM8087, distributed by Profusion. Mouser also stock them, but they are pricey. Texas Instruments have taken over a lot of the National Semiconductor, Analogue Devices and Burr Brown op amps. The TI OPA134 SoundPlus FET series are one of the best, but they’re expensive. Discontinued early Fairchild J-FET op amps (eg, the µA771 and 772) occasionally pop up in late-1970s equipment such as mixing desks from Electrosonic Fig.3. The two-duals-to-quad adaptor board assembled. Fig.4. The two-duals-to-quad adaptor installed in an old Audio Developments mixer channel strip. and Soundcraft. Likewise, the LF351, LF353, LF356, LF441 and TL082 are similar second generation Bi-Fets. All Practical Electronics | May | 2023 Pin 1 for 14-pin socket under board Pin 1 IC1 Pin 1 IC2 Pin 9 for 14-pin socket under board Fig.5. SMD dual-to-quad DIL adaptor close-up. These are dual NE5532s – note that the pin 1 end is identified with a bar. Fig.6. Component overlay of the SMD version of the two-duals-to-quad DIL socket adaptor. Fig.7. Surface-mount version of the quad adaptor shown in a mixer. SMD versions are smaller, so they are often easier to fit. these resemble the TL072 and can be satisfactorily replaced with the NJM8087 (maximum supply voltage ±16V). When all the op amps were replaced with 5532s the hiss dropped 12dB and the distortion dropped to a tenth at 0.002%. However, there were some op amp upgrade side effects, mainly high-frequency instability. This was fixed by putting 47pF capacitors on the feedback resistors and several 100nF capacitors across the power rails from positive to negative. Also, the power consumption for each channel tripled from 12 to 36mA using the current-hungry 5532s. Fig.10, installed in the dual RIAA board described last month. Note the use of solid wire (25 SWG) to raise the adaptor board above neighbouring capacitors. The SMD version is shown in Fig.11 and Fig.12. Practical adaptor – quad devices If you want to replace an old quad ‘741style’ device, such as the LM324, then a two-times-dual-to-quad adaptor board, which simply plugs into the original 14 pin DIL socket, is the solution. Its overlay is shown in Fig.2. An assembled unit is shown in Fig.3, and it is shown installed in an Audio Developments broadcast mixer channel in Fig.4. It produced a marked improvement in the signal-to-noise ratio of the mixer. Quad SMD I’ve also created an SMD version of this adaptor, which is smaller and easier to install, as shown in Fig.5. The overlay is shown in Fig.6 and its mixer installation in Fig.7. Using these boards, I was able to compare the OnSemi SMD versions with their Texas DIL counterparts. Strangely the distortion of the SMD parts was 30% higher. I suspect the die may have been shrunk, a common cost-cutting procedure. If you are interested in comparing op amp distortion figures, Swiss audio engineer Samuel Groner plotted the results for dozens of devices in his epic analysis at: https://bit.ly/pe-may23-ao Two-singles-to-dual adaptor Fig.8. Two-singles-to-dual op amp adaptor board. Pin 1 IC1 IC1 Pin 1 for 8-pin socket under board IC2 Pin 1 IC2 Pin 5 for 8-pin socket under board Fig.9. Overlay for the two-singles-to-dual adaptor board. Practical Electronics | May | 2023 In Fig.8 you can see the little board for adapting a dual op amp to two single op amp ICs. This is a handy option for many applications – for example, if you wanted to replace a dual NE5532 with two NE5534As, which would provide lower noise, lower distortion and double the dissipation it can handle. Do remember that when using the NE5534A with gains below five then it’s necessary to add compensation capacitors across pins 5 and 8. This board is also useful for exploiting the full ±24V rail voltage rating of the single-pack OPA604 because the dual version, the OPA2604, has recently been downrated by Texas to ±20V. The overlay for this design is shown in Fig.9, and you can see it in action in Soldering the adaptors Standard square-pin 0.1-inch headers (eg, BKL from Rapid, part 50-8086) act as the ‘plugs’ into normal leaf spring IC sockets. If turned-pin sockets are used on the ‘motherboard’ then you have to use round-pin headers on the adaptor Fig.10. Two-singles-to-dual adaptor board installed in an RIAA preamplifier. Note the use of tinned copper wire to plug into turned-pin sockets. Pin 1 for 8-pin socket under board Pin 1 IC1 Pin 1 IC2 Pin 5 for 8-pin socket under board Fig.11. Component overlay of the SMD version of the two-singles-to-dual adaptor board. 61 Fig.12. Assembled SMD version of the two-singles-to-dual adaptor board. Compensation capacitors for NE5534 op amps that have been added. Sometimes a power supply decoupling capacitor should be soldered on as well. board (eg, TruConnect from Rapid, part 22-1704). These are also available from Tayda (part number A-4662). I had run out of round-pin stock, which is partly why I had to use the 25 SWG tinned copper wire mentioned above. It fits well, but can bend easily. There is a logical order for soldering the adaptors, such as soldering the headers before soldering the IC sockets on top (see Fig.13). In Fig.2, you can see pins 1 to 7 could end up under the op amps and be very difficult to solder with bits of plastic blocking the soldering iron tip. Sourcing the boards All four adaptor boards are available in packs of five from the PE PCB Service. Fig.13. The best way to solder pins to an adaptor board is to first put them in a socket on the ‘motherboard’. This holds the pins in alignment and provides a stable platform for soldering the adaptor and op amp(s). Note the 8087, which replaced a TL072 in a mixer channel. Desoldering component puller M any electronic engineers make their own little jigs and tools to deal with the odd jobs that come their way. As somebody who deals with studio equipment, I have a vast junk box full of control knobs of all different sizes and shaft diameters. A lot of these are collet knobs, which have a clamping system similar to a drill chuck. These are very useful for holding things such as 3mm drills for stripboard cutting and the de-soldering aid described here. When the curtain twitcher next door died, her hideous net curtains were thrown into a skip. I found a use for the plastic-coated coiled-spring steel wire used to hold them up (see Fig.1). It’s 4mm diameter, so fits perfectly into a standard 4mm Sifam S15 collet knob, as shown in Fig.2. The wire is coated with a plastic sheath which should be peeled off to get a good fit in the knob. Cut about 45mm of wire, then fashion a hook on the end for inserting under a component lead-out (Fig.3). Don’t use your precision Lindstrom side cutters for this job; a tough old electrician’s pair is needed. The tool is made short so it can be pulled against the board using one hand with the soldered joint being heated underneath at the same time, as illustrated in Fig.4. The beauty of this technique is that only two hands are needed, and it stops you getting burnt fingers. 62 Fig.3. Close-up of the hook. Fig.1. Always check skips! This net curtain wire is useful for all sorts of jobs – it’s tough and springy. Fig.2. A 4mm collet knob is attached to the stripped net curtain wire. Fig.4. It’s easy to pull the resistor out of the board with one hand, leaving the other hand free for the soldering iron. (I’m happy to supply knobs, curtain wire and even completed tools.) Practical Electronics | May | 2023