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Notes & Errata Driveway Monitor (July 2015): IC1 is incorrectly listed as an AD723AN in the parts list. It should be an AD623AN as shown on the circuit. This error has been corrected in the on-line edition of the magazine. Next Issue The September 2015 issue of SILICON CHIP is due on sale in newsagents by Thursday 27th August. Expect postal delivery of subscription copies in Australia between August 24th and September 4th. Ultra-LD Mk.4 Amplifier Module, Pt.1 – continued from p39 the exception of C0G/NP0 types, have very high voltage coefficients. As the voltage across the capacitor increases, its capacitance drops. While electrolytics have a reputation for non-linearity, they are nowhere near as bad as these multi-layer ceramics in this respect. It’s so bad that with just 10mV RMS across the coupling capacitor, we were measuring distortion levels as high as 0.1% at 10kHz. Luckily, the same attribute that gives C0G/NP0 a near-zero temperature coefficient means they also have a very low voltage coefficient and so are free of this problem. The output filter capacitor can either be a high-voltage SMD NP0 ceramic or through-hole polypropylene. Its linearity is absolutely critical to performance. Both types are acceptable. However, the NP0 ceramic may be a better bet as we’ve found several different 250VAC polypropylene capacitors with less-than-ideal linearity. We tested several suitably-rated polypropylene capacitors, some of which were X2 types, intended for mains applications. Of these, two introduced measurable distortion of around 0.001% in a simple RC filter (with a 6.8Ω resistor) at just 12V RMS. One X2 capacitor, and the a 400V DC/250VAC type from Epcos/TDK, measured much lower at around 0.0004%. So if you are going to use a polypropylene capacitor we highly recommend sticking to the type we have specified in the parts list. Others may have similarly low distortion but without a high-performance distortion analyser, there’s no way of telling. We do not recommend you use an X2-rated polypropylene as a consequence. Semiconductors In the preview last month, we explained the rational behind changing the small-signal transistors and the advantages of the new parts. Besides replacing the obsolete parts, one of the biggest benefits is that with the input pair in a single package, there will be very little drift in the output offset voltage with temperature as they will track closely. The output transistors, driver transistors and VBE multiplier are identical to those used in the Mk.3 amplifier as these all need to be mounted on the heatsink. The driver and output transistors are among the best available so we didn’t see any point in changing those. By the way, the heatsink mounting arrangement is identical, so it’s easy to replace a Mk.2 or Mk.3 module with the Mk.4 version, by simply replacing the PCB assembly. Next month That’s all we have space for now. Advertising Index Altronics.................................. 80-83 Aust. Exhibitions & Events.............. 5 Av-Comm Pty Ltd........................... 7 Emona Instruments...................... 63 Gooligum Electronics................... 12 Hare & Forbes.......................... OBC High Profile Communications..... 111 HK Wentworth Pty Ltd.................. 64 Icom Australia.............................. 13 Jaycar .............................. IFC,53-60 KCS Trade Pty Ltd........................ 75 Keith Rippon .............................. 111 Keysight Technologies.................. 65 LD Electronics............................ 111 LEDsales.................................... 111 Master Instruments........................ 3 Microchip Technology................... 11 Mikroelektronika......................... IBC Ocean Controls.............................. 8 Premier Batteries Pty Ltd............... 9 Qualieco Circuits Pty Ltd.............. 63 Questronix.................................. 111 Radio, TV & Hobbies DVD............ 25 Sesame Electronics................... 111 Silicon Chip Online Shop.... 104-105 Silicon Chip Subscriptions......... 103 Silvertone Electronics.................. 15 Trend Lighting............................. 111 Tronixlabs................................... 111 Worldwide Elect. Components... 111 Next month we will present the power supply, PCB overlay and photos of the final prototype, along with construction details. We’ll also describe a slightly cheaper, cut-down version of the amplifier for lower power applications, without compromising its SC excellent performance. WARNING! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Competition & Consumer Act 2010 or as subsequently amended and to any governmental regulations which are applicable. 112  Silicon Chip siliconchip.com.au