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If they light up reasonably well at that current, it should be fine to use them without modifying the circuit. If they are too dim, you can change two resistors on the SC200 modules to deliver more LED current. One is 33kW and connects to Q17’s emitter and D6’s anode, while the other is 100kW and connects between Q19’s collector and D5’s anode. Halving their values will double the current delivered to the LED. If you reduce the 100kW resistor below 47kW, it should be changed to a half-watt type. 3. Check the supply outputs; we expect they will be close to ±50V unloaded. That should actually provide some benefits for driving the 4W speakers with the SC200 modules (mainly increased efficiency). That reduced supply voltage will not reduce the continuous power at all and should barely affect ‘music power’. The only effect the lower supply voltage will have on circuit operation is less current through Q6 (about 87% of the design current), but we don’t think that will be a problem. You could change the two 6.8kW resistors at the Advertising Index Altronics.................................35-38 Blackmagic Design....................... 7 Dave Thompson........................ 103 DigiKey Electronics....................... 3 Emona Instruments.................. IBC Hare & Forbes............................. 11 Jaycar..............IFC, 9, 63, 66-67, 93 Keith Rippon Kit Assembly....... 103 Lazer Security........................... 103 LD Electronics........................... 103 collector of Q6 to 6.2kW or 5.6kW each to compensate for that, but that should not be necessary. 4. That’s right; you don’t want to rigidly attach the end with the transistors as it will move up and down slightly with thermal cycling. There’s enough board/lead flex that it’s OK to attach the other end to the chassis, though. and the Isolating High Voltage Probe for Oscilloscopes (January 2015; siliconchip.au/Article/8244). There are also plenty of good commercial offerings, including the relatively low-cost Pintek DP-25. Making & using a mains isolation transformer I want to build another two Ultra-LD Mk3 amplifiers. I have the PCBs, but I am having trouble sourcing some of the transistors that are no longer being manufactured. What can I use instead of the 2SA970, 2SA1837 and 2SC4793? I think I can still get the other transistors. I really appreciate your advice. (D. S., Caringbah, NSW) ● This is very frustrating because the 2SA1837 and 2SC4793 were the transistors we chose to replace the obsolete BF469 and BF470. They have also been discontinued in just a few years, which shows how difficult it is to create a design that can be built years after publication. Also, there isn’t any direct substitute for the 2SA970; all similar lownoise transistors that are available only come in SMD packages (and many of those have also been discontinued lately). It will take us some time to find alternative transistors that are also current and available (if they exist). In the meantime, you can still get the 2SA970, 2SA1837 and 2SC4793 parts. The 2SA970 and 2SA1837 transistors are available from Futurlec (www. futurlec.com). All three transistors are also available from several suppliers on eBay and AliExpress (AliExpress 32882321479 & 32491223824). We haven’t tested those, but the reviews are mostly positive. SC I want to build a 60VA isolation transformer for my digital oscilloscope. What VA rating transformers would I need? (R. M., Melville, WA) ● It is not safe to isolate the supply to your oscilloscope so that it is floating and not Earthed. The equipment you intend to monitor with your oscilloscope should be isolated instead. The easiest way to make a mains isolation transformer is to use two identical mains transformers and connect the secondaries together. The mains supply is applied to one of the transformers, and the output transformer primary (230V AC side) supplies the DUT. For more information, see the YouTube video at https://youtu.be/ WKFFEsIh9Gw The VA rating for the transformers must be sufficient to supply the equipment you wish to measure using the oscilloscope. Alternatively, you can use an isolated or differential oscilloscope probe and keep everything Earthed. The probe’s frequency response needs to suit what you intend to measure using the oscilloscope. We published two projects that might be suitable: the Wideband Active Differential Oscilloscope Probe (September 2014; siliconchip.au/Article/7995) Microchip Technology.............OBC Mouser Electronics....................... 4 PCBWay....................................... 13 PMD Way................................... 103 Silicon Chip Back Issues........... 94 Silicon Chip PDFs on USB......... 71 Silicon Chip Pico Gamer......... 101 Silicon Chip Shop.................98-99 Silicon Chip Subscriptions........ 97 The Loudspeaker Kit.com.......... 10 Wagner Electronics..................... 91 104 Silicon Chip Errata and Sale Date for the Next Issue LEDsales................................... 103 Obtaining transistors for Ultra-LD Mk.3 amp DC Supply Protectors, June 2024: the two through-hole versions specify a maximum current of 7A but the SPP15P10PL-H P-channel Mosfets specified can only handle about 2.5A without heatsinking. A logic-level P-channel Mosfet with a lower on-resistance like the IPP80P03P4L-07 (SC6043) can be used instead without heatsinking (now included in both kits). Fan Speed Controller Mk2, May 2024: in the left-hand column of text on p73, the reference to diode D1 in the fifth paragraph should be to D2 instead. Touchscreen Appliance Energy Meter, August-October 2016: the circuit diagram on pages 30 & 31 of the August 2016 issue had previously been updated in the online version to use the safer ACS718 IC rather than the original ACS712. There were some errors in this updated circuit, mainly with the pin numbers for the Vcc (pin 10) and GND (pins 11, 13, 14 & 15) connections. They have now been fixed in the version on our website. Next Issue: the August 2024 issue is due on sale in newsagents by Monday, July 29th. Expect postal delivery of subscription copies in Australia between July 26th and August 14th. Australia's electronics magazine siliconchip.com.au