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panel? I realise the efficiency might be lower, but maybe other people, like me, have obtained a second-hand 24V solar panel cheaply and wish to charge a 12V system. I have researched 36-12 DC/DC inverters and 24V solar controllers online, but none of them can do the job. With the 24V controllers, the protection circuit kicks in, and a typical inverter has only 12V DC output, insufficient to charge a 12V battery. Any suggestions please? (B. M., East Hills, NSW) • The power conversion circuit in our MPPT chargers isn’t designed to handle such a large voltage step-down ratio (nearly 3:1). That is possible, although not with the existing design, and the efficiency is likely to be poor. Reduced power version of the Studio 350 Can the Studio 350 Amplifier (January & February 2004; siliconchip. com.au/Series/97) be outfitted with two power transistors per rail instead of the four and powered from a lower supply voltage? I was thinking of building a tri-amp system and using different powers for each frequency; for the low frequencies, I would use the 350W version with four transistors per rail, but for the medium frequencies, I would use the lower-power version. Is it possible to do this, or will I have a problem? Also, I am having trouble getting some of the transistors for this design. Is it possible to replace the BF470 and BF469 with MJE350 and MJE340? And what replacements can I use for the 2SA1084? (R. C., Quito, Ecuador) • Yes, it would be possible to build a lower-power version of the Studio 350 Amplifier, although there are better options. You would be better off building the SC200 (January-March 2017; siliconchip.com.au/Series/308) for the outputs that don’t need the full 350W. If you decide to run the Studio 350 with a lower supply rail voltage, some resistor values need to be changed to ensure the amplifier is operating correctly. For example, the 18kW resistor at the collector of Q1 and the 6.8kW resistor at the collector of Q4 would need to be reduced to keep the transistors conducting with the lower supply. Also, the amplifier gain will probably need to be reduced by reducing the 22kW resistor value at the base of Q3. But you have to be careful doing that since lowering the gain of an amplifier can cause it to become unstable! As for the transistors, our recommended substitutes for BF469 and BF470 are 2SC4793 and 2SA1837, respectively – see page 38 of the July 2011 issue. However, note that the pinout is reversed; ECB for the BF469/470 and BCE for the replacements, so the transistors need to be placed in the opposite orientation. The board should accommodate that. The recommended substitute for the 2SA1084 or 2SA970 is the KSA992. This is a direct replacement with an identical pinout. The collector current rating is 50mA vs 100mA, but that shouldn’t be a problem in any audio amplifier as the front-end current is rarely more than 20mA. SC Advertising Index AEEE ElectroneX........................ 7 Altronics...............................23-26 Ampec Technologies................. 81 Dave Thompson...................... 111 Digi-Key Electronics.................... 3 element14................................. 13 Emona Instruments................. IBC Hare & Forbes............................. 9 Jaycar............................ IFC,53-60 Keith Rippon Kit Assembly...... 111 LD Electronics......................... 111 LEDsales................................. 111 Microchip Technology......... 5,OBC Mouser Electronics.................... 11 Ocean Controls......................... 10 PHIPPS Electronics.................... 8 PMD Way................................ 111 SC Vintage Collection DVD..... 110 SC Xmas Ornaments................ 85 Silicon Chip Binders................. 89 Silicon Chip RTV&H DVD...... 108 Silicon Chip Shop.................... 94 Silicon Chip Subscriptions....... 37 Silvertone.................................. 12 Switchmode Power Supplies..... 79 The Loudspeaker Kit.com......... 52 Tronixlabs................................ 111 Vintage Radio Repairs............ 111 Wagner Electronics..................... 6 Notes & Errata Programmable Hybrid Lab Supply with WiFi, May & June 2021: the footprints for transistors Q3 and Q4 on the PCB are incorrect, with the base & emitter pins (pins 1 & 2) swapped. There are two possible solutions to this: either gently bend the pins of these transistors up so that they can be soldered in place upside-down, or trim the leads of two NPN TO-92 package transistors to reach the appropriate pads. Also, there is an error in the parts list; the 150W axial resistor should be 68W, and the 68W SMD resistor should be 150W 0.5W (M2012/0805 size). This error also affects Fig.6 in the June 2021 issue; the 150W through-hole resistor below REG2 should be 68W, and the 68W SMD resistor to the right of REG1 should be 150W 0.5W. High-Current Four Battery/Cell Balancer, March & April 2021: The UM6K34N and UM6K31N transistor types have been swapped throughout both parts of this article. Q7 should have been specified as UM6K34N, while Q8, Q13, Q18, Q19 and Q24 should have been UM6K31N. This is not critical unless the total battery 'stack' voltage can exceed 50V. In that case, you should replace Q8 and Q18 with the 60V-tolerant UM6K34N. Finally, in the second article (April), at the start of page 82 where it refers to dividing a reading by 3.3V, it should instead be divided by 1.65V (ie, half the 3.3V rail, which is the ADC reference voltage). Speedo Corrector Mk.3, September 2013: the BC857 is incorrectly listed for Q3 & Q6 in the parts list, it should be for Q4 & Q6. The circuit and overlay diagram are correct. The October 2021 issue is due on sale in newsagents by Monday, September 27th. Expect postal delivery of subscription copies in Australia between September 27th and October 13th. 112 Silicon Chip Australia’s electronics magazine siliconchip.com.au