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Setting Ultra-LD Amp quiescent current I have a few questions about the Ultra-LD Mk.3 200W Amplifier Module (July & August 2011; siliconchip. au/Series/286). I have already built a module, and it works extremely well. I adjusted the quiescent current as described by you in the magazine. It says, “The voltage across one resistor is then monitored and trimpot VR1 adjusted for a reading of 9.5V – equivalent to a quiescent current of 70mA.” But by 9.5V, it is drawing about 160mA. It works, but then I have more dissipation, and the heatsink gets quite warm. If I lower the voltage on the test resistor to about 3.5V, I have a stable 70mA. Why do I have this discrepancy? I also noticed that the positive side draws a little more current. Is that normal? I have another question about the VAS transistors (2SC4793, 2SA1837) and 180pF capacitors. Is a replacement Advertising Index Altronics.................................25-28 Dave Thompson........................ 111 Digi-Key Electronics...................... 3 Emona Instruments.................. IBC Hare & Forbes............................. 11 Jaycar........................ IFC, 9, 13, 43, ............................. 51, 55, 89, 93, 97 Keith Rippon Kit Assembly....... 111 Lazer Security........................... 111 LD Electronics........................... 111 LEDsales................................... 111 Microchip Technology......... OBC, 5 Mouser Electronics....................... 7 Ocean Controls............................. 8 available? I tried BF470 and BF469 from CDIL, but the THD was much worse than the original transistor from Toshiba. What is the tolerance for the 180pF capacitors? Would 220pF be too high? I can’t find a 180pF polypropylene; currently, I am using ceramic. (B. G., Neu-Isenburg, Germany) ● A reading of 9.5V across each 68W safety resistor corresponds to 140mA per side or 70mA through each output transistor (two per side). We are unsure how you can obtain 160mA when there is 9.5V across a 68W resistor; perhaps your resistors are a bit low in value, closer to 60W. It would be best to check them with a resistance meter. If you can adjust for 140mA and the dissipation is reasonable, that’s good. The heatsinks will get a little warm but should not be too hot in free air. If you can’t handle that much idle dissipation, you can reduce the bias current, but you will have slightly higher THD+N than the figure we published. Once the fuses are in place, the quiescent current can be checked by measuring across each 0.1W emitter resistor. You should get 7-10mV. It is normal for the current to be different for each transistor and to differ between the positive and negative rails. One rail can draw slightly more current than the other, but they should be almost equal once the correct bias level has been established (within a few milliamps). You can replace the 2SC4793/­ 2SA1837 VAS transistors with FZT558 or FZT796A for the PNP transistor and FZT458 or FZT696B for the NPN transistor. However, since these are surface-mount types you need to mount them onto the small heatsinks using clamps, with connecting wires to the PCB. We don’t think using the BF469 and BF470 should be too detrimental to performance as long as they are quality Silicon Chip Binders................ 111 Silicon Chip Shop............ 106-107 Silvertone...................................... 6 The Loudspeaker Kit.com.......... 95 Tronixlabs.................................. 111 Wagner Electronics..................... 12 112 Silicon Chip Errata and Next Issue ROLEC OKW.................................. 4 transistors, but as those parts have been out of production for a while, those left on the market may not be great examples. The 180pF capacitor values are reasonably critical for stability and should be that value. Higher values should work but will slightly compromise the high-frequency performance. Ceramic capacitors are fine as long as they are NP0/C0G types; those are just as good as plastic film capacitors. You can reduce the value of the 220pF capacitor to 180pF by adding 1nF capacitors in series with each. Alternatively, you could use 100pF and 82pF capacitors in parallel, or other combinations that total close to 180pF. Plans for an updated headphone amp? Do you have any plans to present a new headphone amplifier project in the near future? I have the September/October 2011 issues (siliconchip. au/Series/32) and will build that one if there’s no plan to update it (which looks excellent and is by no means obsolete). I also note that you have some parts for that project available in your Online Shop. (P. H., Warwick, Qld) ● We will likely publish another headphone amplifier project eventually, but we don’t have any currently in development. As you say, the September/October 2011 design is still perfectly valid. If we publish a new one, it will probably be a simpler design that doesn’t necessarily perform as well (it still must be good, obviously!) but will be easier and cheaper to build. It’s doubtful we could exceed the performance of the 2011 design anyway. We certainly still have PCBs for that project and it uses standard components that should not be difficult to find. SC AM-FM DDS Signal Generator, May 2022: the 10nF capacitors connected to the A & B pins of rotary encoder RE1 should be increased to 100nF to provide more reliable operation with some encoders. Capacitor Discharge Welder, March & April 2022: in Fig.4 on p31 of the March issue, the 220nF capacitor connected to pin 6 of IC6 should be an MKT type while the other 220nF capacitor should be ceramic. Next Issue: the October 2022 issue is due on sale in newsagents by Thursday, September 29th. Expect postal delivery of subscription copies in Australia between September 26th and October 14th. Australia's electronics magazine siliconchip.com.au