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•  You can reduce the power of this module by removing a pair of output transistors as you suggest and then reducing the supply rails to about ±34V. However, we are inclined to think that using this module to deliver only 50W is a bit of a waste. Have you considered the 50W LM3876 module described in the March 1994 issue? It is cheaper and has short circuit protection. You would be advised to add the reverse biased diodes between the output and the supply rails, as in the 175W module. Amateur band receiver needs alignment I have made up a 3-band amateur receiver (SILICON CHIP, September 1996) and I am having problems. As supplied, my kit is slightly different from the published design. While the circuit specifies a number of 150pF capacitors, the PC board component layout has 68pF capacitors. Also for the F14 balun (T1), the copper wire supplied was not right because I could not get the right turns and had to go to a smaller gauge. Now, when I tune VR2 across the bands I only get one amateur station and one shortwave station all over the bands. Can you please tell me where I am going wrong? (W. S., Christchurch, NZ). •  We note that the PC board in your kit has changed capacitors compared with our article but unless you are unable to set the trimmer capacitors to give the precise oscillator frequencies, these changes are not important. You do not say whether you have been able to correctly align the receiver as outlined in the article. You will also need a fairly good antenna if you are to receive a reasonable number of stations. Have you put up an antenna, as suggested in our article? Is rectifier buzz a problem? I am preparing to build a compact, high performance ampli­fier for studio headphones, which I intend to use as the “heart” of a no-compromise listening system to be packed into a suitcase. It is to be housed in a small metal enclosure and powered from a 16V 1A AC plugpack. This arrangement appeals to me most of all possibilities and I can derive the two different split-rails that are required for the device’s operation “internally”, including rectification, filtering, stabilisation, decoupling, etc. It means, however, that the transformer will be connected to the rest of the power supply circuitry by about 2m of cable. Conventional wisdom implies a danger of “rectifier noise” radiation which could affect the performance of the device, as well as other electronics in the vicinity. How serious is this danger, for the given voltage and cur­rent? What kind of filter could I use and where, to safeguard against this? Would some sort of shielding on the plug­ pack lead be worth considering? And finally, would you suggest a larger case containing a transformer to be a better option – from the noise viewpoint alone? The amplifier will have an active tone control stage which I believe is especially prone to hum pickup. (A. K., Douglas, Qld. •  Conventional wisdom is right as far as rectifier noise is concerned. It is almost impossible to adequately suppress recti­fier noise radiated by a cable and it is even more difficult if the device you are using does not earth the core of the transformer. Having said that, the only way to gauge the serious­ ness of the problem is to give your proposed arrangement a try. It may be quite satisfactory. If not, you will need to resort to a more conventional power supply arrangement. Notes & Errata Stepper Motor Driver With Onboard Buffer, December 1997: the overlay diagram on page 64 shows a .01µF capacitor connected to pins 1 and 4 of IC2. This should be a .001µF as shown in the circuit and parts list. 240VAC 10A Motor Speed Controller, November 1997: while this controller is suitable for power tools with nameplate ratings up to 10A, it is not suitable for appliances such as 2400W radiators which draw 10A continuously. We have also been advised that the mica washers supplied in some early kits have been prone to flashover to the case. To avoid this, we suggest that a minimum of two mica washers be used for both the fast recovery diode and the IGBT. Better still, we suggest that SIL-PAD heatsink washers, a composite of silicone rubber and fibreglass be used, as these have a considerably higher voltage rating. The SIL-PAD 400 (.007) has a breakdown rating of 3.5kV AC. 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 Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable. January 1998  85