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reluctor circuit by changing the 47kΩ resistor connecting to the cathode of zener diode ZD5. Making this value larger will improve sensitivity. Try a value between 47kΩ and 100kΩ or use a trimpot (say 200kΩ) and adjust it until the circuit works. Then select a fixed resistor that is close to the trimpot resistance. Mighty Midget needs low resistance cables I constructed the Mighty Midget power amplifier from the March 2002 issue. It operates as it should except it has a slight problem. When the Bass control is turned into the boost region, the speaker cones (both channels) exhibit a large excursion once every three to four seconds; the current drawn at this point is about 2.5A. This was observed using both a 10A power supply and a car battery. The speakers used were 4-inch, 4-ohm dual cone speakers. All component values and connections have been checked. Seeking enlightenment. (P. L., via email). • The problem is “motor-boating” and is caused by inadequate power supply leads. You need lower resistance power supply cables. Use 4mm auto wire or thicker; the more copper the better. Increasing Woofer Stopper output I have just completed a Woofer Stopper Mk2 (February 1996) successfully but now I want to increase output of this project. I have only one piezo tweeter (KSN 1005A) connected at present but the effect on the dogs doesn’t seem to be enough. Should I be considering the KSN 1177A TD? • The output from the Woofer Stopper is very dependent on the piezo drive. Notes & Errata Sooper Snooper, September 2001: depending on whether the Snooper circuit is built for electret microphone, dynamic micro­phone or RF pickup, the 4.7kΩ resistor should be included or omitted, as indicated in the article. However, if the 4.7kΩ resistor is included, the 1µF capacitor should have its negative side connected to the base of Q1. If the 4.7kΩ resistor is omitted, the 1µF capacitor should have its positive electrode connected to the base of Q1, as shown on circuit but incorrectly shown on the wiring diagram. Alterna­tively, fit a non-polarised 1µF capacitor instead. Audio/Video Distribution Amplifier, November 2001: as presented, the audio stages have a gain of two which will result in excessive audio level with some CDs and DVDs. To restore the gain to unity, remove the 100kΩ feedback resistor from pins 2 & 6 to the 0V line. This makes the op amps in IC2 operate as voltage followers, with unity gain. LP Doctor, January & February 2001: in the text on page 28 of the January issue, the final sentence in the second last paragraph refers to IC5a providing a gentle treble cut at 12dB/octave above 10kHz. Instead it should refer to IC5b (and IC7b). The overlay diagram on page 78 of the February issue shows two trimpots numbered VR8. VR8 shown near IC14 should be VR7. So the KSN1177 twin tweeter which produces 99dB for 2.83V compared to the 1005A at 94dB for 2.83V in will The test procedure (3) on page 82 should read “Monitor Test Point TP4 and adjust VR7 for a 0mV reading.” (Not VR8). Table 3 on page 80 of the February issue should have the heading “How To Set Different Delays For IC3 and IC7 using Linking on IC20” (not delays for IC2 using IC8). Stepper Motor Controller, May 2002: on the circuit diagram on page 77 most of the earth symbols and one resistor failed to print. The “hole” alongside VR1, labelled 10kΩ, should have a resistor occupying it, while all nine of the vertical lines which end with nothing should go to earth (GND). Mighty Midget 70W Amplifier Module, March 2002: this amplifier is very sensitive to dips in the supply voltage and will mute if it goes below about 7V. This may not seem likely but peak currents can be as high as 9A and with thin supply cables, the amplifier will repeatedly mute which can sound like motor-boating. The cure is to use heavy-duty cable. We suggest 4mm auto cable as a mini­mum. 6-Channel IR Remote Volume Control, March & April 2002: the 33Ω 5W resistor in the power supply should be 330Ω 5W. This can be seen in the photos on page 64 of the March issue and page 72 of the April issue. produce much more sound. Paralleling up a few will also increase sound SC levels. 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. www.siliconchip.com.au June 2002  89