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Curing thumps in subwoofer amplifier I have recently built the Plastic Power amplifier module from the April 1996 issue and an active crossover for subwoofers and have coupled them together to make quite a powerful mono subwoofer system. The problem I am ex­periencing is speaker “turn-on thumps”. I noticed Jaycar stocked a kit called a “Universal Loudspeaker Protector” (Cat. KC-5220) which eliminates turn-on thumps and so I bought the kit. I was wondering if you could tell me which wiring diagram I should be following in the instruction guide so as to achieve the result I’m after. I am not sure how to go about hooking up the unit to a mono mains amplifier current. You need a bigger heatsink. The Drill Speed Controller of Septem­ ber 1992 is not suitable because it will not let your router run at full speed (about 80% at no load) and nor will it give a soft start. You need our 10A Speed Control published in November 1997. This will let power tools run at full speed (as well as being variable over a wide range) and will also give a current-limited start which stops the kick from routers, circular saws etc. We can supply the November 1997 issue for $7.70 including postage. Excessive hash from the sine/square generator I recently bought and built the Sine/Square Generator de­scribed in the February 2000 issue. I followed the layouts and instructions to the such as mine. (L. B., via email). •  The loudspeaker protector you have was described in the April 1997 issue of SILICON CHIP and should be assembled as per Fig.5 on page 58 of the same issue. Its power requirements can be obtained from the power amplifier positive supply rail. Connect the protector PC board power connections to the GND and positive supply of the power amplifier. Note that the supply resistor RY should be 220Ω 5W. Since your amplifier is a mono unit, you need only use one side of the relay. Connect the amplifier output to the Amplifier 1 output connection on the PC board. The GND goes to amplifier GND and the Speaker 1 output on the PC board is for the speaker connection. letter but the final result was pretty well unusable for my requirements of testing aspects of my home-built stereo system. There was appalling audible hash and at some frequency set­tings really messy waveforms were produced. Inspection with a CRO showed that the output from the TL071 was dirtier than the input! This was traced to the -5V rail which had nearly 1V p-p hash on it. This was cured with a 47µF capacitor across the 0.1µF which is clearly inadequate on its own. The +5V rail has a 100µF by­pass; why not the -5V rail? This mod basically turned the unit into a usable one, although further reduction of very high frequency hash requires some shielded cabling – which again should really be included in the kit. (M. S., via email). •  The oscilloscope waveforms from WARNING! Fig.3 to Fig.8 in the Febru­ary 2000 issue show that the output from our prototype is rela­ tively clean from hash. Perchance you have a low-spec 79L05 regulator. Having said that, we agree that there could be im­provements made by using shielded cable for the level and range controls and from the output to prevent pickup from the switching circuitry. This would make the circuit more suited to critical audio applications. Combiner needed for two UHF antennas I have recently moved to an area where television is all in the UHF region. I need two antennas pointing in different direc­tions. Is there a preferred method of joining the output from these two? Commercial joiners seem to be UHF + VHF not UHF + UHF. (T. S., via email). •  You need a splitter/combiner. Normally used as a splitter, if you use it the other way, you can combine two signals into one, instead of splitting one signal into two. Get it? When you go to purchase your splitter, ensure that it can also be used as a combiner, eg, Altronics Cat L-1310 2-way. Notes and Errata 12/24 Hour Giant Clock, March 2001: The 10µF capacitor on the overlay adjacent to ZD1 should be a 100µF as shown on the cir­ cuit. Also the LDR should be a Jaycar RD-3480 not RD-3485. The description for easy daylight saving setting is incor­rect. Changing to daylight saving requires the hour switch to be pressed once to set it to the next hour. Returning to standard time requires the hour switch to be pressed until the previous hour is SC selected. 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. 100  Silicon Chip