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car batteries. It is probably not really suitable for AAA cells. However, the method of detecting the end-point for charging should prevent dam­ age to any of your cells or batteries whether AA or larger. We suggest using the thermistor for end­ point detection when charging the smaller NiCd types, to prevent overheating of the cells. Buzzing or squeal in the transform­ er is normal. It can be quietened by potting the windings in epoxy. Using the Champ as a preamp I wanted to ask if I can use the Champ, a 0.5W amplifier described in February 1994, as a preamplifier. What would the frequency response of this kit be? I want to install it between a Dolby receiver and a subwoofer amplifier, as the receiver’s pre-out signal isn’t enough for the subwoofer amplifier. Would I lose the low end using the Champ? (D. C., via email). • You can use the Champ as a preamp. Provided you don’t load the output of the little amplifier with 8Ω but couple straight into your subwoof­ er amplifier, the frequency response should be flat down to below 10Hz. However, you will probably need to set the on-board pot well down because you won’t need all the gain. Blocking capacitor in phone line I am working on a project that re­ quires me to connect to a telephone line. Wanting to “do the right thing”, I purchased a 600Ω isolation transform­ er (Austel approved). My application requires a telephone to be connected Amplifiers wanted for computer surround sound I have been looking for suitable kits to build to provide good sur­ round sound for my computer. My computer has a Sound­Blaster Live card which provides front and rear line outputs. Not a lot of power will be required – say 10-15W RMS per channel for the front and rear – but I have no idea how much power is required for a sub-woofer channel. in parallel with the project for engi­ neering/monitoring purposes before switching to the project. Having built the prototype, I con­ nected everything up and the phone worked fine (I heard dial tone). When I switched in the project, the phone died. I measured the line voltage and found 0.7VDC! When “on-hook” the phone line read 47V and 6V when “off-hook”. When I measured the transformer resistance, I got 21Ω! No wonder there was no voltage to drive the phone. So why do they quote 600Ω for the transformer when it clearly isn’t? As a footnote, I returned to Jaycar and checked out some other ones and came up with similar results. How can I use these so-called isolation trans­ formers and still have a telephone connected and operational? Do I put a 600Ω resistor across the line and use capacitors in series with the transformer leads? (I. B., Monash, ACT). • You need a blocking capacitor in I was thinking of using a couple of your “15W/Channel Class-A Stereo Amplifier” kits for the front and rear and I’m not sure what to use for the sub-woofer. • Have a look at our 50W module described in March 1994. If you don’t want the full 50W, just run it with reduced DC sup­plies. Kits are available from Altronics, Jaycar and DSE. We can supply the March 1994 issue for $7.70 including postage. Don’t use the 15W class A mod­ ule – it is too good and too expen­ sive for your application. series with the primary of the isola­ tion transformer. Try a 10µF 100VW bipolar capacitor. The transformer primary and sec­ ondary winding can be ex­pected to have quite a low DC resistance. The 600Ω rating refers to the impedance of the phone line circuit not the trans­ former. If the transformer winding re­ sistance was high, it would intro­duce large losses into the circuit. Notes & Errata 50W Amplifier Module, March 1994: the 22µF capacitor con­nected to pin 8 on the LM3876 is shown reversebiased on both the circuit and wiring diagram. The 220µF capacitor con­ nected to the negative supply rail is also connected the wrong way on the circuit but is correctly shown on the wiring diagram. Modules that have been running for more than a few hours should have the SC 22µF capacitor replaced. 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. December 2000  89