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Suitability of SC-480 for guitar use I have a couple of questions regarding the 50 watt SC-480 amplifier from the January & February 2003 issues. I am planning to use it as a guitar amplifier. Would it be suitable? If so, what (if anything) would need to be added to it for it to work, such as a preamp? What would I use as a volume control and where? Would this circuit benefit by regulating the power supply? (A. H., via email). • The SC480 is fine for guitar is 5V. He has also purchased the add-on board to set the decimal point and the voltage scale (20V in this case). The main problem is that in the heat of a corrugated iron pump cover out the “back-o-Bourke”, alkaline or NiCad/NiMH batteries suffer and die very quickly. (S. B., via email). • You need a level shifter circuit to accommodate the fact that the LCD meter needs to be connected to a raised (or isolated) earth. Have a look at the 40V power supply in the January & February 1994 issues of SILICON CHIP. Essentially, the relevant part of the circuit (in the January issue) is IC4 which shifts the voltage by an offset. The op amp attenuates the voltage to suit the 2V scale. Note that point X shown for IC4’s 100kΩ feedback resistor can be ground as it is only across the current sensing resistor R1. More current from the DC-DC converter Thank you for featuring the DC-DC Converter in your June 2003 issue. I am an instrument technician and do service work on job sites. My work work. Have a look at the following guitar preamps: 4-Channel Guitar Mixer in January 1992; 2-Channel Guitar Mixer (includes electronic reverb) in November/December 2000 & January 2001. Or if you just want a very simple preamp with 3-band tone controls, you could adapt the Guitar “Widgy” box from the May 2003 issue. We can supply these magazines for $8.80 each, including postage. The preamps listed all have their own volume controls. There would be no benefit from regulating the amplifier’s power supply. requires a laptop computer and I am using an old Toshiba Satellite which requires 15V DC at 3A. Is there a way to upgrade the converter to 3A or 4A? (B. R., via email). • The circuit can deliver up to 4A if the 0.1Ω sensing resistor is paralleled with another 0.1Ω resistor and the fuse rating is increased to 5A. Also, each of the low ESR capacitors will need to be paralleled to increase the ripple rating. Finally, the diodes and Mosfet will need larger heatsinks. Query on the battery desulphator I’m interested in building the battery desulphator described in the Circuit Notebook pages of the February 2003 issue but I cannot understand why the Mosfet source connection goes to +12V and the drain goes to ground. It’s usually source to earth! Also why use a P type when there are more N types available? (D. H., Shepparton, Vic). • Q1 is a P-channel Mosfet, signified by the arrow from the gate to the source. Therefore, source does go to +12V. Perhaps we should have mentioned that in the short circuit description. You could turn the whole circuit upside down to use an N-channel Mosfet but then you would have to swap R1 and R2 to give correct pulse duty cycle from IC1. High-quality AM tuner wanted How about designing a quality AM tuner capable of full bandwidth, with an audio line level output to feed to an existing hifi system? It seems that the majority of AM tuners available are of poor quality and have limited bandwidth etc. (W. N., Casino, NSW). • We have published two high quality stereo AM tuners in the past: the Portable AM Stereo Tuner in September, October & November 1989, and the Wideband AM Tuner in February, March & April 1991. We can supply the 1989 articles in photostat form and the 1991 magazines for $8.80 each, including postage. Protection for DC plugpacks Is there a way to prevent ignorant, clumsy souls like me from destroying my 12V DC plugpacks? Will a zener diode do it? R. L., via email). • You need a fuse. Try connecting a 1Ω 0.25W resistor in series with the plugpack’s output. Hopefully it will blow before any serious damage is SC done. Notes & Errata Gear Indicator, January 2003: the pin 2 & 3 connections for the Hall effect sensor (UGN3503) are shown transposed on the circuit diagram (Fig.7). Pin 2 should be GND and pin 3 the signal output. The overlay diagram is correct. 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. 92  Silicon Chip www.siliconchip.com.au