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Questions on guitar preamp I have a few questions regarding the guitar preamp featured in the November 2000 issue of SILICON CHIP, sold in kit form by Jaycar Electronics, Cat KC-5303. Can you please tell me: (1) To connect the second channel’s PC board to the main board, do I need to connect the points marked ‘TP1’? If not, what are they for? (2) All of the connection points on the back (FX send/rtn, balanced out & unbalanced out) have a ground point, except for the ‘line input’ point. Should this line be grounded somewhere? (3) On the schematic for the power supply, where the leads are earthed to the chassis, a 0.47µF capacitor is shown going from the chassis earth point to a regular earth point. Where should this capacitor go to earth? (P. M., via email). • Do not interconnect the TP1 points on the first and second channels. These are test points to adjust the offset in each amplifier (IC2) using trimpot VR7. The line input shares the unbalanced output ground point. The 0.47µF capacitor connects from signal ground to chassis as shown in Fig.1 of the January 2001 issue of SILICON CHIP. they did not make a rotating magnet assembly for these distributors. The HKZ101 Hall sensor is still available from Jaycar (Cat ZD-1900) but the Bosch rotating vane is not available. This would need to be manufactured using a steel cup-shaped vane with eight slots spaced 45° apart. This vane can be glued to the existing rotor button using high temperature (120°C or more ) epoxy or similar. models available but they are in the “cost you an arm and a leg” price range and I am sure such a design would be greatly appreciated by all of us dads who spend half of our lives repairing our son’s R/C cars. R. H., Yatala Vale, SA). • We described a 50A speed control for R/C cars in the May 2000 issue. We can supply this issue for $7.70 including postage. Matching amplifiers to speakers Converting a UPS to use as inverter I am concerned about possible damage to an amplifier when more ‘powerful’ speakers are connected. For example, if 150W speakers are connected to a 100W amplifier, is there the pos­sibility of the amplifier being damaged? Thinking along the lines of source/ load matching as in antennas it seems to me that, provided the amp/speaker matching is correct, there should be no problem. (N. D., Carine, WA) • There is no chance of damage by connecting 150W speakers to a 100W amplifier. Speakers are passive transducers. Provided their impedance is within normal limits, no damage will result. I have a Chloride 12V computer UPS I want to utilise as an inverter but it needs 240V to be on at the moment of a blackout before the 12V inverter will work. This is of no value to me in the fieldwhen trying to operate a drill or grinder. How can I fix it? (R. M., via email). • Without knowing anything about the circuit, we cannot sug­gest a modification. However, it probably has a stepdown trans­former and rectifier to produce a DC voltage from the mains. When that disappears, the inverter kicks in. You need to measure that voltage and then alter the monitoring circuit to disable it. Speed controller for R/C cars I would like to suggest a project for an electronic speed controller for radio controlled cars. There are commercial www.siliconchip.com.au Notes & Errata Mighty Midget 50W Module, March 2002: the list of parts for the capacitors should be as follows: 2 4700µF 16VW, 1 2200µF 16VW, 2 0.22µF MKT, SC 4 0.1µF MKT. 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