Fullscreen HTML5Med Res (??MB)HTML4

Replacement modules work well Thank you for your help with upgrading my Fisher amplifier (Ask SILICON CHIP, April 1996). I changed the input sensitivity to the LM3876 power amplifier chip as you suggested. The performance is now very clean and very loud, with excellent overall gain – the best Fisher amplifier I have ever heard. I was wondering if the same modification could be made to the 25 watt chip, the LM1875T, as featured in the December 1993 issue of SILICON CHIP. You see, I have another amplifier with the same preamp problem; not enough gain. What would the modifications be? I also want to build two sub­ woofers for my Dolby Pro Logic System. I would like to try using some stormwater piping for the enclosure just as you used in the March 1995 issue. However, that piping is too big for my application as I want them to fit snugly behind the two lounge chairs for that added oomph. I was thinking of using an 8-inch subwoofer supplied by Jaycar (Cat. CW-2136). For a vented enclosure, they say that a 33-litre box with a vent tuning of 39Hz would be a good size. The piece of storm water piping I have is 20cm inside dia­ meter at two metres in length. It is just big enough inside to mount an 8-inch subwoofer. After scraping between the tracks on this very sensitive part of the board it burst into action. After inspecting the so-called wire short it turned out to be a human hair! It sounds far fetched but that is what stopped it from working properly. (M. Chase, 58 Douglas St, Nowra, NSW 2541). Electronic speedo wanted I have a problem with my Ducati and that is that I have changed the size of the front wheel. The speedo was driven from an 18" front wheel which is now 17". There is a commercial unit available but this is just too 94  Silicon Chip Could you please tell me the length of piping I would need to get 33 litres in volume? They also say that the vent needs to be 50mm in diameter and 80mm long, which I would fit at the other end of the piping. Is this correct? Could you please help me with the formula for working out how to calculate internal air volume inside piping as I would also like to do another version using different size piping and woofers for my car subwoofer? I was also very interested in bandpass enclosures and their performance with subwoofers. Is it possible to take a piece of piping and mount the subwoofer driver on a baffle and seal it in the middle of the piping so that there was an enclosure in front of the driver as well as in back, sealed and vented at both ends like a bass cannon? Would this work? I also read in another book just recently that you can now get what they called a “Bass Shaker”. It’s a round device that screws to your walls or floors and it shakes you during heavy bass notes so that you physically feel the action. I would love to hear more about these devices and how they work. Maybe you could publish an article on it and maybe a project on this shaker some time in the future. It seems to be the next logical step towards perfection. (K. S., Morphett Vale, SA). small for viewing quickly in traffic and at high speeds. There were also other problems with this unit; the stated maximum speed was 160km/h but only 100km/h was read. If the unit was left in bright sunshine, the LCD went black then lost some of the display. Now also at night this unit is not backlit so it cannot be seen. What is required is a speedo that works off the wheel via a Hall Effect trigger, with three or four LED sections and a trip­meter so I can calculate the fuel consumption. Also the circuit board area should be as small as possible and run off 12V DC. This of course would not be the only use for the speedo as there are other vehicles around the farm that it could • The method for increasing the gain of the LM1875T module is exactly the same as for the LM3876. To double the gain, simply reduce the 10kΩ resistor at pin to 4.7kΩ. No other changes to the circuit are necessary. To obtain a volume of 33 litres with 20cm ID tubing you would need a length of 105cm. You should also allow for the volume of the loudspeaker itself and so the length should be increased to about 108cm. The formula to use for these calculations is the volume of a cylinder: V = πr2h where r is the radius and h is the height. We cannot give detailed answers concerning vent design or about bandpass systems. To design these you need access to a speaker CAD program, such as BassBox 5.1, reviewed in the June 1996 issue. We have seen references to the Bass Shaker but we can see little to recommend the concept. All that such a unit will do it is to excite various panel resonances around the room so that you will be beset by unwanted buzzes and rattles. It will also disturb your neighbours. If you are using a subwoofer with a response down to 25Hz, it will give you all the physical sensation you could want. You will be able to “feel” the bass – provided the program material does contain really low frequencies. be adapted to. (L. R., Glenorie, NSW). The project which comes closest to your requirements was the digital speedo and fuel gauge described in the October & November 1995 issues of SILICON CHIP. We can supply back issues for $7.00 each, including postage. Kits for this project are available from CTOAN Electronics. Phone (07) 297 5421. • Notes & Errata Digital Voltmeter for Cars, June 1993: the digital readout board has a missing track between pins 12 & 16 of IC3. These must be linked together for the SC circuit to work.