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Changing The Active Crossover’s Quad Op Amp Could you please tell me how much current your Active 3-Way Crossover board (SILICON CHIP, January 2003) requires to function optimally? I am thinking about casing it together with the amplifiers of my system. They already have a PSU, so maybe I could use it to power the crossovers as well, using different regulators. Can you see any potential issue in doing so? Secondly, Texas instruments recommend the TLE2074 as an upgrade to the TL074 – see www.ti.com/ product/tle2074/description or www.ti.com/lit/ds/symlink/tle2074. pdf   They state that: “The TLE207x series of Jet-setting operational amplifiers more than double the bandwidth and triple the slew rate of the TL07x”. However, I have read that a higher slew-rate is not always better for audio applications as it can create artefacts above the spectrum perceived by the human ear which nevertheless affect how we hear the processed sound. Do you think that swapping the TL074 with the TLE2074 would improve the performance of the circuit ? If yes, in which way? Or are there any other op amp options that you know that function better than the TL074s in this design? (J. S., via email). • The power consumption of the total circuit is really quite low since the TL074 only draws about 1.4mA per op amp. Really, the specified 20VA transformer is far too big for the job but it might have been the only suitable one available at the time. So you probably can run the whole board from the supply in your amplifier. There is little point in changing the TL074 op amps. Increased bandwidth can lead to a tendency to supersonic oscillation. There would be no improvement in circuit performance. 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IBC Network Communications.............. 9 Ask SILICON CHIP . . . continued from page 110 but on the primary side (230VAC) rather than on the secondary. Fuses would cause unwanted voltage drop while charging. Most chargers would have an overtemperature thermostat cut-out that is essentially a current limit as the thermostat heats under current flow. A 40A charger with a big battery load like yours should not burn out. (One of our staff has a flybridge cruiser with six batteries and they would easily exceed 400Ah). As far as the alternator is concerned, it too is essentially current-limited (by core saturation), so fuses are not appropriate. We think your charger could have been damaged by some other problem, such as high voltage in a caravan park. Speed control has audible whine I have just built and installed the 12V Motor Speed Controller kit from the June 1997 issue and am having trouble dealing with the noise it creates in the fan motor in my Holden 112  Silicon Chip Rodeo. I have only one of the diodes across the motor labelled D2, as only one came with the kit. I have looked at buying another MUR 1560 diode to see if it makes any difference. Any ideas to reduce the motor noise, as turning the stereo up to mask it is not going to do my hearing any good? If this is common and cannot be fixed I will just have to buy an expensive resistor card; no problem but just a bit disappointing. Would putting a bigger capacitor across the motor smooth out the waveform? The noise is heard throughout the whole of the speed range but is a little quieter at very low speeds. (J. E., via email). • It is common for this controller to produce noise in a motor due to the switching on and off of power to control the speed. The frequency can be adjusted a little and that may reduce the noise when set to a frequency that does not resonate with the motor windings. To alter the frequency, the 10kΩ resistor connected between pin 6 of IC1 and ground can be varied between 50kΩ and 4.7kΩ, using a 50kΩ trimpot with a 4.7kΩ resistor in series. Adjust the trimpot to get the minimum amount of noise. Ocean Controls............................ 11 PicoKit............................................ 6 Quest Electronics....................... 111 Radio, TV & Hobbies DVD.......... 109 Sesame Electronics................... 111 Silicon Chip Binders..................... 32 Silicon Chip Online Shop............. 50 Silicon Chip PCBs...................... 111 Silicon Chip Subscriptions........... 29 Silvertone Electronics.................. 43 Trio Test & Measurement.............. 12 Tronixlabs................................... 111 Vicom Australia.............................. 7 Wabeco Australia......................... 63 Wiltronics...................................... 23 Worldwide Elect. Components... 111 Do not increase the value of C2 connected across the motor as this is there to reduce very high switching noise rather than filtering out the motor-driving pulsed waveform. If the capacitor is made too large, Mosfet Q3 and/or Q4 will be damaged. Fitting a second diode (D2) across SC the motor is unnecessary. siliconchip.com.au