The manual and this group agree, that the QMX+ does like to see 12 V or less to have a long and prosperous live. Available here is 13.8V - as well in my shack, from battery or in the car. The small PA and a tuner which I plan to use in combination with the QMX do use 13.8V so keeping the main supply does make sense for me. Furthermore, I'd like to have an option to regulate the output power, which can be achieved by regulating the input power. Using an readily available switching power supply would have been an option, but that might induce unwanted noise, so I opted for a quiet linear voltage regulator. I couldn't find a ready-to use device, but I still had some time until the QMX arrives, so I build one. The circuit is straight-forward and the scrap-box did contain a nice housing and cooler that could be cut to size, so I got to work.
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Components:?
* MIC29512WT Linear Voltage Regulator from Microchip Technology
* Vishay 534-10K Potentiometer, with a nice? turning knob
* 2x 220?F 35V Elco
* 7.5 kΩ Resistor to VOut
* 75 kΩ Resistor to ground
* 0,28" Mini Digital-Voltmeter 3,5-30 V
* Switch
* 2 Lamp Terminals as strain relief
* 3x2 + 1x3 JST Connector
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The schematic is drawn in the datasheet of the power regulator. Using the two resistors, I get the desired regulation range of 6.5 V to 13 V. Between 11 V and 12 V there is about 3/4 of a turn, so the voltage can be regulated quite precise. I did connect the switch to the enable pin of the voltage regulator, so that input voltage is shown when power is supplied to the device - I plan to use that to control the system voltage. Output power is supplied as soon as the device is switched on. The LEDs, especially the green one are a bit bright for my taste, instead of fiddling with that tiny circuit, I opted to use a double layer of tinting film, which is intended for car windows. That film also covers the small imperfections of the hand-filed slots.
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The soldering work is really not the nicest in the world, but there's no shorts and the connections are good, so it does the job. The MIC29512WT is rated 5A, which should be plenty for the QMX+. It needs 0.7 V dropout, and has a max input of 16 V which does allow a near-empty car battery as well as an overpowered car alternator. One drawback might be, that this device may consume a few watts, in particular on lower output voltages. That's what the cooler is for and I'm not planing for a absolute minimal setup, so I can absolutely live with that.
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Yours, Joe, DJ0CHE
