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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

This kit might be interesting: http://www.qrpkits.com/wallwarttamer.html

Takes in AC or DC and provides clean, adjustable DC output Reuse those surplus wall warts or computer supplies as DC power sources Bridge rectifier, Capacitive filtering and LM317 Adjustable Regulator Recommended Input voltage range of 5-25 Volts Output adjustable from 1.25V to approximately 3V below the input voltage Supplies up to 1.5A current Board size of 2.3” x 4.3”

73 - Pierre - FK8IH

On Fri, Feb 21, 2025 at 04:19 AM, Pierre - FK8IH wrote:

This kit might be interesting

But it is not.

I really do not understand why some of this companyes dont provide schematic.

Will they loose money, hardly noticable

Will they make our life harder, yes

Can we revers engeneer it, yes

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Perfect solution Joe, thanks for sharing. Will keep your idea for when I get my QMX+

Cheers.

Gert

Hi Gert,

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Note that a minimum voltage drop is required.? From the specs:

Output adjustable from 1.25V to approximately 3V below the input voltage

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Three volts below 13.8 is 10.8 volts.? This would be the maximum output voltage regulated from a 13.8-volt supply.

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73

Evan

AC9TU

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