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Thread: DIY rotary DJ mixer

  1. #121
    I decided to rebuild the smaller 2ch mixer which was built from leftover parts and then disassembled to build the xover.

    First I built a 19" enclosure.. the bottom panel is hardboard, sides are 18mm thick pine and the top panel is plywood. The depth at the back is 60mm and 40mm at the front so it's angled, the panel height is 300mm which is 6-7RU.

    The transformer/power supply is from an old tape deck and has 630mA fuses, it provides 30VDC to the preamp and 9VDC to the power on LED. The AC input operating voltage is 230V/50Hz.

    The channel configuration consists of a 6,3mm mono input channel and two RCA stereo channels which do not provide any amplification, ie. they're passive. They're summed into a bus that is preamplified and further fed to the master outputs.

    The summing bus I built from a 12-pin screw terminal that has 4 left, 4 right and 4 ground parallel into L/R/gnd terminals so in other words the ground signals are summed.

    The mono channel is split into L/R via 1kOhm resistors to provide stereo separation between channels. The channel strips use 100kOhm potentiometers for volume control (the mono channel is a single gang, the stereo channels are dual log)

    It also has 2-track in/out, with the input connected to the summing bus and the output taken from the master preamplifier prior the master vol. The master outs are 6,3mm unbalanced via a 100kOhm stereo log, the preamp has trimmers and is wired to the summing bus using a PCB/bus connector (so it's easier to service)

    It's missing knobs atm, I'm also looking to add a headphone monitor section with PFL/master cue (which requires trim pots, a 2-pole 4-pos rotary switch, level pot and an output jack, possibly a headphone amp too), per-channel HPF/EQ and -20dB pad switch as well as balance controls.

    Here's a pic of the internals with the PSU and summing bus/master section visible :



    ..and a pic of the front panel in place :


  2. #122
    Modified the 3RU mixer so it now has a L/R input trim.. basically 1kOhm trimmers in the back panel with a through-chassis hole so they can be adjusted with a flat-head screwdriver. The trimmers are wired between the RCA inputs of the line channels and the corresponding channel preamplifiers.

    This is for adjusting the input level of the source (ie. tape deck/CD player) to prevent the channel preamplifier from clipping/distortion in case the output level of the source is too high as I noticed they may vary greatly between different devices.

    Additionally it allows adjusting the L/R balance if the output levels of the connected device are not equal for one reason or another (output circuitry, cabling etc)

    Another modification I did to the smaller 3ch mixer in my earlier post is a Ch1 mute switch, eg. if it's used as a mic channel since not all microphones have an on/off switch (edit : also added rubber feet for tabletop operation)

    EDIT : here's a pic of the input trim mod from the inside:



    and a front panel pic of the tabletop mixer build:


  3. #123
    Here's a more recent build pic :



    It now has knobs, I also added an internal supply voltage trim to the preamp as well as a 3,5mm headphone output with volume control.

  4. #124
    Added handles too for looks, racking, cable strain relief etc..



    Next I'm looking to add preamps (mainly the mono channel), channel PFL and EQs but I think the main functionality is there.

    Probably ok for stuff like karaoke (wireless mic receivers usually have some sort of built-in output preamps) with background music from external source(s) and in general if there's no need for beatmatching but I haven't tested it yet (it's essentially the same small 2ch device I built a few years ago but it has a mono channel with mute, master cue as well as 2 track in/out and uses a built-in linear PSU)

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