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Thread: DIY rotary DJ mixer

  1. #101
    Quote Originally Posted by efinque View Post
    Well, anyway, I dealt with the pgm cue summing the master/booth outs with the same 10k+10k, the problem is it comes via the output preamp so it could be louder than channel cue, probably needs an additional resistor in it (ie. matched by ear).. also burnt my hand with soldering iron.

    EDIT : the output preamp has trimmers for setting gain though.. I think I'll just put another 10k in the master PFL and call it a day. There's still some labeling etc and other miscellaneous housework to do.

    EDIT 2 : the 10k+10k to another 10k isn't necessarily enough esp. with the equalizer on but I can't be bothered with it as I have no test equipment at the moment (the catch here is not to connect anything of high value into it while testing to avoid possible material losses, the bare minimum is to probe the inputs/outputs with a DMM to see if there's an abnormally high voltage present)
    Yea cue/master switchining in the headphones is going to have that problem, in any mixer where the master pfl comes after the master volume control you will have a change in headphone volume when you switch back and forth. I prefer the way Ecler does it where the feed of master to headphones is before the master pot. But, there is NO good way to do that without an extra stage of amplification. If you connect the master feed to the headphones from the summing point (through a resistor of course, ALWAYS through a resistor to the summit point) then when you switch the headphones to master, you could possibly hear a change in the mixer main output.

    The right way to do this is.. you take all your channels through their summing resistors into a summing amp (this is a simple amp stage with very high impedance input) and just enough gain to make up for losses in the summing resistors. You then take the output of there into both your headphone switching, and into your master potentiometer.. which then feeds another stage that drives your main output.

    Quote Originally Posted by efinque View Post
    Fixed a grounding issue in the master/booth outputs.. they were previously via the master potentiometer, ie. at 0 it feeds the signal to the ground which is a viable configuration in some cases (unless the hot and ground are mixed in the next stage, amplifier etc) but now it's isolated.

    I cut a few corners by using a screw terminal to establish the ground point.
    No, never connect the output of something to the wiper.. that will not work well in this type of circuit. First, you are shorting out your amp stage, which is bad. Second, your fader curve will be completely screwed up.

  2. #102
    Quote Originally Posted by light-o-matic View Post
    No, never connect the output of something to the wiper.. that will not work well in this type of circuit. First, you are shorting out your amp stage, which is bad. Second, your fader curve will be completely screwed up.
    How come? Isn't that how potentiometers are supposed to work?

    All the volume pots are logarithmic, ie. audio pots. If they were used for VCA then linear pots would be a consideration I think.

  3. #103
    Removed the 1MOhm output ground resistors.

    EDIT : another feature I was looking to add is rec/tape out.. also an FX loop was something I was considering. The summing bus issue with the 220R is that it could overdrive the output amp, it had 10kOhm resistors first but no active output stage so I lowered the value in an attempt to get a higher level signal from it; I added the output preamp later but left the 220R resistors in place (I think anywhere between 1k..10k would be ok so as not to overdrive the output stage, the opamps generally have a wide operating range though and as such are forgiving to incorrect impedance matching)

    EDIT 2 : I've been distancing myself a bit from the mixer build since I started working on an EP (I just got my production setup up and running after relocation)

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