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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)

  4. #104
    Fixed the crossover (another project) LF mono sum.

  5. #105
    Fixed the mic channel sum to mono.

    Modifying the phono channels to line and using external RIAA preamps as well as sourcing a fresh headphone amplifier were among some other stuff I was considering.

    Either that or source suitable modules/kits from other manufacturers (or breadboard/assemble my own) as the current line preamps are discontinued I read.

    EDIT : a VU meter would be nice but it requires reworking the front panel as there's no space, I have a separate one somewhere though.

    EDIT 2 : here are some other projects..

    The 2-way crossover internals :

    Front panel :

    ..and the schematic :

    Optocompressor/gate internals :

    Front panel :

    ..and the schematic (the MCU needs code to work) :

    I was thinking of expanding the gate to 4/6ch as the microcontroller I/O allows it and putting it to a 1RU 19" case for a good measure.

    A mono power amplifier (it's essentially the same as the xover comp but it runs off a rectified PSU from a cassette deck which is crap imo but it worked) :

    A step-sequencer/synthesizer (with a drum machine) I was planning :

  6. #106
    I think the master pot resistance should be higher.. this is because the channels have preamps and when you max the trim&vol the only resistance the signal sees is in the summing bus (220ohm) and in the output pre trimmers (the input trim&vol and master/booth are 100kOhm)

    The output preamp was originally meant to drive the master out transformers. Now, the booth outs see a bit different level as they're transformerless.

    So it should be gain staged according to the equipment so that when the channel vol is at max and the master&booth out turned down you shouldn't hear anything.

    A crowbar solution would be to change the summing bus resistors, put resistors in the output (or both) and/or increase the master&booth pot resistance, like 250-500kOhm I guess (edit : or decrease the channel pot R values)

  7. #107
    I looked at your schematics but I have been too busy with work to reply.
    I will post later tho.

    But I see a few things you can make better.

  8. #108
    Quote Originally Posted by light-o-matic View Post
    But I see a few things you can make better.
    Yes there are a couple of things I didn't think or really pay attention to as this was my first mixer build.

    One would be to put all the input modules on a PCB with sockets and corresponding inputs/pinouts to reduce the amount of wiring.

    Another is impedance matching.. which is closely related to my previous post (dealing with R values and levels in different stages of the signal path)

    There are a few things to tackle like the mono PFL, and overall I had to come up with solutions to problems I didn't even know that existed until I encountered them during the build. (edit : a bad practice is also the way I wired the PFL as I should've used the ch vol pot input instead of the trim, had there been a control voltage to the opamp it would've gone straight to the headphone amp, I checked the schematic first though.. the channel volume pots were harder to solder which is my laziness, they had pins instead of pads though and in my defense they were kind of in a tough spot when I began sorting out the PFL)

    Then there's the optional stuff like cue/pgm, VU meter, channel/master balance, tape/rec out, FX loop/aux send.. per channel EQs would be nice or even HPFs but then again I had problems getting even the master EQ to work. A crossfader would be a neat addition due to the circuitry involved but I rarely use one myself, and imo it's more of a turntablist thing (with more than 2 channels I think it requires a very sophisticated routing system with 3 master buses, crossfader A&B and bypass but don't take my word for it as I've never built one)

    You can easily rule out the per channel EQs though as there's simply not enough space in a 3RU panel.

    I was thinking of ordering a couple of modules and more components to build a tabletop mixer with faders.. or make a 4-10ch studio mixer with EQs, comp, auxes and spring reverb (I built a 16-24ch frame with a leather wrist rest but it was binned due to relocation, it was rather large anyway and a fairly complex project too I guess, I still have the prototype channel module though)

    The 2 channel mixer I built had me thinking of a battle mixer too.. it lacked cue though.

    Fascinating as well as I have absolutely no idea what people want from a battle mixer, stuff I've seen is adjustable CF/upfader curves&assign, lots of space around the faders, 2-band EQ.. as to how the curves are done I think the faders are either physically different from normal ones so that they contain a number of resistive tracks and you pick one via a switch.. or the fader does VCA into some hoodoo circuitry (voltage multipliers or something)

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