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Thread: DIY rotary DJ mixer

  1. #111
    Modified the master section preamp as well to improve serviceability.. it has internal trimmers and can be bypassed similarly by removing the line card and jumping the correct wires (it has the ground mono'ed as the master EQ does it anyway)

  2. #112
    Added a potentiometer to blend between cue/pgm (on the right side, next to the cue select and HP vol)



    EDIT : here's what it looks on the inside as of now :



    The input modules are on the left (mic, phono 1&2, line 1&2), the blue things connected to them are the bus connectors (the two in the front panel are mic on/off and EQ bypass switches).. the summing bus is the yellow "bar" in the middle, next to it is the master output preamp on the right and the output transformers (small aluminium boxes). The green PCB on the far right is the PSU, the blue PCB attached to the front panel is the EQ.

    EDIT 2 : next I'm looking to add the HP amp module (and possibly split cue).. I'm out of hookup wire and short on all kinds of components though, as well as space (both working space and chassis)

    EDIT 3 : also noticed a few wires loose.. they do that because of mechanical stress from opening the front panel and sorting out wires etc

  3. #113
    Added the headphone amp with cue blend and split cue.

    What it does (or is supposed to do) is the cue select goes to the left channel of the amp module and the pgm to the right, then the cue/pgm pot distributes the output between L/R.. with the split engaged the L/R channels are separate so the tip is cue and ring is pgm, and the split disengaged the L/R channels are summed (so the cue/pgm are heard from both cups)

    EDIT : the split turned off (=hp outputs summed) could in turn introduce crosstalk.. the HP amp shouldn't however pass signals from the output to the inputs (the master channel cue from which the pgm is wired from has summing resistors, I could use a couple of diodes but I'm afraid they have a fairly high forward voltage)

  4. #114
    Here's a close-up of the modified monitoring/master section :


  5. #115
    I'm contemplating whether to add an FX loop.. basically I'd break the connection between the master output preamp, put RCA outs around the back and another pair back in via an A/B switch to toggle the input to the volume controls (master&booth) between FX out and in.. the FX out could be used as an additional insert or a rec out when not in use (or pre-fader metering)

    Or just breakout to RCAs and use jumpers between them to close the loop when it's not needed.

    EDIT : I have no need for such feature though since it'd require a stereo FX unit so just a rec out would do.. the catch here is that plugging FX to the master out leaves the booth out unaffected (there was a discussion about this in reddit, the workaround is to plug into a system via a zone mixer that does the level control between separate outputs)

  6. #116
    Moderator pete's Avatar
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    I've got a lot of respect for what you are doing!
    Keep posting on the build.
    bored, curious, deaf or just bad taste in music?
    finally a mix by me
    and what's this, another shoddy mix...another dull mix

  7. #117
    Quote Originally Posted by pete View Post
    I've got a lot of respect for what you are doing!
    Keep posting on the build.
    Thanks!

    I'll try to update as much as I can.. I actually took parts for the mod from the 2-way crossover I built as it was a prototype I put together out of boredom from a 2ch mixer build that had no cue (which in turn was built from surplus parts I had for this mixer)

    I'll probably order some vital parts and finish the mixer, then rebuild the crossover (possibly as 3-way with hi/lo/fullrange, I have no need for one though but they're somehow exciting to build due to the fairly straightforward design) or make a smaller tabletop mixer.

  8. #118
    Moderator pete's Avatar
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    I recently got a valve balanced headphone amp.
    I'd be tempted to slap it into a mixer build if I ever do one (very unlikely).
    Cos, you know, rotary...valves...mixer/audio p0rn.
    bored, curious, deaf or just bad taste in music?
    finally a mix by me
    and what's this, another shoddy mix...another dull mix

  9. #119
    Quote Originally Posted by pete View Post
    I recently got a valve balanced headphone amp.
    I'd be tempted to slap it into a mixer build if I ever do one (very unlikely).
    I wouldn't if I were you.. valve preamplifiers are an audiophile thing imo as they tend to colour the sound, if anything I'd use them for the master out (ICs and discrete opamps are much easier to work with in my experience, don't take my word for it though as I've never used valves for anything, however I've seen the insides of a tube radio and once tried servicing a reel-to-reel deck which had an output buffer/VU indicator tube)

    Valves tend to generate both light and heat.. something you don't necessarily want inside a case. They're also somewhat fragile, so they aren't exactly road-friendly (much sought-after guitar/bass amps have tubes and they're considered sounding warmer but transistor/solid-state devices have largely replaced them, there are valve synthesizers too but they're very rare in my experience)

    Then they need to be constantly serviced and biased which can become expensive in the long run unless you know how to do it yourself (and finding NOS tubes, esp. matched pairs can be difficult)

    I too was considering them for analog VU meter lights but ran out of space.. basically any incandescent/LED would do, with the latter being a better choice as LEDs generally have a longer lifespan (unless you give them too much voltage) and the light bulb filaments/shell can break from moving the unit around.

    There's no doubt valve technology has changed over the years but like stated they're an audiophile/connoiseur curiosity, not a "pro audio" standard (many studios have tube mics/preamps and even valve mixers but for DJing they're too delicate, maybe in an install DAC or something they could be ok)

    Compare it to an electromechanical relay for instance.. designing a logic circuit with them was the only option before the invention of transistor so it's more of a novel reason (they are however still widely used these days in some applications) but solid-state relays and TTL/CMOS technology has superseded them in favour of improved performance and smaller footprint.

    EDIT : but hacking together stuff is always a plus in my book.. if you know how to design something with valves you probably know how to do it with ICs too (I can already tell you building a 2ch line mixer isn't much else than a couple of potentiometers and an output stage, the rest is up to you.. then you just get into the details of impedance matching, crosstalk, summing, THD, SNR etc)

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