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Thread: DIY rotary DJ mixer

  1. #91
    Well, I modified the other phono channel too.. I was thinking 1k..4k7 first but the initial configuration had 10k resistors in the summing bus so I knew it'd go through, and headphones aren't quite like amplifier/loudspeaker stages impedance-wise (ie. you get away with lower signal levels because of shorter wire lengths and smaller loads)

    It's missing the headphone amp though which I guess is a must in loud environments and with small impedance headphones (ie. large diaphgram drivers which require more power) as well as it could duck the main pgm levels when switching to cue as the PFL now is driven by the channel preamps alone.

    I'll modify the line channels today probably.. other thing I fixed was the mic volume pot as it was connected to left channel only in the wiper.

  2. #92
    Did the same 10k+10k resistor trick for the line channels' mono sum to PFL.. haven't tested it yet though.

    EDIT : this means it's virtually "stereo" I guess.. I think it could benefit from a mono/stereo switch in the output too. I've yet to sort out the HP amp as well.

    EDIT 2 : also now in theory the mic channel on and at full volume forms the mono sum as there are only the 220ohm summing bus resistors as a shunt load between L/R.. this can be prevented by keeping the mic volume pot at 10k (I personally don't use the mic that much but I consider it an integral part of a DJ mixer so I included it in the build)

  3. #93
    220 ohms is far too low.
    As I said, it is all about keeping the output impedance low before the summing resistors and high at the input to the next stage. So if you have a summing/output amp module.. that most likely has a high input impedance, you want ALL the summing resistors to have a high impedance as well. If even one of them is low it results in loss on all other channels and ruins the whole thing.

    How is your mic pot wired in?

  4. #94
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    Can you scan and post the schematic? That would be most helpful and I'm curious.

  5. #95
    Quote Originally Posted by light-o-matic View Post
    How is your mic pot wired in?
    Preamp out to wiper sum and separate output to L/R in the summing bus (it's a dual gang pot)

    It's not fully stereo yet though, as the EQ module mono sums the signal in the PFL. I'll look into it tomorrow, I noticed it because I didn't label the back panel connections and began sorting them out.

  6. #96
    Quote Originally Posted by SWS Productions View Post
    Can you scan and post the schematic? That would be most helpful and I'm curious.
    I have an up-to-date schematic in which I've been adding minor fixes as I'm sorting it out.

    I can scan it at some point once I get the master PFL from EQ/bypass fixed (I was to some extent aware of this back in 2018 when I was putting it together)

  7. #97
    Quote Originally Posted by efinque View Post
    Preamp out to wiper sum and separate output to L/R in the summing bus (it's a dual gang pot)

    It's not fully stereo yet though, as the EQ module mono sums the signal in the PFL. I'll look into it tomorrow, I noticed it because I didn't label the back panel connections and began sorting them out.
    Ok.. so I'm not clear on how you have it...
    If you have the signal from the mic preamp going to the wiper, that's not right.
    You want the signal from the preamp to go to the top of the pot, the bottom of the pot goes to ground. Then the wiper goes to the summing resistors (which, has to be pretty high).

    Since you have a dual-gang pot, you could put the mic signal to the top of both gangs, and then 2 summing resistors.. one from the wiper for each gang.
    BUT, you can do it with just a single gang or using just one gang of the pot, and take both resistors from a single wiper.. and just don't connect the other gang.

    Your summing resistor should be at least 10x the resistance of your pot, more is better.
    If your summing resistance is too low relative to the pot, then you will get not just leakage between channels, but also, you will get interaction between channel volume (turning down the volume on one input will tend to also bring the level down on other inputs).

  8. #98
    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)

  9. #99
    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.

  10. #100
    The current build is also missing a safety ground.. this is due to the IEC connector which melted during soldering and the prong fell off.

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