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Thread: DIY rotary DJ mixer

  1. #21
    Quote Originally Posted by light-o-matic View Post
    Looks nice. Why do you think you need a pair of preamplifiers? How many channels does each phono preamp board have?
    And, how exactly are you connecting the potentiometers? I've built mixers and lots of other audio circuits, I can help...
    Well, the phono cards are stereo.. they have an opamp in the circuitry but they let no sound through for one reason or another which is why I'll probably have to put a normal line preamp after them to get some gain (they're 35dB@1Khz whereas the line preamps are 40dB.. the phono input impedance is 47kOhm and the lines output are 1kOhm)

    The gain pots in the front panel are wired from the circuitry trimmers (which are post opamp, the originals in the kit were 1K) to 100K dual log pots so the range is a bit questionable, but due to a number of reasons I can't be bothered to switch them to 1K at the moment. Funny that you mentioned potentiometers; the ALPS ones are actually nice but they're PCB mount (they have no wire holes in them) which means they have to be soldered with care.

    Anyway, I managed to break the headphone amp which has been a lot of trouble, due to desoldering etc.. those kit PCB's are small but they're also prone to all sorts of damage. Actually I was desoldering a LED from the board to the front panel so as of now the monitoring is "kaputt".. I'll try to fix it at some point. (edit : I've also been looking at this as an alternative to the current headphone amp)

    EDIT : I actually put a bigger LED in the headphone amp circuitry with jumper wires.. it's a bit brighter with 6V (the headphone amp board operates at 4,5V) and it's also modified to mono (I was thinking of putting a resistor or two in the summing)

    EDIT 2 : I also applied to an electronics polytechnic.. although I highly doubt I'll make it in this time either (this is the 3rd time I've been taking part in the polytechnic application process) since they're most likely not interested in a hobbyist with a black plastic box with a red LED and a few potentiometers in it.

  2. #22
    If you're getting no output from the phono preamp then you have a problem that needs fixing. Adding another gain stage is not necessary. I suggest testing your preamp in isolation outside the mixer.

  3. #23
    And what I mean by how do you have the potentiometers wired.. I mean exactly what wire from what board to what pin on the potentiometer. And 1K is def too low for this purpose. 10K is generally a safe choice if you aren't sure.

  4. #24
    Quote Originally Posted by light-o-matic View Post
    And what I mean by how do you have the potentiometers wired.. I mean exactly what wire from what board to what pin on the potentiometer. And 1K is def too low for this purpose. 10K is generally a safe choice if you aren't sure.
    I think the 1K is a lot more accurate... however, the line preamp kits have an option to switch one of the resistors to 100K pre opamp to match the impedance... I haven't tested it though.

    Also, there are some quirks. When the unbalanced booth out is plugged the overall noise reduces. The master pot on the other hand has little to no effect on the overall sound level (it's as if it was a worn pot, or an S-taper), whereas turning the booth out pot creates a low-pass filter effect.

    EDIT : 100K log pots have 1K resistance when turned approximately 10% and 10K at about 50%.

    EDIT 2 : Anyway, I didn't see this coming.. I knew there'd be some troubleshooting with the assembly and stuff but I wasn't prepared that I'd damage one of the PCBs.. I guess I should've made my own from the start.

  5. #25
    What do you mean by more accurate?

  6. #26
    Quote Originally Posted by light-o-matic View Post
    What do you mean by more accurate?
    Well, the "usable" resistance range of 0 to 1K which has any real effect on the signal resides in the last 10% of the taper.. the 99K before it are sort of useless. (edit : but it works)

    EDIT : As for the DIY PCB's I did see some sodium persulfate for sale when looking for parts.. but then I'd also need a few sheets of bare PCB and an oven or a UV light box and a lot of time as well as a lot of different components (and other tools)

  7. #27
    Quote Originally Posted by efinque View Post
    Well, the "usable" resistance range of 0 to 1K which has any real effect on the signal resides in the last 10% of the taper.. the 99K before it are sort of useless. (edit : but it works)
    That is because you don't have them wired correctly.

  8. #28
    Quote Originally Posted by light-o-matic View Post
    That is because you don't have them wired correctly.
    Well, at first I had the front panel pots wired in series... which is useful only if one decides to play with the volume pots at 10.

    EDIT : I know that people used to play rotaries with the pots at 6-7 and the rest was the gain..

  9. #29
    The headphone amp works to some extent as of now... I accidentally soldered the LED (which serves as a power on indicator too) the wrong way around

    EDIT : this is because I had the power supply the wrong way around and the + was in the ground; the LED lit up but there was no power in the board itself.

  10. #30

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