DIY rotary DJ mixer

[light-o-matic]

Tested the RIAA preamp through a line pre... couldn't still get a sound.

Right.. well I do not know the specs of your phono preamp.. but it is pretty typical for them to give you an output around -10dBu nominal with a typical cart. That is the level expected by the line inputs on most hifi equipment, and is PLENTY of output such that if you were to plug the output of your phono preamp into the line input of your DJ mixer (the one you have that works.. not the one you are building), or a powered speaker or a stereo receiver aux input.. you would hear it nice and clear, with no problem. If you cannot, then it's not working and adding extra gain will not help you, you need to find the problem and fix it.

As far as gain in general.. you need to decide what nominal level you want coming from each channel into the mix bus, and get all your inputs to give you that level nominal .. for the line inputs that probably means zero gain at the middle position on the gain knob. Or even negative gain, as CDJs for example output 0dBv nominal, whereas your mix bus may be operating at -10. So, once you decide that, then you will know how much gain you need in your master output stage. Your actual mixer section, if you use let's say a 10k pot, then you will want a 100k or higher resistor from each of those into your summing amp, which itself should have a very high input impedance. The higher the summing resistor, the less interaction you will have between channels. If you put an additional buffer stage between each pot and the mix bus, then you can eliminate interaction, but at greater cost.

[efinque]

Right.. well I do not know the specs of your phono preamp.. but it is pretty typical for them to give you an output around -10dBu nominal with a typical cart.

They're 35dB@1Khz which I believe is the standard RIAA frequency.

I haven't analyzed them though.

EDIT : if I understood correctly they should do something like this :



EDIT 2 : I remember there was the Order of Magnitude rotary which was crazy in terms of build and sound quality..

[efinque]

Switched back to the Akai PSU.. one of the input wires shorted the mains and it burnt a 10A fuse.

I tested one of the line preamps with some 30VDC and some smoke came from the circuit but it still works with 12VDC. (the actual output values of the PSU are 38..42V and 12..14,5VDC when tested with a multimeter; the power supply is from an old tape deck)

EDIT : I tested the mic channel too with a line level source and it *should* be functional as well.. I've yet to test it with a dynamic mic though because the ones I have in my studio require a 48V phantom.

[light-o-matic]

Why are you connecting any power supply voltage other than the correct one? If something is designed for 12VDC then connect 12VDC and nothing else. No wonder things aren't working for you!

And.. once something smokes, you can pretty much assume that it's broken.. even if it works.

[efinque]

Well, I re-tested the mic channel with a line source and it doesn't work (I wired the gain pot from the PCB trimmer)... it's not that I need one for home use though but if I make the mixer reliable enough I may throw a gig with it (and it's good practice)

[light-o-matic]

I'm starting to think you're just trolling us.
Why would you test a mic input from a line source? Not that it's likely to destroy anything the way that connecting the wrong voltage power supply or connecting the power supply in the wrong polarity will destroy things.. but it certainly isn't a good test.

If I were you, I'd take every module you've got and test it with the right input connected, the right output load connected, the right power supply connected.. and do nothing else until you have them all working perfectly.

[efinque]

If I were you, I'd take every module you've got and test it with the right input connected, the right output load connected, the right power supply connected.. and do nothing else until you have them all working perfectly.

That'd require a very advanced testing environment (I'm actually soldering and assembling the mixer on a sofa table)

EDIT : if I was to make these for a living I'd probably build a faux test chassis with different input and output connectors and a power supply but as this is a one-off I can't be bothered.

[light-o-matic]

That'd require a very advanced testing environment (I'm actually soldering and assembling the mixer on a sofa table)

False.

It would require:

* A reliable power supply.. which, you NEED ANYWAY since you're building a mixer. So that costs nothing.

* A digital multimeter.

* Some kind of line level monitor.. a cheap computer speaker would do, with some alligator leads. Put a .1 uF capacitor in series with the input to protect it from accidental DC connections.

* Signal Sources: If you're testing a phono preamp, you probably already have a turntable. If you have a mic preamp, you probably have a cheap mic, if you are testing a line stage, you probably have some line level sources. You just need to make a cable to go from whatever you have to the input terminals of whatever you are testing. Again, alligator leads are helpful.

And that's it.. that's all you really NEED.

Now, if you wanted to get "fancy", a signal generator would be helpful, but again you do not need to buy one. It would be useful to own one, but you can get apps for your phone or programs for your computer that do the job. Just, again, solder up a simple stereo mini to bare wire (or alligator leads) cable with a .1uF or so capacitor in series to protect the phone/computer from DC.

So basically, everything you need, will cost you no more than the cost of a few connectors and some wire.

Now, if you wanted to actually have the gear necessary to build something like this with MUCH less guesswork, you could get a full setup.. bench power supply, pocket oscilloscope, and signal generator.. if you are willing to buy from Chinese sellers or used gear, you can have all that for $100-150.. which when you consider that the oscilloscope is also a voltmeter and some of them are full multimeters. It's not a lot of money considering you've already spent at least several hundreds on parts and at the rate you are going you'll be spending a couple hundred more.

EDIT : if I was to make these for a living I'd probably build a faux test chassis with different input and output connectors and a power supply but as this is a one-off I can't be bothered.

Well, it seems to me that your project is going pretty badly and that you have already lost some money on boards that you have damaged. You should add up the cost of what you have spent so far and then ask yourself whether it's worth a little bit of extra time to get the power supply right before you blow it all up.

Just a little advice from someone who's actually built a working mixer or three in my life.

[efinque]

* A reliable power supply.. which, you NEED ANYWAY since you're building a mixer. So that costs nothing.

I'm using wall warts.

* A digital multimeter.

I broke my multimeter.

* Some kind of line level monitor.. a cheap computer speaker would do, with some alligator leads. Put a .1 uF capacitor in series with the input to protect it from accidental DC connections.

I'm running the sources through and back to an Allen & Heath Zed10FX.. I've been using headphones for monitoring the outputs mostly but I did play some material back through studio monitors (Fostex PM1mk2's)

* Signal Sources: If you're testing a phono preamp, you probably already have a turntable. If you have a mic preamp, you probably have a cheap mic, if you are testing a line stage, you probably have some line level sources. You just need to make a cable to go from whatever you have to the input terminals of whatever you are testing. Again, alligator leads are helpful.

I've used Reloop RP-4000m3d's with an Ortofon Concorde Pro S for phono. As for mics I tested T.Bone SC-300 and an AKG C2000B and a line source from the main out of the A&H mixer.

Now, if you wanted to get "fancy", a signal generator would be helpful, but again you do not need to buy one. It would be useful to own one, but you can get apps for your phone or programs for your computer that do the job. Just, again, solder up a simple stereo mini to bare wire (or alligator leads) cable with a .1uF or so capacitor in series to protect the phone/computer from DC.

I've already considered the possibility of using an oscilloscope; I believe there's one at the school I used to study in (at least there used to be, they've rebuilt the whole facility since), however getting a permission to work there is another thing.

[efinque]

Here's a rough circuit diagram/schematic...



Plot made using circuits.io

There may be some stuff that's not 100% correct because I drew it from the top of my head and it may be subject to change in the near future. Also there's some troubleshooting with the modules and overall functionality but I think it's coming along nicely..

EDIT : fixed a few things

EDIT 2 : added a master cue (it's not included in the current build though)