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Old 16th November 2010, 06:37 PM   #1
laki101 is offline laki101  Serbia
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Default diy dj mixer

so cause of the price of a off the shelf mixer and the fun of making something by yourself i want to make a diy dj(sort of) mixer

so what I need is 4 stereo inputs from a PC or cd player or whatever whit volume, bass-trable control, a cross-fader between the 2 channels and 1 mic input and it all goes to 1 stereo output that goes to my amp and finally to the speakers

Click the image to open in full size. i found this and I was thinking of making one of them per channel, Click the image to open in full size. using this mic amp (now can somebody tell me how to wire this cause i cant figure out what is a XLR pin) and then combining them into one with this Click the image to open in full size.

now what I also need is a cross fader with one extra function and that is chanall select so you have i dial to select one of the 4 channels on one side, and the other and than having the fader fade between those 2 channels

now I have an idea how to make the channel selector with a simple 4 position dial-switch but have no idea how to make the fader itself

and if you think I forgot something please tell me and do you think it could work like this
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Old 17th November 2010, 03:28 AM   #2
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Disco mixers sold here usually have RCA jacks for the turntables and a 1/4 phone jack for the mike. No phantom power for the mike, so they assume dynamic mike. RCA jacks have the ring connected to the case unless mounted on a plastic board, so the two turntable inputs send the shield of the cables out in two divergeant directions, plus the sheild on the cable to the CD player or radio, so it makes a really big ground loop and can pick up a lot of hum. XLR connectors sold at newark.com, Farnell United Kingdom | world-leading distributor of electronic and maintenance, repair and operations products. (I didn't type the ad, that came from them, but they are closer to you than North Carolina) and elsewhere, have 3 pins and a shell. You can use twisted pair with shield cable from your mag phono cartridges on XLR 3&2, with the shield only connected at the turntable end to 1 and the headshell of the cartridge, and carrying no current. Using T.P. really cuts the area of the ground loop, and if you also hook 3 & 2 xlr to the + and - inputs of the op amp in a balanced gain fashion, causes low hum current coming in, which, because it is common mode to the op amp, really cuts the gain on the hum.
My RA88a has high gain on the dual mag phono inputs, gain of 2 on the cd player input, and high gain again on the mono mike input. The outputs of the various input op amps are run through slider pots to ground (middle of +- voltages) and the resulting 4 signals are run into 10 k resistors all into one input of the last op amp, a low gain setup, that boosts them back up and out the rca jack to the power amp.
Your second drawing shows the mixer amp plus a bass trebel cut op amp mine doesn't have, then out. the lower drawings would go into that.
PA mixers tend to have 1/4 phone for midgain inputs like guitar amps, and xlr on the same op amp with more gain for mikes, plus DC phantom switchable power on the xlr's. They have no RIAA roll off on any of the inputs, so they have the gain for mag phono but are useless for it. All the disco mixers and PA mixers I have looked at schematic of have noisy 4558 or worse op amps, so you are starting off at an advantage for quiet sound with TL072. I'm upgrading a $15 disco mixer for a hub for my home hi-fi. It has superb sliders, and low noise op amps have tamed the hiss to acceptable levels, but I'm still chasing the hum. This week I'm trying a pi filter ahead of the 1000 uf power supply capacitors, which as bought come right off the rectifiers from the C.T. power transformer. If pi filter is not enough, next step-drilling the PCB and installing regulators. Not low hum enough, next step, ungrounding all the RCA inputs from the case, if I have to, requires sawing the case open and putting a plastic board and new RCA jacks in.
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Last edited by indianajo; 17th November 2010 at 03:45 AM.
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Old 17th November 2010, 12:17 PM   #3
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Too late to edit. Correction: The third drawing above shows the mixer op amp with the 4 resistor inputs , volume and to the input of the first drawing. You don't need a 4 to 1 switch. The second drawing shows the mike input amp with a tone control after it, which is odd. My disco mixer on mike has pretty heavy treble cut standard, also bass roll off, as the mike on disco mixer is for talking, not music. It also has a switch that shorts the other 4 input resistors through a master resistor to ground to attenuate the music while the DJ is talking. If you don't need the tone controls and if TL072 has enough drive, you could change the output resistor and pot of the third drawing to 2 k or so and run it out to the power amp, if it is close (6' or less) and has 100k input resistance. My disco mixer is missing the 1st drawing stage entirely.
You don't have a drawing of a mag phono input stage, because there aren't enough capacitors on any input for bass boost and high filter.There are 3 more inputs to the 4 resistors in the third drawing besides the mike stage shown, 2 mag phono, one CD player, . I'd post the input of the Herald RA88a that I traced out, but I can't figure out how to use this sun drawing freeware I have to make .jpg drawing. Sorry.
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Last edited by indianajo; 17th November 2010 at 12:24 PM.
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