DIY rotary DJ mixer

[efinque]

Tested the mic pre... nothing but hum.

I've yet to wire it correctly though (the volume pots are in series, because the trim is a dual log)

EDIT : the mic pre works with line level source.. I guess I'd need a dynamic mic to test it.

EDIT 2 : wired a pot from the mic pre trimmer to the front panel.. haven't tested it yet though (the trimmer was 220k, it's a dual log 100k now because I ordered a stereo pot by accident)

 

[efinque]

Tested the RIAA preamps through the line preamps... didn't work.

EDIT : I mentioned earlier using an Arduino along with an LCD screen as a level meter... however I'd like to keep it classy. I think one option would be to use a separate meter out (maybe with a L/R switch with cue in one and pgm in the other channel) as I have the Omnitronic DB-100 laying around unused.. there's simply no space in the front panel due to the master/booth out placing.

 

[efinque]

Switched back to the Akai power supply... I tested one of the line preamps from a 30VDC output and some smoke came from the PCB but I tested it with 12VDC and it still works.

The headphone amp works too but it's really quiet.

EDIT : the PSU burnt a fuse because the one of the AC wires was loose and it shorted the mains.

 

[efinque]

Any ideas for the front panel finish? I was thinking maybe just a white marker and some lacquer would do... although I did find some printable decals while looking for parts.

EDIT : as for the summing bus it's partially copied from the NYD/Forssell designs, only with 10K resistors on both channels.

 

[efinque]

Aight, here's a revised version of the schematic..

EDIT : plot made using circuits.io

EDIT 2 : there are a few things that are wrong there and I'm too lazy to fix them but you get the idea..

EDIT 3 : fixed.. the headphone amp out/trim was wired incorrectly. I've yet to figure out the phono channels though..

EDIT 4 : fixed the outputs and a few other things.. it's still missing the headphone out grounds and stuff. The mic channel has been tested with a line source only (it worked prior to the trim mod but it's not functional as of now, and it needs a mute switch)

EDIT 5 : added a master cue.. it's not included in the current build though.

EDIT 6 : fixed

EDIT 7 : There's actually a summing resistor in every channel, so in the sense the schematic is incorrect.. I'll upload a revised version later.

 

[efinque]

Anyway, I've been contemplating the power supply and narrowed my choices down to either a) a transformer with 2 secondaries and two 1A linear power supply kits or b) a switched-mode power supply with 5VDC/12..24VDC outputs.

I think option a would be more sound DIY ethic but it has it's risks.. although I assume that linear power supplies are more "cleaner" and a bit more efficient.

The switched-mode PSU I've been looking at has an efficiency of approx. 70% according to the spec sheets but the rest 30% would be converted to thermal energy, ie heat inside the chassis which isn't necessarily a good thing (although I don't think it's going to be a problem)

Cost-wise the SMPS is half of what the linear solution would be though..

EDIT : the EQ module needs 15VDC so I was thinking a SMPS with 5VDC/24VDC and a DC/DC converter to bring it down to 15VDC.

EDIT 2 : the reason for this contemplating is because I ran out of wire, broke my multimeter and burnt a few fuses so there isn't much I can do about this project for now other that clean up the mess that I made...

EDIT 3 : I've been also thinking an external PSU with a SMPS and a linear power supply with a transformer (a hybrid of the two) and a D-Sub connector or similar insulated with bakelite or something because the input boards have been tested with 12VDC, the headphone amp with 4,5V and the EQ module that's on it's way operates at 15VDC... the only problem is that I already mounted the power connector and a switch to the chassis.

 

[efinque]

Also, how would one go on about changing the IC's in the circuit to, say, Burr-Brown opamps?

EDIT : the reason I'm asking this is because now it's basically a black gain box with a red LED and a few potentiometers in it, although it's one worth doing well. With some +40dB gain on the inputs, and a 10K resistor choke in the summing not to mention the Lundahls in the output there's a chance that the signal distorts at the gain stage. Now, a TL072 has a THD of 0.003% whereas Burr-Brown OPA2134 has a THD of whopping 0.00008% so there is a theoretical difference between the two, but I believe the problem comes with the operating voltages..

EDIT 2 : I also applied to an electronics vocational school.. and a polytechnic in digital systems & design (but this thing has little to do with it..)

 

[efinque]

Some of you might find it odd that a digi-native like me is building a mixer with a circuitry design that dates back to the 70's.. it's a one part sentimental build but I think also an introduction to the very basics of mixer design learning curve-wise.

Anyway, I'm eagerly waiting for the EQ kit to arrive, meanwhile I'm sourcing some parts for the PSU and cleaning up the wiring and the chassis to house the module and the power supply.

EDIT : if I did get the RIAA preamps working I guess I should also mod a potentiometer in the circuit... any ideas how to accomplish that?

 

[efinque]

I just had a look at the original Urei 1620 schematics... 47K summing resistors? 61dB gain in the phono amps? (edit : and only 20dB gain in the auxiliaries... although I must admit it's an old design and line sources were probably a bit different back then)

Although they're very clearly laid out.

 

[DBenjolin]

Hello GroupDIY,

and excuse me for being new here.

Anyway, I've been dreaming of making a rotary DJ mixer for about 10 years now. I've searched the web and doodled a bit on paper but I came here asking for some additional advice.

As for some background ; I've been DJing since 2006 and played my first gig in 2007. I've owned and used several mixers over the course of years as well as serviced and repaired them so I have some knowledge on how they work. (edit : basically the design is something along the lines of Bozak/Urei and the boutique stuff like E&S)

Here's the initial sketch of how it would look :

(edit : the width is the standard 19")

Basically, I'd build the preamps from kits as modules with wiring as well as the (optional) isolator and level meter and the headphone amplifier.

For potentiometers I was thinking ALPS 50K stereo log and Alpha 100K dual log for trim controls. (edit : then there are Bourns pots also which are excellent from what I've read) As for the cue it'd be a rotary switch.

I found a 3-way linear phase filter/isolator kit from Analog Metric

I'm a little concerned about the PSU... I've been looking at one that's specs state 30VA, 230V AC / 18V + 18V / 0.833A + 0.833A but I don't know if it packs enough punch to power the whole thing (preamps/headphone amp + isolator)

Another thing is balanced outputs...

What are your thoughts?

-ef

EDIT : as for the isolator it'd be post-master (possibly bypassable)

EDIT 2 : I was also thinking Sifam VU meters but they're pricey albeit very classy looking.

EDIT 3 : Sorry I missed the headphone level control and cue. Updated

EDIT 4 : as some of you might have noticed I'm obsessed with disco stuff.. I've been designing and building a classic dance stack (the "finished" stack is on page 11 but I haven't worked on it since apart from making a driver hole in one of the scoops)

EDIT 5 : Stuff I already have/can make use of : soldering iron, multimeter, a bit of free time... I also have an Arduino Uno board and some components too. Financially speaking it wouldn't be a big deal I guess, back in high school when I had the idea I didn't have any resources to begin with... now there's a slight chance of making one.

EDIT 6 : Sorry for the messy picture but I hope you get the idea.

EDIT 7 : I've also been scheming a studio mixer on Arduino forums.. I guess it'd be better to start from a project of this scale.

EDIT 8 : I'm not an avid DIY-er when it comes to electronics.. I built a radio back in school but that's about it. I did pull a headphone amp from a Presonus FP10 audio interface to my Mackie D4 because I broke a plug inside the connector and couldn't get it out in one piece without damaging the terminal... it works like a charm. I've also been thinking of servicing my Formula Sound PM-90... I asked the company for spares and the costs would've been astronomical, and seeing the mixer was installed at a venue back then I postponed it..

EDIT 9 : I'd guess some would prefer pan controls instead of gain.. which is something I've also considered as it is a rotary. One thing I didn't consider was a cue/pgm switch.

I have purchased a set of PCBs to do the Bozak mixer and could get hold of the gerbers, etc. to do more PCBs if you are interested, but we need at least 10 people in order to get a decent price for them.

 

[efinque]

Here's the tone control module assembled.. (edit : I screwed up because I had to desolder some of the caps without using a pump so they're a bit off I guess, however I may have to change the two..)

EDIT : those are Dale resistors and Wima & Nichicon caps.. I've yet to test it though. The opamps are TDA2320A.

EDIT 2 : tested with 12VDC although it's designed to operate at 15VDC.. the sound comes through really distorted. I don't have a suitable PSU for the module as of now and there isn't much I can do about mounting it in the chassis because of breakout wiring for the pots (I ran out of wire while doing the master channel..)

EDIT 3 : and the reason for the breakouts is the crowded front panel.. I can barely fit the pots in there, but the module has to be mounted somewhere else. Another reason is that and the knobs are too wide for the current PCB layout.. and for the wiring; I've already peeled some 10m of audio cable so there's about 20m in total.

EDIT 4 : then there's the Integral Sound SDX 3700.. I don't know about the price but it hurts to think.. about the EQ-module; it actually has 220nF caps where there should be 4.7nF ones.. I don't know if it affects the overall performance as I've yet to test it with the correct voltage.

 

[efinque]

Here's the *almost* finished build with the EQ module..

Sorry for the crappy quality webcam photo..

EDIT : the EQ knobs are a bit wonky but oh well.. they're aluminium btw because I thought they'd look cool.

EDIT 2 : and I've yet to wire the damn thing, among a few other problems..

EDIT 3 : some would argue that the tone controls should be the other way around (lo, mid, hi) but that's because I flipped the PCB over so that it fits there... this could be done with the breakouts so it's not completely ruined though (I think I should've spaced them more wider apart like in the picture, however that wouldn't leave any room if I choose to add phono/line switches to the first two stereo channels)

EDIT 4 : one way would be to put smaller knobs in there too..

 

[efinque]

Aight, ordered a new PSU and some other stuff that I thought might be useful.

EDIT : also a few spare line preamps and a desoldering pump.

 

[efinque]

It works to some extent with the reworked SMPS/linear PSU combination..

EDIT : tested the mic pre with 15VDC (the PCB states 12VDC but the tech specs are 10...30VDC) with a line level source as I don't have a dynamic mic.. I've also yet to test the phono channels and the EQ and wire the power supply..

 

[efinque]

Here's a picture of the insides with the new SMPS.. there's the linear PSU module (edit : it was prebuilt to my surprise.. I thought it was a kit) behind it.

EDIT : I've also been thinking of an isolation transformer in the AC input... there's no room for one though, maybe outside the chassis but it'd have to be insulated somehow.

EDIT 2 : tested with all the modules except the EQ.. works but there's something wrong with the headphone amp, plus I think I damaged one of the line preamps in the transformer frying incident (the trafo still works to my surprise, one of the secondaries shows ~6,7V and the other is kaputt)

EDIT 3 : I also ordered a small black plastic box in case I make the PSU external.. it could house an isolation transformer but there's a 630mA fuse in the 3 pin IEC socket (I took off the yellow-green pin though)

EDIT 4 : soldered an on/off-switch in the mic channel too..

 

[efinque]

Here's a picture of the back panel...

EDIT : and the updated front panel.. it's still missing the EQ knobs and some functionality but I think it's coming together nicely

 

[efinque]

One thing I noticed (apart from the outputs being sort of quiet) is a LOUD hum from the outputs when nothing is connected to the inputs. Could it be prevented with a loop connector of some sort?

EDIT : now it's nothing but hum.. I've also been thinking of pulling out every module and testing them individually, although it'd take a while.

EDIT 2 : I need to add the EQ module to the schematic.. I actually have one version in circuits.io which has it labeled but I'll tidy it up later.

 

[efinque]

Aight the line preamps should be here later this week... I don't know whether to wire them to the phono channels (with an a/b switch) or use them to drive the outputs (the phono channels don't work, at least with my turntables.. a line signal passes through however)

The problem is the mixer is really quiet... I checked the signal paths with a multimeter so it shouldn't be an interference/crosstalk issue.

EDIT : I think it's the EQ module.. I tried measuring the voltage but the meter didn't show any readings. I set the voltage from the linear PSU to around 14,8VDC and ran the wires in series starting from the mic channel to the phonos and the line channels, and from there to the EQ (ie. minus to plus and so forth) so the load could be too much for the power supply (the idle current draw of an input module is 5mA according to the spec sheets)

EDIT 2 : then I proceeded to adjust the voltage to 15,8VDC which yielded a nice hum from the outputs so I guess there's something happening

 

[efinque]

Wired the modules in parallel.. no difference whatsoever. I actually measured some of the voltages when wired in series and the first module (mic) showed ~18VDC and the next one was ~15VDC, the third one didn't show any readings.

Running the RCA input plugs via the ground goes through nicely though.

EDIT : there's a significant advantage in running the modules in parallel; if one of them dies the rest still continue to work.

EDIT 2 : added a preamp post-EQ... the signal is louder now but so is the noise floor.. it's probably because of the SMPS, and I don't currently have a suitable transformer to make a linear PSU

 

[efinque]

Wired the Lundahls in serial-serial.. no noticeable difference whatsoever.