DIY rotary DJ mixer

[efinque]

Finished the rest of the channels and also got the summing stage in place

 

[efinque]

Well... I powered it up, first using a wall wart for one of the line channels. Lots of hum but I did hear something coming through though.

Then I switched to the transformer.. I didn't have a fuse for the power socket but I tried anyway. After powering it up a few sparks flew from the trafo and it blew one of the fuses from the fuse panel.

 

[kooma]

how was it connected?
do you have any kind of schematics for those modules/your psu/etc?

 

[efinque]

how was it connected?
do you have any kind of schematics for those modules/your psu/etc?

The normal way I guess.. from ground to switch and hot through a fuse box. (edit : I had the transformer wired the wrong way, the spec sheet states brown/blue but I had it wired red/black which is the secondary)

There was no fuse though but it still went through. I guess the transformer is fried (the plastic covering the copper wiring melted), I have another one from an old Akai tape deck which is 25V but I don't know if the preamps can handle it.

 

[kooma]

I'm sorry, but those pictures are quite blurry
and since I dont know anything about those modules(apart from some of the being velleman)
and since there 's no schematics it's really hard to  know what you are doing.

what kind of power/psu/regulation are you using?

 

[efinque]

I'm sorry, but those pictures are quite blurry
and since I dont know anything about those modules(apart from some of the being velleman)
and since there 's no schematics it's really hard to  know what you are doing.

what kind of power/psu/regulation are you using?

It's a toroidal 230V 50Hz/2x15V/15VA.. I'll probably just buy a new one and tidy up those wires meanwhile. I have an EQ module en route from China which should be here in a few weeks.

 

[kooma]

It's a toroidal 230V 50Hz/2x15V/15VA.. I'll probably just buy a new one and tidy up those wires meanwhile. I have an EQ module en route from China which should be here in a few weeks.

how do you regulate/rectify it or where do you connect that?
do you need bipolar transformer?
if example your RIAA-module is this;
https://www.velleman.eu/products/view/?id=8985
you can power it with regulated 10-30Vdc.

and on safety note; if you dont have fuse in place you shouldnt have sparks flying.. everything is grounded ok(safety ground)
and you have your fuse in 230v side?

 

[efinque]

how do you regulate/rectify it or where do you connect that?
do you need bipolar transformer?
if example your RIAA-module is this;
https://www.velleman.eu/products/view/?id=8985
you can power it with regulated 10-30Vdc.

and on safety note; if you dont have fuse in place you shouldnt have sparks flying.. everything is grounded ok(safety ground)
and you have your fuse in 230v side?

I have a 250V/630mA fuse inside and the secondaries show some voltage readings on a multimeter but I think it's gone for good.

There's no space for a PSU except for an external one. (edit : I ordered a SMPS which I think fits there.. although it may cause interference with the opamps)

EDIT : and I've yet to wire the cue select switch, the headphone amp and the booth out.. there's about 10m wire inside the case in total.

 

[kooma]

are you feeding AC to your modules?

 

[efinque]

are you feeding AC to your modules?

I guess..

I tried with an old Macbook PSU that I had laying around around which is 16,5V but it's still nothing but hum.

EDIT : The plan was to wire all the modules in series btw.

 

[KrIVIUM2323]

Hi,
What your saying is that you sent the output of psu transformer to circuit board directly (without rectifier or capacitors between)?!
And all that without fuse on primary side of psu transformer?

If that is that, please do stop anything mains related and try to find someone (with experience or ability to do that kind of things) which could help you with that part of your circuit/project.

I don t want to sound rude but you are seriously threaten your own health (and the one of your family if they live in the same place as where you are doing your experiments). If something bad happen you could even KILL yourself. And i don t think this is your goal.

Except your safety issues (which should be your main concern!), you should try to see the basis of electronics if you try to power a circuit using non rectified and filtered b+ ( i am not even talking about regulation). You probably fried everything connected to your 'psu' during your rush.

Seriously, take a rest about your project go reread some basic books or article then go back to what you want to do... it is not worth injury (or kill yourself, or burn your home...).

And please don t ever try some tube circuits until you have a great understanding of safety when playing with electricity! I regret to have talked about that kind of circuits previously.

 

[efinque]

So would a DC PCB transformer be a better solution that a toroidal one?

 

[KrIVIUM2323]

Efinque, it seems obvious you don t have some basic knowledge about electronic.

I repeat, it is DANGEROUS TO DO MAINS RELATED MANIPULATION WITHOUT UNDERSTANDING WHAT IT IMPLIES FOR YOUR OWN SECURITY.

So please get in touch with someone which knows what he/she is doing to help you.

Nevertheles we all started to learn one way or another so... make a search about 'non regulated linear power supply', then once you did understand what it is all about study some schematic using regulator, or find datasheet of an lm7815 or lm317 and check the typical application.
You should find video explantion on youtube easily.

But i repeat, don t do things mains related without supervising of someone with understanding and knowledge about security.

And remember that if you make your house burn or injury badly yourself your insurance may not cover anything if you didn t follow basic safety rules.

You can hurt you badly mate, don t do something foolish!

To answer your question, a toroidal transformer is only a type of transformer, by itself alone it has nothing to do with dc... you need other components to convert ac to dc. This whole circuit including transformer is called a psu (a linear psu).

 

[kooma]

But i repeat, don t do things mains related without supervising of someone with understanding and knowledge about security.

+1 to this!
If I were you I would step back and take a minute to think.
I like to draw things before I build anything, so I would make some sort of drawing -marking all the modules
you are planning on using and what connects to what.
If you list the modules you can then look  what kind of power you need;
looks like most of those velleman kits only need 10-30vdc, so you could power them with wallwart/smps/something where you dont need to touch 230v.

And please take warnings in line voltage seriously!make sure you know what you are connecting where and triple check that everything is ok. but for now, just figure out what you need!

Mixer projects may seem simple and jolly, but there is actually lot to think about.
as an attachment is kind of drawing where I would start working on.

 

[KrIVIUM2323]

If I were you I would step back and take a minute to think.
I like to draw things before I build anything, so I would make some sort of drawing -marking all the modules
you are planning on using and what connects to what.

I agree, for a project this big (yes it is a relatively 'big' project with many circuits implied, probably with different needs for psu) you need to proceed with order and already thought electrical/connection layout and requirements.

What kooma propose is the way to go imho.

Do a detailed diagram of each modules/parts of your mixer and how you want them to be organised, with audio in/out and an other one with psu. And keep this posted in front of you to have a quick way to look at it.

From then, you could start to test each module individually (and not already mounted in the rack, this will help you to make progress being sure everything works before linking everything together and don't know where to find fault if they occurs).

If you list the modules you can then look  what kind of power you need;
looks like most of those velleman kits only need 10-30vdc, so you could power them with wallwart/smps/something where you dont need to touch 230v.

Yes! And if you want help from here it make sense to have a list of all the module you use here for others to have a look at it and give you advice/idea.

I do think kooma is right, a self contained ac/dc wallwart to do prototyping is the way to go from now.
It will give you opportunity to go step by step into your project and meanwhile you study and learn what is needed for a psu, then you'll end up knowing what you REALLY need in term of power/voltage requirements and how to achieve this.
About that i do think you'll end up using something external to your rack for the psu. I know this is not what you want but this have a lot of advantage and given the number of circuits you are going to use this will be the easiest way imho. And aas you plan to use it at home mostly this isn't going to be too much of an issue anyway.

 

[efinque]

Ok, I've been iterating the problem areas..

And I did get a clean sound through it with a wall wart using one module and a single pot through the Lundahl output trafos, but that was one channel only.

I've also been thinking of giving up on the upper "gain" knobs since they don't do much else than act as an additional resistor in the circuit.. I could mount them on the back though and get linear pots for panning. (edit : as I remember reading somewhere that logs aren't good for that type of stuff..)

 

[KrIVIUM2323]

Efinque,
happy to know you decided to play safe and forget the main/line experimentation for now...

Could you make a sketch of what you have done?

Because from what i understand you used an riaa preamp to drive a pot which drive a transformer.
You'll have sound but this is far from ideal... not enough overall gain to drive line output, not enough current too. At least you'll need a buffer stage before the transformer but even with that i doubt this would work as expected...

Usually an riaa for MM cartdrige (moving magnet, the one used by  most dj) will give you something in the 40/45db gain range.
A more or less standard MM dj cartdrige will output something around 5mv (0.005 volt).
With only an RIAA max output will be around -4 to 0 dbu max output (and so without headroom: 0.5 to 0.77 volt!).

I've also been thinking of giving up on the upper "gain" knobs since they don't do much else than act as an additional resistor in the circuit..

Have you ever studied a mixer schematic?
Because... to have gain in a circuit (beside the use of a step up transformer but forget that for a moment) you need an active stage (something with a transistor, an opamp or a tube) to provide the needed gain.

The pot you want to discard is usually placed after the riaa module and before or around* an opamp to give something around 20db gain before going to (eq or in your case a) fader.

*: before or after will depend of the kind of topology used for the gain stage for opamp.

Take a look at the attached schematic for example:

for the 4th chanel (the upper most on the schematic with phono4 input) you have tl072 ic113 1/2 which is the Riaa preamp from which the output is send to tl072 ic114 1/2 which is the gain stage which will drive the eq part which in turn drive the fader (well in this case it don't drive a fader but a vca but let's assume this is a fader for easier understanding of the principle).

The pot r116 1/2 is a dual 50k pot which determine the gain of this stage. This is the pot you want to discard and as you can see this is not passive and it needs gain...

 

[efinque]

Have you ever studied a mixer schematic?
Because... to have gain in a circuit (beside the use of a step up transformer but forget that for a moment) you need an active stage (something with a transistor, an opamp or a tube) to provide the needed gain.

I've looked at the old Bozak CMA-10-2DL schematics, and Formula Sound PM-90 (after contacting them about spares I was emailed a quote and the schematics)

The pot you want to discard is usually placed after the riaa module and before or around* an opamp to give something around 20db gain before going to (eq or in your case a) fader.

*: before or after will depend of the kind of topology used for the gain stage for opamp.

Take a look at the attached schematic for example:

for the 4th chanel (the upper most on the schematic with phono4 input) you have tl072 ic113 1/2 which is the Riaa preamp from which the output is send to tl072 ic114 1/2 which is the gain stage which will drive the eq part which in turn drive the fader (well in this case it don't drive a fader but a vca but let's assume this is a fader for easier understanding of the principle).

The pot r116 1/2 is a dual 50k pot which determine the gain of this stage. This is the pot you want to discard and as you can see this is not passive and it needs gain...

The "first" pot is for ease of use.. I don't use a "real" gain pot apart from the line modules (+40dB) having trimmers, the RIAA modules are around +37dB (edit : 35dB@1kHz) according to specs.

I just modified a PSU from an old tape deck that outputs 18VDC and a 12VDC, but in contrast it's huge.

 

[efinque]

Aight.. here's a (edit : rough) schematic of what's in the box atm.

Plot made using circuits.io

EDIT : except for the PSU side of things, that is. And it's missing the booth out and the headphone section too but I can't be bothered to finish/implement them when there are already so many problems to solve.

EDIT 2 : The "clean" sound I got from it had a nice low frequency response (which I didn't expect as the line preamps are 40-20kHz) although I have no frequency analysis data to back up the observation; ie it's based on my hearing (which could be getting worse but I don't usually use EQ to correct while listening) and knowledge of the equipment I've used for years. Of course the EQ module will bring it's own colouration and problems along with it, if there's any space in the chassis anymore..

EDIT 3 : The 2nd pot in the chain is actually an ALPS RK27 100K (it should say stereo log) except in the mic channel they're both Alpha because they're a lot cheaper.

 

[KrIVIUM2323]

I don't use a "real" gain pot apart from the line modules (+40dB) having trimmers, the RIAA modules are around +37dB according to specs.

The riaa module is fixed gain. In practice you may need more output voltage (gain) to have a proper SNR in the whole circuit.

+40db for line in? This is way too much gain for typical line in!
Please define what YOU call line (if possible reference level, if you don't know which kind of gear you'll be using).

As you can see from the schematic i posted typically in 'modern' mixer there is no gain in line in, this goes to a gain stage which is common to both riaa and line (you switch between one or the other. In the Ecler schematic this is S116 in chanel 4).

I just found the bozak schematic you talk about, will have a look at it but not at the moment.

About your schematic:
_ if you want other to comment please follow rules for drawing a schematic (example:  label components clearly  like "POT1 Level 1/2  dual 50k log"  in the schematic, use the posted Ecler schemo as a guideline, for clarity don't draw psu line in the same page, make a page dedicated to that only)
_ there is multiple things that won't work or does nothing...

First except the first pot in line after each module all others don't do anything are are drawn in such a way i can't figure what they are doing, maybe other can but sorry drawn that way i can't read it...

The transformer are not wired as they should and you do NEED an active stage to drive them.