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Planing power for my audio mixer

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q-bertsuit

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Hi,

I have taken out the electronics of an old mixer and Im building a new case and adding some stuff to it. One of the things Im adding is a small screen. Problem is, the mixers adapter gives 10V AC and the screen driver needs 12 DC. Theres a 5 volt regulator on the original board where Im planing to get the power for some other stuff Im adding. I though about just putting the 10VAC adaptor and a 12 V DC adapter inside the mixer and just using a regular power cord to get power from the wall socket. This would work but it would take up alot of space, and it wouldnt really look neat inside. Are there any other solutions? Step up voltage chip or something?

Thanks for taking a look
 

Unless the basic mixer electronics uses a bipolar voltage supply and expects an AC input voltage to generate it, 10V AC can be perfectly substituted by 12 V DC from a wall adapter.
 

Use a standard AC transformer to get from 110/220V to 24Vac, then another (smaller, hand-wound?) transformer to get down to 10Vac. Also, from the 24Vac output, go through a diode bridge rectifier, a ripple cap, then into a linear regulator (LDO) to drop the 20-some volt DC signal down to +12Vdc for your display.

In all cases, you need to get from 110/220 Vac to 10Vac and 12Vdc. It's easiest to convert AC to AC, then AC to DC. It's much more complex to convert DC back into AC, or low voltage DC to a higher voltage DC.
 

What is the mixer? A 10VAC adaptor means there is a rectifier and capacitor inside the mixer. So, if you connect the power the correct way round you can probably use a 12VDC adaptor for both provided it can supply enough current.

Keith.

---------- Post added at 18:18 ---------- Previous post was at 18:16 ----------

There might be an issue with the grounds if you connect between the two units and they aren't isolated. It depends on what is inside the mixer.
 

Thank you all for answering.

Yes, there is a recitifier and capacitors inside the mixer. After that I can see at least a 5V regulator, and two similar (regulators I think, but it might be something else) ICs that are difficult to read whats on. On further inspection I found a point on the PCB that said +12 volt and one that said -12V. Im a bit scared to use these since Im not quite sure what ground to use, or if I even should use it.

In any case, would it be ok to use a 12V DC adapter instead of a 10 VAC, and then steal some power for my screen from it?

Thanks again!

---------- Post added at 21:04 ---------- Previous post was at 21:04 ----------

Sorry forgot to say, its a Numark 1002X two channel mixer

---------- Post added at 21:14 ---------- Previous post was at 21:04 ----------

I bent them back a bit and found out that they are 7912c and 7812c voltage regulators. So i have 5V 12V and -12V. Would it be ok to use the +12V to power the screen?
 

What current does the screen need? What is the current rating for the 10VAC adaptor?

Keith
 

The current rating is 500mA. Im not really sure how much the screen needs, from what I can gather from the data sheet its 20mA, but that seems a bit low dosnt it?
**broken link removed**
 

You have confirmed by guess about a bipolar supply, thus feeding DC instead of AC isn't and option. I'm not completely clear about the cicruit, but if the transformer has two terminals, one terminal would be connected to ground and the other feeding two halfwave rectifiers for +12 and -12V.
 
The datasheet I have found for the display shows 20mA for the backlight and 8.6mA for the display itself. This is using 3.3V for the display and 19.8V for the backlight. That seems to agree with the demo board you are using so I guess you should allow for maybe 50-60mA from the 12V supply (to allow for a switching regulator to generate the higher backlight voltage). That doesn't seem an excessive amount to pull off the mixer power supply. I would hope it wasn't already running within 50mA of its maximum rating.

Keith
 
Thank you very much for your inputs guys! Much appreciated. I'll try hooking it up and hopefully nothing blows up.

I have an unrelated question I thought I'd ask while I'm here; two of the screws on some of the metal casing that covers the PCB have some blue glue under them. It almost feels like plastic. IS there any clever way of getting this off? Why would they use screws AND glue? seems silly to me..
 

It is tricky to say without seeing it. If it is in the screw threads it could be threadlock.

Keith
 

Ok thanks, I'll try just giving it some force and see if it gives. One last question; Is it possible to use some other adapter that would give 10VAC but more amps? What would happen if I used say 10VAC and 1500mA ? Would it blow up?
 

It wouldn't make any difference. The circuitry will only draw what it needs so if it needs 100mA, that is what it will take even if you use a 10A power supply.

Keith.
 
Does that mean you think this is the best approach?

Thanks again

---------- Post added at 13:36 ---------- Previous post was at 13:34 ----------

As apposed to what I mentioned in the first post, about pulling a chord directly from the wall and having two separate adapters inside the mixer casing.
 

I think that, as your display doesn't take much current, stealing a bit of power from the mixer seems reasonable. The only risk is if the extra current makes the 12V regulator overheat. That is easy to check. You could probably add a bit of heatsinking if necessary.

Keith
 

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