12V DC Regulated Power Supply

OK, I want to build myself a decent quality PSU for my DAC to replace the SMPS wall wart. The DAC has a standard "wall wart" style 12VDC input (centre pin is positive). I just want to replace the wall wart, I don't want to start messing about with getting rid of the regs on the DAC.

I was just going to get a Scott Nixon PSU (scroll down the page a bit) but it's fairly pricey and he hasn't responded to my e-mail enquiry so I've decided I'm going to build my own.

Only trouble is I don't really know where to start! I know the PSU basics (toroidal transformer, bridge rectifier, smoothing caps) but how do I regulate the output voltage to 12V? I thought about buying this thing from LC Audio. It's only 53 Euros inc. toroidal. However, it's a dual rail output (+12V, GND, -12V) - can I just use the +12V rail and GND and ignore the other one?

If I just wanted to "roll my own" completely does anyone have a circuit diagram I could use?

Michael.

 

How do you regulate? Easiest thing to do is to use an IC regulator. You can get both switch-mode and linear regulators. A decent 12V reg will run about £5.

I can design one for you... not difficult to do, you already know most of it.

Yes, you can use just one rail of a dual-rail supply.

 

have a look at the avondale audio website les may have some helpful stuff on there. also it may be worth looking into the russ andrews power pak's they are switch mode jobbies and are pretty cheap - 40quid http://www.russandrews.com/product....&customer_id=PAA0866126004998QVNRXVHJDODEIPPG

but you'll need an iec cable too.
cheers


julian

 

why not speak to Andy Weeks about his super regulator.
It comes in a tad above a basic single lm chip, but gives remote sensing and you can set the voltage with 2 resistors.
If you did decide to go for it, i can supply Andy's recomended resistors, i used all the caps tho, but may have a few of the transistors left.
Its a very small pcb and with a toroid and a pair of caps would fit in a quite small box.

 

just use a 7812,about 50p each

 

Cheers for the info guys. I'll probably just get one of those LC Audio boards. Add an IEC socket, case and a captive 12V DC output lead and I'm sorted. Will ask Andy Weeks about the super regs over on PFM too

Michael.

 

The 'SuperReg' would be rather wasted upstream of the internal IC regulators. It's arguable in this case whether you need regulation at all but an LM317 will allow a little bit of design before soldering and a 7812 will let you get straight on with it.

Applying the SuperReg internal to the DAC just for the 1543 chip would be a thing to try.

Paul

 

There are no IC regs in the DAC, but appear to be some discrete regulation circuits.

 

LM323t's **** the 7812's

 

Well it would be nice to know exactly what it is but there's still regulation inside the DAC. FWIW I think the supplied wallwart is an SMPS? Might be worth looking out an alternative just to try.

FWIW you can get 7805/8/9/12 etc in TO92 cases. Perhaps Michael could read what's on the package?

(I think the LM323 is a 5v job so wouldn't be appropriate as a direct replacement for a 7812...)

Paul

 

I won't have a chance to take a proper look at what's on the TO92 cases until the weekend and I'll let you know what they say. The MSN Groups page of the guys is here with various pics of the DAC and the regs.

Surely if the 12V DC input is well regulated then the quality of the 3 regs downstream in the DAC is less critical?

The supplied wallwart is an SMPS, I was hoping to replace it with a linear PSU rather than another wallwart.

Michael.

 

I'd have thought that the internal regulators essentially defined the quality.

I probably have a 12v linear wall wart lying about, but that doesn't really help you...

Paul

 

Paul,

Quite right its a 5v'er, ideal for standard off the shelf upgrades (T220 package) great for that 8412 I believe.

 

Hi Michael.

As others have told you the regulation must be done with a voltage regulator. The quality of the regulator is not very important , because in the box there are local voltage regulators and they are only working as pre-regulators.

Use a EI transformer (Classic iron style) and not a toroidal ,because the former has a much less capacitance between primary and secundary and put less supply in the grounding.In a source component that noise is very important.

Cheers :MILD:

 

TD,

I'm curious as they generate 40 times the EMI of a torrodial. I would agree for space issues & where psu's not too fussy over siting in non senstitive placement areas with delicate circuitry then ideal yes.
Else planning on siting it a good couple of meters away? in a nuke proof box? . depends on the implementation of the regulated supply too, why have regs on the that dac board anyway?(Apart from compactness) possibly this just leads to more contimation of the alreday suseptable circuirty ? . Can't say I noticed any difference when the board was reconfigured, to take this into account .

 

Tony,

for clarification, Tube dude recommends the use of an EI transformer, but you say they generate 40x more noise than a toriodal. Hence your recommendation of a toroidal.

Seem like there is a difference of opinion here on which is best. Or did I miss something?

 

OK, just had a look. The TO92 packages for the two 5V supplies VD+ and VA+ for the CS8412 have "TL431C Y819" and "TL431C Y810" written on them. I just measured the voltage and they were both at 4.98V

The TO92 package supplying the TDA1543 has "431L 885" written on it, the measured voltage there is about 7.4V.

Each of the TO92 packages appears to be part of a group of components which is presumably the discreet regulation circuit. The components are:

- a (non electrolytic) cap (blue case with 104J100V written on it)
- 2 small blue resistors
- 1 big brown resistor
- the TO92 package
- one 47uF electrolytic cap (Black Gates no less).

The only difference in the circuit supplying the TDA1543 is in the value of the resistors (presumably because a different voltage is required).

So, presumably those are discreet regs?

Michael.

 

The TL431 is an integrated 'shunt' regulator. There's a data sheet here, if you're interested.

Paul

 

Thanks for the datasheet Paul. I think I'm going to leave those regulators alone and just stick with making a better quality 12V DC supply.

Michael.

 

An EI transformer has more external magnetic field than a toroidal. It is also bigger for the same VA rating. Neither of these things is important if you are placing the transformer external to the rest of the circuit. As tubedude said, the primary and secondary are less 'connected' for RF [ie less capacitance] so less kak from the outside, [and viceversa] into the psu.

I would put any stiffening components like decoupling capacitors as close to the relevant chips as possible, and regulators not far behind. I cant see the point of regulating something a long way from where its needed, you just add unwanted resistance and inductance with the distance.

IIRC [but a long time back..]

104J100V = 10 x 0000 pF = .1 uF

j = [anybody??] 10%?

100v = 100v