AC to DC question

impulz2002

Member
2004-09-26 7:51 pm
NL
Hello,

Sorry for this n00b question, I looked several places here but couldn't find the complete answer to my question.

I have a nos-dac which operates on a 12v DC simple transformer. I'm not sure if this is the maximum.

Now I would like to buy a better power supply with a larger transformer and some diodes and some caps.

If i'm right, when you convert ac to dc, the ac rates multiplies by 1,4.

So if I need 12v DC should I, for example, need a 9v AC or can I also buy a 12v AC which than goes beyond the 12v DC.

By the way, the chip is a TDA1543.

I also read somewhere I could use a linear DC voltage regulator, but is this good/bad for the sound? I would like to do it only with diodes and cap's as for example also with the gainclones.

Thank you
 

Bobken

Member
2002-12-23 11:22 pm
UK
Hi,

You are generally thinking along the right lines here.

When using a full-wave bridge rectifier arrangement, the peak DC voltage output will be 1.414 times the AC voltage input to the rectifiers, but this does need to be reduced by 2x the rectifier 'losses' to give the final voltage level for the circuit.
This will depend upon the rectifiers used, but as another generalisation, 0.6V would be average here for each diode, so the final output from such an arrangement would be approx. 1.2 volts less than the calculated 1.4-ish times the AC from the transformer's secondaries.

There are other matters to take into account though, such as the actual load compared with the regulation factor of the transformer, and if the transformer is only lightly loaded, you will very likely end up with a higher final result. Similarly, your local mains voltage may not be the same as the transformer primaries are specified for, and this can easily increase or reduce the overall outputs by 10% or so.

If you intend to use a regulator which will (in every similar case I have experienced) be a substantial improvement, you need to allow some 'headroom' because regulators 'drop' some voltage (this value depends upon the type of reg used) during operation, so for say a regulated output of 12V, the required input voltage will probably need to be at least 15V DC in.

This is a little involved, but not too complicated, matter if you wish to get the best results out of your trials, and I am sure that there is a lot of information available if you do some more-specific searching on these topics. You really should make up your mind which way you intend to go before purchasing anything, or you could end up with either too much, or inadequate voltages for your intended purposes.

For the chip which you mention, for example I use 10V transformer secondaries which when regulated down, will provide something in excess of 9V DC max., which is slightly beyond the safe operating voltage for these TDA chips anyway. I believe that Peter Daniel recommends and sells 8V secondary transformers with his kits, so something in the region of 9V secondaries should be a reasonable ball-park figure here. However, if you end up with too little headroom for your chosen regulator, it won't regulate properly, and if you end up with excessively high voltages, there will be more heat to dissipate which the regs might not handle happily.

I hope that this helps, but some more reading and thought first, will be of undoubted assistance to you in this connection.

Regards,