2x12 transformer sufficent for +-12V out?

[DRÄPPEL]

Hi!

I'm about to order some parts for the MBFM project and ran into something that made me scratch my head. For the +/-12V power supply needed to power the TL074s it seems like you need a 2x12V transformer (actually I think it's alright to power them with anything from +-5 to +-18 but i want to power a AOUT module with the same psu)  according to http://www.ucapps.de/mbhp/mbhp_opl3_psu.pdf, but (here comes the confusing part) doesn't the bridge rectifiers and voltage regulators drop the 12v supplied by the transformer to something below 12v? In which case I would need a 2x15V transformer. Is there something I'm missing or am I right?

Thanks

[Synapsys]

Hi Drappel,

When you apply full-wave rectification to an ac signal the dc output level is actually higher than the ac level. The general formula is that the dc output in volts will be 1.414 times the input. Thus you can expect to have about 12*1.414 or 16.97 volts dc feeding the regulators. In actual practice this voltage will be somewhat lower due to the diode drops, you mentioned, as well as other losses in the transformer. A good rule of thumb is that the ac voltage of the transformer secondary should be about 0.8 times the expected unregulated dc output from the bridge.

Hope this helps...

Regards,

Synapsys

[DRÄPPEL]

That explains everything, thank you!