Electronic – Use AC-DC adapter instead of AC-AC adapter

actransformer

I'm building an O2 amp, which according to the designer requires an AC-AC adapter (Triad WAU 12-200) to convert regular wall power to 12VAC. By looking at the schematic for the amp however, it seems that I might be able to use a regular AC-DC converter (also 12V) instead.

According to this site, an AC-DC adapter uses a transformer to stop down the voltage, rectifier diodes to make the current DC, and caps/regulators to smoothen it. Now, the datasheet for the WAU 12-200 shows the following as the adapter's schematic:

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(Credit: Triad)

It would seem that the WAU 12-200 merely acts as a simple transformer. Meanwhile, the following equipment is present inside the O2 amp:

enter image description here

(Credit: NwAvGuy)

After entering the O2, the AC power goes through a set of diodes and caps/regulators just like what would happen in a AC-DC adapter. Not that I don't plan to use batteries in the amp at all (strictly wall powered) so the "battery power" section of the amp in the schematic should be omitted.

I've got a 12V AC-DC adapter that delivers 200mA (same as the WAU 12-200). Is it possible for me to omit the diodes and caps on the O2 board and feed it 12VDC power instead?

Best Answer

First of all the 7812 (positive voltage) and 7912 (negative voltage) regulators need about 2v higher input than the output in order to operate properly (that is at least 14v for the 12v regulator and -14v for the -12v regulator) .

The AC voltage in the input is rectified by two diodes to generate a symmetric unregulated supply (that is a positive and a negative voltage with reference to the ground) which feeds the regulators inputs and you get +12v and -12v regulated rails at the output. If you feed the input with 12v DC there is no way for the circuit to generate the negative rail voltage so this can't work.

Here is a link that explains the operation of the diodes in the circuit you show
http://metroamp.com/wiki/index.php/Half_Wave_Voltage_Doubler

enter image description here

I've got a 12V AC-DC adapter that delivers 200mA

Depending on the internal implementation of the AC-DC adapter it may be possible to use it.
If it is the classic transformer type rather than a switching type (you can easily judge it by size and weight) then you can omit the output circuitry (you'll need to open the adapter case to do that) and connect the AC output of the transformer to the AC input of the O2 power supply circuit.

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