Use AC-DC adapter instead of AC-AC adapter

Question

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:

(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:

(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?

Answer 1

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

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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.

Answer 2

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?

No. Not without adding some stuff in return. Not knowing how much current your circuit actually draws, or the type of regulation it requires, it's hard to even make a suggestion. The use of a AC-AC adaptor, with two linear regulators (one positive one inverted) results in a very quiet supply, which is what you want for Audio devices. Prevents noise. Using a random AC-DC adaptor will inject an unknown amount of switching noise.

If we ignore the noise and extra regulation, you still only have a positive supply. You would need a voltage inverter, which would add multiple parts, a design change, and inject even more noise (as another switching regulator).

The nice part is that as the battery section and post regulation diodes show, the Amp can work with 8.4v - 0.7v (Diode drop) = 7.7v Battery power (Positive and Negative). Even less when the NiMH batteries start going low (Probably down to ~7 Volts). You could replace the 7812 and 7912 with 7810 and 7910 for +-10V, which would smooth out some of the noise from the inverter, and work with 12v input.

Answer 3

In this circuit the diodes on the input are not connected as a simple rectifier, they are connected as a Delon (bridge) type voltage doubler. A voltage doubler is fed with AC voltage. The doubling effect of the circuit is how it is able to supply the higher voltage needed by the voltage regulators. It will not function properly with a DC input, in fact only one of the regulators would get any input voltage as the other would be blocked by one of the diodes.

Answer 4

Using a DC supply is equivalent to using batteries. Your schematic already includes circuitry to operate for batteries, and designates the part of the circuit that are not required if battery-only operation is desired. You could simply build as if for battery-only operation, but use an equivalent DC supply in place of the batteries.