In many analog audio and op-amp circuits, a dual power supply is required so signals can swing both positive and negative around ground. Traditionally, this is achieved using a center-tapped transformer or two separate supply rails. However, those approaches increase cost, size, and transformer complexity.

This circuit provides a simpler alternative by generating positive and negative supply rails from a single AC secondary winding. Using only two diodes and two filter capacitors, it creates a split supply with a midpoint ground reference. This approach is attractive for small-signal applications because it allows dual-rail operation without requiring a special transformer.

This technique is commonly used in:
- preamplifiers
- headphone amplifiers
- op-amp circuits
- active filters
- small analog audio projects

where the current demand is relatively low and perfect rail symmetry is not critical.

Its main advantages are low cost, simplicity, and the ability to obtain bipolar supply rails from a basic AC wall adapter or standard transformer. However, these benefits come at the expense of poorer regulation, higher ripple, and reduced performance under uneven loading compared to conventional dual-rail power supplies. The circuit is shown below.

Fig. 1: a simple way to get a +/- bipolar supply from a single transformer winding

This circuit is a simple way of generating dual power supply rails (+V and −V) from a transformer that has only a single AC secondary winding - such as a plug-pack, or wall adaptor. It is often used in small audio circuits where a split supply is needed but a center-tapped transformer is not available.

The transformer T1 isolates the mains and steps the voltage down to about 12VAC. The secondary winding produces an alternating voltage, meaning the polarity continuously reverses.

The middle connection between capacitors C1 and C2 is used as the circuit ground reference (GND). This point becomes the midpoint of the supply.

During the positive half-cycle of the AC waveform, terminal 4 of the transformer becomes positive relative to terminal 3. Diode D1 becomes forward biased and conducts current into capacitor C1 and charges with its positive terminal at the top, creating the positive supply rail:

+V = Vpeak − VD

where:

Vpeak is the peak AC voltage and VD is the diode drop (~0.7V)

At the same time, diode D2 is reverse biased and does not conduct.

During the negative half-cycle, terminal 4 becomes negative relative to terminal 3. Now D2 becomes forward biased and conducts, charging capacitor C2 with opposite polarity. This creates the negative supply rail:

−V = −(Vpeak − VD)

while D1 is reverse biased. As a result C1 stores the positive half-cycles and C2 stores the negative half-cycles. The midpoint between them acts as ground, or 0V.

If the transformer is rated at 12VAC RMS, the peak voltage is approximately:

Vpeak = 12 × 1.414 = 17V

After subtracting the diode drop, the unloaded rails become roughly:

+V = +16.3V, −V = −16.3V

although actual voltages depend on load current and transformer regulation.

The capacitors smooth the rectified waveform and reduce ripple voltage. Since each capacitor is charged only once per AC cycle, ripple frequency is equal to the mains frequency for each rail; 50Hz ripple on 50Hz mains or 60Hz ripple on 60Hz mains.

This is different from a full-wave bridge rectifier, where ripple frequency doubles.

The main advantage of this circuit is simplicity. It allows dual rails to be generated from:
- a single secondary winding
- only two diodes
- no center-tapped transformer

This makes it inexpensive and convenient for small low-current analog circuits.

However, the circuit has several disadvantages.

The biggest disadvantage is poor regulation and load balance sensitivity. The positive and negative rails depend on the load currents being reasonably symmetrical. If one rail draws significantly more current than the other, the ground midpoint shifts.

This causes:
- unequal supply voltages
- increased ripple
- possible hum or distortion in audio circuits

Another disadvantage is higher ripple compared to full-wave rectification. Each capacitor is refreshed only once per mains cycle rather than twice, so ripple voltage is larger for the same capacitance and load current.

The ripple voltage approximately follows:

Vripple = I / ​f * C

where:
- I is load current
- f is mains frequency
- C is filter capacitance

Because f is relatively low, large capacitors are usually needed.

Another disadvantage is poor efficiency under heavier loads. The transformer utilization is not as efficient as a proper full-wave center-tapped or bridge supply. The charging currents are also more pulsed and uneven.

The circuit is therefore best suited for:
- op-amp circuits
- preamplifiers
- small headphone amplifiers
- low-current analog circuits

It is generally not suitable for:
- power amplifiers
- high-current loads
- circuits requiring tightly balanced rails
- precision analog systems

For higher-performance dual supplies, designers usually prefer:
- center-tapped transformers
- bridge rectifiers
- regulated split supplies
- active virtual-ground circuits.

This circuit demonstrates a simple and economical method of generating dual supply rails from a single AC secondary winding. By using two rectifier diodes and two filter capacitors, the circuit produces positive and negative DC voltages with a midpoint ground reference, making it suitable for powering low-current analog and audio circuits that require bipolar supplies.

Although this approach offers advantages such as reduced cost, fewer components, and the ability to use a standard non-center-tapped transformer or AC adapter, it also has several limitations. The supply rails are more sensitive to unequal loading, ripple performance is poorer than conventional full-wave supplies, and regulation is limited under higher current demands.

Despite these disadvantages, the circuit remains a practical solution for small-signal applications such as preamplifiers, headphone amplifiers, and op-amp circuits where simplicity and low cost are more important than high-current capability or precision regulation.