• Inexpensive
• Small footprint – doesn’t use huge transformers
• Higher efficiency

• Complex circuitry
• Susceptible to radio frequency interference (RFI). Requires shielding.
• Susceptible to electromagnetic interference (EMI). Requires filtering.
• Produces high-frequency noise – Could backwash back into AC mains.
• Greater ripple voltage
• Slower transient response
• Potential reliability issues

• Reliable
• Simpler circuitry
• Not as susceptible to EMI and RFI
• Much quicker transient response
• Better isolation from mains

• Expensive
• Low efficiency – Sometimes less than half of SMPS
• Larger footprint
• Gets hot – requires heatsinks

The job of the transformer is to step up or down the AC voltage via electromagnetic induction. The ratio of windings determines the voltage difference. For example, if we wanted to step 120V down to 5V, there will be 24 more copper windings on the primary than the secondary (24:1).

There are many types of transformers (E-core, HF, etc). LPS for audio components typically use toroidal transformers. They’re more expensive and bulkier but have the following benefits:

• fewer stray magnetic fields
• more efficient
• better regulated
• low hum/distortion/noise

Transformers are rated for VA (volt-ampere). With purely resistive loads, this value translates to watts. So 500VA = 500 watts. The maximum output current is the VA rating divided by the desired output voltages.

A larger VA rating also means a larger core and a higher impedance. As far as LPS design for audio, it’s generally preferred to have separate mains transformers for each output (rather than having secondaries wound on a larger transformer). Of course, this adds bulk and incurs a much higher cost.

Some transformers could be arranged in a balanced configuration, which will require more windings and a larger core. This design should reduce noise even further.

The rectifier could take the form of four discrete diodes or a GBU bridge rectifier (or a more powerful bridge). Its job is to take the AC signal from the transformer and convert it into DC.

It does this by forcing the signal to go in the “positive” direction – essentially the absolute value of the AC signal. As expected, this DC isn’t a straight line on an oscilloscope but rather pulses or ripples. In this state, the power isn’t quite usable just yet.

Also, diodes have a forward voltage drop (typically 0.7V) so there will also be a lower voltage at the output of the rectifier.

Voltage ripple is dependent upon capacitance, AC frequency, peak voltage, and load current. In order to smooth out those ripples from the rectifier, a filter capacitor is used. Since it temporarily maintains a charge, it helps to keep the voltage across a load more constant. The higher the capacitance, the greater the voltage and the longer the duration of the charge. Ideally, you would want to use an “infinite” capacitor to get a perfectly straight line on the o-scope – but they don’t exist.

The common types of capacitors used for LPS:

• Electrolytic – The e-cap is the most common type of capacitor.
  • Wet – cheaper but not as reliable
  • Solid – better performance and reliability but expensive.
• Supercapacitors
  • have lower voltage limits but store large amounts of power.
  • more robust and deliver charge much quicker than batteries.

Some designs will add decoupling capacitors before the rectifier to reduce mains noise.

At this point, the voltage is unregulated. The output voltage could still vary due to the demands of the load, capacitance, and the input voltage across the transformer. This is fine for non-critical applications but rarely acceptable in audio components where these voltage ripples could be problematic. We want the output voltage to be constant regardless of these fluctuations.

That’s the job of the voltage regulator.

Through a feedback network, voltage dividers, NPN bipolar transistors – a voltage regulator tries its best to maintain a certain voltage level. Regardless of temperature, load demand, etc. They come in set voltages (12V, 5V, etc) or could be adjustable. They typically require more voltage at the input than the voltage required at the output.

Some of the most popular, 3-pin, off-the-shelf voltage regulators (typically in TO-220 packaging):

• LM78XX (where XX is usually the voltage)
• LM317 (adjustable)
• LM338
• LT1085 – An improved 317
• LT3045

For audio, these regulator characteristics are most important:

• Low noise – lower low-level signal interference
• Quick transient response
• Quick settling time
• Low output impedance (10 milli-ohms or better)
• Wide operating bandwidth – to deal with RFI and allows better control of load current fluctuations at higher frequencies (e.g., from DACs). These MHz current pulses will test the speed of the regulator and may add noise at the supply output.