Choosing a Toroidal Transformer for an Audio Amplifier

Most discrete and chip amplifiers (TDA7294, LM3886, TDA7293 and the many Class AB boards sold in India) run from a split supply: a positive rail, a negative rail and ground. A centre-tapped toroidal transformer (written as 24-0-24, 30-0-30 and so on), followed by a bridge rectifier and filter capacitors, is the standard way to build one.

Step 1: Secondary voltage → DC rails

After rectification and smoothing, each DC rail charges to roughly the peak of the AC voltage, minus a diode drop:

V_dc ≈ V_ac × 1.414 − 0.8 V (per rail, bridge rectifier) 24-0-24 → 24 × 1.414 − 0.8 ≈ ±33 V

Two effects make real measurements differ:

At idle the rails read higher. The transformer's rated voltage is specified at full load. With no load it can be 5–15% higher, depending on size (smaller transformers regulate less tightly).
Under heavy load the rails sag. This is caused by transformer regulation, rectifier conduction and capacitor ripple.

Always check the amplifier's limit

Every amplifier IC or board has a maximum supply voltage. Choose a transformer whose unloaded rail voltage, on your highest mains voltage, stays below that limit with a margin. Indian mains can sit well above 230 V at night.

Common transformers and the rails they give

Transformer	DC rails (approx.)	Ceiling into 8 Ω	Ceiling into 4 Ω
18-0-18	±25 V	~30 W	~60 W
24-0-24	±33 V	~55 W	~110 W
30-0-30	±41 V	~90 W	~180 W*
35-0-35	±49 V	~130 W	~250 W*
50-0-50	±70 V	~280 W	Needs a heavy-duty output stage*

The "ceiling" is the theoretical maximum clean power per channel, (V_rail − 3 V)² ÷ (2 × R_load). Real amplifiers deliver somewhat less. *Only with an output stage (enough transistor pairs, heatsinking) designed for that current.

Vaani lists 24-0-24, 30-0-30 and 50-0-50 toroids in several VA ratings. For other voltages, use a custom request.

Step 2: VA rating

VA (volt-amperes) is the transformer's power rating. Music is not a continuous sine wave, so a Class AB amplifier does not draw its full rated power all the time. The widely used rule of thumb is:

Transformer VA ≈ 1.5 to 2 × (total rated output power of all channels)

Amplifier (Class AB)	Suggested VA
2 × 50 W	150–200 VA
2 × 100 W	300–400 VA
2 × 150 W	450–600 VA
2 × 200 W	600–800 VA

Go towards the upper end if the amplifier will be driven hard for long periods (PA, DJ use, subwoofers). A bigger transformer also has tighter regulation, so the rails sag less on bass transients.

Step 3: Check the current

With a bridge rectifier and capacitor filter, the transformer's AC current is peaky, so its RMS value is higher than the DC current drawn. Use:

Usable DC current ≈ 0.62 × transformer AC current rating AC current rating per secondary = VA ÷ (total secondary voltage) Example: 300 VA, 30-0-30 (60 V total) AC rating = 300 / 60 = 5 A Usable DC ≈ 0.62 × 5 ≈ 3.1 A per rail

Class A, AB and D

Class AB: use the rules above.
Class A: draws its full current continuously whether or not music is playing. Size the transformer for the continuous DC power (rail voltage × bias current × number of rails) plus a healthy margin, typically 4–5 × the rated audio output.
Class D with a linear supply: the amplifier itself is efficient, so around 1.2–1.5 × output power is usually enough. Check the board's maximum voltage carefully. Many Class D modules are rated for lower rails than Class AB designs.

Quick checklist

Find the amplifier's maximum and recommended supply voltage.
Pick a centre-tapped secondary whose rails land in that range with margin: V_dc ≈ 1.414 × V_ac.
Size the VA at 1.5–2 × total output power (Class AB).
Fit a slow-blow fuse on the primary, and consider a soft-start above ~300 VA. See inrush current.
Mount it correctly, away from the input stage. See mounting.