Topic Audio/Design1

Designing the audio transformer for loudspeaker 25W, 16_Ohm

Input parameters

Input Voltage 100V, 50-17000Hz, sine wave
  Wire Cu, double insulated for small winding capacitance, wound outside
Output Nominal DC output voltage 20V @ 50Hz, min 14.1V @ 17000Hz (-3dB)
  Nominal DC output current 1.25A @ 50Hz on 16 Ohm
  Wire Cu, double insulated round wire for small winding capacitance, wound inside
Core Steel M6, annealed
  Type Gaped EI core with 5 mil between E and I, for blocking amplifier DC bias current and the fine tuning of the resonance frequency
Bobbin Insulation class Single section bobbin
Design Criterion of design <10% regulation

General rules

1. In order to keep the leaking reactance low normally you’ll use a single section bobbin.

If the resonance frequency is lower than the maxim operating frequency (in this design 17000Hz) then you can reduce the capacitance (and increase the resonance frequency) using a double section bobbin.

In that case 50% of the primary and 50% of the secondary are wound in each section.£

Ceneral rules

2. For a low amount of the winding capacitance you’ll use a double or heavy insulated round wire. In some cases you’ll also need to use the layer insulation.

3. Typical core assembly of a loudspeaker audio transformer is gaped EI or C cores with annealed grain oriented steel. In order to avoid the output voltage distortion through

the current harmonics of the magnetizing current the maximal induction should not exceed 1.3T in an annealed EI core or 1.7T in a C core. In that case the third harmonic

of the magnetizing current (no load current) will not exceed 10%-15% (in our design approx. 3mA, view “Results at 50Hz without gap”)

With a gaped core assembly you can manipulate the resonance frequency changing the value of the no-load inductance. Additionally the gap will block the influence of the amplifier DC bias current.

4. At 17000Hz the output voltage is 15.85V (14.1V) and the phase delay 28.1°. If you would like to improve this operation mode then you need to reduce the eddy current

losses (using litz wire) and the leaking reactance (using a long EI core with bigger Fe cross section). Note that the best results can be achieved using a toroidal core with

minimum 8 sectors and one layer windings within each sector.

Results at 50Hz with gap

Results at 50Hz with gap Input and Circuit CORE/BOBBIN/STEEL/CASE Ceneral Data

Results at 17000Hz

Results at 17000Hz

Results at 50Hz withot gap

Results at 50Hz withot gap