Archive for August, 2007

F4 – Amplifier

Wednesday, August 8th, 2007

This is my F4 amplifier completed in August 2007:

  1. I designed the PCB and Casing myself and get China factory to custom make for me.  The casing size is 350mm (W) x 420mm (D) x 200mm (H) with front plate 10mm larger than the casing.
  2. R3 and R4 still 22 ohm, and Q1 and Q2 are using 2SK170BL and 2SJ74BL for matched Idss to within 9.8 to 10mA.
  3. C1 to C4 use 330uF 35V Panasonic FC, gate resistors R10 to R15 are 100 ohm, Source resistors R16 to R21 are 0.51R 3W.
  4. IRFP240 and IRFP9240 are matched Vgs within 5mV which give variation of bias current within 5% (measured voltage at 0.51R 5% resistor) among all the mosfets.
  5. Use two 18V x 2 250VA transformers and 4 birdge rectifiers to parallel the secondary voltages to about +/-21.8Vdc with 1.45A bias for each channel.
  6. Filtering capacitors are Nichicon 68,000uF 63V x 2 + 0.47R//4 pcs + Mallory 33,000uF 35V x 4.
  7. Heat sink size is 420mm x 200mm x 50mm for each channel (side).  Temperature rise is about 17 deg C (i.e. 47 degC for room temp of 30 degC).
  8. The DC offset is stable to be within 10mV after warm up for 1 hour.
  9. Measured distortion is 0.02% at 1W output for both Channels.  Noise is mainly at low frequency band from 25Hz to 1kHz.
  10. The F4 amplifier has been playing for 4-5 hours a day in the pass week and the sound is already stable.  The sonic is skid to soft type bass (due to limited power) and clean and clear mid and high frequency.
  11. Overall the sound is very good and satisfying.

Some photos below:

FFT measurement at 1W and 20W:

F4 FFT2 1W 8ohm

F4 FFT 20W 8ohm