freq - frequency in Hz
 7.40 - excitation from two batteries in series (7.40V)
15.00 - excitation from four batteries in series (hypothetical, power supply is used)

Audio amplifier TDK2040 is tested. 
A single cascade is used with 30 capacitors and 60 (30x two-in-series) diodes 1N4007.
Power supplied by RIGOL DP832 at the frequency for max output DC voltage is measured, too.
Sinusoidal excitation so that audio goes into saturation. 
 
 ETD49 core with 4 isolated layers as secondary, no gaps and 3-turn primary 

 freq   7.40   15.00
 ------------------
   1k   1.94   3.37
   2k   3.67   7.17
   3k   4.73   9.51
   4k   5.55  10.93
   5k   6.07  11.85
   6k   6.38  12.55
   7k   6.55  13.03
   8k   6.63  13.44
   9k   6.64  13.73
  10k   6.62  14.00
  15k   6.31  14.51
  20k   5.92  14.53
  25k   5.58  14.38
  30k   5.24  13.93
  35k   4.85  13.11
  
  Max power  @9kHz / 7.4V = 0.64W
  Max power @10kHz /15.0V = 2.01W