Audio: low THD two tone generator
@2012/9/14
these 2 weeks i play the 2 tone generator with good THD performance, I had build one audio generator with THD around 3%, and had a factory Rig built long away ago with NIXIE frequncy display, that's cool and very Big. THD about 0.3%.
Wein oscillator stable by JFET, i can get total THD < 0.1%, output amplitude around 50mV, relatively low, a additional gain stage is needed.
simple design plan
1. use mica 6800pF as bridge capacitor for best performance
2. match 5% resistor: get close value as you can
700HZ: 33K + 6800pF
1800HZ: 13K + 6800pF
Power supply
use op-amp to provide a +VCC and -VCC. a additional LDO stage is suggestion.
LOW THD WEIN oscillator
quick built
following section is added @2011/11/25
696Hz THD: 0.03%
1.856Khz THD: 0.01%
Buffer and gain / selector
@2012/11/25
buffer following the oscillator and a additional gain stage provide amplifier for the signal cause the oscillator output level is so low. (50mV-- 100mV).
then use a double 3 position switch select the A , B or A+B signal. and use a 2k pot as balance adjust.
after mixer, a extra gain stage is added to get 500mVpeak level signal. (note finally i have only a high output, 0.47uF series with a 560R resistor).
buffer build block:
balance and Attenuator:
Balance borrowed from F2DC's 2 tone generator. (low impedance driver + high input impedance buffer)
Mixer of audio use this one:
here is my combination quick prototype:
all done in Box
@2012/11/25