Odibu - headphone (line) amplifier (preamplifier)
It is simple yet very powerful amplifier
consisting of high quality operational amplifier and discrete diamond
X1 should be audio-grade operational amplifier. I used LT1122 (good price and quality with litte noise - does matter only with sensitive IEM), other good OAs are: LT1115, OPA627A, AD825, LME49710, AD797, LT1363, LT1360, AD844, AD810, OPA134 or just common NE5534.
Schematic shows just one channel, input IN should have potentiometer for volume control (max. 10kOhm). C1 is adjusted to LT1122 (C1 should be increased if instability occurs with another OA). R22 and R23 or R7 and R8 sets quiescent current (if not used as amplifier in ZERO DAC you can increase quiescent current). R9 and R10 sets gain: A=1+R10/R9. R4 and C5 is low-pass filter.
PWR+ and PWR- is symmetrical +-15V PSU (just transformer + LM317/LM337 + capacitance multiplier or high quality symmetrical PSU like this).
table shows measurements when used as headphone amplifier in modified ZERO DAC (input
with log.potentiometer 10k, THD @ 1kHz, IMD @ 60Hz+7kHz, measured with
AKG K240 MKII as load and directly on amplifier PCB, SNR calculated from peak-peak values):
some other measurments:
THD @ 1kHz and max
spectrum @ max volume
and USB input, noise
spectrum @ max volume and coaxial input and @ half volume, THD @ 1kHz and half volume,
THD @ 3kHz and
half volume, THD
@ 42Hz and
half volume, IMD
60+7000Hz @ half volume and @ full volume, IMD 250+8020Hz @ half volume,
@ 15kHz and half volume, THD
@ 15kHz and full volume,
output impedance is smaller than 0.2Ohm through whole audio spectrum.
Author: Luboš "Amarokcz" StÝíteský