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Odibu - headphone (line) amplifier (preamplifier)

It is simple yet very powerful amplifier consisting of high quality operational amplifier and discrete diamond buffer.

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).

Following 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):

And some other measurments: THD @ 1kHz and max volume, noise 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, THD @ 15kHz and half volume, THD @ 15kHz and full volume, output impedance is smaller than 0.2Ohm through whole audio spectrum.
Parameters of "big" version vith different operational amplifier [THD 1kHz / THD 15kHz / IMD 250Hz+8kHz]: TL061 0.003%/0.006%/0.003%, LM741 0.003%/0.015%/0.005%, NE5532 0.004%/0.005%/0.005%, LT1122 0.002%/0.003%/0.003%, measuring device limits 0.002%/0.003%/0.003%.

PCB in 300DPI suitable for milling, dotted extension applies only when using as headphone amplifier in ZERO DAC.
And photo of completed amplifier before putting into ZERO DAC

Stand-alone amplifiers

"Big Odibu"
This version contains of high quality power supply with toriodal transformer (INDEL 40VA 2x24V), all in KK07-231 box.
Frequency response DC to hunderds of kHz, THD over whole acustic frequency range and IMD (250Hz + 8kHz) <0.003% (value limited by measuring device). Maximum output with supply +-15V is 8.5VRMS, noise (20Hz to 16kHz) at full volume is 40uVpp (SNR 115dB at full voltage, calculated from peak-peak values).
And another two numbers from more precise measurement: THD 1kHz = 0.0004% a THD 15kHz = 0.0007%. In this case THD is limited by LT1122, LME49710, AD797 or OPA627 would perform better.

"Small Odibu"
Following photos shows amplifier built-in U-AH311 universal box with very simple PSU (EI transformer + LM317/LM337).
Stand-alone amplifier parameters: frequency response DC to hunderds of kHz, THD over whole acustic frequency range and IMD (250Hz + 8kHz) <0.005%.

Author: Luboš "Amarokcz" Stříteský

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