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Old 13th September 2015, 03:00 PM   #1
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Default Headphone amp with 2SK1058

This is a headphone amp with a 2SK1058. It is for headphones with impedances from 33Ohm to 50Ohm. It is class a with no feedback and a current output. The phase is inverted. Below is the schematic of the amp.
From left to right, input is from a tablet pc. The attenuator is shunt type and optional. The potentiometer works as variable resistor. The value should be 2,2k or below.
The input capacitor is an MKP type 1F/400V-. Input resistor is 22k. The adjustment pot is a 25turns type rated at 100k.
The coil is the secondary winding of an transformer with the following data.
Transformer: 30VA, primary 2x115V, secondary 2x18V.
Both windings in series R=3,8Ohm and L=0,16H (measured without current).
With current L will be decreasing.
Small heat sinks for the transistors are recommendable.
The resistor R parallel to the headphones matches the headphone impedance to the coil and lowers the gain.
The value of R is between 15Ohm and 22Ohm. It depends on the sensitivity and impedance of the headphone.
The voltage source Vcc is LiIon cell with 3,7V.
Quiescent current is adjusted to I=0,2A, so voltage drop across the coil L is U=0,76V.
My listening test was done with a Fostex T50 with an impedance of 50Ohm
The value for R is 22Ohm. The gain is 13dB. The -3dB point is approximately at 17Hz.
Sound quality is improved compared to the output of the tablet without the amp. There is enough space around the musicians.
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Old 11th June 2017, 03:31 PM   #2
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The first stage with the LND150 mosfet inverts the phase. Gain is approximately one. The bias current is 4mA.
The 1F and the 3.3F capacitor are MKP types, Potentiometer 100k (25 turn) and 10k (25 turn).
The second stage has two 2SK1058. The upper transistor works as mosfet Diode. The 12 Ohm resistor is 5W type. Bias current is 160mA.
Gain is approximately 10dB. This stage is similar to a voltage mirror.
Output capacitor is 1000F/16V Aluminium Polymer type.
The capacitor C1 are 17 (or more) parallel 1000F/16V Aluminium Polymer. They are loaded by the 22 Ohm resistor.
A switch or a relay bridges the resistor after loading.
Power supply is a 12V sealed lead accu.
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