This audio headphone amplifier is a so-called minimum type since it consists of only two field effect transistors. Due to its construction, it runs in class A. The input transistor is a JFET, while the output transistor is a medium power MOSFET with a relatively high idle current. The prototype uses Toshiba JFET, which is no longer produced, but the transistor from the American semiconductor company Linear Systems can be used instead.
This headphone amplifier has replaced my previous headphone amplifier, described here.
An advantage of the new one is that a simple power supply is
sufficient, as long as it can supply enough power. A 10 W Mascot AC /
DC converter has therefore been used as an external power supply of 24
V for both channels. The following description applies to a single
channel. In the prototype, a volume potentiometer was used for each
channel. A stereo potentiometer can of course be used, but remember
that this is a headphone amplifier, so tracking between the two
channels must be very good (if a balance potentiometer is not used in
addition).
The circuit diagram is shown below. As mentioned, an external unregulated voltage supply is used, which is supplied to pins J5 (and J6 for 0 V). Therefore, two low-pass filters (RC filters) have been added to increase the ripple suppression of the amplifier. Of course, there is no disadvantage in using a regulated supply.
The input resistance is mostly determined by the parallel connection between R1 and R4 and is thus in excess of 100 kohm. Since the volume potentiometer is in front of the input here, the value should not be greater than 20 kohm. In the prototype, this size was used, but 10 kohm should also be a good choice.
JFET Q6 (2SK170 / LSK170) runs at a current of about 2 mA, which is
largely given by the gate-source voltage of the output transistor Q9,
MOSFET IRF9610. Using the potentiometer RV17, the DC voltage at the
output (drain at Q9 and the top of R11) is determined. This voltage is
usually set to about half of the supply voltage, in my case about 12 V.
This gives a working point of Q9 of 12 V voltage and 120 mA current. It
is therefore advisable to let the transistor have a heatsink.
The gain in Q6 is about 25 times (28 dB), this is mostly given by the size of R5, R7 and the transconductance (in excess of 20 mS). See also the article JFET amplifiers for calculation of gain in JFET. The gain in Q9 depends on the load and is about 24 dB for a resistive load of about 30 ohms. This provides an open-loop gain of about 52 dB for a load of 30 ohms. The closed-loop gain is given by 1+ R11 / R5, i.e. about 3 times (9.5 dB).
The value of C12, shown here as 300 uF, determines the lower cut-off frequency. This is just under 20 Hz for a 30 ohm load. It is adviseable to check your own headphone impedance before choosing the size of this capacitor. For 8 ohm headphones, 3000 uF will be the minimum. In the prototype, 3 film capacitors were used, each of 100 uF. For the electrolytic capacitors, a good rule of thumb is to choose the size so that the cut-off frequency is well below 20 Hz. Note that the plus pole of the capacitor (s) must be drain on Q9 (top of R11). The upper bandwidth of the amplifier is close to 1 MHz, set by C10.
The output impedance depends on the size of C12, since it will rise for the lowest frequencies. At 300 uF it was measured to about 5 ohms at 100 Hz and falling to less than 0.5 ohms at 1 kHz. With C12 = 3000 uF, the output impedance drops to one tenth at 100 Hz, so the moral is: For very low-ohmic headphones, it is necessary to use high values of C12.
The prototype distortion was about 0.05% at an output voltage of 1 V RMS in 30 ohms. The distortion is relatively constant in the audible range, but be aware that volume potentiometers of 50 kohm or higher will increase the distortion slightly at the highest frequencies (in the kHz range). The distortion at 8 ohms was not measured. Due to the amplifier design, the distortion is dominated by the 2nd harmonic.
Bill of Materials (BOM) is shown below. The amplifier is well suited for your own adaptations. When replacing, remember to take into account changed physical dimensions and pin configurations.
0.6 W metal film resistor with 1% tolerance is used. It is quite possible to use a different MOSFET for Q9 than IRF9610, but it has not been tried. Remember that the power dissipation of the transistor is so high that a heatsink should be used.
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