Headphone Amplifier

Introduction

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).

Circuit Diagram

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.

Power Supply

As mentioned, an unstabilized external 24 V DC voltage has been used. This consists of fuse, transformer, rectifier and electrolyte capacitor. The ripple at maximum load was specified to less than 30 mV. Of course, there is nothing wrong with using a stabilized power supply. Although the amplifier is designed for a supply voltage of 24-30 V, lower supply voltage can be used.

PCB Layout and wiring

The PCB Layout of the amplifier is shown in the figure below. As mentioned, the Q9 should have a heatsink, it is not absolutely necessary to use the one shown. Remember that the right side of the C12 should be a minus on electrolyte capacitors, if these are used.


The phono input is connected to the volume potentiometer and further from there to the amplifier circuit board. The shield on the phono cable is connected to the amplifier board, to the point marked J2. The hot end of the phono cable is connected to the amplifier board marked J1. From the headphone jack, the two conductors are twisted and fed to the amplifier board to the points marked J3 and J4, where the latter is ground. From the power supply inlet, the two conductors are twisted and routed to J5 for plus and to J6 for ground..

With RV17, adjust the output voltage on the drain to Q9 (common with R11) to the half value of the supply voltage, approx. 12 V for 24 V supply. If possible, use an oscilloscope to look at the output, there should be nothing but noise here if everything is in order.

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.

BOM

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.

R1 220k
R2 1k
R4 220k
R5 33
R7 2,2k
R8 220
R11 68
R13 10k
R16 10k
R20 22

RV17 100 k Potentiometer Bourns 3296W

C3 1u MKT P5.08 (P5.0 mm)
C10 2.2n MKT P5.08 (P5.0 mm)
C12 3x100u Radiell Film L41.5mm W20.0mm P37.5 mm See text
C14 1u MKT P5.08 (P5.0 mm)
C15 4700u 35 V D18 mm P7.5 mm
C18 10u Radiell Film L18.0mm W9.0mm P15.0mm
C19 4700u 35 V D18 mm P7.5 mm

Q6 LSK170 JFET TO-92
Q9 IRF9610 MOSFET TO-220

J1-J6 Solder terminals
 
Heatsink L35 mm P25.4mm, e.g. Fischer SK104

Please notice:
This project description is for non-commercial use, only. Using this document on a site and charging a fee for download is vialation of non-commercial use and prone to demand for payment. So, for commercial use, contact me for agreement of terms. This page, however, can be downloaded for own use, and linked to, not violating term of non-commercial use.


 
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Knut Harald Nygaard