PM-H2 headphone amplifier
1. Introduction
PM-H2 is a kind of modification of my Dispre 2-JFET preamplifier. It was designed in 2011. The main difference is that PM-H2 uses one pair of Toshiba 2SK170 devices in the differential input stage, not the complementary-differential input quadruple that was used in the Dispre 2-JFET. The main reason of this change was a global lack of 2SJ74 Toshiba JFET devices, that became very hard to buy and genuine parts are almost unobtainable.
PM-H2 is basically used as a headphone amplifier.
PM-H2 has single differential JFET input topology biased by constant current source.
Output stage idle current is set at about 25mA, and it works in a class A up to 5V into 100 ohm load. For standard load 600 ohm, the output stage works in a class A for the whole range of output voltage up to clipping.
To remove output DC voltage component, DC servo is used.
PM-H2 has wide bandwidth and high slew rate.
2. Circuit description
Drawings of “PM-H2” show circuit diagrams of the preamp. Let us speak about left channel only, the right channel is identical and its part numbers are same as in the left channel +100.
X1 terminal block is an input of audio signal. It is followed by an input divider R1, R2. To get overall preamp gain of +1, please make R1=8k2, R2=2k7. In case you need voltage gain (up to 5x), please follow further text.
P1 potentiometer - it strongly recommend to use a good quality pot, as is the blue Alps.
Then there is an input RC filter (R3, C1), followed by input JFETs Q1, Q2. They constitute differential N-JFET input stage. Its operating current is set by multiple current mirror Q3, Q4, Q6 at 4mA per pair, i.e. 2mA per transistor.
Output current from this input stage is converted to voltage by Q5 loaded by active load Q6 – constant current source. This Q5, Q6 circuit has quite high output impedance, so it is followed by diamond buffer, Q7 – Q12, with very high input impedance and very low output impedance. Output impedance of the whole preamplifier is defined by R26 at 2.7 ohm.
R14 and R13 create quite strong global negative feedback, and they define gain of the preamp referred to input of R3.
Output voltage of the preamplifier is connected to both terminal blocks X2. The output is single-ended. Sheet 2 shows DC servo.
Comments on parts used and assembly
JFETs are used with Idss range from 6mA to 12mA for Q1, Q2 (BL grade). For best results, selected/matched are these 2 devices to have as same Idss as possible. I recommend matching at +/-0.2mA tolerance. No worse than +/-0.5mA, this would be the worst-case limit. Setup fixture for JFET Idss selection is described on my web page
http://web.telecom.cz/macura/testfet.pdf
No special adjustment or resistor selection is needed, except for input divider – see later.
Input divider
R1 and R2 (input divider) are selectable. The maximum gain is +5, with R1 shorted and R2 unused. The following table shows possible values of R1 and R2 and the resulting overall gain. The table is valid only for 10k potentiometer.
Output stage – diamond buffer
To check idle current value, voltage drop across R24 resistor is measured. Voltage drop (in mV) divided by 2.7 (R24 value) gives idle current in mA. To increase idle current, slightly decrease R18 value. To decrease idle current, increase R18. Idle current to be measured 5 minutes after power supply turn on. Before you set up idle current, please solder all the components including DC servo, and check that DC output voltage (X2-4 terminal) is close to 0V, no higher than several mA.
3. Further comments
Potentiometer P1
Recommended is Alps 27mm, type RK27112, 2 x 10k log.
Caution: body of Alps pot must be connected to PCB ground pin named as LSP1, located right from potentiometer. This can be done by short wire placed under potentiometer nut. The opposite end of this wire is to be soldered into LSP1 hole. If these directions are not followed, much worse inter-channel crosstalk will be obtained and the pot will be an antenna for RF/EMI signals.
DC servo (IC1, IC2)
OPA134, OP177, OP07 were verified. TL071 can be used in case of misery.
Heatsinks (KK1 – KK4)
Heatsinks used for BD139/BD140 transistors are SK 95 25 2 x M3 type, supplied by Fischer Elektronik. It is not absolutely necessary to use this type, but it is an optimum for mechanical fixing.
Power supply voltage
2 x 15 V stabilized. Current consumption per board is 2 x 100mA (standby), power supply should be able to deliver at least 2 x 200mA. The ZT15VA power supply with 70053K transformer is a good choice.
Signal and supply connection
INPUT
X1-1 left channel live
X1-2 left channel ground
X1-3 right channel ground
X1-4 right channel live
OUTPUT
X2-4 left channel live
X2-3 left channel ground
X2-2 right channel ground
X2-1 right channel live
SUPPLY
X3-1 +15V
X3-2 0V (ground)
X3-3 -15V
Technical parameters
Measurements
Voltage swing into 50 ohm load
THD(+N) vs. output voltage at kHz into 50 ohm and 20 kohm load. Measurement limited to 1V because of not enough precision of my system.
Full swing step response (square wave response)
Response to 40 Hz square wave
1. Introduction
PM-H2 is a kind of modification of my Dispre 2-JFET preamplifier. It was designed in 2011. The main difference is that PM-H2 uses one pair of Toshiba 2SK170 devices in the differential input stage, not the complementary-differential input quadruple that was used in the Dispre 2-JFET. The main reason of this change was a global lack of 2SJ74 Toshiba JFET devices, that became very hard to buy and genuine parts are almost unobtainable.
PM-H2 is basically used as a headphone amplifier.
PM-H2 has single differential JFET input topology biased by constant current source.
Output stage idle current is set at about 25mA, and it works in a class A up to 5V into 100 ohm load. For standard load 600 ohm, the output stage works in a class A for the whole range of output voltage up to clipping.
To remove output DC voltage component, DC servo is used.
PM-H2 has wide bandwidth and high slew rate.
2. Circuit description
Drawings of “PM-H2” show circuit diagrams of the preamp. Let us speak about left channel only, the right channel is identical and its part numbers are same as in the left channel +100.
X1 terminal block is an input of audio signal. It is followed by an input divider R1, R2. To get overall preamp gain of +1, please make R1=8k2, R2=2k7. In case you need voltage gain (up to 5x), please follow further text.
P1 potentiometer - it strongly recommend to use a good quality pot, as is the blue Alps.
Then there is an input RC filter (R3, C1), followed by input JFETs Q1, Q2. They constitute differential N-JFET input stage. Its operating current is set by multiple current mirror Q3, Q4, Q6 at 4mA per pair, i.e. 2mA per transistor.
Output current from this input stage is converted to voltage by Q5 loaded by active load Q6 – constant current source. This Q5, Q6 circuit has quite high output impedance, so it is followed by diamond buffer, Q7 – Q12, with very high input impedance and very low output impedance. Output impedance of the whole preamplifier is defined by R26 at 2.7 ohm.
R14 and R13 create quite strong global negative feedback, and they define gain of the preamp referred to input of R3.
Output voltage of the preamplifier is connected to both terminal blocks X2. The output is single-ended. Sheet 2 shows DC servo.
Comments on parts used and assembly
JFETs are used with Idss range from 6mA to 12mA for Q1, Q2 (BL grade). For best results, selected/matched are these 2 devices to have as same Idss as possible. I recommend matching at +/-0.2mA tolerance. No worse than +/-0.5mA, this would be the worst-case limit. Setup fixture for JFET Idss selection is described on my web page
http://web.telecom.cz/macura/testfet.pdf
No special adjustment or resistor selection is needed, except for input divider – see later.
Input divider
R1 and R2 (input divider) are selectable. The maximum gain is +5, with R1 shorted and R2 unused. The following table shows possible values of R1 and R2 and the resulting overall gain. The table is valid only for 10k potentiometer.
Output stage – diamond buffer
To check idle current value, voltage drop across R24 resistor is measured. Voltage drop (in mV) divided by 2.7 (R24 value) gives idle current in mA. To increase idle current, slightly decrease R18 value. To decrease idle current, increase R18. Idle current to be measured 5 minutes after power supply turn on. Before you set up idle current, please solder all the components including DC servo, and check that DC output voltage (X2-4 terminal) is close to 0V, no higher than several mA.
3. Further comments
Potentiometer P1
Recommended is Alps 27mm, type RK27112, 2 x 10k log.
Caution: body of Alps pot must be connected to PCB ground pin named as LSP1, located right from potentiometer. This can be done by short wire placed under potentiometer nut. The opposite end of this wire is to be soldered into LSP1 hole. If these directions are not followed, much worse inter-channel crosstalk will be obtained and the pot will be an antenna for RF/EMI signals.
DC servo (IC1, IC2)
OPA134, OP177, OP07 were verified. TL071 can be used in case of misery.
Heatsinks (KK1 – KK4)
Heatsinks used for BD139/BD140 transistors are SK 95 25 2 x M3 type, supplied by Fischer Elektronik. It is not absolutely necessary to use this type, but it is an optimum for mechanical fixing.
Power supply voltage
2 x 15 V stabilized. Current consumption per board is 2 x 100mA (standby), power supply should be able to deliver at least 2 x 200mA. The ZT15VA power supply with 70053K transformer is a good choice.
Signal and supply connection
INPUT
X1-1 left channel live
X1-2 left channel ground
X1-3 right channel ground
X1-4 right channel live
OUTPUT
X2-4 left channel live
X2-3 left channel ground
X2-2 right channel ground
X2-1 right channel live
SUPPLY
X3-1 +15V
X3-2 0V (ground)
X3-3 -15V
Technical parameters
- Output voltage max. 27 Vp-p / 600 ohm (9.54 Vrms)
- Output voltage max. 8.54 Vrms / 50 ohm (P = 1.46W)
- Load impedance - can drive even 24 ohm headphone
- S/N at 50 mV / 600 ohm 80 dB (BW = 24kHz)
- Freq range (-3dB) 0.1 Hz – 170 kHz
- THD at 1Vrms/50 ohm - less than 0.002% through audio range
- CCIF IMD 19+20kHz at 1Vrms/50 ohm - less than 0.002%
- Slew rate >30 V/us
- Input RC filter prevents from slew rate limitation
- Gain - adjustable from 1x (0 dB) to 5x (14 dB) by resistors in a voltage divider
- Input impedance – 10 kohm
- Output impedance – 2.7 ohm
- Supply/consumption - 2 x 15V stabilized, 2 x 100 mA
- Power supply requirements - 2 x 15V stabilized, at least 2 x 200 mA
Measurements
Voltage swing into 50 ohm load
THD(+N) vs. output voltage at kHz into 50 ohm and 20 kohm load. Measurement limited to 1V because of not enough precision of my system.
Full swing step response (square wave response)
Response to 40 Hz square wave
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