Headphone amplifier and buffer ECC88

It is a headphone amplifier, which power transistors are complementary: BD139, BD140 and the preamplifier was made of integrated power amplifier. Burr Brown OPA2134 chip was used. The PCB and system of amplifier were designed so that the signal strength can be easily set using dip switches.
By changing their position of respectively: +10dB, +18dB, +20dB, you can drive more „resistant” headphones. Buffer consists of two popular duotriodes ECC88 operating in WK-WA system.



Button on front panel selects the operation mode (switching system is 4017):

• headphone amplifier (operational amplifier and transistors)
• preamplifier ECC88
• headphone amplifier and preamplifier ECC88

Those features make the system universal enough to be used for both headphones and preamplifier in an audio set.

The amplifier is a module system:

• power supply
• audio
• DC protection

Some components used for the construction:

• 1% resistors Vishay, Beyschlag
• capacitors Wima, Rubycon, Nichicon, Mant
• relays Relpol, Takamisawa
• ultrafast diodes ER802
• audio cables: Alphard
• precise potentiometers BOURNS
• efficient toroidal transformer 100VA, shielded

Power supply:
The system of power supply gives +/-15V, stabilized, precisely set under load, +150V of anode voltage (with 30 seconds delay system), about 6,3V for tubes glow. In addition, you can take 5V from the transformer.

The power supply was designed so that it provides voltage to those components which are currently in use.

DC protection:
The system is to protect headphones against DC at the output, what is indicated by a red LED. DC has also a function of delayed turn on the headphones, so you do not hear an unpleasant sound of ***** while turning on and off the amplifier.



Housing:
Housing/base of the amplifier was made of an oak board. It is hard wood, covered with transparent lacquer. Its dimensions are 31x24x4cm.

Front:

• power switch glowing with orange ring
• button for change of the operation mode
• three LEDs for currently working operation mode
• Jack 6,3 input for headphones
• volume potentiometer BOURNS 2x50k

Back:

• audio input and output of the preamplifier
• golden chinches
• mains socket

Pictures:




Link to original thread - Hybryda - Wzmacniacz Słuchawkowy + bufor ECC88 DIY