Output circuits of standard signal sources as CD players suffer from limited capability to drive capacitive load and low impedance load. Depending on signal cable length (capacitance) and amplifier input resistance/capacitance there is an interaction between output circuits of CD player and the cable. This may result in a loss of resolution of high frequencies, harsh (grain) sound and soft undefined bass. The following circuit may solve the problem and can be also used as a high quality headphone amplifier. It has high input resistance and low capacitance and low output resistance. It is able to deliver output current of some 250mA. The frequency range is far beyond audio needs. The distortion is very very low, order of 0.0001%. This circuit should be connected to the CD output by coaxial cable no longer than some 10 - 15cm.
Following image shows original circuit of Audio buffer:
As op amp X1, OPA134PA, LT1122, OPA627AP and AD825 were tested. The best results were obtained with OPA627AP on X1 position, but this op amp is 10x more expensive than OPA134PA. The second amp X2 is Burr-Brown fast buffer BUF634T. The whole circuit has a voltage gain of +1, acts as a buffer amp and the influence of output cable is eliminated. The load resistor R12 (terminating resistor) is optional. Voltage gain with R12 termination is +1, without termination +2. For proper location of R10 and R12 resistors please see following image:
The R12 resistor should be placed as a terminating resistor at the end of buffer output cable.
The circuit as shown operates at quiescent current of 8mA. This current can be increased by reducing R11 to 0R (wire jumper), then Iq=15mA and buffer mostly operates in class A even for 50 + 50 ohm load.
Printed circuit boards
Please click here to see the PCB.
Printed circuit boards are not available now.
Bill of materials
Power supplies connection
The same circuit can be used as a headphone amp. R12 is to be omitted. The R4 can be enlarged up to 24k to obtain the gain of some 20dB. You should use a potentiometer of 10k before C1.
The recommended schematics of headphone amplifier is as follows:
C10 and C11 are optional.
January 2020 headphone amplifier update
I am updating my recommendations for a headphone amp. The best results are with R10 completely omitted, output impedance is then close to zero, with AD744 at X1 it is 0.056 ohm. This eliminates influence of headphones impedance to resulting frequency response. I have Sennheiser HD598 headphones with impedance plot as below and the improvement when omitting R10 is substantial.
With R10 set to zero, not every opamp may be used and stability is to be checked. My current setup is with R3 = R4 = 1k5 and X1 = AD744. The difference of R10 = 0 compared to R10 = 100 ohm or 50 ohm is stunning, re sound accuracy from headphones.
created: May 2002
last updated: February 10, 2012 and January 2020.