Audio Buffer
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 leads to a loss of resolution of high frequencies, harsh (grain) sound and soft undefined bass. The following circuit solves 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, under 0.001%. This circuit should be installed into the CD player or connected to the CD output by cable no longer than some 10 - 15cm.
First version - with lower Iq
Following image shows original circuit of Audio buffer:
First op amp X1 is Burr-Brown OPA134PA type. LT1122 and OPA627AP were also tested with perfect results, AD825 may be used. 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 current load (R10 and R12) of the output is a must, R12 must be connected at the end of the output cable.
Quiescent current of the BUF634T can be set by R11 resistor. For the value of 220 Ohms buffer's Iq = 8mA and no heatsink is needed. After long-term listenning tests it was observed that it is better to increase Iq of the buffer, this results in :
Second version - with Iq = 15mA
According to the BUF634T datasheet it is possible to use R11 =
0 Ohm (jumper connection). Iq = 15 mA and the buffer operates in
class A until 3V peak at the buffer output (for R10 = R12 = 50
Ohm). C10 capacitor was added to optimize step response of the
circuit with OPA134 or OPA627 chips. BUF634T should be placed on
the small heatsink with this Iq setting.
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.
Printed circuit boards
PCB's are not produced now, but new version is prepared. Click
here to
see the older PCB.
Headphone Amplifier
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:
Pavel Macura
last updated: April 25, 2006
