Audio Buffer – unity gain link stage/headphone amp with ultimate output drive capability
I have been using a similar circuit in measuring instruments for HV tests since 1980's and later decided to use it for audio, for its excellent output drive capabilities. I described the reasons in my web page in the year 2002
http://pmacura.cz/buffer_en.html
“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.”
Output impedance of the CD players was usually between 200 ohm and 1 kohm and output stage was a low cost commercial opamp. This has not changed much till nowadays for DAC converters, so the issue often remains the same.
My solution is a small box with unity gain, input impedance of 100 kohm, output impedance of 50 ohm, which is able to drive load down to 50 ohm and almost unlimited capacitance.
Here are some measurements
Maximum output voltage is 9Vrms into 10k load, 4.5Vrms into 50 ohms.
Response to 10kHz square wave
Response to 10kHz square wave, 50 ohm load
Response to 10kHz square wave, 47nF capacitive load
Response to 1kHz square wave, 1uF capacitive load
Output HF noise up to 10MHz
THD at 1kHz / 1.8Vrms / 1kohm load (LF noise rise is an issue of my measuring gear)
THD at 1kHz / 0.9Vrms / 50 ohm load (LF noise rise is an issue of my measuring gear)
THD vs. frequency at 1.8Vrms / 1kohm
THD vs. amplitude at 1kHz and 1kohm and 50 ohm load
Conclusion
This circuit eliminates influence of the link cable to output of CD players or DACs. It can be loaded with as low impedance as 50 ohm and capacitance up to 100nF will not degrade its audio parameters. Capacitances up to 1uF (and more) may be driven, at the expense of high frequency roll off.
Most of the distortion measurements of this unit are already affected by resolution of my measuring gear.
I have been using a similar circuit in measuring instruments for HV tests since 1980's and later decided to use it for audio, for its excellent output drive capabilities. I described the reasons in my web page in the year 2002
http://pmacura.cz/buffer_en.html
“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.”
Output impedance of the CD players was usually between 200 ohm and 1 kohm and output stage was a low cost commercial opamp. This has not changed much till nowadays for DAC converters, so the issue often remains the same.
My solution is a small box with unity gain, input impedance of 100 kohm, output impedance of 50 ohm, which is able to drive load down to 50 ohm and almost unlimited capacitance.
Here are some measurements
Maximum output voltage is 9Vrms into 10k load, 4.5Vrms into 50 ohms.
Response to 10kHz square wave
Response to 10kHz square wave, 50 ohm load
Response to 10kHz square wave, 47nF capacitive load
Response to 1kHz square wave, 1uF capacitive load
Output HF noise up to 10MHz
THD at 1kHz / 1.8Vrms / 1kohm load (LF noise rise is an issue of my measuring gear)
THD at 1kHz / 0.9Vrms / 50 ohm load (LF noise rise is an issue of my measuring gear)
THD vs. frequency at 1.8Vrms / 1kohm
THD vs. amplitude at 1kHz and 1kohm and 50 ohm load
Conclusion
This circuit eliminates influence of the link cable to output of CD players or DACs. It can be loaded with as low impedance as 50 ohm and capacitance up to 100nF will not degrade its audio parameters. Capacitances up to 1uF (and more) may be driven, at the expense of high frequency roll off.
Most of the distortion measurements of this unit are already affected by resolution of my measuring gear.
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