Skip to content
LearnTronics menu

LearnTronics

Audio Op-Amp Buffer

Tutorial Index

Recipe #20 — buffer the passive mixer

The passive mixer works, but its output level depends partly on what is connected to it. An op-amp voltage follower can be placed after the mix node to make the following stage much less able to load that node.

PASSIVE MIXER FOLLOWED BY AN OP-AMP BUFFER INPUT 1 R1 10 kΩ INPUT 2 R2 10 kΩ PASSIVE MIX NODE + − BUFFERED OUT U1 voltage follower op-amp power connections omitted here for clarity
What changed? R1 and R2 still provide the source isolation. U1 does not increase the voltage in this follower connection; it gives the mix node a very light load and provides a lower-impedance output for the next circuit.

High input impedance, low output impedance

The op amp draws very little current from the passive mix node compared with a heavy load. Its output can then drive the next stage more firmly.

Another way to think of it

The passive mix node whispers the voltage to the op amp. The op amp repeats that voltage without asking the mix node to provide all the current needed by the next stage.

This fixes loading, but not the passive loss

The follower prevents the next stage from dragging the mix node down, but the mixing resistors still form a passive network and still lose some level. The next step is to let the op amp become the actual summing device.

A practical circuit must also provide the op amp with a suitable power supply. For a beginner-friendly signal drawing, the supply pins are omitted here.

This is the same basic voltage-follower idea introduced earlier, now used for a specific reason inside an audio mixer.

Try these yourself

What is the main reason for adding the op-amp follower here? PASSIVE MIXER FOLLOWED BY AN OP-AMP BUFFER INPUT 1 R1 10 kΩ INPUT 2 R2 10 kΩ PASSIVE MIX NODE + − BUFFERED OUT U1 voltage follower op-amp power connections omitted here for clarity
The follower presents a light load to the mixer and gives a lower-impedance output to the following stage.
What is the voltage gain of an ideal voltage follower?
A voltage follower is wired for unity gain: the output follows the input voltage.
Do R1 and R2 still have a purpose after the buffer is added?
The two mixing resistors still keep the source outputs from being tied directly together.
< Passive Tone Control   |   Op-Amp Summing Mixer >