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§ 3.10 - AC-coupled Mono Dual Buffer or Amplifier

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Before you attempt this build, please:

This will help ensure that you will have the best possible experience, resulting in a build that works the way you expect it to.

This Mixer Block build is an AC-coupled, unity- or higher-gain, line-level, single-channel, mono, dual buffer.

It has one signal input, and two buffered or amplified outputs of that signal, one on the L output, and one on the R output. This provides for driving up to two sources without compromising signal levels or introducing crosstalk. If desired, one of the outputs may remain unconnected.

For instance, you might want to drive both an audio input, and a metering circuit from the same signal, but you don't want noise from the metering circuit to get into the audio, and you want to maintain the same level of drive.

Input impedance of this buffer/amplifier build is 10 k-ohms. However, the input resistors (R3, R4) are 20 k-ohms; this is because two inputs are in parallel, and the impedance is established as two, 20 k-ohms resistors in parallel into a (virtual) ground, hence the resulting 10 k-ohms total input impedance. Because of this, when building a gain amplifier, make sure that you provide the Amplifier feedback resistance calculator with 20000, not 10000.

If you build the Mixer Block for this role with 10 k-ohm input resistors, the input impedance will be 5 k-ohms. 10 k-ohms is standard for line-level signals.

Tip: To power this build of the Mixer Block with the Power Block, use the +/- 12 regulated Power Block build. If this is all you'll be powering, you won't need heat sinks on the Power Block, as the power requirements for the Mixer Block are very low.

Build Steps:

  1. C1—C2: .1 uF ceramic capacitor
  2. C3—C4 optional: ceramic capacitors, only needed for op-amps that require compensation;
    TLO-72 and TLO-82 op-amps do not require compensation. Refer to the specific op-amp's datasheet for design values
  3. C5: — 2.2 uF ceramic or other non-polarized type of capacitor (see frequency response calculator)
  4. C21—C22: 10 uF ceramic or other non-polarized type of capacitor
  5. Choose Unity Gain Buffer or Gain Amplifier build, then:
    1. If buffer: R1—R2: 20 k-ohms 1/8th watt resistors
    2. If amplifier: R1—R2: Amplifier feedback resistance calculator (use 20000 ohms as the input impedance)
  6. R3—R4: 20 k-ohms 1/8th watt resistors
  7. U1: TLO-72 low-noise op-amp, can substitute TLO-82, or similar. Observe polarity. An IC socket is strongly recommended.
  8. Install 3-pin jumper J1 — use .100 inch (2.54 mm) header pins; do not close
  9. Install 2-pin jumper J2 — use .100 inch (2.54 mm) header pins; close with a header jumper cap
  10. Install 2-pin jumper J3 — use .100 inch (2.54 mm) header pins; do not close
  11. Install 2-pin jumper J4 — use .100 inch (2.54 mm) header pins; do not close
  12. If you will be using a 2-pin input connector, mount it now at the dark blue box near C1; otherwise, connect the input and signal ground wires now
  13. Install connectors, quantity 4, optional: 5.08mm (.200 in) Spacing, 2-pin PCB Mount Screw Terminal Blocks. Or you can just wire the output from the indicated connections.
  14. Lastly, install M3 mounting hardware at all 4 mount points
Tip: If you would like to DC-couple the outputs while retaining the AC input coupling, place jumper caps across J3 and J4.
See also:Recommended Tools
Mounting to Wood Surfaces
Mixer Block Parts Sources
Wiring to Terminal Blocks


AC-coupled 1-channel Mono Dual Buffer, Build Guide


Mixer Block Schematic

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