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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesThis project builds a small mono analog mixer around an op-amp summing amplifier. Each source is attenuated, isolated through a resistor, and combined at the op-amp’s inverting input. It is a useful line-level learning circuit, but it is not a complete commercial console: microphones may need preamps and power, and the output normally feeds powered speakers or a separate power amplifier.
The reference project is documented at All About Circuits. Its five-channel concept and two-channel physical demonstration should not be confused: the published example parts are not a complete five-channel bill of materials.
What the mixer does
A mixer combines several voltage signals into one output while letting you adjust each channel. A practical signal path is:
Source → coupling capacitor → microphone preamp or buffer (if needed) → channel level control → mixing resistor → op-amp summing node → optional master volume → output buffer or power amplifier.
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Do not connect source outputs directly together. Their output stages can load or fight one another, causing distortion or damage. Separate resistors and the op-amp’s virtual-earth summing node combine signals in a controlled way.
Passive or active mixing?
| Approach | Advantages | Limitations |
|---|---|---|
| Passive resistor mixer | Few parts and no op-amp supply | Signal loss, source interaction and usually a required buffer |
| Active inverting mixer | Adjustable gain or attenuation and better channel isolation | Needs a suitable supply, decoupling and headroom |
| Per-channel preamp plus mixer | Handles weak sources consistently | More parts and more opportunities for noise |
| Commercial mixer | Enclosure, connectors, protection, monitoring and controls included | Costs more and teaches less circuit design |
An active summing stage is generally the practical choice for a two- to four-input hobby line mixer.
The summing-amplifier circuit
For channel voltages V1 through Vn, input resistors R1 through Rn, and feedback resistor Rf:
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Vout = −Rf × (V1/R1 + V2/R2 + … + Vn/Rn)
With equal input resistors, Vout = −(Rf/Rin) × (V1 + V2 + … + Vn). Increasing Rf increases every channel’s gain; increasing one channel’s resistance reduces that channel’s contribution. The negative sign indicates inversion, normally unimportant for one mono output but relevant when stages or stereo channels are combined. The equation assumes the op-amp stays in its linear range.
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Inputs and controls
Channel level potentiometers
Use an audio-taper (logarithmic) potentiometer for volume. Human loudness perception is approximately logarithmic, so a log control feels more even than a linear one. A linear pot works electrically but usually bunches most audible change near one end. Choose resistance compatible with the source and circuit; poor pots can be scratchy, mismatched or mechanically unreliable.
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Microphones are different from line sources
Phones, tablets and music players normally provide consumer line or headphone-level signals. Dynamic microphones produce much less voltage and commonly need a preamp. Electret microphones need the correct bias arrangement. Condenser microphones may require external power. Battery-powered microphones may not. Balanced professional microphone inputs also require a different connector, input impedance and protection scheme.
This circuit is not phantom-power capable merely because a microphone is connected. The source discusses 12, 24 and 48 V phantom systems, but requirements vary. A safe phantom input needs balanced wiring, voltage generation, current limiting, blocking capacitors, switching, protection and deliberate shield grounding.
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Coupling capacitors and frequency response
Each input capacitor blocks DC that could bias the op-amp toward a supply rail. Together with the resistance it sees, it forms a high-pass filter:
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fc = 1/(2πRC)
Calculate C for the desired low-frequency cutoff and actual effective resistance. The proof-of-concept used 10 nF capacitors and reported a lower −3 dB point of about 4.1 kHz, removing bass, low vocals and much of the audible range. Do not copy that value for full-range audio. Depending on impedance and cutoff, 1–10 µF is often a more appropriate design range, but it is not universal. A polarized capacitor must have correct DC polarity; use a suitable non-polar or bipolar part where polarity is uncertain.
Parts: original example versus a new build
Values in the original two-channel build
- LF347 op-amp
- 47 kΩ audio-taper potentiometer
- 100 kΩ linear potentiometer
- 470 kΩ resistor
- Two 10 nF capacitors
- ±12 V DC supply
Those values describe the author’s physical example, not a universal recipe or complete five-channel BOM. The conceptual design described four microphone inputs plus one music input.
General supplies
- Op-amp compatible with your supply, signal range, noise target and load
- One channel potentiometer, coupling capacitor and mixing resistor per input
- Feedback resistor and optional master-volume stage
- Breadboard, short connecting wires and regulated or suitably filtered supply
- Powered speakers, amplifier, recorder or other high-impedance line input
- Local supply-bypass capacitors near the op-amp power pins
- Optional channel switches, buffers and microphone preamps
Many general-purpose op-amps can demonstrate the principle, but “any op-amp” is not a safe design rule. Check supply-voltage range, common-mode range, output swing, bandwidth, noise, bias current, load stability and output current. The LF347 was used in the source build; other hobby designs use LM324 for convenient single-supply operation, with higher noise.
Best Value
- Mixes 3 signals into one output
- A passive circuit, so no extra power draw on your system
- Width: 2HP
- Bill of materials and assembly instructions can be found at the Eurorack Hardware website.
Power-supply choices
Split supply
The ±12 V example lets bipolar audio sit around ground without an artificial midpoint. It still requires correct op-amp pinout, rail decoupling and a current-limited initial startup.
Single supply
A single-rail mixer needs a quiet virtual-ground or bias reference, usually applied to non-inverting inputs, so audio remains inside the op-amp’s input and output ranges. A 12 V single-supply design can work, but headroom is constrained and the reference must be well filtered. Do not assume a 12 V adapter suits every op-amp or signal level.
Build procedure
- Define the use case. Choose channel count, mono or stereo, source types, frequency range and downstream equipment.
- Choose the topology. Give every channel a coupling capacitor, level control and series mixing resistor into an inverting summing amplifier.
- Calculate values. Set gain with
Rf/Rin, check summed peak voltage against output swing, and calculate capacitor values from the cutoff equation. - Wire the op-amp. Put a DIP package across the breadboard center gap, verify the exact datasheet pinout and connect rails before signals.
- Add bypassing and grounding. Place bypass capacitors at the power pins; keep input leads short, shielded where possible, and away from output wiring.
- Build one channel first. Add a switch or mute only after the basic channel works. A second inverting stage can provide master volume.
- Connect the output correctly. Use powered computer speakers, a standalone amplifier, recorder or suitable interface input. An op-amp output is not a passive-speaker drive.
- Power up safely. Use current limiting, verify rail polarity, continuity, capacitor polarity and output DC voltage, and check for heating before connecting valuable equipment.
- Test one low-level line source. Confirm smooth level control, correct mute behavior, clean output and sensible downstream level.
- Add channels gradually. If clipping begins, lower source levels, reduce feedback gain, add attenuation or buffering, and check the waveform with an oscilloscope.
Connector and stereo wiring
On a standard 3.5 mm stereo plug, tip is left, ring is right and sleeve is ground. Wire colors vary, so test continuity rather than trusting color. To make mono from stereo, feed left and right through separate resistors; never tie them directly. A mono plug in a stereo jack can make different contacts depending on the jack and device. Multiple powered sources can also create ground-loop hum.
Troubleshooting
| Symptom | Likely cause | Fix |
|---|---|---|
| No output | Wrong pinout, missing rail or broken ground | Verify the exact datasheet, rails and continuity |
| Distortion with several channels | Summing node or output clipping | Lower inputs, reduce feedback gain, attenuate or increase safe headroom |
| Very weak output | Passive loss, excessive resistor values, wrong pot wiring or loading | Add gain or a buffer and check the capacitor value |
| No bass or muffled sound | Coupling capacitor too small | Recalculate the cutoff; do not reuse 10 nF blindly |
| Hum or buzz | Ground loop, ripple or unshielded breadboard wiring | Use shielded leads, deliberate star grounding, filtering and short returns |
| Scratchy volume | DC on the wiper or poor potentiometer | Provide a correct DC path, inspect wiring and replace the pot if needed |
| Microphone inaudible | Mic level too low or missing bias/preamp/power | Add the appropriate microphone input stage |
| Oscillation | Missing bypassing, long wires or capacitive load | Shorten wiring, decouple locally and isolate the output if required |
What this project does not provide
- Complete microphone preamps or phantom power
- Balanced XLR inputs and outputs
- Power amplification for passive speakers or headphones
- Stereo panning, EQ, effects, USB recording or monitoring
- Specified maximum output, distortion, noise, impedance or channel count
- Enclosure, connector protection and finished-product safety
For a finished unit, use shielded connectors, strain relief, a deliberate enclosure ground, power switching and current protection. Keep mains voltage physically separated from low-voltage audio circuitry.
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When to use a commercial mixer
Choose a commercial mixer when you need reliable connectors, balanced microphone inputs, phantom power, low noise, EQ, metering, headphone monitoring, stereo routing or immediate plug-and-play operation. Choose this project when the goal is learning op-amp summing with a few line-level sources and a powered output system.
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