perfboard · beginner
Wire a two-input passive audio mixer on perfboard
Learn why a passive mixer loses signal, draw its summing network, simulate one driven input, and follow a checked perfboard build with screenshots.
The app opens the current example chooser. Choose Open to create your own account copy. For this guide's exact v2, download and import the project backup if the catalog version has changed.
A passive mixer lets two inputs feed a common node through isolation resistors. The price is signal loss and interaction with the connected sources. Here you will draw a two-input network, study a deliberately one-source testbench, and reproduce its 10 × 8 perfboard connections. There is no battery or active buffer.
Parts: Two 10 kΩ summing resistors, one 220 nF output coupling capacitor, A100K level pot and three 6.35 mm TS jacks. The project download preserves the exact fitted parts and terminal mapping.
Before starting: have a CircBench account and use a copy for experiments. This is a software build and simulation walkthrough; the images are actual application screenshots, not photographs of assembled hardware. Every numbered step has a detail image and a link to its unmodified full context PNG. Open full size when comparing small pin labels, especially on a phone.
Draw and understand the schematic
1. Open the exact reviewed example
Use the example link above, sign in if requested, and confirm Two-input passive mixer, version 2 in Explore an example. Choose Open to create your own account copy. The link alone does not create a project. If the catalog changes, import the versioned backup above to follow this precise guide. All screenshots show the actual reviewed copy, with completed wiring as a reference for your own drawing.
2. Draw the two input branches
Inspect R1 and confirm 10 kΩ, then inspect R2’s matching value. In a new drawing, place each resistor after its own input tip. Their far ends will join at the summing node. Do not connect the two input tips directly: the resistors provide the isolation and form part of the attenuation network.
3. Couple the summing node to the output
Inspect C1 and confirm 220 nF. Connect the joined resistor ends to one capacitor terminal and its other terminal to the top of the level-control track. This nonpolar coupling capacitor passes changing signal while blocking DC between the summing node and output control. It does not buffer either input.
4. Finish the level control and returns
Fit the schematic. Connect the pot bottom to common return and its wiper to output tip. Join all three jack sleeves to the same return. Trace both separate input paths before their junction. Use wires that terminate on actual pins or junctions, not a crossing that only appears to join.
Configure and interpret the simulation
5. Configure the one-source experiment
Choose Simulate → Setup. Drive input A with a 100 mV-peak 1 kHz sine through 1 kΩ; load the output with 1 MΩ, level full, at 25 °C. Reference is the common return. Input B is held at zero volts by an ideal test source. This is a termination, not a battery or a second audio stream.
6. Confirm the DC reference
Run DC operating point and wait for current result. With zero DC source offsets, the passive network settles near zero volts. If a node floats, check jack sleeves, reference and source negative connections. There is no active bias to establish, and adding a supply would change the lesson rather than fix a missing return.
7. Read attenuation and phase
Run AC frequency response. The resistor network and terminated B input reduce the signal; the coupling capacitor affects the low end. Read the phase in its separate frame. Gain is relative to the selected input node, while generator amplitude is set before its source impedance. Keep that distinction when comparing an analytic divider with the chart.
8. Inspect the settled waveform
Run Transient waveform. The output remains a sine with lower amplitude: approximately 45 mV peak for this 100 mV generator and the stated terminations. The dashed input and solid output show attenuation, not amplification. Read several settled cycles. A sample at the cursor is not the peak of the waveform.
9. Explain what input B is doing
Reopen Setup and inspect B’s zero-volt termination. B is electrically present and loads the summing node even though its signal is zero. This explains why unplugging or changing a real source could change the mix. Save the same bench; this launch lesson does not claim to simulate two simultaneous independent audio streams.
Build the physical layout
10. Orient the 10 × 8 perfboard
Open Perfboard build and Fit board to window. Ten columns and eight rows give labeled holes A1–H10. The finished example is a reference for reproducing the layout. Individual pads start isolated; their proximity cannot substitute for an explicit copper or wire connection. Internal r3c5 corresponds to displayed C5.
11. Fit the resistors and capacitor
R1 spans B1–B5, R2 D1–D5 and C1 C5–C7. Confirm both fitted resistors remain 10 kΩ and C1 is 220 nF. Their summing ends align at column 5 but are not joined merely by alignment. Keep the output side of C1 at C7 for the level-control harness.
12. Create the summing and return paths
On the copper face, add B5 → C5 → D5 as one summing solder path. Join the return pads G1 through G4 with the authored return path. The copper view mirrors the component face, so use labels. A solder path joins intervening pads; an insulated jumper only joins its intended endpoints.
13. Connect all three jacks and the pot
Input tips go to B1 and D1, sleeves to G1 and G2. Pot top goes to C7, bottom to G4 and wiper to E9; output tip also attaches to E9 and output sleeve to G3. Follow the complete schedule below. Keep panel terminals distinct from pads even when the rendered hardware sits beside the board.
Exact reviewed connection schedule
The following tables come from the same versioned backup. Terminal identities belong to the fitted part; letter/number contacts belong to the board. In the copper view the picture mirrors, but these labels retain their identity.
| Part | Logical terminal → board contact |
|---|---|
| R1 | 1 → B1; 2 → B5 |
| R2 | 1 → D1; 2 → D5 |
| C1 | 1 → C5; 2 → C7 |
| J1 | T → B1; S → G1 |
| J2 | T → D1; S → G2 |
| J3 | T → E9; S → G3 |
| RV1 | 1 → G4; 3 → C7; W → E9 |
| Insulated connection | Endpoint A | Endpoint B |
|---|
| Solder path | Contacts in order |
|---|---|
| 1 | B5 → C5 → D5 |
| 2 | G1 → G2 → G3 → G4 |
Check, repair and preserve the project
14. Run a fresh independent wiring check
Choose Check wiring and wait for the completed match. CircBench extracts the represented board connections independently and compares them with the schematic. If your copy differs, reveal each reported open or short, verify both endpoints and check unplaced parts or fitted mappings. A previous or pending result is not a fresh match.
15. Introduce a deliberate wiring fault
Select and delete the B5–C5–D5 summing solder path on the copper face. Run Check wiring again. The resulting open separates resistor/capacitor contacts that still look neatly aligned. Reveal the finding and trace the isolated pads. The correct schematic cannot supply copper that is absent from the physical build.
16. Undo the fault and prove the repair
Choose Undo to restore the deleted connection or cut. Run Check wiring again and wait for a fresh match. Undo alone is not verification: the completed check proves the represented topology was restored. Keep this repaired version for the following persistence and download checks.
17. Save, reload and check again
Wait for the normal save indicator, then reload the page. Open the same build and run Check wiring again. Confirm a fresh match after reload. If saving reports a problem, open save recovery and export a backup; do not assume remote account storage succeeded merely because the editor still displays the project.
18. Inspect and export the parts list
Open Bill of materials, review the fitted parts, values and quantities, then use Export CSV. Compare it with the versioned parts list above. A BOM lists represented parts; it does not select manufacturer voltage ratings or prove that a purchased package has the same terminal order. Resolve substitutions before physical assembly.
19. Export a portable project backup
Open project Settings, go to backup and choose Export project. Keep the resulting .circbench.json file. This preserves the schematic, fitted build, definitions and saved bench/model settings. Raw simulation vectors and uploaded audio are separate outputs and are not retained in this project backup.
20. Import the backup and recheck
Return to Projects and Import the backup file. The imported copy appears in your library; explicitly Open its schematic, then its build. Run Check wiring and confirm a new match. If an import is rejected, keep the original file and compare its schema/version with the app rather than editing integrity fields by hand.
What this result establishes
This is a one-driven-input model with B ideally terminated at zero volts. It demonstrates neither two-stream playback nor buffering, galvanic isolation, gain or compatibility with arbitrary powered outputs.
The screenshots show current solver results and fresh wiring checks for this exact example version. They do not assert that hardware was assembled, measured or qualified. Use the supplied source, load and stimulus settings when comparing results.
CircBench authored this standard resistive summing teaching circuit and independent perfboard layout. Use the exact v2 backup to preserve its source/load choices and physical harness.
Your next project
Explore a different passive loading problem in the treble-cut guide, or add active gain with the breadboard boost.
