board
Diode-Bridge Voltage-Controlled Attenuator
Public Unreviewedby John Lauer
Two-layer through-hole audio attenuator: four 1N4148s as a current-biased shunt bridge, 0 to 2 V control, designed by Saif Abdessayed with Adom Hydrogen
main
John Lauer
Saif Abdessayed's diode-bridge VCA: KiCad project, Gerbers, BOM, simulations, reports, and the earlier RC filter test
45f3ff6
11d ago
Assembly notes
All populated parts are through-hole and suitable for hand soldering. Use the BOM’s exact MPNs where polarity or pitch matters.
- Inspect the bare PCB against
images/board-topandimages/board-bottom. Verify the 58 × 40 mm outline and that J1 silk reads+12 GND -12. - Install resistors first. R5 and R9 use vertical board-orientation footprints; do not confuse their values.
- Install D1–D4 from one production lot if practical. The glass-body cathode band goes to square pad 1. The custom vertical diode footprint places cathode pad 1 at the lower end in the board file.
- Install U1 with pin 1 at the square pad/notch mark. A quality 8-pin DIP socket is optional but is not included in the BOM.
- Install Q1 with onsemi 2N3904BU’s flat-face E-B-C order aligned to pads 1-2-3. Do not substitute a device with a different TO-92 lead order without rewiring.
- C1 and C2 must be Nichicon UES bipolar parts. They have no positive/negative installation orientation.
- C6 is polarized: positive to +12 V, negative to GND. C7 is polarized in the less intuitive direction: positive to GND, negative to -12 V. The board silk calls this out.
- Fit terminal blocks with wire-entry openings facing the board edge. J2 is audio input/GND, J3 is control/GND, J4 is audio output/GND.
- Clean flux and inspect every PTH joint for wetting and bridges. Check resistance from each rail to GND before inserting U1.
- First power-up should use a current-limited, regulated dual supply. With no input, verify approximately +12 V at U1 pin 8 and -12 V at pin 4 relative to GND. Do not reverse J1.
Suggested prototype test order: 100 mVpp at 1 kHz, confirm 0 V gain, measure the five-point control sweep, sweep 20 Hz–20 kHz, then increase input amplitude incrementally while watching THD and clipping. Do not begin at 12 Vpp.
# Assembly notes
All populated parts are through-hole and suitable for hand soldering. Use the BOM’s exact MPNs where polarity or pitch matters.
1. Inspect the bare PCB against `images/board-top` and `images/board-bottom`. Verify the 58 × 40 mm outline and that J1 silk reads `+12 GND -12`.
2. Install resistors first. R5 and R9 use vertical board-orientation footprints; do not confuse their values.
3. Install D1–D4 from one production lot if practical. The glass-body cathode band goes to square pad 1. The custom vertical diode footprint places cathode pad 1 at the lower end in the board file.
4. Install U1 with pin 1 at the square pad/notch mark. A quality 8-pin DIP socket is optional but is not included in the BOM.
5. Install Q1 with onsemi 2N3904BU’s flat-face E-B-C order aligned to pads 1-2-3. Do not substitute a device with a different TO-92 lead order without rewiring.
6. C1 and C2 must be Nichicon UES bipolar parts. They have no positive/negative installation orientation.
7. C6 is polarized: positive to +12 V, negative to GND. C7 is polarized in the less intuitive direction: positive to GND, negative to -12 V. The board silk calls this out.
8. Fit terminal blocks with wire-entry openings facing the board edge. J2 is audio input/GND, J3 is control/GND, J4 is audio output/GND.
9. Clean flux and inspect every PTH joint for wetting and bridges. Check resistance from each rail to GND before inserting U1.
10. First power-up should use a current-limited, regulated dual supply. With no input, verify approximately +12 V at U1 pin 8 and -12 V at pin 4 relative to GND. Do not reverse J1.
Suggested prototype test order: 100 mVpp at 1 kHz, confirm 0 V gain, measure the five-point control sweep, sweep 20 Hz–20 kHz, then increase input amplitude incrementally while watching THD and clipping. Do not begin at 12 Vpp.