Attiny85 timer

THT 3.3V Stereo 3-Band Equalizer & Preamp

An ultra-compact, high-fidelity, 100% Through-Hole (THT) stereo audio equalizer and headphone/line preamplifier designed for 3.3V battery or single-supply operation.

Zero surface-mount components. Optimized for hand soldering, DIY audio gear, portable headphone setups, and custom instrument tone shaping.

Highlights & Specifications

Circuit Architecture

[3.5mm Stereo IN] 
       │
       ▼ (1 µF AC Coupling)
[VOL Potentiometer (50k Log)] 
       │
       ▼
[Active 3-Band Baxandall Filter] ── MCP6004 (U1A / U1D)
  ├── BASS  (100k Linear) ~100 Hz shelf (±12 dB)
  ├── MID   (100k Linear) ~1.0 kHz band (±10 dB)
  └── TREB  (100k Linear) ~10 kHz shelf (±12 dB)
       │
       ▼
[BAL Potentiometer (100k Linear)] (Stereo cross-attenuation)
       │
       ▼
[Output Driver & Buffer Stage]   ── MCP6004 (U1B / U1C)
       │
       ▼ (100 µF AC Coupling + 47Ω isolation)
[3.5mm Stereo OUT]

1. Dual Rail-to-Rail Op-Amp (MCP6004 DIP-14)

• Sections A & D: Left and Right inverting active tone stages. Placing the tone network in the negative feedback loop ensures low distortion and predictable boost/cut curves.
• Sections B & C: Left and Right output driver stages configured for clean buffering with 47Ω series output resistors (R_STAB_L, R_STAB_R) to prevent oscillation when driving capacitive cables or low-impedance headphones.

2. Virtual Ground Bias (VBIAS)

• Single-supply audio requires referencing AC signals to mid-rail ($V_{CC} / 2 = 1.65\text{ V}$).
• Created by a low-noise resistive divider (R_BIAS1, R_BIAS2 = 100 kΩ) heavily decoupled by C_BIAS1 (47 µF electrolytic) and C_BIAS2 (100 nF ceramic).
• Virtual ground connects to the non-inverting pins (Pins 3 and 12 of U1) with dedicated traces and zero ground loops.

3. Five Control Knobs (14.0 mm Pitch)

Five dual-gang Alpha RD902F vertical potentiometers line the top edge for convenient front-panel integration:

1. VOL (RV1): 50 kΩ Dual Audio (Logarithmic) — Input gain and volume attenuation.
2. BASS (RV2): 100 kΩ Dual Linear — Low-frequency shelf response.
3. MID (RV3): 100 kΩ Dual Linear — Mid-frequency vocal and instrument presence.
4. TREB (RV4): 100 kΩ Dual Linear — High-frequency air and sparkle.
5. BAL (RV5): 100 kΩ Dual Linear — Left-to-Right stereo pan balance.

Bill of Materials (BOM)

All 51 components are through-hole parts readily sourced from LCSC, Mouser, Digikey, or local electronics suppliers:

Assembly & Soldering Sequence

To ensure easy physical access for your soldering iron tip, solder components in ascending order of height:

1. DIP Socket (Optional): If using a 14-pin DIP socket for U1, solder it first.
2. Resistors: 1/4W axial metal-film resistors (flat against PCB).
3. Ceramic & Film Capacitors: 2.2 nF, 22 nF, 47 nF, 100 nF, 1 µF disc/box capacitors.
4. Power Header & LED: 2-pin power header J3 and 3 mm indicator LED D1 (note cathode flat side).
5. Electrolytic Capacitors: 47 µF (C_BIAS1), 100 µF (C_PWR1, C_OUT_L, C_OUT_R) — observe polarity (+ marked on silkscreen).
6. Audio Jacks: 3.5 mm TRS connectors J1 and J2.
7. Potentiometers: 5x Alpha dual-gang pots RV1–RV5 (snap bracket lugs into mounting holes and solder).
8. IC Insertion: Insert MCP6004 into socket or PCB (Pin 1 oriented toward notch).

PCB Fabrication & Ordering Guide

The design is 100% compliant with standard prototype manufacturing specs (JLCPCB, PCBWay, OSH Park):

• Production Package: Upload jlcpcb_production/tht_opamp_eq_gerber_jlcpcb.zip directly to the manufacturer's order page.
• Recommended Options:
  • Dimensions: 88.0 mm × 61.0 mm (detected automatically from zip)
  • Layers: 2 Layers
  • PCB Thickness: 1.6 mm
  • Copper Weight: 1 oz (35 µm)
  • Surface Finish: HASL Lead-Free (RoHS) or Regular HASL
  • Solder Mask: Green, Matte Black, Blue, or Red
  • Silkscreen: White
  • Drill Minimum: 0.3 mm / 0.8 mm via

Project Structure

tht_opamp_eq/
├── README.md                      # Complete documentation & specifications
├── tht_opamp_eq.kicad_pcb         # Master production PCB file (KiCad 9)
├── tht_opamp_eq.kicad_sch         # Schematic source file (KiCad 9)
├── tht_opamp_eq.kicad_pro         # KiCad project file
└── jlcpcb_production/             # Final manufacturing release bundle
    ├── tht_opamp_eq_gerber_jlcpcb.zip   # Gerber & Excellon drill archive
    ├── tht_opamp_eq_bom_jlcpcb.csv      # Formatted BOM for JLCPCB / sourcing
    ├── tht_opamp_eq_cpl_jlcpcb.csv      # Component placement list (XY coordinates)
    ├── tht_opamp_eq_render_top.png      # High-res photorealistic 3D top render
    └── tht_opamp_eq_render_bottom.png   # High-res photorealistic 3D bottom render