MaiAo Original Design DIY Amplifier Circuit Kit

Product Name: Professional Tube Preamp Circuit Board (DIY Kit)
Package Includes: Full set of loose electronic components (requires assembly) + vacuum tube + power transformer

Function and Role of a Microphone Preamp:
A microphone preamp is often thought of as simply amplifying the microphone signal, but its primary role is actually to enhance and refine the sound coming from the microphone, making it more pleasing to the ear. A good preamp adds weight and cohesion to the low end, smooths out harsh highs, and rounds out the midrange. It can also improve a microphone’s ability to handle high sound pressure levels. In short, a preamp is a tool that adds polish and character to the microphone’s tone—a staple in any professional recording studio. Conversely, a poor-quality preamp may amplify volume but can make the sound harsh, lose texture, or even introduce distortion and clipping. Therefore, in professional recording, a quality preamp is essential—its importance is on par with the microphone and audio interface. If a microphone is worth tens of thousands, a good preamp holds equal value.

Circuit Features:
This preamp uses a hybrid tube and transistor amplification design. The tube stage adds warmth and richness to the sound without muddying the lows, while the transistor stage maintains punch and power in the low end. The midrange remains honest and full, and the highs are harmonically balanced with low distortion and minimal noise. Many budget tube preamps (¥1,000–3,000) use op-amp and tube combinations, whereas high‑end models (¥10,000+) typically employ discrete transistor and tube circuits. The downside is larger size, more components, and higher cost. The upside is lower noise, higher fidelity, and a more natural sound.

Component Selection:

• Vacuum tube: Low‑noise, high‑transconductance 6DJ8/6922/ECC88

• DI channel: High‑impedance FET front‑end, followed by tube stage

• Coupling capacitors: German WIMA, Japanese Rubycon

• Power supply: LM317/337, 7806 for regulated DC

Board Specifications:
Dimensions: 13 × 20 cm
Over 200 components; values printed on PCB for easy assembly
Four transistor models have been sanded off for circuit protection
Resistors should be verified with a multimeter before soldering
Tubes are tested before shipping; other components have very low defect rate
If the unit does not work after assembly, it can be returned for inspection
No returns are accepted without valid reason

Technical Specifications:

• Minimum gain: 7 dB

• Maximum gain: 67 dB

• Frequency response: 10 Hz – 100 kHz (±2 dB)

• Equivalent input noise: –128 dB (60 dB gain, 10 Hz – 20 kHz, 50 Ω)

• THD: <0.006 % (10 Hz – 20 kHz, +22 dBu output)

• CMRR: >64 dB at 60 dB gain; >75 dB at 30 dB gain (50 Hz – 20 kHz, 100 mV input)

• Max input level: +22 dBu (without pad, 8 dB gain, 20 Hz – 20 kHz)

• Max output level: +34 dBu (20 Hz – 40 kHz, into 10 kΩ)

• Mic input impedance: 5 kΩ (2 × 2.5 kΩ)

• HI‑Z input impedance: 1 MΩ

• Phantom power: +48 VDC (±1.5 VDC)

• Power consumption: 8 W

• Mains power: 220 VAC, 50/60 Hz