Audio Preamplifier Design Calculator

Design high‑performance audio preamplifiers: compute gain (dB & V/V), feedback resistor values, input coupling high‑pass filter, and visualize frequency response.

Feedback Network (Gain)
Voltage Gain: 11.00 V/V   |   Gain: 20.83 dB
Input High‑Pass Filter
Low‑cutoff (-3 dB): 15.92 Hz
fc = 1 / (2π × Rbias × Cin). Ensures subsonic roll‑off and DC blocking.
?️ Line Stage (gain 10 dB)
? Microphone Pre (40 dB)
? MM Phono (34 dB / RIAA eq optional)
? High Gain (60 dB, low noise)
? Hi-Fi Flat (gain 15 dB)
Local & private: All computations run inside your browser. No data is uploaded.
? Preamplifier Performance Summary
Midband gain (V/V): 11.00
Midband gain (dB): 20.83 dB
Feedback factor β: 0.0909
Input impedance (Zin): ≈ 10.0 kΩ (dominated by Rbias)
Output impedance (Zout): ≈ < 0.1 Ω (closed‑loop, typical for audio op‑amps)
Corner frequency fL: 15.92 Hz
Frequency response (low‑frequency roll‑off) – input high‑pass filter effect
Magnitude response normalized to midband gain. First‑order high‑pass characteristic with -3 dB at fC. Frequency range: 1 Hz – 200 kHz.

? Non‑Inverting Op‑Amp Preamplifier: Core Principles

The non‑inverting amplifier is the gold standard for audio preamplifiers because of its high input impedance, low output impedance, and stable gain set by two resistors. Gain equation: Av = 1 + Rf/Rg. The input coupling capacitor Cin together with the bias resistor Rbias creates a high‑pass filter that blocks DC offset and defines the low‑frequency cutoff: f-3dB = 1 / (2π Rbias Cin). Proper selection ensures subsonic noise rejection without audible bass loss.

Av (dB) = 20·log₁₀(1 + Rf/Rg)   |   fc = 1/(2π·Rbias·Cin)

For ultra‑low noise, choose metal film resistors (0.1% tolerance for precision) and a high‑quality op‑amp such as NE5532, OPA2134, or LM4562. The calculator assumes ideal op‑amp behaviour; in real circuits, slew rate and bandwidth limitations appear at high frequencies, but for audio (20 Hz – 20 kHz) most modern op‑amps perform excellently.

⚡ Practical Design Steps & Trade-offs

  • Gain staging: Keep total gain appropriate for the source: line level (0 dB to 20 dB), microphone (40-60 dB), moving magnet phono (34-40 dB before RIAA).
  • Rg selection: Low values (100 Ω – 1 kΩ) reduce Johnson noise, but ensure the op‑amp can drive the feedback network without excess current.
  • Input capacitor: Use film capacitors (polypropylene) for lowest distortion; value ≥ 0.47 µF for 10 kΩ bias to keep fc below 10 Hz.
  • Power supply: Dual ±12V to ±15V maximizes dynamic range. Decouple with 100 nF ceramic and 100 µF electrolytic capacitors.
Case Study: Low‑Noise Microphone Preamplifier for Dynamic Mics

A dynamic microphone (e.g., Shure SM58) requires roughly 40–50 dB gain to bring the signal to line level. Using Rg = 100 Ω and Rf = 10 kΩ gives Av = 101 (40 dB). Setting Rbias = 2.2 kΩ and Cin = 22 µF yields fc ≈ 3.3 Hz – no bass roll‑off. The calculator confirms the numbers; the chart visualizes the filter response. This preamp configuration can be built with OPA1611 for extremely low noise (0.9 nV/√Hz).

?️ Component Tolerance & Real‑World Implications

Resistor tolerances (1% or 5%) affect actual gain. Use E96 series for precision. The high‑pass cutoff is dominated by electrolytic capacitor tolerance (±20%). For critical subsonic filtering (e.g., vinyl playback), calculate using worst‑case values. Modern preamps often include a servo or additional high‑pass switch to eliminate rumble.

Application Recommended Gain (dB) Typical Rg (Ω) Typical Rf (Ω) fc target (Hz)
CD / DAC buffer 6 – 12 1k – 2k 2k – 8k < 5
Moving Magnet Phono 34 – 40 100 – 330 4.7k – 33k 10 – 20 (with RIAA)
Dynamic Microphone 40 – 60 100 – 470 10k – 100k 2 – 10
Instrument / Guitar 20 – 30 470 – 1k 10k – 33k 10 – 30

Why Input Impedance Matters

The non‑inverting input impedance is extremely high (≥ 1 MΩ for JFET op‑amps). However, the actual input impedance for AC coupling is determined by Rbias. This resistor sets the input loading for the source. For microphones, 1–10 kΩ is standard; for guitar pickups, you need ≥ 500 kΩ to avoid treble loss. The calculator shows the effective Zin = Rbias (ignoring negligible op‑amp bias current effects). Adjust Rbias accordingly.

Expertise reference: Design principles derived from Douglas Self’s "Small Signal Audio Design" (3rd ed.), Texas Instruments Audio Design Handbook, and operational amplifier application notes by Analog Devices. This tool follows industry best practices for gain accuracy and filter synthesis. Last revision: May 2026.

❓ Frequently Asked Questions

Usually between 6 dB (2×) and 15 dB (5.6×) to drive power amplifiers. Higher gain may introduce unnecessary noise. Our calculator helps adjust R1/R2 to match your system.

Cin = 1/(2π × Rbias × fc). For fc = 10 Hz and Rbias = 10kΩ, Cin ≈ 1.6 µF. Use 2.2 µF film or bi‑polar electrolytic for affordable designs.

This version focuses on the non‑inverting topology due to its higher input impedance. For inverting designs, gain = -Rf/Rg; you may adapt the resistor values manually.

At gains > 40 dB, parasitic capacitance and poor layout cause instability. Use a small capacitor (10–47 pF) in parallel with Rf and follow proper grounding. The calculator assumes stable conditions.

If powered by a single supply, output coupling is needed. For dual supply, typically DC‑coupled output is fine if the next stage has no DC offset sensitivity.
References: Self, D. "Small Signal Audio Design" (2020); TI Application Report SLOA011; "Op Amps for Everyone" (Ron Mancini). Analog Devices Audio Design Center.