Choosing appropriate phono preamplifer gain for a phono cartrage

Choosing the right phono gain is about amplifying the cartridge's tiny output voltage so it reaches approximately the same level as other line-level audio sources without overloading the phono stage or your preamp.

The basic calculation

Use this formula:

$$\text{Gain (dB)} = 20 \times \log_{10}\left(\frac{\text{Desired output voltage}}{\text{Cartridge output voltage}}\right)$$

Where:

• Cartridge output voltage is the manufacturer's rated output (usually measured at 1 kHz, 5 cm/s).
• Desired output voltage is typically around 300–500 mV for a line-level input.

Example 1: Moving Magnet (MM)

Cartridge output: 5 mV

Desired output: 500 mV

$$20 \times \log_{10}(500/5) = 20 \times \log_{10}(100) = 40\ \text{dB}$$

So a 40 dB phono gain is a good match.

Example 2: High-output Moving Coil (HOMC)

Cartridge output: 2.0 mV

Desired output: 500 mV

$$20 \times \log_{10}(250) \approx 48\ \text{dB}$$

A gain setting around 46–50 dB works well.

Example 3: Low-output Moving Coil (LOMC)

Cartridge output: 0.4 mV

Desired output: 500 mV

$$20 \times \log_{10}(1250) \approx 62\ \text{dB}$$

A gain of about 60–64 dB is typically appropriate.

General guidelines

Cartridge output	Typical gain
6–7 mV	36–38 dB
4–5 mV	40–42 dB
2–3 mV	46–50 dB
1.0 mV	54–56 dB
0.5 mV	60 dB
0.3 mV	64 dB
0.2 mV	66–70 dB

Fine-tuning

The calculated value is a starting point. You may want to adjust by a few decibels based on your system:

• Too little gain: You'll need to turn the volume control much higher than usual, and noise may become more noticeable.
• Too much gain: The sound may become harsh, compressed, or overload the input of the next component.

It's common to choose a gain that's within ±3 dB of the calculated value, depending on your preamp's available settings and the sensitivity of the rest of your system.