Subsonic filter frequency

In a phono preamp

A phono preamp takes the small signal from a moving-magnet (MM) or moving-coil (MC) cartridge, amplifies it and applies playback equalization: the frequency correction needed for records. It supplies a line-level signal for an amplifier’s line input, such as AUX. Within this function, a subsonic filter reduces unwanted very-low-frequency energy before it reaches the rest of the system. Pro-Ject Phono Box S2 manual, pp. 2–5.

Record warps and mechanical movement can create low-frequency signals that make a woofer move even when little sound is audible. Reducing that energy can reduce unnecessary speaker movement and preserve amplifier headroom—the remaining capacity before overload. This is a possible benefit, not proof that every moving cone indicates a warp or that a filter will cure every problem. Bandwidth Audio Kaskode One manual, §1.9.

The trade-off is wanted low bass. With otherwise comparable curves, raising the cutoff extends attenuation farther into the bass. Stronger filtering can therefore remove something you wanted to hear. A higher frequency is not a quality ranking. Analog Devices: Analog Filters, pp. 8.7–8.8.

Filtering also does not replace cartridge matching. Gain is the amount of amplification; loading is the electrical load presented to the cartridge. Set these from the cartridge and phono-stage documentation. Changing a low-cut setting does not correct unsuitable gain or loading. Pro-Ject Phono Box S2 manual, p. 2.

Compare a documented setting

This is a practical comparison framework; the equipment manual determines which actions are available.

1. Check the applicable manual for the active input/output path, filter control and starting setting. Follow its power and disconnection instructions before changing settings or connections. Use only documented user controls; do not open the case or move cables while the equipment is powered.
2. Compare the same affected record passage at a modest, unchanged listening level. Observe unwanted slow woofer movement or rumble and listen to the bass balance.
3. Keep the setting if it reduces the unwanted effect while retaining acceptable bass. If it brings no useful improvement or removes bass you want, restore the previous setting or the manual’s baseline, following the same precautions.
4. If the problem persists, stop increasing the filtering in search of a cure. Consult the manual, manufacturer or a qualified technician to investigate the cause.

The comparison combines the documented purpose of filtering with manual-led setup precautions; it is not a reported listening test. Bandwidth Audio Kaskode One manual, §1.9; Pro-Ject Phono Box S2 manual, pp. 2 and 4–5.

An IEC equalization label can be associated with low-frequency filtering, but the label alone does not state a numeric cutoff. Look for an explicit frequency and response specification. Bandwidth Audio Kaskode One manual, §1.9.

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In an optical-cartridge equalizer

Here the specification may be named Low-Cut Frequency. The filtering principle is the same, but the cartridge connection and the way you select a response need particular attention.

In the DS Audio optical-cartridge system described here, an LED and light sensors detect stylus movement. The compatible equalizer supplies power to the cartridge’s LED and applies the different playback equalization that this cartridge needs. Its audio signal is analog—a continuously varying electrical signal; “optical” here does not mean a Toslink digital-audio connection. An MM/MC-only phono input cannot substitute for the required optical equalizer. DS Audio: Optical cartridge explanation.

Low-cut filtering can suppress signals from record warps and tonearm resonance—the arm and cartridge’s low-frequency mechanical oscillation. DS Audio identifies these as reasons to filter its optical-cartridge output; that explanation does not establish how another optical design behaves. DS Audio: Phono-Equalizer Circuit Technical Information, p. 3.

Read the switch and output together

The DS003 manual provides a concrete example: a 30/50 Hz switch and two output pairs produce four response curves. These are not four distinct cutoff frequencies. Record both the switch position and the output pair when interpreting a setting. Its second output uses combined filtering; reducing every curve to one assumed cutoff-and-slope pair would lose information. DS003 manual, p. 3, controls 3–4 and graphs a–d.

Before connecting, confirm that the exact cartridge is supported by the selected optical input or module. Do not connect an MM/MC cartridge to a powered optical-only input. Check the destination too: the DS003’s line output feeds an appropriate line input such as CD or AUX, not another phono input. Matching RCA plugs do not establish compatibility. Follow the manuals for the actual equipment. DS003 manual, pp. 3–4.

Choose among documented curves using the comparison above. The higher frequency can reduce more wanted bass, so it is not automatically the best setting. Equal cutoff numbers on two devices say nothing about compatible cartridge power or equalization. Compare the complete response and the required connections, not just the Hz value. DS003 manual, pp. 3–4; DS Audio: Optical cartridge explanation.

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