The Sidechain Filter on the Master Bus Compressor - What It Does, Why It Matters

Master bus compression can tighten your mix, glue everything together and add polish - but it can also drag things down if you’re not careful. One of the most useful but under-used controls is the sidechain filter and understanding what it really does can unlock a cleaner, more balanced mix without over-compression.

Let’s break it down - technically and practically - so you can use it with intention.

Why Use a Sidechain Filter on the Master Bus Compressor?

On most compressors, including the SSL Native Bus Compressor 2, the S/C HPF (Sidechain High-Pass Filter) doesn’t change what gets compressed - it changes what the compressor listens to when making decisions.

• The compressor duplicates your audio signal internally.

• That duplicate is filtered (high-passed) before hitting the detection circuit.

• The actual compression is still applied to the unfiltered full-range signal.

So when you roll off the lows in the sidechain, you’re telling the compressor:

"Don't react to the subs and kicks - make your decisions based on everything else"

This avoids overreacting to loud low-end hits that often dominate the energy of a mix.

Why the Low-End Dominates Compression

Most of the raw power in a mix lives below 250 Hz. Kicks, subs and low bass lines push the overall level more than mids or highs. That means:

• The compressor starts working early - often before the mids and highs would trigger it.

• The result? Over compression, pumping and energy loss.

A sidechain filter changes this behaviour. By filtering out that low-end from the detection circuit, the compressor becomes more responsive to the mids and highs, giving you more frequency-consistent and musically useful gain reduction.

Using the S/C HPF on the SSL Native Bus Compressor 2

If you’re using the SSL Bus Comp 2, that bottom-left knob is your S/C HPF. It runs from OFF to 185 Hz. Here’s how I approach it:

1. Start with it OFF.

2. Play your full mix and listen for excessive ducking or “pumping” when the kick hits.

3. Bring it up to 60–100 Hz - this is the sweet spot for most mixes.

4. If the low-end still feels heavy-handed, try 120–150 Hz, especially in bass-heavy genres.

5. Always A/B with and without the filter - sometimes the difference is subtle but meaningful.

This process helps you avoid letting the kick dominate the compression, while still gluing the mix together.

What Happens When You Use It

Think of it as shaping the behaviour of the compression — not the tone of the mix.

• Low-end elements are still compressed, but they don’t trigger the compressor.

• The mix retains more transient clarity and punch.

• The dynamics stay stable, especially when the low-end gets busy.

• You get a more natural-sounding glue that doesn’t squash your groove.

  
  

On the master bus, that usually means aiming for around 1–3 dB of gain reduction 

Just enough to add cohesion without flattening the energy.

In electronic music - especially house and techno - letting the bottom of the mix sit free from compression often helps the kick and bass hold the groove in place. When the low-end isn’t constantly triggering compression, it stays tight and grounded, becoming the anchor for the whole track. The rest of the mix can move and breathe around it, but that low-end pulse stays solid.

Wait - So It Still Compresses the Bass?

This is where it often gets a bit fuzzy - and it’s totally understandable. You might assume that by using a high-pass filter on the sidechain, you’re somehow leaving the low-end untouched by the compressor.

But that’s not quite it.

The sidechain filter doesn’t change what gets compressed - it changes what the compressor reacts to. The entire mix, including the low-end, still gets compressed when the compressor kicks in. The difference is: the compressor is no longer being triggered by the bass.

So yes - when a snare or vocal peak causes compression, the kick and bass still get turned down too. But if the kick hits hard and nothing else is happening, the compressor might not react at all - because that frequency was filtered out of the sidechain.

This can feel counterintuitive at first. You might think, “If I don’t want my low-end dipped, why would I allow it to be affected when something else triggers compression?” And in some cases, you wouldn’t - but that’s where context matters.

In genres like house, techno, or anything groove-driven, the low-end often acts as the anchor of the track. By removing it from the sidechain, you’re not isolating it, but you’re letting it sit more confidently, undisturbed by its own energy. The rest of the mix still compresses around it, which keeps everything cohesive, but the kick and bass no longer dominate the dynamic decisions.

So the bottom end isn’t getting a free pass - it’s just not the one driving the bus anymore.

Why This Isn’t Multiband Compression

This is worth pointing out: sidechain filtering doesn’t split the signal into bands and compress them independently (like multiband comp). It just filters the signal that tells the compressor when to engage. The full-frequency signal still gets processed as one.

That’s why it sounds more cohesive than multiband in many cases - and also why it’s more subtle.

Genre-Specific Recommendations

Your setting will depend on the genre and the role of the low-end:

• EDM, Hip-Hop, Bass Music: try 100–120 Hz.

• Rock, Indie, Pop, Acoustic: try 60–80 Hz.

• Cinematic/Electronic Hybrid:Consider automating HPF across sections.

Advanced Application: Automating the HPF

This is often overlooked: automation.

Say your track opens with sparse pads, then drops into a full beat. You might want more compression reactivity in the intro, but keep the low-end filtered when the drop hits. Automate the sidechain HPF to adapt to your mix.

It’s a subtle detail - but if you’re already riding levels and gain-staging carefully, this just gives you finer control over dynamics.

Pitfalls to Avoid

Here are the big mistakes people make when using sidechain filter on the master bus:

1. Thinking it filters the audio output - It doesn’t. It filters the signal the compressor reacts to.

2. Ignoring how much low-end affects compression - Bass carries energy. A kick can trigger 6 dB of gain reduction on its own.

3. Overcompressing anyway - If your threshold is too low and your ratio is too high, the sidechain filter won’t save you. Keep it subtle (1.5–4:1 ratio, ≤6 dB reduction).

4. Applying it blindly across genres - Not every mix needs aggressive sidechain filtering. Trust your ears.

5. Forgetting to A/B test - Always compare with and without the filter to hear how it’s shaping your dynamic response.

6. Not automating - Static settings don’t always suit tracks with evolving arrangements. Adapt the filter to follow the energy.

   
   

Bonus: Other Places to Use It

While the SSL Bus Compressor 2 only gives you a high-pass filter for the sidechain, that alone can be hugely effective beyond the master bus.

• Drum buses - Filtering out the kick lets the compressor respond more to the snare and overheads, which often gives a tighter groove without the whole kit ducking every time the kick lands.

• Parallel compression - On a drum or instrument group, a subtle high-pass in the sidechain can keep the compression focused and musical.

• Bass-heavy subgroups - If a synth bass and a kick are sharing a bus, the HPF helps prevent the kick from driving all the compression.

More advanced compressors (like FabFilter Pro-C 2) offer full sidechain EQs where you can also tame high frequencies - but with the SSL, you’re working specifically with low-end filtering to control how the compressor reacts.

Final Thoughts

The sidechain filter on the master bus compressor is one of those subtle controls that separates clean, controlled mixes from squashed, energy-drained ones. It’s not just a technical trick - it’s a musical decision.

Used well, it keeps your mix breathing, lets the groove shine through, and gives you that polished “glue” without killing the dynamics.

It’s not about what it changes. It’s about what it protects.