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Sidechain filter in compressors (LA3A, SSL) - when to use?

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Sidechain filter in compressors (LA3A, SSL) - when to use?

Postby stormymondays » Mon Mar 16, 2020 10:32 pm

What are your go-to uses for sidechain filters in classic compressors (and clones) like LA3A and SSL-style boxes?

I think I'm guilty of using the filters by default without thinking too much. And yes, I know, we have to use our ears to decide. Here's what I'm doing, comments welcome.

-Golden Age Comp 3A (LA3A clone). For some reason, my sidechain filter knob ended by default in the middle position, so it compresses the high frequencies a little more. I normally use it for guitars (electric and acoustic) and it usually sounds great right off the bat. So - I never tweak the sidechain knob.

-TK BC1-S (SSL style). Here I always engage the 150 Hz sidechain filter so the bass content doesn't make the compressor pump. Sounds good to me. However, I think the "classic" SSL buss compressor sound wouldn't have the sidechain filter, right? Again, since I mix right into it, I never bother changing the setting.

-Tree Audio The Branch II (opto compressor included in the channel strip). It has a 250Hz sidechain filter that I always engage when tracking vocals - only because I read somewhere that it was its intended use! :-S What would you do here?

-Drawmer 1968. The "big" and "bigger" filters do what they say, and since I use it on the drum buss, I usually have "bigger" enabled. For some reason this one seems easier!

While it all works for me, some insight would be great, such as what to listen for when deciding whether to engage the filter or not.
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Re: Sidechain filter in compressors (LA3A, SSL) - when to use?

Postby CS70 » Tue Mar 17, 2020 12:15 pm

Well, you answered yourself - if the filter is a high pass, it will make the compression disregard lower freqs and focus on the highs.

As of why doing so, it's a matter of type of music and timbre/effect you want to achieve.

Lows carry a lot of energy (and thus a lot of level). A kick drum (and at times bass) tend to produce a short peaky sound and generally they tick along with the rhythm of your son.

If your compressor "sees" that sound, it will react to it at every kick beat (or bass peak) and - if the attack is fast enough and the release short enough - lower the volume of the entire track and quickly raise it again ("pumping").

If you filter out the low freqs, the compressor will react to the higher freqs only, which have usually a different character (way more decay and lower short term dynamic changes) so the result is more natural.

On the other side, sometimes pumping is good and you want it - so it really depends on what you're after.
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