TECH TIPS: Cali76 Compact Bass HPF Control – Why It Works

If Origin Effects is anything approaching a household name among the guitar community, it’s because of compression. We like to think we know a fair bit about it and our Cali76 compressor pedals are popular enough to suggest that we do. But let’s not overlook one crucial element to that popularity: bass players! 

Yep, bass players know even better than guitarists how important a good compressor is. However, we often get asked by potential customers to compare the Cali76 Compact Deluxe and Compact Bass models. So, this month’s Tech Tips is all about the one big, fat difference between the two pedals – the Compact Bass’s HPF (High-Pass Filter) control.

First, let’s start with the similarities. Both pedals are based on the industry standard and highly regarded UREI 1176 studio compressor, featuring discrete FET compression and very high headroom. There’s a DRY control which adds your clean signal in parallel as well as low-noise buffered bypass. The Compact Deluxe has separate ATTACK and RELEASE controls, whereas the Compact Bass combines these on one knob (don’t worry, all the most useful sounds are still there), making room for the HPF control. This simple control works really well for bass guitar, preventing the “steamroller effect” of too much compression on bass, and it does it by filtering bass frequencies out of the sidechain. But what is a sidechain and why does this work? 

In a compressor, the signal is split into two paths: the Signal Path and the Sidechain. The signal path is the bit you hear, the part where the compression is being applied. The sidechain, on the other hand, is where the compressor “looks at” the volume of the incoming audio (in other words, which are the loud bits that need to be compressed). Normally, this sidechain is a full-range signal, no different from the signal at the input of the compressor. Most of the time, this is fine, and it’s useful for the compressor to “see” the whole signal so it doesn’t miss any loud frequencies that need a good squashing. 

With bass guitar, though, there is a huge amount of low end present in the signal. Furthermore, this low end is consistently louder than the other parts of the signal that contain information about how hard you’re playing. When this bottom-heavy signal shows up at the sidechain input, the compressor simply thinks “wow, this is all really loud, so I need to compress everything”. The result of this is that your bass sound gets indiscriminately compressed, with your playing dynamics and articulation getting lost in the process. 

But you can’t really blame your compressor. The poor thing can’t hear whether you’re playing softly with your thumb or digging in with a pick. It’s too busy listening to that ever-present low-end information. We could try and combat this by just turning down the amount of compression but that wouldn’t quite solve the problem – you wouldn’t have those high compression levels when you really need them. The best solution is to remove low frequencies from the sidechain. 

Just to be clear, the actual signal path stays full-range, meaning that all your bottom end will be there at the compressor’s output, it just won’t be reacting to it. Also, any compression that is applied will still apply to the whole signal – there’s no multiband funny business going on here. All that’s happening is that the low frequencies are being filtered out of the sidechain, allowing high frequencies to pass through (High-Pass Filter, remember?). This means that the sidechain can now ignore all that low stuff that causes excessive compression, instead concentrating on whether you’re sitting back in the pocket or slapping away like your gig depends on it! 

Because the sidechain is now free to “listen to” these more important, high frequency parts of your sound, you’ll have a compressor that is much more responsive to the dynamics in your playing, even with higher levels of compression. It will compress harder when you play harder and vice versa, instead of fighting with your compressor for articulation. 

Of course, there’s nothing wrong with excessive compression on bass. In fact, we rather like it – that’s why we made this High-Pass Filter variable from the Compact Bass’s HPF knob. As you turn it up, you’re increasingly freeing the sidechain from more and more overwhelming low end, breathing valuable dynamics back into your compressed bass tone. 

Bass players, eh? Don’t say we ain’t good to you!