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How Do Analog Stereo Compressors Work?

I’ve over simplified the question in the title above about stereo compressors, but that gets to the essence of the discussion between myself and a smart gentleman, who I thank for sending in such a high quality set of questions.

how do stereo compressors work?

Thanks to all of the people writing in. We can’t get back to everyone, but we’re doing our best. We’re performing triage over here, so the goofier the question, the less likely we are to respond. And we’re absolutely not responding to all you self-promotional bloggers, so knock it off. On to the questions:

Stereo Compressors on the Group Bus


I recently read your article on Bus Compression and just had a question. You mentioned that if you’re using two mono compressors, in order to get equal compression in both channels they need to be stereo linked.

Otherwise you’re doing dual-mono bus compression. If I’ve understood correctly, this is where (even if both compressors were set to the same settings) both channels would react differently depending on the dynamics of each side / channel.

Obviously some bus compressors only have one set of controls, like the SSL Bus Comps. But excluding those, if you have a dual compressor with two separate channels and a link capability, is it simply stereo compression when it is linked (so both channels react the same), and when it’s unlinked it’s working as a dual-mono bus compressor? Or are there exceptions?

I ask because I recently watched a video where the guy was using a Chandler Zener Compressor / Limiter (a two channel stereo compressor). And he was dialing in each channel individually to different settings, plus there was a switch that said mono/stereo, which I assume is the channel link when its in stereo, but when I talked to him and asked whether he was compressing in stereo or dual-mono, he said that it was stereo.

So was he just confused, or are there some compressors that you can control both sides and if you set both sides the same that they will react exactly the same just like a linked stereo compressor, Or when you are dialing in both sides separately would it always be dual-mono?

Also, how do you determine if a mix or master could benefit from dual-mono compression vs. stereo? Because isn’t a drawback of stereo compression that when there is a floor tom hit that’s panned fully left and a gentle guitar panned fully right, when that floor tom strikes, then that guitar is also getting compressed the same amount and for the same amount of time as the floor tom.

Whereas in dual-mono, maybe during the part that the floor tom hits the guitar stays unaffected. Are there any benefits or merit to that thinking or technique? Or any other reasons that dual-mono compression may be a consideration on a mix or master?



James, thanks for emailing in. This is a whopper. Let’s strap in and take the ride. Remember, this is all in the context of master bus compressor or on a group bus. Either way, we’re dealing with a group of instruments or a full mix, with instruments panned all around.

Stereo Linking Dual Compressors

There are basically two types of dual analog compressors. There are those are sold separately as mono channels, usually taking up one rack unit each. These each provide a stereo link jack that can be connected with a patch cable to turn them into a stereo comp.

stereo compressor
Typical Two-Channel Compressor in One Chassis

There are also bigger two and three rack unit models that include two channels of compression in one chassis, like the Chandler Zener Compressor you mentioned. These are wired internally so you can turn them into a stereo compressor with a switch rather than having to deal with a patch cable behind the rack.

Controlling Stereo Compressors

For dual-mono units with stereo link jacks, when connected with a patch cable, they will become stereo linked and the controls of one of them will become inactive. Often, you can choose with a switch which one is the master (meaning its controls remain active) and which one is the slave (where the knobs stop working and it follows the lead of the other).

For instance, I have a pair of DBX 163X’s in my rack next to me as I type this. They’re cheap and I bought them to compress the audio of movies when I watch at my computer, since down-mixing from 5.1 usually sounds horribly imbalanced in terms of volume in 2.1 without a center channel.

When I stereo link them on the back with a patch cable, I also choose which is the master. Both have an LED on the front indicating whether or not stereo linking is enabled and which one is the current master. It’s that simple.

On bigger dual compressors in one chassis, when you flick the switch for stereo linking, sometimes these will automatically enter a master/slave relationship where the left channel becomes the master. Other times, like with the Chandler Zener, you’ll have to set the controls of both sides to the same settings. Make sure to check the user manual of whatever compressors you’re using to know.

The key to remember as we continue this discussion is:

  • Dual-mono compressors will still fire independently and act differently on each channel of the audio. This is perfect when desired and a disaster otherwise. They’re not to be used on mixes or buses.
  • Stereo-linked compressors will both fire together regardless which channel gets activated, and they need to perform the same operations at the same time. This is forced with a master/slave wiring setup or you have to set both compressors to the same settings manually.

This may not seem to make sense on the surface, but let me explain why you’d have to tweak the settings on both sides below.

How Stereo Compression Works

As mentioned, sometimes there won’t be a master/slave set-up so you need to set the controls of both sides to the same setting. This is so both sides will react the exact same way to the audio input.

When two compressors are set up as mono channels, they will each respond independently of the other. Using your example, if the floor tom in the left channel exceeds the threshold, only the left channel compressor with activate. Same goes for the guitar in the right channel.

When they are stereo linked, if the left-panned floor tom exceeds the threshold, both compressors will be activated and compress both channels at the same time, even if the right-panned guitar never exceeds the threshold. This is preferable when mastering when you don’t have stems.

Whether or not the guitar gets compressed totally depends on how deeply you have threshold set. If set to a higher decibel level, the guitar may pass under the threshold and not be affected.

The guy in the video you’re talking about was either confused or was trying to do some magic where both compressors are triggered at the same time but one compresses more than the other. I’m 99% sure he was confused in his response to you, because otherwise it completely defeats the point of stereo compression.

The core problem he’d be experiencing is a shifting of the center of his mix. If the left side compresses more than the right side, the perceived loudness of the left channel will increase and the center of the mix will shift to the left.

Then it’ll snap back to the center (how quickly depends on the release setting). This is obviously an unwanted effect, one you can’t escape if both compressors aren’t using the same settings. In stereo compression, you will always use the same settings, which is why the master/slave setup often exists or you have to set both sides to the same settings.

Dual-Mono vs. Stereo Compressors in a Mix

That last paragraph kind of deals with your final question. There’s no reason to use dual-mono compressors on a full mix, whether in mastering, on the master bus, or a group bus.

You’ll move the center of your mix left and right, wobbling back and forth. Mono compressors are used during mixing on individual mono instruments, but never two of them on a whole mix.

In mastering, generally you’ll use a stereo compressor to barely compress at all. You’re just providing glue to the mix and using a very high threshold simply to “push down” the tips of the waveforms.

This is simply to buy you more headroom so you can normalize the track afterwards to get more volume (for the loudness wars). Some will even snip the tips completely with a limiter as long as there’s no perceivable distortion. The key is to keep the compression / limiting sounding musical.

The Panned Floor Tom / Guitar Dilemma

Back to your original example. Let’s say you’re a mastering engineer and you’re delivered a mix where the floor tom panned to the left is entirely too loud. You may try to use an equalizer to target it, but since we’re talking about compression let me tell you about another tool, called Multiband Compression.

It works just like a parametric equalizer. You’ll be able to target the exact frequency range you want to reduce in volume and even set the frequencies of the threshold separately. This lets you isolate instruments, effectively so since in your case the floor tom is panned hard left like in LCR mixing.

The truth is though, if anything is that bad out of whack, it needs to get sent back to the mixer to be fixed. There’s some things you simply can’t fix in mastering, even with stereo compressors and multiband compressors. Mastering is for a polish and unity across an album, not surgical fixes in the mix.

I hope this helps. Hit me back if needed,