You are here

Warm Audio Bus Comp

Stereo VCA Compressor By Matt Houghton
Published March 2022

Warm Audio Bus Comp

This homage to a famous console compressor is keenly priced and offers some thoughtful extra features.

As its name suggests, the Bus Comp is Warm Audio’s nod to the classic SSL 4000 G‑series console’s VCA bus compressor. That processor has acquired a reputation as a ‘glue’ compressor on the stereo master bus, and it has spawned a host of rackmount, 500‑series and software imitators available at a vast range of prices, each seeming to offer slightly different sonic and functional touches to distinguish it from the crowd. Consistent with their mission to bring professional products within reach of the average home and project studio engineer, perhaps the biggest selling point of Warm’s version that it is priced more keenly than most of the hardware competition. But it does offer some useful features too.

Outside & In

The Bus Comp is built into a 19‑inch 1U rackmounting chassis which extends about 24cm behind the rack ears. On the rear, the XLR balanced input and output pairs are duplicated on TRS jacks for convenience, which is a welcome touch, and these are joined by another XLR (no parallel jack this time) for the mono external side‑chain input, which allows you to use sources other than the one being processed to trigger the gain reduction. Mains power comes in through the usual IEC inlet, which is switchable for US or European voltages and is accompanied by an external chassis grounding point.

Moving to the front, the brushed‑steel panel is populated with six knurled metal knobs, three large backlit buttons, a moving‑coil gain‑reduction meter and an on/off rocker switch, all laid out in largely similar fashion to SSL’s own rackmount version of their famous processor. The threshold and make‑up gain controls are pots, whereas the ratio, attack, release, and HPF (side‑chain filter) knobs turn rotary switches. The buttons engage/bypass the compression circuit (including the make‑up gain stage), toggle between the internal and external side‑chains, and optionally route the output signal through a pair of Cinemag transformers — another thoughtful touch that adds to the tonal versatility. First impressions are that it seems solid and well built; with no wobbly knobs, loose switches or anything like that there don’t appear to have been any corners cut here.

On the inside, a toroidal power transformer steps down the mains AC voltage and a single main PCB hosts most of the circuitry, including for the power‑supply regulation and the signal path. This board also hosts the rear‑panel I/O but not the heavy output transformers, which sit off to one side and are mounted on the chassis. Most front‑panel controls sit on a vertical daughterboard that connects to the main PCB using a ribbon connector, though the meter and on/off switch are fastened to the front panel individually and have their own wires to the main PCB. The PCB is populated with traditional through‑hole components, which may not be inherently better than SMDs in terms of performance, but I do approve since it makes access for future repairs rather easier. Everything appears to have been assembled with the requisite care and attention.

The more upmarket products of this type tend to use four THAT 2181 VCA chips for the gain reduction, in a parallel configuration which lowers the noise floor by 3dB each time you double the number of chips. This is one point of departure in Warm’s implementation: a single THAT 2180C in each channel’s signal path takes care of the gain reduction. The 2180 is a pre‑trimmed version of the 2181, and they’re graded A (best) to C at the factory according to the distortion measurements. To put a sense of scale on this, the C version has typically 0.03% THD compared with 0.005% for the 2180A. The 2181A manages 0.0025% but requires slightly more complex circuitry and individual alignment. But while the Bus Comp doesn’t have the bragging rights when it comes to comparing THD+noise specs, it’s by no means terrible, and for a good 90 percent of prospective users the difference won’t matter in any practical sense. The noise floor doesn’t really become an audible issue unless you really crank the gain reduction and make‑up gain, and I can’t say I’d ever want to do that for the intended subgroup and ‘mix glue’ applications, where you’re dealing with fairly high signal levels and modest amounts of gain reduction. The distortion will also be moot for most users; what distortion there is is subtle and benign, and pales into insignificance compared with the colour added by the transformers. So while some users may decide to pay more for a quieter device, I reckon this is a pretty good way to be keeping the asking price low.

The main analogue I/O (apart from the external side‑chain input) are duplicated on XLR and TRS jacks for easy integration into your studio.The main analogue I/O (apart from the external side‑chain input) are duplicated on XLR and TRS jacks for easy integration into your studio.

Test Drive

To test the Bus Comp in the ‘real world’ I used it on a few mix projects over the course of several weeks, on subgroups (drums plus bass, and electric guitars), the master stereo bus and a few individual sources, including bass and acoustic guitar — worth mentioning since a lot of people forget that despite this being a stereo processor, you can use a single channel to process mono sources! I also took the opportunity to compare it with my Serpent Audio SB4001 and the Wes Audio ngBusComp that I happened to have in for review at the same time (both of which cost rather more), as well as some plug‑ins.

In terms of general gain reduction, the Warm Audio Bus Comp behaved just as I’d hoped. It performed admirably in gelling things together on the mix bus, with a low (1.5:1 or 2:1) ratio and just a couple of dB of gain reduction tickling the meter into action. On the drum bus, I could go more aggressive, with a slow attack, fast release, 4:1 ratio and transformers switched into the signal path, and 60Hz side‑chain HPF helping an uptempo rock part smack beautifully. (A nicer effect still when followed in the chain by a nice tape emulation). As when using most such compressors aggressively on drums, I found that I wanted to tweak the level balance of the multitracks feeding the bus after I’d put the compressor in the signal path, most notably to lift a fairly snappy‑sounding snare drum higher up in the mix that was hitting the compressor.

It performed admirably in gelling things together on the mix bus, with a low (1.5 or 2:1) ratio and just a couple of dB gain reduction tickling the meter into action.

This wouldn’t be my first choice of compressor for processing individual sources, but it proved more than capable of reining in a rather dynamic acoustic guitar performance, and in getting that part and a vocal in the same song to gel together. The transformers weren’t so nice on the guitar part, muddying the low end a little. But on bass, it worked well as a limiter and the transformers sounded great, thickening and enhancing the sound nicely.

The switchable transformers are definitely a useful feature, then — not a gimmick — but I wonder if Warm might have missed a trick in not including some means of attenuation after the transformers so that you can drive them hard and still have a sensible level hitting the next device in the signal chain.

In terms of control, it’s a tad less versatile than my Serpent unit, which just has more options on board in terms of ratios, release times and so on. But whether you miss any of those features depends very much on what you want to use it for. The feature I missed the most, which appears on several similar compressors now, was a wet/dry mix control, which allows instant parallel compression and extends the versatility of the processor on different sources considerably. But not everyone likes to work that way (you can set up parallel compression on any mixer or in a DAW and have greater control), and you get plenty of features for your money here.

Whatever the specs might say, the aforementioned ‘noise’ wasn’t an issue in real‑world situations: I had to turn the make‑up gain knob clockwise past the 12 o’clock position (about +15dB of gain) before any hiss became a distraction at sensible monitoring levels, and that was when the track wasn’t playing back; when it was, I just didn’t notice it. Moreover, that gain setting just wasn’t one I needed to get close to when compensating for the gain reduction I actually wanted to dial in.

On the whole, then, I’m happy to recommend the Bus Comp. It ‘does what it says on the tin’ and, while it might not offer quite as much control or quite as stellar on‑paper specs as some of its pricier competitors, none of that gets in the way. It certainly doesn’t sound or feel ‘cheap’ and it offers the very appealing bonus of those Cinemag output transformers. I’d have no qualms about leaving this on my mix bus.


  • It does that VCA ‘glue’ compressor thing nicely.
  • Switchable output transformers add pleasing character.
  • External side‑chain input.
  • Keenly priced.


  • Not as feature‑laden as the competition.
  • Noise floor could be lower.


This competitively priced VCA compressor is very happy sitting on a drum or mix bus, and the output transformer coloration option is welcome.


£679 including VAT.


Warm Audio +1 (512) 348 6585.