DBX Compressors, What do they sound like and how do they work?

Dbx, synonymous with compression in every studio for the last 40 years. But what does it sound like and how do they do it?

The company was primarily the brainchild of David Blackmer and in 1971 he developed a circuit for addressing the then pressing issue of noise reduction as this was the era of tape. He did this by compressing the audio at the recording stage and then played back with an expander, this made softer sounds louder at playback at further away from the levels of tape hiss thus giving a greater ‘signal to noise’ ratio.

The compressor part of this process was achieved by a clever array of transistors dubbed the Blackmer Gain Cell. This original four-transistor design ultimately formed the basis of every iteration of the Blackmer gain cell, including the seven-transistor 200 “silver can” VCA used in the 160 and 161 compressors. The 160 also features the 208 RMS detector.

The attack is designed to be as fast as possible and the attack trajectory depends on the internal impedances of the transistors. This means that the attack curve for the dbx 160 is highly nonlinear, with an attack time that increases as the attack progresses.

Today, the 160's popularity continues, at least in part, because of the eccentricities of its RMS 208 detection circuit.

Two units that were aimed at the consumer hifi market, the 119 and 117 also utilise the same 208 rms detector circuit and the 200 vca, the other main consumer unit the 118 share a similar architecture but not these exact same components.

This basic design later evolved into the 8 transistor 202 VCA which was the heart of their flagship studio compressor the 165.. With this they also added attack and release controls. The original 160 was a strict hard knee compressor the clamped down on the signal as soon as it recached a certain threshold whereas the 165 offered a soft knee or in their terms ‘overeasy’ form of compression that started to be applied before the threshold was reached.

The 202 VCA went through a few iterations with incrementally improved gain control and total harmonic distortion. a later gold version (the 202C) ended up being used by SSL for their famous 4000 series bus compressor.

Moving away form discrete components, the company introduced the 163x using 2150 IC, which was the first monolithic implementation of the Blackmer gain cell. This would also be the heart of the 160X.

Obviously the trend of the more cost effective ICS continued eventually replacing all of the discreet VCA’s. fFor the most part, they sound very similar.but people still claim that the older VCA have a certain charm which keeps these older units fetching good money on the used market.

For instance the 165 will still set you back at least $1500. There are plug ins of course and the less expensive consumer series I mentioned earlier. These are unbalanced and inherently less HI FI than their studio siblings but if its charm your after then these units have it in spades.

I’m currently using a 118 for this purpose and I also have a pair of 163X;s and a 166 to draw some comparisons with the flagship 165.

Here is comparison between the units I have in the studio and 165A from Access Analog that I use remotely. I’ve also thrown in some plugins as well. He compression ratio will be about 5 to 1, the 163x has no ratio control so I’m just going by gain reduction amount and by ear. I’ll start off with some obvious but not excessive compression and then lower the threshold for some more aggressive character inducing goodness.

As the 165 has attack and release, when I applied heavier compression then the differences became much more apparent with a drum loop as the processes signal. To counteract this, I used a transient designer to help even out the limited implementation of these compressors in attempt to see if I could obtain similar sound through these less expensive units