In a Variable “Vari”-Mu design the gain is being controlled by a vacuum tube. The first types of compressors were Vari-Mu and were designed for use in broadcast. The need came from trying to level out inconsistent speech on radio. Normally going from a whisper to yelling would require an engineer to ride the fader live and human reflexes are only so fast. It wasn’t until later that the studio adopted the use of compressors.
The unique quality of Vari-Mu compressors is the ratio of the gain reduction is actually increased as you hit the unit harder; this is musically pleasing when you lay into it more aggressively. Vintage Variable-MU compressors are also great for saturation as the tubes and large iron transformers all have unique qualities. Popular vintage Vari-Mu include RCA BA-6A, Altec 436, UA-175. The most popular modern Vari-Mu is the Manley of the same name which is used very prominently in mastering studios across the world.
Diode bridges have been used in radio for automatic level control for a long time. Diode bridge audio compressors offer fully configurable parameters. A number of compressors designed in the 60’s used this basic design, but you don’t see many used in modern compressors. If the unit isn’t designed well they can be noisy due to the low signal level allowed by the didoes.
Diode bridge circuits allow for the compression curves and the attack and release to be designed independent of the compression element. These compressors are tonally distinctive, because of the diodes harmonic distortion. These designs are flexible and can add a pleasant, colorful character.
Famous examples include the Neve 2264,Neve 33609 and newer examples include the Rupert Neve Shelford Channel Strip compressor.
Fun fact: The 33609 we have at Sabella Studios was actually taken from HBO studios when they decided to throw them all out and go digital.
I admittedly didn’t even know what a PWM compressor was before writing this article. Gregory Scott, a compression guru from Kush Audio, recommended I add a section on Pulse Width Modulator circuits. After a bit of research I found out the reason I had never heard of a PWM or used one is because not that many that exist. According to Scott, “there are only a few because you truly have to be a rocket scientist to design one, aka Dave Hill or whatever geek at Pye did theirs.”
A Google search turned up an essay written by Dave Hill of Crane Song. In the essay Dave explains that all compressor circuits have good and bad characteristics. He goes on to say, “in designing a compressor with as little artifacts as possible, the gain control choices are limited. PWM has been used in vintage compressors and also modern devices. If one takes that idea and uses the latest technology it is possible the build a compressor with very little negative sonic artifacts.”
VCA’s let a percentage of energy through depending on the control voltage. The problem is that when the control voltage is changed you start to hear bad sounding artifacts. If there was a switch to control the energy on the output you could give a more accurate average control voltage leading to a more sonicly pleasing unit. The PWM circuit turns the control voltage into an incredibly fast variable width switch that controls the energy that is being outputted.
What is the difference between hard and soft knee?
Soft knee compression gradually attenuates the signal after it has passed the threshold while hard knee compression attenuates the signal immediately after it has passed the threshold. Soft Knee compression is thought to be more musical because it’s not as abrupt and abrasive; hence the name soft.