admin 3 January 2024

Analog Audio Compressor

The idea was to design and build a modern, studio-grade, fully-analog VCA audio compressor with a somehow 'vintage' look & feel. The objective was to get as close as possible to something looking as a real product.

It is based on several design notes from THAT Semiconductors®, the main one being Design Note 115.

Yet, functionalities have been well extended with:

A pre-amplifier stage to offer both Mic/Line balanced inputs
48V Phantom power for capacitor Mic biasing
Side-chain external input for ducking applications
100Hz high-pass filter for audio/side-chain with selectable path
Analog VU-Meter
Driver for dual Line-level balanced outputs

The heart of the compressor is a THAT4301 dynamics processor including both VCA and RMS detector.

1. Schematics Analysis & Simulation

Main Board Schematics

Power Supply Board

Power Supply & Phantom

Compression Analysis

The audio input signal is routed to both the VCA and the RMS envelope detector. Let-us focus on the way gain controlled is applied.

The RMS detector provides a rectified output voltage, which is a logarithm function of the audio RMS amplitude. It is therefore proportional to the input dBu level:

On the above graph, the measured RMS detector gain is in agreement with the specified 6mV/dBu.

$V_{RMS} \approx 6 \times dBu_{in} + 60$

The RMS envelope signal V_RMS is then summed up with a DC voltage controlled by the Threshold potentiometer V_th, acting up like a tunable offset applied to V_RMS:

$V'_{RMS}=-{\left({k}_{{1}}{V}_{{{R}{M}{S}}}+{k}_{{2}}{V}_{{{t}{h}}}\right)}$

Where k₁ and k₂ are set by resistors ratio.

Because the summing stage is also an inverting-rectifying stage, only negative part of V'_RMS are kept. In other terms, gain correction is only applied when:

$V_{RMS}>\frac{{{k}_{{1}}}}{{{k}_{{2}}}}{V}_{{{t}{h}}}$

That's where the Threshold operates!

Next, the V'_RMS signal is applied to a resistor bridge including the Ratio potentiometer. The resistor network apply a variable gain {0→1} which is a non-linear function of the potentiometer position:

$\displaystyle{V}_{{{c}{t}{r}{l}}}={G}_{{{r}{a}{t}{i}{o}}}\times V'_{RMS}$

$\displaystyle{G}_{{{r}{a}{t}{i}{o}}}\in{\left\lbrace{0}\rightarrow{1}\right\rbrace}$

The plot below show the static V_ctrl (V) resulting from input signal (dBu) obtained with:

Threshold potentiometer @ 30%
Ratio potentiometer @ 80%

Next, the Attack and Release stage is basically an asymmetric rate (speed) limiter for V_crtl. It is made of two tunable current sources charging (Release) and discharging (Attack) a capacitor at dialed rates. The whole stage is loop-closed so that output voltage V_{ctrl_AR} tries to 'follow' V_ctrl.

V_{ctrl_AR} is finally summed up with another DC voltage sets by the Makeup potentiometer, before being send to the inverting input ec- of the VCA gain controller.

$V_{ec-}=-\left( k_3 V_{crtl\_AR} + k_4 V_{makeup}\right)$

Again, k₃ and k₄ are set by resistors ratio. V_makeup brings the desired offset to V_{ctrl_AR} thus modulating the static gain reduction, and therefore the output average volume after compression. After the 4301 datasheet, the global VCA gain reduction constant on ec input is about 6mV/dB.

The plots below show the global static gain reduction for various Ratio and Threshold settings. Makeup is set to zero. It basically shifts the results vertically when engaged.