What Is A Diode Bridge Compressor & How Does It Work?

The diode bridge compressor type is a lesser-known type of compressor but is worth knowing about when studying audio production and the tools of the trade.

In this article, we'll discuss diode bridge compression in detail, covering the technology and theory involved and also go over some characteristics and applications for this type of compressor.

A Primer On Compression

Before getting to the main discussion of diode bridge compressors, let's quickly go over the basics of compression.

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Dynamic range compression (compression) is defined, as the name would suggest, as the process of reducing the dynamic range of an audio signal. Compression, then, is an audio tool used for compressing/minimizing the difference in amplitude between the highest and lowest parts of an audio signal.

Technically, a compressor will attenuate only the “loudest parts” of the signal (rather than bringing the quiet parts up, which is considered “upward compression“).

Two important questions govern how a compressor will perform:

• What constituted the loudest parts?
• By how much should the loudest parts be attenuated?

These two important questions are answered by a compressor's threshold and ratio controls/parameters, respectively.

What is the threshold of a compressor? The threshold of a compressor is a set amplitude limit that dictates when the compressor will engage and disengage. As the input exceeds the threshold, the compressor kicks in (with its given attack time). As the input drops back down below the threshold, the compressor disengages (according to its release time).

What is the ratio of a compressor? The ratio of a compressor compares the number of decibels the input signal is above the threshold to the number of decibels the output signal is above the threshold. In other words, it is the relative amount of attenuation the compressor will apply to the signal.

Other compressor parameters worth mentioning are the following (I've added links to in-depth articles on each parameter):

• Attack Time: the amount of time it takes for a compressor to engage/react once the input signal amplitude surpasses the threshold.
• Release Time: the amount of time it takes for the compressor to disengage (to stop attenuating the signal) once the input signal drops below the threshold.
• Knee: the transition point around the threshold of the compressor where the output becomes attenuated versus the input.
• Makeup Gain: the gain applied to the signal after the compression takes place (typically used to bring the peaks of the compressed signal up to the same level as the peaks pre-compression).

All compressors have a gain reduction circuit that effectively compresses the audio signal in response to a control signal. This control signal (also referred to as the sidechain) is derived from the input audio signal (common) or via an external audio signal (less common). It is manipulated via the aforementioned compressor parameters.

So every compressor will have two critical signal paths:

• The audio signal path which passes through the gain reduction circuit and gets compressed.
• The control signal (sidechain) path that reads manipulates the sidechain signal (input or external) and controls the gain reduction circuit.

In the case of diode bridge compressors, the gain reduction circuit is centred around a diode bridge circuit.

With that primer, let's get into diode bridge compressors and how they act to compress the dynamic range of audio signals!

What Is A Diode Bridge Compressor?

A diode bridge compressor, as the name would suggest, is a compressor that utilizes a diode bridge at the core of its gain reduction circuit.

What is a diode bridge? A diode bridge (also known as a diode ring) is an arrangement of 4 (or more) diodes in a bridge circuit configuration that provides the same polarity of output for either polarity of input. Diode bridges most commonly act as rectifiers (convert an AC input into a DC input).

This conversion is made possible due to the fact that diodes only allow current in one direction.

A diode bridge rectifier provides full-wave rectification from an AC input, meaning that it rectifies the negative components of the input voltage, turning them into positive voltages before converting the AC into DC (pulse current).

Typically, two pairs of diodes are set up in a diamond array. The audio signal is applied across two opposite corners, while the control signal is applied across the other two.

A very basic diode bridge would look something like the following.

In a diode bridge compressor, the sidechain control signal is a rectified (DC) version of the input audio signal. The input audio signal is balanced, meaning that the same signal is applied to each of the “audio” corners, though in the opposite polarity (when one is positive, the other is negative and vice versa).

As mentioned, a diode generally only allows current to flow in one direction. Typically, they either do not conduct (at low voltage) or conduct fully (at high voltage). However, they have a small region where their conductance varies according to the voltage applied across it.

This oddity allows diodes to be used, in the case of diode compressors, as voltage-controlled attenuators.

By varying the resistance of the diodes within this special range, we can alter their conductance. More specifically, we can control the level of the signal passing through the gain reduction circuit.

By maintaining a “bias voltage” (from the sidechain) within this range, the resistance of the diodes will be altered and will have an effect on how much signal is passed through.

By increasing the input signal level, the sidechain control signal level is also increased, and the diode bridge will cause more attenuation to the signal.

This can be explained with a simple voltage divider with a diode:

Where we have the following general equation:

V_{\text{out}} = V_{\text{in}} \cdot \frac{R_2}{R_1 + R_2}

• V_out: Audio Out
• V_in: Audio In
• R₁: resistance of the resistor (audio circuit before the diode)
• R₂: resistance of the diode

As the resistance of the diode decreases, the diode allows more signal to run to ground and effectively attenuates the output. As the bias voltage level of the sidechain increases on the diode (due to increasing input signal level), it decreases the resistance of the diode and effectively attenuates the output.

That's essentially how diode bridge compression works.

One thing to mention is that this region is small and generally requires low-level signals. There should be an attenuation stage before the compression circuit. Similarly, there should be a gain stage after the gain reduction circuit of a diode bridge compressor to bring the overall level of the output signal up to a useable level.

Unfortunately, this low-level compression circuit may pick up significant noise that will then be amplified before the output. To help reduce this noise, diode bridge compressors are designed with balanced circuitry (similar to push-pull amplifiers).

In other words, the input signal is split into two identical compressor paths. The second path, however, acts upon a reverse-polarity signal. As the compressor passes audio, both paths pick up the same amount of noise.

Then, at the output, a differential amplifier or transformer sums the differences between the two paths, thereby adding the two signals (regular and reverse polarity) and cancelling out noise common to both lines (common mode rejection).

So for a diode bridge compressor to work properly, it must have:

• Matched diodes (to reduce an otherwise high level of distortion)
• A balanced signal throughout the compressor (due to the pairs of diodes)
• A signal within a very small dynamic range (to act within the variable portion of the diodes’ transfer curves)

That’s a lot to design, and it’s no wonder these compressors aren’t so popular.

Unless an external sidechain is used (rarely the case), it's the audio input that the sidechain rectifies, manipulates (for time, threshold and ratio controls) and sends to the gain reduction circuit.

Here's a simple signal flow chart to visualize the compressor sidechain.

Most often, the level detection circuit detects the peak (maximum amplitude) and produces a DC voltage of the same value.

Diode bridge circuits allow for the compression curves (ratio, threshold and knee), along with the attack and release time parameters, to be designed independently of the compression element.

These parameters are described with a bit more detail below (I've provided links to in-depth articles on each control):

• Threshold: the amplitude limit that dictates when the compressor will engage and disengage.
• Ratio: the ratio of input signal amplitude above the set threshold to the output signal amplitude above the threshold.
• Attack: the amount of time it takes for a compressor to engage/react once the input signal amplitude surpasses the threshold.
• Release: the amount of time it takes for the compressor to disengage (to stop attenuating the signal) once the input signal drops below the threshold.

Transformers are used to balance and set the signals' levels, which, along with the diode characteristics and the compressor circuit itself, add distortion to the signal. This distortion is often harmonically rich and adds pleasant colour to the signal.

Characteristics Of Diode Bridge Compressors

In this section, we'll consider a few of the typical characteristics of diode bridge compressors:

• Very fast attack and release times
• Non-linear compression that adds character via harmonic distortion
• Requires low-level input signals
• Requires more output gain, which often raises the noise floor

Before moving on to the diode bridge compressor examples, I'd like to point you toward my video on the seven different compressor circuit types. You can check it out below:

Diode Bridge Compressor Examples

Before we wrap things up, it’s always a great idea to consider some examples. Let’s look at 3 different diode bridge compressors to help solidify our understanding of this type of compression.

In this section, we’ll discuss:

• 500 series diode bridge compressor: Rupert Neve Designs 535
• 19″ rack unit diode bridge compressor: Rupert Neve Designs 5254
• Diode bridge compressor plugin: Lindell 254E

Other notable diode bridge compressors include:

• Neve 33609
• Neve 2254
• Chandler Limited Zener
• Heritage Audio Successor

Rupert Neve Designs 535

The Rupert Neve Designs 535 is a diode bridge compressor packed into a 500 series unit. Its basic design is based largely on Rupert Neve's original 2254 compressor and includes advanced controls.

This versatile unit offers a unified Timing control that alters both the attack and release times of the compressor. Choose between 2 modes (Fast and Slow) with 6 options each: Fast, Medium Fast (MF), Medium, Medium Slow (MS), Slow and Auto. That's a total of 12 unique time constants!

The 535 has the typical threshold (31-detent knob from -25 dB to +20 dB) and ratio (1.5:1, 2:1, 3:1, 4:1, 6:1, 8:1) controls along with a makeup gain knob.

Parallel compression is made easy with the 535 via its 31-detent blend knob, which can be set to 0% (fully uncompressed) and 100% (fully compressed). The unit also features a selectable sidechain high-pass filter at 150 Hz.

With custom transformers and class-A output amplifiers, the 535 offers superior sonic performance while maintaining the harmonically rich tonality that diode bridge compressors are known for.

Though the Rupert Neve Designs 535 is a single-channel unit, multiple 535s can be linked together to act on stereo signals.

Rupert Neve Designs 5254

The Rupert Neve Designs 5254 is another excellent diode bridge compressor. Like the aforementioned 535, the 5254 is also inspired by the classic compressor of Rupert Neve's early days. This time, however, it's a stereo unit in a rack-mount form factor.

The controls are largely the same as the 535 on each channel.

Notable differences include the variable sidechain high-pass filter from 20 Hz to 250 Hz and the fact that each channel has its own VU meter with the option to meter output levels or gain reduction. Independent external sidechain signals can also be used on either channel of the 5254 and can be selected by pressing the S/C Insert button of either channel.

The 5254 can be run in either dual-mono or stereo configurations.

Lindell 254E

The Lindell 254E is, again, inspired by the legendary Neve 2254 that was first introduced in 1968. This compressor plugin is programmed to recreate the magic of the real thing and offers extra versatility with additional features.

This diode bridge compressor plugin sounds great and is very easy to navigate.

The Lindell 254E features metering for input and output levels as well as for gain reduction. It has a mix control for parallel processing, a selectable sidechain high-pass filter, and slow and fast options for the compressor and limiter virtual circuits (which can be run simultaneously).

The compressor portion of the 254E has the typical threshold, ratio and recovery (release time) controls. The limiter portion is defined by a limit level control and limit recovery (release time) control. The output/makeup gain of the plugin also has its own knob control.

Related Questions

What are the different types of audio compressors? The term “type” can have a few meanings so let's look at a few different “types of compressors.

In terms of circuit topology, compressors will generally fall into one of the following types:

• Variable-Mu (Tube) Compressor
• FET Compressor
• Optical Compressor
• VCA Compressor
• Diode Bridge Compressor
• Pulse Width Modulation Compressor
• Digital Compressor
• Compressor Plugin

In terms of how a compressor will perform when compressing an audio signal (and the typical tasks it will be set to do), we can think of the following types of compression:

• Multiband Compression
• Peak-Metering Compressoion
• RMS-Metering Compression
• Feedback Compression
• Feedforward Compression
• Upward Compression
• Limiting Compression
• Parallel Compression
• Bus Compression

Should compression be used on every track? As a general rule, compression should be used with intent and, therefore, only be used on every track in the case that every track would require it. More often than not, there will be certain tracks in a mix that sound perfectly fine (and better) without dynamic range compression.

Once again, the typical benefits of using compression on a track include (but are not limited to) the following:

• Maintaining a more consistent level across the entirety of the audio signal/track
• Preventing overloading/clipping
• Sidechaining elements together
• Enhancing sustain
• Enhancing transients
• Adding “movement” to a signal
• Adding depth to a mix
• Uncovering nuanced information in an audio signal
• De-essing
• “Gluing” a mix together (making it more cohesive)