What Is A Diode Bridge Compressor? How Does It Work? And What Should You Use It On?

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COMPRESSION DEFINITION || OPTICAL || FET || VARI-MU (Tube) || VCA || DIGITAL || DIODE-BRIDGE || MASTER BUS COMPRESSION

While compression is fundamentally the process of balancing the loudest and softest parts of an audio signal, different types of compressors operate in different ways. The five main types of audio compressor circuitries serve as the basis for the majority of the plugin emulations found in today’s DAWs. One of the less-known of the five types is the diode bridge compressor. Diode bridge compressors convert AC to DC with the help of a combination of rectifying diodes, to bring about audio compression. In this diode bridge compressor article, we will delve deep into the inner workings and applications of diode bridge compressors. We shall compare their results to FET compressors utilising sound examples created with top diode compressor plugins.

What Is An Diode Bridge Compressor? (TL;DR)

Since the diodes in the bridge configuration are forward-biased, the level of attenuation or gain reduction depends on the exponential resistance of the diodes. 

Being cyclic in nature, this exponential resistance is configured by the amount of bias passing through the diodes at a given point in time. Following a feedback circuit, the current from the side chain going to the gain reduction unit determines the bias current.

The Diode Bridge Configuration:

The bridge configuration uses four diodes so that it can segregate and rectify the two half-cycles of a sine wave. As we can see in the diagram below, the load is connected to one side of the diode bridge network, and the winding of the transformer is connected to the other side.

You can read more about electronic engineering here.

The best way to understand this configuration is by studying D1/D2 and D3/D4 separately.  During a positive half cycle, D3 and D4 remain reverse-biased, allowing the diodes D1 and D2 to conduct.

In the second cycle, the polarity is reversed. Now D1 and D2 remain reverse-biased, allowing D3 and D4 to become forward-biased.

The circuit is able to convert the alternate current (AC) input voltage into a pulsating direct current (DCC) output voltage as the load voltage is always consistent. This means that no matter what the Input inputity is, the load voltage maintains the same load current and polarity in a specific direction.

That being said, you never get a perfect full-wave voltage spanning the load resistor. The diode doesn’t actually get activated until the source voltage hits 0.7 V. 

Since the bridge rectifier only runs D1/D2 or D3/D4, at any given moment, the remaining two diode drops (0.7V x 2) belonging to the source are always sacrificed.

So the general equation for the peak output is:

Vp(out) = Vp(in) – 1.4V

where,

Vp : peak voltage

In order to get a consistent DC voltage, we need to filter out the full-wave signal. The pulsating DC voltage output signal isn’t usable in its primary form as it rises to a maximum value and then suddenly goes down to 0.

As a way of bringing consistency, a smoothing capacitor is added across the load resistor. Their forward-biased cycling. By the time the input value hits its peak, the capacitor voltage hits Vp.

After hitting its peak, the input voltage starts to decline. As the input goes below VP, the capacitor voltage switches off the diodes.

In its off state, the capacitor provides load currents till the upcoming peak has arrived. Upon arrival, D3 and D4 recharge the capacitor back to its peak value.

What Goes On Inside A Diode Bridge Compressor?

We will go ahead and explore the circuitry of the legendary NEVE 33609, After which a Lot of Diode bridge Compressors are modeled.

While entering the unit with a balanced input signal is quite common with a lot of compressors,  the unique thing about this circuit design is that the signal stays balanced even after going through the gain reduction unit. 

After passing the gain reduction unit, this balanced signal continues to flow through the buffer and eventually makes its way to a circuit transformer where it finally gets unbalanced.

The unbalanced signal now passes through the all-important BA340, which is the gain block. Being a powerhouse the BA340  propels the Output transformer ahead, giving us a balanced output again.

The side chain for the compressor is directed before the BA340, while an additional branch, after the gain block, leads to the limiter.

The compressor side-chain section hosts all the important functions like the attack, release, makeup gain, and threshold. This is where the rectifier transforms the audio signal from AC to DC. 

After getting converted, the signal goes back to control the initial gain reduction unit, through which the balanced input had passed.

When you choose the limiter, The signal runs to the side chain after the BA340. By making use of the small makeup gain pot house below the BA340, you can crank the signal going to the limiter.

Feedback Style Compressor Design:

Being a feedback-style compressor, the side chain is the fed signal after the gain reduction. This means that the gain reduction unit is monitoring its own results as they get fed back into the control signal via the side chain.

The best part about this feedback design is that the detector spends a lot of time reacting to the compressed signal, which leads to smoother results.

While this helps the compressor make up for the non-linearities within the signal, it loses its ability to precisely control the attack and release times, like compressors using the feed-forward design.

You can read more about the research done on compressor components here.

Voltage Divider Section:

In most compressor types other than the VCA, we are generally using a voltage divider to control the balanced signal. The voltage divider consists of positive and negative resistances, which get attenuated by the gain reduction unit.

Similar to how a FET and a light-dependent resistor (LDR) are used to change the resistance in FET and Opto compressors, the resistance of the voltage divider gets altered by diodes in this case.

Both the positive and the negative resistors are connected to individual diodes, which take the signal to the ground.

For instance, the positive resistor and the diode both control the amount of signal going to the ground. As they are proportionate, less resistance at the diode means higher levels of signal get through to the ground.

As we proceed, it’s important to know that diodes can only conduct electricity at a specific voltage. They are also uni-directional, which means they only conduct electricity from positive to negative. 

Being binary in nature, they only conduct electricity when their specific voltage is hit. The diodes in this example carry a voltage of 0.7 volts. So the diodes only conduct electricity to the ground when they receive 0.7 V. Any voltage below that leaves them in the “off” position.

But similar to a 0-ohm resistor, the diode utilizes an exponential curve before the trigger point. This means that the incoming signal travels along a sudden steep curve as soon as the incoming current is about to hit 0.7 V. 

So the compressor waits for the current to enter the exponential curve, where the DC bias in the diode takes over. The compressor now generates a DC control voltage from the sidechain area.

Though the current is less than 0.7 volts, which is quite small, it is enough to surface on a threshold, which alters the resistance of the diodes themselves.

Gain Reduction:

Similar to the LA4 and 1176, there is a variable resistor (the diode) inside the circuit attenuator that controls the voltage divider inside the diode bridge compressor. The level attenuation occurs with a step-down transformer and a built-in attenuator within a resistor, placed just before the diode.  

Lower levels of input are generally sent through diode bridge compressors, as we need to be mindful of the 0.7 V mark. In fact, if we were to crank up the levels above 0.7 V, it could start to resemble a distortion unit.

While Opto and FET compressors would begin to distort after a certain level too, their clipping points are way above 0.7V.

While the diodes are generally very well matched for keeping distortion to a minimum, some interesting compression anomalies can occur with unmatched diodes too. In the case of unmatched diodes, you might end up with some asymmetrical distortion.

As we are dealing with a low-level signal at the input, the make-up gain pot below the BA340 is instrumental in driving the gain in the unit. 

How Does An Diode Bridge Compressor Sound?

Unlike the FET which is edgier, they glue the sound from top and bottom, adding roundness to the sound.

Why Does Diode Bridge Compression Sound Unique?

The use of discrete amplifier circuitry and transformers at both the input and output results in a sound that is peculiar to diode bridge compressors.

While the transformers generate characteristic distortion artifacts, the class ‘A’ discrete amplifier circuits bring out the subtleties of the overtone series, creating a lush array of odd and even upper harmonics.

You can see more diagrams of the Neve 33609 C here.

In the diagram above, you can notice how smoothly the odd and even harmonics transition from 6 kHz to 22 kHz. This becomes highly useful when dealing with compressing room mics. The most expensive and sensitive mics in a studio’s arsenal are generally used to mic the room during recordings.  

The sound of a room is generally captured between 12 kHz and 20 kHz. This frequency spectrum creates the image of the room in the listener’s head due to the late reflections it captures. 

The diode bridge compressor’s smooth harmonic overplay in this area preserves these frequencies very well. The class “A” amplifier provides lush saturation to this frequency range, creating the iconic diode bridge sound. 

If you look at the gain reduction plot of the Neve 33609 C diode bridge compressor below, it shows how the frequency response is consistent through different levels of amplitude. 

You can notice how the low end remains the same at all input levels. As we start entering the midrange frequencies, we notice the compression affecting the gain structure. Even when we reach the high frequencies, there is only a difference of around 4 dB.

With such a consistent frequency response curve, the mid-level presence is maintained at all degrees of compression. Also, the nonlinear response curves of the diodes in the circuitry, result in a compression that is smooth, natural, and musical.

How To Make Use Of Diode Bridge Distortion?

Once we hit high volume levels, the signal alters the amount of current flowing through the diodes. As the signal takes the diodes away from their bias points, the exponential gain starts to change, which results in noticeable distortion.

The gain in the circuit is dependent on the audio signal and the bias current, which is in turn dependent on the side chain. This means that the distortion is also dependent on the audio level and the moment-to-moment gain reduction occurring in the circuit.

When the hardware is not in gain reduction mode, the diodes’ exponential resistance is quite low. So if you were to pass or if you were to crank up the input signal, the distortion would still be quite low.

The sweet spot in diode bridge compressors is between 3 dB and 7 dB when the diode’s resistance is on par with its neighboring circuit elements. In between 3dB and 7dB, the total harmonic distortion can reach 8% to 10%. 

While 8% to 10% of distortion is common in most cases, the diode bridge compressor’s non-linear properties and frequency curve make distortion sound extremely musical. This makes diode bridge distortion extremely sought-after.

Scenarios Where Diode Bridge Compression Is Useful

Parallel Drum Bus:

We started tweaking the right side before the left, as that is the natural flow of signal inside diode bridge compressors. 

As you can hear in the sound example, the dry drums lack definition. You cannot hear the top skin of the snare separately from its body. The rhythmic drive created by the open and closed hi-hats is lost within the kick-snare beat. The body of the kick is too wide for the high hat to cut through.

Our goal with parallel compression was to hear the drum stick’s attack on the top skin of the down being lowered to 3:1 and release set to 800ms making the snare more cohesive, resulting in a gain reduction of -18 dB.

Now going to the limiter section, we started off with a fast attack and release of 800 ms.  Scrolling through the limiter threshold, we settled at +12 dBu, which stayed consistent with our compressor’s gain reduction of -18 dB.

Parallel Drums DRY

Parallel Drums (WET) Fast limiter

Parallel Drums (WET) Slow limiter

As you can hear, the fast attack defines the top skin of the snare and gives shape to the kick. While we were going for an aggressive sound, the snare seems to have lost its body. 

Setting the attack to slow allowed enough frequencies to pass through, which made the snare more wholesome.

Bass:

You can notice how the attack of the notes is accentuated. You can also notice that the low frequencies have more weight while the higher harmonics of the bass taper off slowly.

In the second sound example, we’ve set the ratio to 2:1 and cranked the input all the way up to 13.8 dB. We took the output down to – 4.2 D and set the analog at 25%

With the limit of section deactivated, you can notice how the gravity of the 2nd sound sample is more focused on the lower mids as opposed to the lows.

You can feel the bass being a bit lighter than in the first sample. That being said, there is more intent in the higher frequencies of the second sample as compared to the first. 

While it only changed the threshold by one unit, the primary difference comes from the input-to-output ratio. As with all analog emulation plugins, attenuation circuitries have a huge impact on the behavior of the compressor. 

Bass (DRY)

Bass (WET) Ratio 2:1

Bass (WET) Ratio 3:1

Mix Bus:

Since we’re dealing with the mix bus, we’ve also added an FET compressor comparison to accentuate the unique roundness that diode bridge compressors provide. 

If you noticed the dry sample,  you know how the guitar dominates the vocals. While the guitars sound nostalgic, we want to add more chest to the vocalist’s voice so it can ride on top of the guitar.

Mix Buss (DRY)

Mix Buss (WET) Analog Knob-100%

Mix Buss (WET) Limiter OFF

Mix Buss (WET) Limiter ON

Mix Buss (WET) FET Comparision

We began by pulling the input down to – 2DB and raising the output to 9.7 DB. We set the ratio to 4:1  And the release to auto in the compressor section. 

As you can hear in the sound example above, we set the analog to 100%.

It is important to listen to your diode bridge distortion carefully, as it’s not always the best option for capturing micro-dynamics. For example, something like a Fairchild660 would have been able to capture the pitch change perfectly.

In the final example, we dialed down the analog by 25%, making the nuance more noticeable. 

In the second example, we added the limiter section. Dialing the limit the level all the way up to 12DBU, We set a slow attack and an auto release. We switched off the analog dial too.

Now, quickly testing this example with a FET 1176, show us how edgy it could sound. Notice how the guitar transients pop up during the chord changes, and the vocals Get more nasal in nature.

The second sound sample is a good example of using the diode bridge compressor for transparency. You can hear how the edginess of the vocal transients is pulled back, providing a controlled but transparent sound.

Popular Diode Bridge Compressors

Here’s a short list of popular Diode Bridge Compressors that have become a studio staples over the years:

1. Neve 33609/N Discrete Stereo Compressor/Limiter
2. Neve 2254/R Limiter/Compressor
3. Chandler Limited TG12413 Zener Limiter

1. Neve 33609/N Discrete Stereo Compressor/Limiter:

Compatibility: macOS 10.14+ Mojave onwards. UAD, UADx, LUNA, UA Connect compatible with M1/M2 processors. Windows 11+. Windows 10 Anniversary Update necessary for Thunderbolt connections with UA devices. 64bit only. 
Price: Universal Audio Teletronix LA-2A Classic Leveling Amplifier $3995 (for hardware)UA NEVE 33609C BUSS COMPRESSOR $149 (for emulation)

2. Neve 2254/R Limiter/Compressor:

Compatibility: macOS 10.15.7+ Windows 10+. Windows 21H2. 64bit only. AAX Native, AudioSuite, VST, AU, SoundGrid.
Price: Neve 2254/R Limiter/Compressor $2795 (for hardware) Waves V-Comp: $29 (for emulation)

3. Chandler Limited TG12413 Zener Limiter:

Compatibility: macOS 10.14+ Mojave onwards. UAD, UADx, LUNA, UA Connect compatible with M1/M2 processors. Windows 11+. Windows 10 Anniversary Update necessary for Thunderbolt connections with UA devices. 64bit only. 
Price: Chandler Limited TG12413 Zener Limiter $5150 (for hardware) Chandler Limited TG12413 Zener Limiter: $149 (for emulation)

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