Heritage Audio Successor

Based on a classic design and bursting with transformers, the Successor from Heritage Audio combines a vintage sound with serious flexibility.

I'm not quite sure why, but of all the vintage approaches to compression, the diode-bridge circuit seems to be the one which has been least exploited by the many manufacturers creating either hardware clones or analogue-modelling plug-ins. It's all the more surprising given that the most notable examples of this technology — the Neve 2254 mono and 33609 stereo compressors — have proved enduringly popular in a number of roles, not least the 33609 as a stereo bus compressor. However, that situation seems slowly to be changing (see the 'Alternatives' box), and Madrid-based Heritage Audio are one of the latest companies to breathe new life into this old technology: their Successor, which clearly draws on the Neve approach, is a 1U rackmountable stereo bus compressor, manufactured in the EU.

Controls & Connections

A cursory glance and the Successor's front panel instantly reveals its muse, the painted, battleship-grey fascia and winged knobs giving it a particular vintage appearance that wouldn't look out of place in a studio from the 1970s. Though there are two channels, this is a true stereo compressor — so there's a single set of controls, which stretches across the width of the unit in an unbroken line from the all-black meter on the left to the power switch on the right. After the meter, which displays the amount of compression on its 0-20 dB scale, comes the Dynamics In switch, and in addition to activating the compression circuitry this also switches on the meter illumination — a very neat alternative to the customary green or red LED.

The Threshold control ranges from -20 to +20 dBu and, like the Successor's other rotary control (Blend), is detented to ensure repeatability of settings. Ratio is a six-position stepped switch, selecting between compression ratios of 1.5, 2, 3, 4, and 6:1, or a limiting ratio of 20:1. The Attack control is similarly stepped, accessing attack times that are initially pretty speedy (50 s to 0.2 and 0.5 ms), and then increase in ever-larger steps to 2, 5 and 20 ms. Release is another six-step switch, its first four positions covering times of 25, 50, 100 and 400 ms, and its final two steps switching in the A1 and A2 automatic release timings, which are fast when peaks are being compressed and slower when more sustained program material is being dealt with.

The side-chain filter is an intriguing implementation of a feature that has become increasingly common in recent years, and provides (in addition to an Off position) high-pass filters at 80 and 160 Hz, bell-shaped pre-emphasis boosts at 800Hz and 3kHz, and a high-pass filter at 5kHz. A smooth roll-off below 50Hz is also fitted to the side-chain in order to prevent sub-bass frequencies from affecting the compressor. In addition to all this, an external send-and-return loop allows the use of external filtering or other processing to either replace or complement the internal filters, for more precise control.

The final six-position switch is the Gain Makeup control, and this offers 0-10 dB of makeup gain in 2dB steps. The front-panel control layout is completed by the detented dry/wet Blend control that provides simple, one-knob access to parallel compression, and this can be switched in and out of the circuit for instant comparison between the dry and parallel-compressed outputs.

On the rear, you'll find a four-pin locking XLR connector for the external power supply, the balanced XLR audio I/O and the send and return jacks for the unbalanced side-chain insert loop.

The Successor already offers generous side-chain shaping capabilities, but it also boasts unbalanced insert send and return jacks — making it possible to use external processors to manipulate the control signal.

Inside

As attractive as it might be on the outside, internally, the Successor is a work of art. A single, beautifully laid-out PCB carries the twin pairs of input and output balancing transformers and the Heritage Audio '73 Class-A output stages. At the heart of all analogue compressors sits a voltage-controlled gain-reduction block which, in the case of a diode-bridge compressor, is made up of four diodes, arranged in a diamond shape to form what's called a 'balanced bridge'. The incoming balanced audio signal is connected to one pair of opposite corners of this diamond, and a control signal is connected across the other pair of corners. This balanced arrangement ensures that any DC offset from the control voltage can be cancelled out in the following stages by unbalancing and then rebalancing the compressed signal before it reaches the output XLR connectors — it's the reason why, unlike most other designs, diode-bridge compressors normally have fully balanced signal paths. The Successor, like all diode-bridge designs I've encountered, employs both input and output transformers, and those here are made by Carnhill — and are intended to contribute to the device's vintage sonic character.

Many of us think of a diode as a device whose sole purpose is to allow current to flow in a single direction. But while they do achieve this in their normal operating range, there is a small area of a diode's operation where its conductance varies in proportion to the voltage being applied across it, and it is this variable conductance that allows a diode to be used as a voltage-controlled variable attenuator. In order to keep the balanced-bridge diodes within this area of their operation, and to reduce the level of distortion arising from their non-linearity, the audio signal entering the gain block needs to be attenuated by around 40dB. This means that the compressed signal then has to be amplified back up to compensate. However, since the Gain Makeup stage is actually a 10dB attenuator, where the level of attenuation is reduced as the 'gain' level is 'increased', that means that there is 50dB of gain and a balancing transformer, placed in the signal path between the gain cell and the balanced outputs. It is this attenuation/compression/amplification process, plus the effect of the transformer, that creates the harmonically rich, upfront sound quality that makes a diode-bridge compressor sound so pleasing to the ear.

In the Successor, Heritage Audio have employed a pair of matched gain cells to ensure the solid left-right tracking that's essential in a bus compressor to avoid shifts in the stereo image arising from differing amounts of compression in each channel. The tracking itself is described by Heritage Audio as 'Oxford' mode — a term derived from the way the two channels' side-chain control voltages are linked in the original SSL stereo bus compressor (there are some partial clones whose design cuts corners with less expensive, less sophisticated side-chain circuitry).

In Use

The first point to note once you've hooked up the Successor is that while the Dynamics In switch merely moves the compressor in and out of circuit, it does not bypass the input transformers, or the 1073-style '73 amplifiers, or the output transformers. I'm more than happy with that arrangement myself, as it allows you to use the unit for a little character without the gain reduction, but your mileage may vary if you favour pristine clarity over character.

Vocal compression is all about enhancing the character and presence of the voice, and the Successor made that an intuitive and extremely rewarding process.

Diode-bridge compressors are all about fast attack, and in the 2254's heyday (the pre-VCA and pre-FET era), it had one of the fastest attacks around. However, the Successor's 50 s attack is actually twice as fast as its spiritual ancestor's 100 s, and this, alongside its fast release timings, make it ideal for drum-bus duties; its speedy attack and fast release will allow you to shape or crush transients as required. Used more modestly, there's a sense of an increase in presence and detail — and although this is accompanied by a reduction in punch, the Blend control can be used to counter that, as it allows you to layer the transient punch of the uncompressed dry signal back on top of the solidity and detailed presence of the compressed one.

Superb though it is at controlling an entire drum bus, the Heritage Audio Successor is also capable of considerable subtlety, and its particularly well-featured side-chain has a lot to do with that. The side-chain's 50Hz roll-off is a useful backstop in preventing any sub-bass transients from influencing compression. Staying with the drum bus, switching in the 80Hz high-pass filter will help ensure that the bass drum has a similarly reduced level of influence, allowing it to move further forward in the drum mix. Switching up an octave to 160Hz will tend to prevent both bass and snare drum from overly dominating compression, enhancing the presence of both those drums in the mix. Move over to the 800Hz bell-shaped boost and you're essentially making the unit's detector circuitry see more around this centre frequency, which can help deal with an over-present snare, whilst the 5kHz high-pass filter confines compression triggering to frequencies above that point, giving you targeted control of the splashy cymbal sector.

As you might expect, the Successor can produce superb results when strapped across a guitar bus too, whether you're aiming to smooth out the recorded response of a clean electric or acoustic guitar, keep a clean-to-crunch track from jumping around in the mix or gluing together a layered, distorted wall of sound. Crunchy and distorted electric tones tend to swamp the character that the Successor brings to the party, but feed it with a stereo acoustic guitar bus, add a lower-ratio compression and experiment with threshold level and attack and release times and you'll find a smooth compression with an added warmth and definition that can be alarmingly addictive. When it comes to recording bass guitar, a transformer-balanced preamp feeding a transformer-balanced compressor can do no wrong as far as I'm concerned, and the Successor gave me no reason to change my mind about that.

Vocals are another area where the Successor proves its worth. The 800Hz bell boost is a well-targeted tool for taming honky vocals, and the 3kHz boost and the 5kHz high-pass filter proved adept at taming sibilance. However, what impressed me the most was the character and presence that the Successor brought to vocals and, in particular, to a male lead vocal. To me, vocal compression is all about enhancing the character and presence of the voice, and the Successor made that an intuitive and extremely rewarding process — it could, potentially, be my new favourite vocal compressor!

Overall

As you can probably tell, I was extremely impressed with the way in which Heritage Audio's Successor delivers its compression. The convenience of intuitively being able to set up a suitable level of compression, find a wet/dry ratio that gives me what I want to hear, and then switch between that blend and the dry signal to check things out without having to switch the actual compression in and out was the icing on the cake. Its overall audio performance — a bandwidth of 15Hz-30kHz ±0.5dB, and inputs and outputs capable of handling in excess of +22dBu — was equally impressive.

The diode-bridge approach to compression is, inherently, slightly noisier than other more familiar designs, but it's not usually a problem in the intended applications — indeed, the Successor's less-than-75dBu self-noise didn't cause me any issues in use.

Ultimately, the Heritage Audio Successor is a compressor with a distinctly vintage character. It won't necessarily work as an 'only compressor', but if you're already covered for more general bus-compression duties (whether via software or hardware), it's nice to explore different characters — and the character imparted by the Successor is well worth exploring. It delivers that certain something and, for a stereo compressor of this quality, at a decent price too. I'm personally sorely tempted by its delights, and I'm pretty sure that I won't be alone in that.