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PC-2A Compressor In-depth

by Phil Taylor

The PC-2A is a stompbox compressor based on the vacuum tube circuitry found in the vintage LA-2A studio compressor – like the LA-2A, the PC-2A has an all-tube signal path and photo-optical attenuator. This compact pedal was the brainchild of over two years of research, experimentation and design and is Effectrode’s best selling effects pedal. Here’s the story behind this fabulous little tube pedal…

PC-2A close-up of front panel

For some time I’d been considering the idea of designing and building a tube stompbox compressor but it was the world-renowned fingerstyle guitarist, Adrian Legg who finally catalysed me into serious action when he made an enquiry about his need for a true hi-fidelity compressor back in 2008. He was searching for a tube compressor that would improve and enhance the over-bright and brittle tone of the piezo bridge pickup on his guitar. It’s a common problem and a tube input stage is often the perfect tonic to this as it handles the transients exceptionally well and retains the natural timbre of the instrument.

Another motivation was that I wanted to offer guitarists a compressor that was a genuine improvement over the numerous boutique stompbox compressors on the market, which were either exact copies or variations on the Japanese CA3080 transconductance op amp Ross design. Being a tube nut, I’d already been doing my homework on vintage compressors/levelling amplifiers and had an idea in mind for creating something exceptional, unique and very special. I was determined that the Effectrode compressor should incorporate several of the essential elements found in a real studio compressor, including a ‘proper’ control side-chain, optical attenuator and all-tube signal path, a signal path that would utilise pro-audio grade components throughout, including polyester coupling capacitors, precision metal-film resistors and a triode vacuum tube.

Only one tube was required for the design and this led me to consider utilising a N.O.S. mil-spec subminiature tube to make the pedal as compact and robust as possible. These little tubes were originally designed for use in guided missile systems in WWII and their reliability and audio performance are outrageous – they just don’t make tubes like this anymore.

LA-2A Inspiration

Teletronix Engineering LA-2A Leveling Amplifier

Teletronix Engineering LA-2A Leveling Amplifier

So, I began an outline detail for my own circuit design from the ground up, taking the classic LA-2A studio compressor as my inspiration. I recollect visiting the APRS (The Association of Professional Recording Services) show in 1994 whilst working at BSS Audio, and checking out Universal Audio’s replica of the Teletronix LA-2A compressor at one of the exhibitor booths. This compressor processed music material with a light touch, wrapping the signal in velvet – a beautifully smooth and rich tone that was full of depth – it was impossible to get an unusable or bad sound from it. The front panel was pleasingly minimalist, straight-forward and uncluttered, with just two large bakelite knobs labeled ‘Peak Reduction’ and ‘Gain’ and a big, bold analogue VU meter. This was the antithesis of BSS gear with it’s hi-tech look and densely populated front panels, which appear formidable, intimidating even, looking equally at home on the USS Enterprise. Two lessons learned: keep it simple and make it sound nice.

So there are just two controls on the PC-2A front panel – ‘Peak Reduction’ and ‘Gain’. It’s the opinion of this designer (and guitarist) that controls for ‘attack’, ‘release’, ‘knee’, etc are more appropriate for studio compressors where an engineer may want to tailor these parameters to use the compressor for use with different instruments or for different applications. Adding too much flexibility to a guitar compressor, or any guitar effect for that matter, can be a bad thing – there are some places you just don’t want to go as they will lead you to ‘the dark side…’. For example, fast attack and release times can lead to ‘pumping’ where the compressor sounds distorted because it’s attempting to track the envelope of the input signal. In extreme cases this sounds like clipping – but not nice tube clipping distortion. It sounds terrible.

The LA-2A Sound

resistive photocell

The photo-resistive attenuator in the classic LA-2A studio compressor is one of the key components responsible for natural and musical compression.

The LA-2A is argueably one of the most popular compressors ever made and its legendary reputation for natural, transparent compression has firmly established it as one of the industry standard compressors for recording studios. It’s transparency and natural sound is, in part due to the well considered tube signal path, but also the gain control component. This is a photo-optical attenuator consisting of a fixed resistor and shunt cadmium sulphide photocell controlled by an electro-luminescent panel. The attenuator is passive, as opposed to the more common active AGC (Automatic Gain Control) compressor circuits and has vanishingly low harmonic distortion and an exceptionally pure sound. It doesn’t impose an electronic signature on the signal and thus preserves the tone and character of the original sound. Some ‘make-up’ gain is required to adjust the output level so that it matches the original level. This is accomplished with an all-tube fixed gain pre-amp circuit which has low noise and wide frequency response, which is essentially flat at 0.1dB between 30 – 15,000Hz.

The compression sustain and release characteristics are pretty much dictated by the properties of the photocell and electro-luminescent panel which allows for a fast attack – meaning it will catch tranisent bursts of high signal levels effectively working as a signal limiter. This gain control component, coupled with tube architecture (based on a gain stage and White cathode follower) design makes this an intrinsically great leveling amplifer for studio applications and almost certainly overkill for electric guitar. However, it’s the transparency and capability for fast attack that made me consider it as a good starting point for a guitar compressor over, say some of of the other studio compressors out there. Additionally, the photocell response characterstic is inherently musical to the ear, where the LA-2A has no adjustment for attack and release – it just relies on the physics of the gain attenuator components for its compression characteristic. The unique electro-optical attenuator system allows instantaneous gain reduction with no increase in harmonic distortion – an accomplishment at the time, still appreciated today. The all-tube signal path and optical attenuator are a killer combination.

Adapting the LA-2A for use with Guitar

Fletcher-Munson equal loudness curves

Fletcher-Munson equal loudness curves show how the human ear responds to sound pressure levels (click on picture for more details)

The side-chain circuit in the PC-2A is based on feedback control circuit, just like the original LA-2A. With this circuit topology the signal that is used to drive the side-chain is affected by the gain-reduced signal. The larger the input signal, the lower the gain, resulting in a reduction of dynamic range or compression of the signal [Note: Conversely, an expander increases the dynamic range of a signal]. This type of topology seems to be preferred over feed-forward for its more musical characteristics.

An additional advantage of side-chain control is that it can be tailored to be whatever you want it to be. For example, a side-chain can be frequency selective, say at around 6KHz so the compressor can minimise sibilance and act as a desser on vocals. In the case of the PC-2A there is some pre-emphasis in the 2KHz region, similar to an “A” weighted filter curve. This curve approximates the way the human ear responds to varying sound pressure levels and one of the reasons the PC-2A sounds so transparent – it is difficult to discern whether it’s your ear or the compressor that is reacting to volume level changes. The “A” weighted filter also works to help reduce ‘pumping’ that can occur in a compressor when it’s attempting to process music material with lots of low frequency content. This means the PC-2A is very fast and ‘tight’ and works well with ‘difficult’ instruments such as active 5-string bass, NS stick and instruments with extended lower register.

Electro-luminescent panel

An electro-luminescent panel is utilised as a light source in the LA-2As photo-optical attenuator because it’s faster than a filament bulb and smoother than a neon tube.

The photo-resistive attenuator in the original Teletronix LA-2A studio compressor dictates its attack and release characteristics. The electro-luminescent panel inisde the attenuator was developed in the early 1960s to eliminate the attack speed shortcomings of neon and filament photo-optical attenuators. The photo-resistive device, a cadmium sulphide photocell is specially selected for fast attack of less than 1ms. The release time is also entirely determined by the photocell. The photocell has a desirable two-stage decay characteristic where it releases within 40 to 80ms to approximately half its off resistance when light is absent. The remainder of the release then takes place over as much as several seconds.

The electrical performance of the attenuator in the PC-2A is a very close match to the original, however although the dynamic response of a photocell lends itself well to audio processing – its fast attack and slower release time ensure that it’s relatively free of artifacts such as pumping – it’s not perfect. To improve on this the side-chain in PC-2A has a little additional smoothing to further extend the release time and the attack time can also be slowed too. All this refinement and ‘tweaking’ serves a purpose: to reduce artifacts such as ‘pumping’ to vanishing point so that compressor operation is completely transparent.

Summary

Whew… glad you stayed with me on this. It got a little technical in places, but we made it through! The PC-2A really is a very nice little compressor pedal and totally unique – built using a tube, not a chip! But please don’t take my word for it, check out our sound clips, videos or give one a test drive at your nearest Effectrode dealer.

Special thanks to Adrian Legg and Phil Taylor (Gilmour Music) for their invaluable help with audio tests during the development of this pedal – they made it possible to take the design of the PC-2A Compressor far beyond the horizons I could have reached myself. To find out more about the magic of tubes you might also want to take a look at Eric Barbour’s excellent article ‘The Cool sound of Tubes’ originally published in the August 1998 issue of IEEE’s magazine.