PC-2A Compressor In-depth

by Phil Taylor

The PC-2A is a miniaturized version of a vintage studio compressor in a pedal – the brainchild of over two years of research, experimentation and design. For some time I’d been kicking the idea around of buliding a compressor with all-tube signal path and photo-optical attenuator to obtain exceptional sound quality in a compact stompbox. Eventually, it was guitarist, Adrian Legg who provided the additional energy to catalyze me into serious design activity on this project, when he made an enquiry about using the tube compressor with acoustic guitar to warm and smooth the over-bright and brittle sound of piezo pickups. I was determined that the PC-2A should have all the important components found in a classic compressor, including a ‘proper’ side-chain, optical attenuator and all-tube signal path – a signal path that would utilize pro-audio grade components throughout, including polyester coupling capacitors, precision metal-film resistors and triode vacuum tube. Only one tube was required for the design and this led me to consider utlizing a NOS mil-spec subminiature tube to make the pedal as compact as possible. Simply stated, I did not intend to cut any corners.

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LA-2A Inspiration

I’d already been doing my homework on compressors/levelling amplifiers and had an idea in mind for making a unique and special compressor pedal. I wanted to create an alternative to the numerous boutique stompbox compressors on the market, which were either exact copies or variations on the Japanese CA3080 transconductance op amp Ross design. So, I began an outline detail for my own circuit design from the ground up – my approach was to take pro-audio and vintage studio compressors as my inspiration. I recollect visiting the APRS pro-audio 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 warm tone – it was impossible to get an unusable or bad sound from it. The front panel was pleasingly minimalist, straight-forward and uncluttered, with just’Peak Reduction’ and ‘Gain’ knobs and a big analog VU meter. This was the antithesis of BSS gear with it’s hi-tech look and densely populated frontpanels which were a little intimidating, looking equally at home on the USS Enterprise. Two lessons learned: keep it simple and make it sound great.

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 the industry standard for recording studios. It’s transparency and natural sound is, in part due to the well considered tube signal path, but more importantly the gain control component which is based on a photo-optical attenuator, which consists of a fixed resistor and shunt cadmium sulfide 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. This means it doesn’t impose an electronic signature on the signal, therefore preserving 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 characteristic are defined by the photocell and electro-luminescent panel also allows fast attack. This gain control component, coupled with tube architecture (based on a gain stage and White cathode follower) design makes this an intrinsically great levelling 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. A 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.

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Adapting the LA-2A for use with Guitar

The Side-dhain circuit in the PC-2A is based on feedback control circuit, just like the LA-2A studio compressor. With this circuit topology the signal that is used to drive the side-chain is affected by the gain-reduced signal. This type of topology seems to be preferred over feed-forward for its more musical characteristics. There is also some pre-emphasis on the side-chain similar to an “A” weighted filter curve. This curve characteristic approximates the response of the human ear. Here it works to help reduce ‘pumping’ normally associated when a compressor is processing music material with lots of low-frequency content. The larger the input signal, the lower the gain, resulting in a reduction of dynamic range or compression of the signal. Conversely, an expander increases the dynamic range of a signal. Gain Reduction Circuit. The photo-resistive attenuator is the heart of the compressor and determines the attack and release characteristics. The photo-resistive device is specially selected for fast attack of less than 1ms. This is important for minimizing undesirable artifacts such as ‘pumping’. The electrical performance of the attenuator is a close match to the electro-luminescent (EL) panel used in the Teletronix LA-2A studio compressor. The EL panel was developed in the early 1960s to eliminate attack speed shortcomings of neon and filament photo-optical attenuators. The release time of the compressor is entirely determined by the cadmium sulfide photocell in the attenuator. The cell 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.