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The PC-2A: A Studio Compressor in a Pedal

The PC-2A compressor pedal is based on the Teletronix LA-2A studio Leveling Amplifier. Like the LA-2A, the PC-2A possesses an all-tube signal path, photo-optical gain attenuator and a beautifully musical, smooth tone. This luscious sounding vacuum tube pedal is the result of over two years dedicated research and experimentation—a labour of love—and is Effectrode’s BIGGEST selling effect! Here’s the PC-2A’s story…

The Beginning

The year: 2008. The place: the Effectrode workshop, a.k.a. my bedroom. I was thinking. Thinking about making a tube compressor pedal. I’d been thinking about it for a long time. Too long. Thinking about it; and doing nothing. Then the phone rang, and my train of thoughts to nowhere got derailed.

It was Adrian Legg—that’s right, the Adrian Legg, the world-renowned, famous fingerstyle guitarist. He began asking questions. Serious questions. Questions about a true hi-fidelity stompbox compressor. It was time to get my act together: Quit thinking and start doing!

Adrian wanted a compressor pedal, but not just any old, common, off-the-shelf compressor though—no, of course not—he wanted something special. He wanted something to smooth out and mellow the over-bright, brittle tone of the piezo bridge pickup on his guitar. It’s a common problem: piezo pickups put out some pretty nasty transient spikes, momentary high level signal bursts, that solid-state op-amp based preamps and compressors really don’t get on with. Adrian thought a tube input stage might be the answer.

He thought right. Being high voltage devices, tubes possess stacks of headroom. Any high level signals generated by the piezo would be within limits of the electronics, ensuring the signal peaks aren’t clipped off, a.k.a. overload. To put it another way, no harmonic distortion can take place, so the natural timbre of the instrument is preserved. A tube compressor might just be the perfect tonic for Adrian’s peaky piezo pickups.

And a tube compressor would give bassists something new; something genuinely different to all the solid-state stompbox comps on the market. Many of these pedals are copies or variations on the Japanese CA3080 transconductance op amp Ross design—nothing special really. They certainly don’t stack up against a vintage rack-mount studio machine, a machine like the Teletronix LA-2A. Now, that vacuum tube leveling amplifier is something special, magical even.

Then an idea surfaced in my mind: What if that magic of the could be embodied in a guitar pedal? That had to be worth doing. It would be a great leap forward in tone for guitarists and bassists. But I had doubts: Would musicians even want such a thing? And, would they buy it? Ah, what the hell, let’s do it anyway! My thoughts refocused and converged on a mission: To build an LA-2A, in a pedal.

LA-2A Inspiration

I began sketching out the circuit design, from the ground up. The Effectrode compressor must incorporate the essential elements of the LA-2A: a ‘proper’ control side-chain, photo-optical gain attenuator, and… an all-tube signal path. I had to squeeze all this circuitry into a die-cast aluminium box—a very small die-cast aluminium box. Was it doable? I didn’t know. But I did know what the LA-2A could do…

Olympia Exhibition Centre London — Olympia Exhibition Centre, London

I recall, back in ’94, visiting the APRS (The Association of Professional Recording Services) international exhibition in Olympia, London. At the time I worked for BSS Audio, as a test engineer, and the company actively encouraged employees to attend—they even handed out free tickets and let us take the day off work to go. Seems incredible today, but that’s how it was.

This event was a real circus. Huge crowds of people eddied and swirled about exhibitor stands beneath the vast glass canopy of Olympia’s grand hall. There must have been hundreds of companies exhibiting their latest technological wares in that mighty space. The excitement and energy was palpable. It was novel, exciting, and then, as the day wore on, exhausting, and ultimately, mind-numbing.

But, amongst all the mayhem and noise, one exhibitor’s booth caught my attention. There, beneath bright halogen spotlights, I could see a Universal Audio LA-2A Leveling Amplifier mounted in a floor-standing 19″ rack unit. I moved towards it, automatically; unconsciously; my instincts had taken over. I had to have a play.

la_2a_leveling_amplifier — Teletronix Engineering LA-2A Leveling Amplifier

On the old-school, battleship grey front panel were two big, bold Bakelite knobs, labelled ‘PEAK REDUCTION’ and ‘GAIN’, and in the centre was an impressively large analogue VU meter. That was it. This look was not so cool in those days; it was as if the LA-2A was out of its time, but it did have a certain charm… and class. It was simple, straight-forward and uncluttered; like the age it came from.

The exact opposite of BSS gear, with its high-tech, slate-grey instrument panels, densely populated with multi-coloured knobs, push buttons and even backlit dot matrix LCD screens. BSS gear is formidable to say the least, intimidating even. It looks more like something you’d see installed in the helm of the USS Enterprise than in a recording studio.

BSS DPR-402 is a 2-channel compressor/limiter — *BSS* DPR-402 is a 2-channel compressor/limiter, high frequency de-esser and wide band de-esser with peak limiting

I put on the pair of headphones dangling on the side of the rack, began fiddling with knobs, and listened… Humph, it doesn’t seem to do much, I thought. Was it even connected? But wait! It was doing something. The effect was subtle, however the more I listened, the more I heard; and the more I heard, the more I liked. The LA-2A was caressing the musical material with a feather-light touch, wrapping the signal in silk—a beautifully smooth and rich tone; full of depth and clarity. This compressor was the business! It was impossible to get an unusable sound from it. What was its secret?

Let There Be Light… Controlled Compression.

Clairex CL505 photocell — Slow release: The LA-2A's 'T4' photo-optical gain attenuator contains two, now obsolete, *Clairex* photocells.

The Teletronix LA-2A is one of the most popular compressors ever made. The magic it imparts can be heard on almost every record ever pressed. And the LA-2A was used by the big broadcast companies, RCA and CBS. Its sound is legendary. Sound engineers and musicians covet its smooth compression and warm, natural tone. Today this machine is one of the industry standard compressors for recording studios. It lends itself well to processing all kinds of instruments: piano, bass, acoustic guitar, drums, vocals, sweeten up a mix, mastering, you name it. It’s a useful tool to have around, if you can afford one. That legendary tone comes at a cost with vintage units carrying four, or even five figure price tags.

The LA-2A’s pleasing sound quality is in part due to its tubular architecture (based on a grounded cathode amplifier gain stage and ‘White’ cathode follower) and UTC input/output transformers. However the photo-optical gain attenuator—the bit that does the compressing—plays arguably the most vital role in creating that coveted, expensive tone.

The gain attenuator is made up of a fixed series resistor and two Clairex photocells connected in parallel. The resistance of the photocells is altered by varying light from a Grimes electro-luminescent panel. Grimes were one of the first aircraft lighting design and manufacturing companies and their product range included electro-luminescent panels, which were utilised for illuminating the instrument panels on aircraft. The Clairex photocells and Grimes EL panel are mounted on a Bakelite octal tube base and enclosed within a rectangular metal shroud approximately 3″ tall, a.k.a. the ‘T4‘ optical attenuator module [US Patent 3,258,707].

Teletronix chose an EL panel because its light output is uniform and diffuse. Not only that, its light intensity changes smoothly and quickly with applied voltage. In comparison, filament light bulbs have a slower response time, meaning their light output does not change instantaneously as the applied voltage changes. And the light output from neon lamps varies unevenly and erratically with applied voltage. The Grimes EL panel and photocell attenuator introduced far less harmonic distortion into the audio signal than the variable bias compressors of the time—less than 0.35% according the Teletronix instruction manual, whereas the Altec 436C is up at around 2%.

electroluminescent_panel — Fast attack: The 'T4' photo-optical gain attenuator also contains a *Grimes* electro-luminescent panel.

The photo-resistive device was chosen for fast attack of less than 1ms. The release time of the LA-2A is also determined by the photocell. The photocell possesses 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. This is not dissimilar to the way the human ear responds to high sound pressure levels: its sensitivity clamps down quickly when exposed to loud music, such as a RUSH concert at Birmingham NEC, and then recovers more slowly after the event has finished.

The rapid reaction time of the photocells and the EL panel within the T4 module mean the LA-2A is capable of instantaneous gain reduction. Any transient signal bursts, or momentary high level signals, entering its input are reduced before being output to tape or broadcast. This prevents overload and clipping distortion—the LA-2A is an excellent limiting amplifier. Some ‘make-up’ gain is required to boost the output level back up so that it matches the original input level. This is accomplished with an all-tube fixed gain pre-amp circuit, which has low noise and wide frequency response—essentially flat at 0.1dB between 30 – 15,000Hz.

The T4 photo-optical gain attenuator is ‘passive’, as opposed to the ‘active’ VCA (Voltage Controlled Amplifier) solid-state circuitry found in more modern compressor designs. These modern VCA compressors come with a few gremlins hidden within their electronic guts. Problems like distortion, noise, offset change with gain and noise modulation. And there are other factors that influence performance, such as stability, device geometry, physical layout, and the semiconductor manufacturing process [see Gary K. Hebert’s paper An Improved Monolithic Voltage-Controlled Amplifier].

Being purely resistive, the photocells in the T4 optical attenautor manifest none of these problems. The upshot of this is that the LA-2A sounds beautifully transparent and natural when compared with VCA based studio, and stompbox, compressors. And the LA-2A’s harmonic distortion is significantly lower than vintage variable bias tube compressor amplifiers. By nature of their operation variable bias (vari-mu) compressors introduce a degree of harmonic distortion. As the control signal from the side-chain alters the biasing to the tube, its bias point moves into the non-linear region. The gain changes, but so does the level of harmonic distortion.

The LA-2A has none of these shortcomings. It doesn’t impose an electronic signature on the signal and preserves the tone and character of the original sound. And its transparent gain reduction and purity were a real achievement at the time—qualities that are still appreciated by sound engineers and musicians today. The PC-2A is based on the same kind of photo-optical and vacuum tube technology. The performance of the gain attenuator is a close match to the T4 optical module and, like the T4, its free of any unpleasant sonic artefacts.

And a Side-chain

The side-chain circuit in the LA-2A is based on a ‘feedback‘ topology. The audio signal from the output of the compressor is fed back to the gain attenuator through the side-chain. The side-chain rectifies, smooths and amplifies the audio signal turning it into a control signal. With this circuit topology the control signal driving the side-chain is affected by the gain-reduced signal from the output. 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].

Feedback side-chain compressors are favoured by many sound engineers over feed-forward types because they sound sweeter and more musical. Almost all of (Rupert) Neve’s earlier designs, such as the 2254, were based on feedback side-chain topologies of one sort or another. Other examples of vintage vacuum tube feedback compressors include the Altec 436, RCA BA-6A and legendary Fairchild 660.

Feedback compressor block diagram — Fig. 1 Feedback compressor.

Feed-forward compressor block diagram — Fig. 2 Feed-forward compressor.

Feedback compressors were ubiquitous back in the early days. Feed-forward designs didn’t gain popularity until later. Here’s my best guess as to why. Conceptually, feedback control makes more intuitive sense than feed-forward control. After all, that’s how sound engineers operate when they’re ‘riding the fader‘. They listen to sound coming from the speaker (the output) and then adjust the volume level of the playback amplifier as the track plays. Could it be that the first compressors were simply mimicking this operation?

The advantage of side-chain control is that it can be tuned to be frequency selective, and therefore the compression is frequency selective. For instance, say at around 6KHz, so the compressor operates as a de-sser on vocals to minimise sibilance, as with the DPR-402. The LA-2A has a similar thing going on; there’s an adjustable high-pass filter in its side-chain, which gives up to 17dB of boost at 15KHz. The filter reduces the amount of low frequency content entering the side-chain, resulting in compression that’s less sensitive to those frequencies and more sensitive to high frequency content.

The designers of the LA-2A did this to minimise the effects of the over-modulation caused by the pre-emphasis in FM broadcasting and TV aural transmission systems. This type of side-chain pre-emphasis is not advantageous, or useful, for electric guitar at all, but there are types of pre-emphasis that are.

"A" Weighting Too

Like the LA-2A, the side-chain circuit in the PC-2A is based on a ‘feedback‘ control circuit, however there’s pre-emphasis added in the 2KHz region. This is similar to an “A” weighted filter curve, which approximates how the human ear responds to increasing sound pressure levels. The idea is to make the PC-2A compress in the same way as the human ear does. Well that’s the theory, but what about the practice?

In practice, it’s not bad, not bad at all. In fact, it’s difficult to determine whether it’s your ear or the PC-2A that’s compressing any volume level changes. The overall impression is that the PC-2A sounds transparent—because it’s knocking out high volume levels in the same way our ear does. And the “A” weighted filtered side-chain significantly reduces ‘pumping’ when the compressor is working hard to process music material containing a great deal of low frequency content.

fletcher_munson_curves2_320px — Fletcher-Munson equal loudness curves show how the human ear responds to sound pressure levels (click on picture for more details)

Ever searching for better guitar tone, I wondered: Might it be possible to enhance the PC-2A’s performance still further, so that it handled the dynamics of stringed instruments more effectively? A slower attack time would make the compression effect more subtle, even more transparent; and extend the PC-2A’s flexibility—a good thing.

I made a few simple modifications to the side-chain, adding a handful of passive components and an internal trimpot. Now the attack time could be adjusted to slow it down. This makes the PC-2A a great tool for enhancing the tone of acoustic guitars. Just 1-3dB of compression and an attack time of around 10-25ms really helps an acoustic sit well in a mix, without losing too much of the instrument’s natural clarity and dynamic response—my work was done. Then the phone rang.

And Parallel Compression

David Gilmour’s guitar tech, Phil Taylor, was on the line. He wanted a compressor, but not just any old compressor; he wanted something like an LA-2A in a pedal. “No problem”, I gasped, trying to sound cool, calm and collected, “we’ll get a PC-2A pedal to you”.

Sam posted the pedal to David Gilmour’s Astoria studio in Hampton Court. But s few days later I got another call from Phil. As he spoke, I got that sinking feeling. Apparently the PC-2A was not cutting it in the studio: the attack was too slow. He suggested he send back for modification to see if I could accelerate its compression performance. In hindsight, I was smart enough not to argue with him about it.

After all these years you’d you’d think, after all these years working in audio industry I would have got used to it, but you never do. I was back at square one.

I put my social life on ice, what little there was of it, and prepared to spend a few long, lonely nights burning the midnight oil in the Effectrode workshop. It was business as usual.

The next batch of PC-2A’s (serial numbers over 300) feature a second internal trimpot: To blend in the unprocessed ‘dry’ signal with the compressed ‘wet’ signal. The effect of blending the two signals in parallel is a reduction the compression ratio, and the sharpness of the ‘knee’, that is, the threshold at which the PC-2A begins compressing the signal. All this obsessive ‘tweaking’ serves one purpose: to reduce pumping to a vanishing point to create silky smooth, transparent—practically invisible—compression.

Sound engineers have used this parallel compression technique in studios since the 1970s, possibly earlier, to add audible detail to music. Typically, compressors attenuate the peaks (‘downward’ compression) in the program material to reduce the dynamic range. However, blending dry signal with compressed signal also reduces the dynamic range, by amplifying the quietest sounds (‘upward’ compression). See the difference? The technique also goes by the name of “New York compression“, as it was used extensively by sound engineers based in New York City to enhance drums, vocals and bass. When applied to guitar, parallel compression helps preserve pick attack and the natural dynamics of the instrument. In use, the PC-2A sounds fast, ‘tight’ and effortlessly handles ‘difficult’ instruments, such as active 5-string bass, NS stick and instruments with extended lower register.

But Keep It Simple

As with the LA-2A, there are just two knobs on the PC-2A: ‘PEAK REDUCTION’ and ‘GAIN’. No ‘ATTACK’, ‘RELEASE’, ‘KNEE’ or ‘RATIO’ knobs, and no adjustable side-chain filters or other gizmos. Just those two knobs, which is not a bad thing. Personally, I find too many options a distraction from playing. And distractions will hold your playing back. Don’t get me wrong, there is a place for gadgets & gizmos: on a studio compressor. So sound engineer can tailor these parameters to use the compressor with different instruments: to tighten up the low end on bass guitar, to get harder-hitting kick-drums, tame cymbals, de-ess vocals to remove sibilance, enhance reverb tails for a bigger sound, compress the dynamic range of musical material before cutting a record or broadcasting, that kind of thing.

A guitar compressor’s job is simply to level out playing dynamics, or smooth out the difference in output levels from the strings. Adding too much flexibility to a guitar compressor, or any guitar effect for that matter, is a bad thing. Why? Because there are some places you do not want to go. Fast attack and release times result in ‘pumping’ as the compressor tries to closely follow every peak and trough in the envelope of the input signal. In extreme cases this sounds like clipping; but not nice tube clipping distortion—it sounds rough. Two lessons learned: Keep it simple and make it sound sweet.

Wrapping it Up… in Velvet

The last vital element in the PC-2A’s construction is the quality of its parts. Within its tough aluminium alloy enclosure there’s an all-tube analogue signal path made up of top-notch audio-grade components. High quality resin-dipped polyester coupling capacitors and instrumentation-grade precision metal-film resistors ensure distortion and noise are kept to an absolute minimum. And then there’s the tube…

At the heart of the PC-2A there’s an vintage American black-plate subminiature tube. The last word in tone and reliability. These diminutive tubes were developed by Sylvania Electric for use in guided missile systems in WWII; their reliability is out of this world—they can handle anything working musicians might throw at them, and more.

And the tube operates at high voltage—just like the LA-2A—for unmatched clarity and headroom. Solid-state or VST modelling compressors do not come close… despite what those over enthusiastic salesmen might tell you. The PC-2A leaves ’em way back along the road to tone, lost in digital dust! It’s the real McCoy: A compact compressor pedal for working musicians seeking the real Teletronix LA-2A sound for their guitar, bass or keyboards.

A big thanks to Adrian Legg and Phil Taylor (Gilmour Music) for helping audio test the PC-2A during its development. They made it possible to take the design of the pedal far beyond expectations to new horizons—I could not have got there alone. To find out more, check out our sound clips, videos or test drive a PC-2A at your nearest Effectrode dealer.

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The PC-2A: A Studio Compressor in a Pedal