Pendulum Audio
ES-8 Tube Limiter
A New Take on a Venerable Limiter Design
 by Dave Martin - Recording April 2001

   If you want to pick nits, a 'tube limiter' is a limiter that uses a tube as the gain reduction element. That means an LA-2A is not a tube limiter-it's an electro-optical limiter. There are tube-based amplifier circuits in the input and output stages of the box, but that doesn't make it a tube limiter. You could correctly call it a 'tube leveling amplifier' without error if you so choose.

Why does this matter? Well, one important reason is that there are tube compressor/limiters, and the term is most appropriate when talking about them.

During the 1950s, several different manufacturers began using the rather common GE-6386 tube in a variable gain configuration for their broadcast limiters. The circuit works with the bias of the dual triode tube, and therefore the output is changed by side-chain control voltages.

Among the devices based on the 6386 were broadcast limiters by Gates, Altec, Collins... and Fairchild, who designed a stereo limiter and continued to use the 6386 tube in a variable gain circuit. The result, the Fairchild 670, was a widely used mastering limiter that later became a widely used mixing limiter.

The 670 is long out of production and has become possibly the most expensive piece of outboard audio gear in the world, with prices well north of $25,000. In fact, the 6386 tube is no longer made and has become fairly rare (and fairly expensive if you can find them).

The folks at Pendulum Audio decided to build a tube limiter with the same characteristics as the as the venerable Fairchild 670 - not only its time constants (attack and release times) but also the way its compression ratio increases with gain reduction. The design goal was to maintain the 'dynamic signature' of the 670 but come up with a more accurate and less colored signal path.

Initially, they used the 6386 as a gain control tube, and then coupled that with class-A amplification that had an active-balanced output stage rather than the transformer output used in the original. The result is the
Pendulum 6386 ($3995), for users who insist on the real thing where the tubes are concerned.

Realizing that the 6386 tube is becoming much harder to find, they set out to find another tube that could duplicate the Fairchild compression curve and time constants.They found it in the 6ES8, which led to the introduction of today's subject: the ES-8 Variable Mu Tube Limiter.

Swinging back and forth

The Pendulum ES-8 is a stereo/dual mono limiter. When linked for use as a stereo limiter the threshold and time constants are controlled entirely by Channel 1; in the dual mono mode each channel is completely independent.

The solid-state side chain offers three modes of processing. Its Fast mode is self-explanatory, with attack and release times of 0.5 ms and 50 ms respectively. The Preset mode offers program settings identical to the 670 (more on the program settings in a bit), and the Manual setting gives you control over both the attack and release times to suit the application.

Controls

The input control is actually a passive attenuator placed before the input transformer. Its normal operating position is at it maximum unless the overall input level is overloading the input transformer or gain stage. TheThreshold control determines how much gain reduction is applied to the source material, and the Output control is for gain make-up after compression.

Each channel also has a meter switch, allowing you to monitor the input level, output level, or gain reduction. The manual attack and release controls are active when the Mode switch is set to Manual; the attack times
are adjustable from 1 ms to 100 ms and the release times from a tenth of a second to 2 seconds.

One setting on the Mode switch is Presets, which accesses six settings that are identical to the time constants of the Fairchild. These Presets are labeled by their release times, though the attack times vary as well. The first four settings offer fixed attack and release times, while the last two have a quick initial release followed by a longer decay time to zero gain reduction. (Interestingly, this type of release is a characteristic of electro-optical compressors like the LA-2A.)

The available release settings are 0.1 s, 0.3 s, 1 s, and 2 s, with 1 to 4 seconds and 0.5 to 20 seconds on the program-dependent settings. All controls, including a hard-wired bypass switch, are duplicated for the second channel. And when the two channels are linked, Channel 2 is also bypassed by the Channel 1 switch -a neat idea.

The rear panel has the both XLR and TRS I/O (the TRS plugs can be run unbalanced), along with inserts for an equalizer into the sidechain detector circuit. There is an IEC connector for AC power, a power switch, and fuse holder.

Observations

The ES-8 is an interesting box. On one hand it aims to duplicate the compression characteristics (a large part of the sound) of the Fairchild 670, while avoiding the use of output transformers (also a large part of the Fairchild sound). So rather than attempting to do any sort of a comparison-which would really be hard, since I don't happen to have the $25 grand to purchase a 670 of my own!-we'll discuss it on its own terms. And as a high quality compressor/limiter, it works extremely well.

When I needed to control single instruments and lead voices without noticeable compressor artifacts, the Fast setting was often just the ticket. And the program-dependent settings (especially number 5, with a 1
to 4 second release) were great when I wanted just a bit of an optical vibe. But most of my play time was spent using the ES-8 as a stereo compressor/limiter. To my ears, that's where its strength lies.

When set within the typical operating parameters of the unit (that is, without going for huge amounts of added gain), the ES-8 is sonically as neutral as any compressor I've heard. When you wish to rein in the dynamic range of a mix without adding any coloration, this is the box to do it with. Even with dramatic amounts of gain reduction, the output maintained a remarkable transparency.

I'm a firm believer in Bypass switches for signal processing devices, and a hardwire bypass that routes the input directly to the output (bypassing all electronics) always gets bonus points. While I usually use the bypass switch to make sure I'm not making the sound worse, with the ES-8 I kept having to check to make sure I was indeed compressing the signal!

The flip side of that is that sometimes you want a less transparent limiter. But it would be silly to applaud a piece of equipment for meeting its design goals so well and then turn around and complain that it meets
its design goals so well.

Thus

The Pendulum Audio ES-8 is an extremely high quality tube limiter that I'd recommend to anyone looking for transparent level control without the sonic artifacts typically associated with variable mu limiters. At $ three and a half grand the ES-8 could hardly be called inexpensive, but then the good stuff seldom is.

Price: $3495

More from: Pendulum Audio, Inc., P. O. Box 339, Gillette, NJ 07933. vox/fax
908/665-9333, www.pendulumaudio.com.