Compressors Denver Colorado Pendulum Audio ES-8 ES8 Pendulum Audio Quartet Neve Mastering Buss Bus Compressor Neve MBP Denver Colorado High End compressors

Compressors and Limiters

Pendulum Audio ES-8 – $3,500

Pendulum Audio ES-8

If you had to have just one compressor… The ES-8 was my first compressor and as I added to my collection in a very big way, I appreciated the ES-8 even more. Of course, many people use the ES-8 for tracking vocals running in and out of one channel and into the other in mono. The ES-8 gives you flawless transparency and sometimes you can’t believe it’s not in “bypass” even when you are seeing a tremendous amount of gain reduction. On room mics as well the ES-8 adds so much to a drum mix or to a live recording. The ES-8 “glues” the track together. The ES-8 is well thought of and used by quite a few folks that have multiple platinum records. And it is a standard limiter to use on the two bus.

*Classic feedback compression with the same compression profile as the Fairchild 660 and 670
*The 6386 uses the same gain control tube (remote cutoff triode) as the Fairchild 660 and 670.
*The ES-8 uses a more readily available (and less expensive) tube
with the same compression characteristics as the 6386.
In all other aspects, the two units are identical.
*Modern class A, solid-state gain make-up stage for wide bandwidth and low distortion
*Soft-knee design with smooth transition from compression to limiting
*Fast attack (0.5ms) and release times ideal for tight program compression or tracking
*Low noise and superior transient response
*Three sidechain operating modes: Fast, Presets and Manual
*Two completely independent channels with stereo linking
*Tubes calibrated for precise stereo balance
*Hard bypass switches for each channel
*Sidechain inserts for frequency-dependent compression and de-essing
*Transformerless balanced output stage capable of delivering +27dBu
*Soft-start warm-up with outputs bypassed until all voltages are stabilized
*Gold-plated relay contacts, I/O connectors and tube sockets
*Fully regulated high voltage (250V), filament and bipolar power supplies
*Custom toroidal power transformer with shield for minimum hum
*Polypropylene capacitors and metal film resistors
*Limited production, hand-built product

The ES-8 is modern two channel limiter designed to have the compression characteristics of a vintage unit, but with a higher level of sonic accuracy. Rather than trying to recreate the past, Pendulum Audio has taken an entirely different approach. The 6386 uses the gain control tube (remote cutoff triode) found in the vintage Fairchild limiters to provide the classic feedback response unique to the characteristics of this class of tubes. The ES-8 uses a different remote-cutoff triode with the same dynamic characteristics. Subsequent amplification is accomplished with a Class A, balanced transformerless line amp, rather than the push-pull transformer output stage used on vintage variable-mu units. The result is a new class of limiters that possess the ‘dynamic signature’ of a classic tube compressor, but with a more accurate, less colored signal path. The two units are identical, except for the tube used for gain reduction.

The soft-knee design used in the ES-8 offers effortless dynamic control with a smooth transition from compression to limiting. In addition, both channels can be operated independently, or linked with the flip of a switch. When linked, the threshold and attack/release parameters are controlled entirely by channel 1, assuring precise stereo balance over the entire range of gain reduction.

For maximum versatility, the ES-8 incorporates a solid-state sidechain to provide a wide range of control with three modes of processing: In the FAST mode, the limiter works extremely fast (less than 0.5ms attack time), and is remarkably free of the ‘pumping’ artifacts normally encountered with slow variable-mu compressors. It’s the ideal choice for invisible dynamics processing, either for tracking or tight program compression.

The PRESET mode consists of six classic attack/release time settings (identical to the Fairchild 670) that are optimized for program compression, including two settings with program-dependent release times.

The MANUAL mode offers total control over the attack and release times, which is particularly useful for creative compression effects or for processing bass guitar.

Most important, the hard bypass switches on the front panel conveniently allow a quick determination of exactly what the ES-8 is, or in most cases, isn’t doing to your signal.

Rupert Neve Designs Master Buss Processor – $3,995

Portico MBP

Based around the high voltage, discrete and class-A signal paths made famous in the 5088 Mixer and the Portico II: Channel, the Portico II Master Buss Processor is creative tool that redefines the boundaries and limitations of traditional 2-channel compression and limiting.

With input and output transformers designed and implemented by Mr. Rupert Neve, the high-voltage 72V topology found in the Master Buss Processor (MBP) will integrate flawlessly with virtually any system. Additionally, the MBP incorporates mastering grade detented pots throughout to fine tune its revolutionary dynamics, tone, and stereo field controls. This new topology is a significant evolution of Mr Rupert Neve’s classic designs with appreciable benefits to headroom, dynamic range, distortion, noise, slew rate, bandwidth, and accuracy while still providing for the sweet, musical performance that has been a part of countless recordings.

The Compressor

The MBP’s two compressor sections allow virtually limitless possibilities in dynamics for either dual mono or stereo sources, with controls for ratio, threshold, attack, release, blend, side chain HPF, limit and make up gain. With the stereo link control engaged, Ch. A settings act as the master control for convenient operation. When engaged, the compressor section can be used in both feed-forward and feed-back modes to provide a transparent modern response (feed-forward), or a smoother more musical vintage response (feed-back). Peak mode alters the compressor’s attack to react to peak transients with a roughly .1ms response time. When the Peak switch is disengaged, the compressor responds to the RMS signal in conjunction with the attack and release settings. SC HPF inserts a high pass filter at 250 Hz into the side chain to deal with intense low frequencies that may skew the response of the VCA with certain songs and instruments. “Blend” creates a parallel mix between the compressed and dry signals. By mixing the compressed and dry signals, it is possible to increase the volume of quieter elements in the source material (for instance, delicate snare brushing on a track with much louder hits), while maintaining a natural dynamic feel for the louder elements.

To further control the side chain, there is also an insert “send” and “return” that may be paired with an external EQ or other filters for additional manipulation. The “return” may also be used as a “Key” input for ducking one signal to another (for instance, a voice-over keying the compressor to duck a background music track).

The Limiter

The Portico II Master Buss Processor also features an extremely versatile, transparent and musical limiter. At first glance, one might scoff at the single knob operation, however this limiter is extremely intelligent, knowing how to appropriately respond to the various signals presented to it. Our new Adaptive Release Technology is behind this revolutionary performance. Using a blend of release time constants, this limiter will simultaneously respond quickly to transient material (such as the “snap” of a snare drum) and slowly to more sluggish signals (such as a bass guitar). This configuration allows the limiter to grab a transient and let go just an instant later, while also dealing with more constant signals in a slower, more musical way. In this manner, the MBP Limiter can provide a much more aggressive amount of limiting than typically possible, while maintaining the essential character of the music and remaining free of the modulation distortion usually found in a fast acting limiter.

Typically there is a tradeoff between how fast limiter can react and the amount of modulation distortion in the lower frequencies. This is due to the lower frequencies finding their way into the side chain signal, triggering the compressor on and off very quickly, which ends up modulating the overall signal. This is interesting to look at with sine waves, but sounds quite undesirable with music. The MBP does not have this tradeoff, and one is able to have the best of both worlds: a quick, snappy response while maintaining the integrity and smoothness of the low end. In addition to the adaptive time constant circuitry, the release time is also varied with the position of the knob. As the knob is turned counter clock-wise, the release time is increased accordingly, as typically one would want a longer release time with a larger amount of reduction.

The new limiter found in the MBP is designed to respond as fast as .03 mS in order to reduce the first half of a 20 kHz waveform over the threshold. It has a “medium knee” initial ratio and within 3 dB of the threshold attains a better than 10:1 ratio. A soft clipper circuit catches transients that may have been in the “knee” when the threshold knob is set quite high. Both the limiter and soft clipper are switched out of circuit with the knob is fully clockwise. The release times are fully automatic and adjust depending both on the average depth of limiting and the duration of the transients above the threshold. The limiters share the same discrete, class-A gain module and VCA with the compressors, so using the Limiter does not introduce more stages that the music would have to pass through. This combination of features provides exceptionally transparent limiting, and often allows twice as much gain reduction compared to other limiters before objectionable artifacts become apparent.

The Stereo Field Editor

The stereo field editor on the MBP takes traditional M-S techniques to new heights with width, depth and corresponding bandpass filters.

The width control enables the user to increase or decrease the width of a stereo image (wide/mono) and adjust the amount of ambience inherent in the recording. As the width control is rotated toward wide, the amount of difference material is boosted, often resulting in more ambient material, and accentuated stereo reverbs. Conversely, the stereo field is contracted when rotated to mono, and, if the left and right channels are highly coherent (i.e. both channels include closely similar material that is in phase), this mono content is enhanced. If the phase of one of the input channels is then reversed the mono content may be virtually eliminated. Because the amount of effect the width control has is entirely dependent on the amount of stereo information in the original source material and the interplay between the stereo field editor′s other controls, listening and experimentation are essential for the best results.

The depth control of the MBP adjusts the spatial positioning of elements in the sound stage. Center-panned elements like solo instrument or vocal can be brought forward in a mix, in relation to supporting instruments. In many cases, these same elements may be virtually eliminated without adversely affecting the music bed. Used in conjunction, the depth and width controls effectively alter the perceived room ambience and dimension.

To fine tune the SFE, there are individual filters that allow a fine tuning of what information is reintroduced from the width and depth circuits, thus tailoring each effect to a specific bandwidth. For example, if one wanted to increase the amount of low frequencies in the center image, engaging the SFE Depth and Depth EQ, set to LF, would filter out everything in the Mid signal except what is below the filter point (in this case, 250 Hz), and once reintroduced to the original would result in a perceived increase in the low frequencies in the center image. It is also possible to do the same thing with the Width EQ, except instead of boosting the width, cutting it, which removes low frequencies from the Sides, tightening up the low frequency perception in the center. Using the Width EQ again, this time set to HM (or LM as the case may be), increasing the amount of band-passed Side information can provide a wonderful spreading of instruments, reverberation and background vocals, giving the illusion that the sounds are spread further out, enveloping the listener.

Another technique available on the SFE is routing the Mid and Side signals to the Channel A and Channel B compressors, respectively. Now whatever amount of Depth or Width is introduced is first routed to the compressors, allowing the user to utilize the compressor features on the Mid and Side information. Now it is possible to not only increase the side information, but to utilize the compressor to bring up some of the low level side information, or allow the user to tame an overly expressive lead singer. With the addition of using the EQ section on the Depth and Width, a wide range of tools is available to the engineer.

Pendulum Audio PL-2 – $2,450

Pendulum Audio PL-2

The PL-2 is a stand-alone two channel version of the very popular peak limiter found in the Pendulum Audio Quartet II Tube Recording Channel. The PL-2 uses a novel approach to brickwall limiting in the analog domain, one that is fast enough to prevent digital overs without using time delay or phase-shift delay techniques. This allows us to keep the analog signal path short, and keep the limiting circuit ‘switched out’ of the signal path below the peak limiting threshold. Distortion for all levels below the the limiting threshold is low, and does not rise as the threshold for limiting is approached. Unlike analog peak clippers, which chop off the top and bottom of the waveform, our limiter retains dynamic information at the top of the peak.

The PL-2 has two modes of peak limiting, using either junction field-effect transistors (JFET) or metal-oxide semiconductor field effect transistors (MOSFET) as the control elements. While both are equally capable of brickwall limiting, each type has its own characteristic sound. The JFET tends to be a stiffer, harder mode of limiting, while the MOSFET has a softer, more compliant response. The resulting waveforms look different, and each device has its own characteristic sound.

In addition, Input and Output gain controls allow the user to scale the gain structure for a wide range of applications, from driving the front end of an A/D converter at +22 dBu, to inserting it as an effect into a +4 dBu patch bay. Several units can be linked for multi-channel applications.

Each channel also has an expanded 13 segment LED display which accurately indicates peak reduction.

*JFET (hard) or MOSFET (soft) peak limiting modes
*Two independent channels with stereo link
*Precise readout of peak reduction
*Stepped input and output controls – ideal for mastering or session recall
*Limiter circuit out of the signal path when below threshold
*Transformerless Class A solid-state design

Pendulum Audio OCL-2 Electro-Optical Compressor Limiter – $2,900

Pendulum Audio OCL-2

The OCL-2 is a two channel electro-optical compressor/limiter designed for the utmost in transparency, detail and versatility. Our short signal path design uses a custom optical input attenuation network in front of an all tube class A gain stage. Since the signal path is entirely transformerless, we’ve eliminated the coloration inherent in traditional transformer-coupled tube designs. The result is an open, detailed sound with an expanded sound stage and clarity that can only be achieved with modern tube circuitry.

The OCL-2’s soft-knee design offers smooth, effortless dynamic control with the ratio adjustable from light compression to limiting. Our unique sidechain circuit linearizes the optical gain element to reduce distortion, and achieves much faster attack and release times than encountered with conventional optical compressors. And stereo linking is accomplished with the flip of a switch .

In the FAST mode, the optical cell operates with very fast attack and release times, which are peak-averaged by the rapid operation of the cell. This mode is remarkably free of pumping artifacts even at high compression levels. It’s the ideal choice for ‘invisible’ dynamics processing, either for tracking or tight program compression.

The PRESET mode consists of six classic attack/release time settings that are optimized for program compression, including two settings with program-dependent release times.

The MANUAL mode offers total control over the attack and release times, which is particularly useful for creative compression effects or for processing bass guitar.

Most important, the hard bypass switches on the front panel conveniently allow a quick determination of exactly what the OCL-2 is, or in most cases, isn’t doing to your signal.

*Fast electro-optical input attenuator with an all tube signal path
*Soft-knee design with fully adjustable compression ratio
*Transformerless high voltage class A signal path
*Low noise, wide bandwidth and superior transient response
*Custom optical cells matched for precise stereo balance
*Three sidechain operating modes: Fast, Presets and Manual
*Two completely independent channels with stereo linking
*Hard bypass switches for each channel
*Sidechain inserts for frequency-dependent compression and de-essing
*Transformerless output stage capable of delivering +35dBu
*Soft-start warm-up with output muting
*Gold-plated switch contacts, I/O connectors and tube sockets
*Fully regulated high voltage (300V), filament and bipolar power supplies
*Custom toroidal power transformer with shield for minimum hum
*Polypropylene capacitors and metal film resistors
*Single-sided audio pc boards to minimize capacitive interaction

Manley Variable Mu Stereo Limiter Compressor$3,960

RND 543

The MANLEY VARIABLE MU® LIMITER COMPRESSOR has been our best selling product for many years. It is one of the very few compressors that has become a real standard in Mastering studios and contributed to most hit records over the last decade and probably the next. “Mu” is tube-speak for gain, and Variable Mu® is our registered trademark for this limiter compressor. It works by using the “remote cut-off” or re-biasing of a vacuum tube to achieve compression. The precious vintage Fairchild 670 also uses this technique and is one of few all-tube compressor to do so, that we know of. Even the side-chain has glowing rectifier bottles. How’s it work? The unique 5670 dual triode is at the center of the peak-reducing and compression action constantly being re-biased by the vacuum tube rectified side-chain control voltages which cause this tube to smoothly change its gain. Just like that.

The COMPRESS mode is soft-knee 1.5 to 1 ratio while the sharper knee LIMIT mode starts at 4 to 1 and moves to a more dramatic ratio of 20 to 1 when limiting over 12dB. Interestingly, the knee actually softens as more limiting is used. Distortion can be creatively used by turning up the Input and turning down the Output while using very little or no compression.

You might notice that the Variable Mu® Limiter Compressor has a ganged input control, but do not jump to conclusions that it is mono-unfriendly. Track away! There are separate threshold and output controls to make compensations with plus you can always adjust your individual source levels elsewhere, right? The advantage of the stereo input control becomes dramatically clear when you switch to LINK mode, and that’s what our Variable Mu® Limiter Compressor does better than anything else: final mix, 2-track, or mastering limiting and compression. Like one reviewer put it: “It’s like pouring a bowl of sweet cream over the mix.” Mmmmmm. Yummy. Give your music a big hug.

Features and Specifications

MANLEY input & output transformers with nickel laminations in mu-metal cases with flat frequency response from 20Hz-25KHz
Fully differential ALL-TUBE circuitry using one each 5670, 5751 (or 12AX7), 7044 (or 5687), & 12AL5 per channel
Independently regulated B+ and Heater supplies
Hard-wire BYPASS switch
Stepped switch INPUT attenuator as of 10/2011. (older units use Silent dual-ganged potentiometer)
RECOVERY 5 steps: 0.2s, 0.4s, 0.6s, 4sec., 8sec.
Variable ATTACK: 25msec-70msec
Continuously variable THRESHOLD
LIMIT (4:1 to 20:1) or COMPRESS (1.5 to 1)
HP SC: High Pass Side Chain Filter -3dB @ 100Hz
Large ILLUMINATED Sifam METERS (older units before serial number MSLC61642 shipped before 12/2003 use: 26V 1.2W FESTOON LAMPS;
Manley’s Part Number: VAR016B) Order spare bulbs using our parts order form. (newest units after serial number MSLC61642 shipped after 12/2003 use white LED lighting)


Several units can be linked for Surround (custom order )
Maximum gain: 35dB
Max. output: +30dBu (26Vrms) 26dB Headroom
<0.1% THD @ 1KHz Noise floor: -85dB typical
Power Consumption (120/240VAC): 80 watts
Unit is factory set for 100V, 120V or 220-240VAC operation for original destination country’s mains voltage.
Operating Mains Voltage changeable with power transformer changeover switch and fuse value change.
Mains Fuse Value for 100~120VAC operation: replace with 1.25A as of 10/2011
Mains Fuse Value for 220~240VAC operation: replace with 0.6A (600mA) as of 10/2011
Mains Fuse Type: MDA or MDL SLO-BLO Time delay 1 1/4″ x 1/4″
Mains Voltage Frequency: 50~60Hz
Dimensions: 19” x 3 1/2” x 10” (chassis occupies 2u) Power transformer protrudes 3.5″ out the back of the chassis.
Shipping Weight: 23 lbs.
Unit Weight: 14 lbs.


Proudly from our beginning, we at MANLEY LABS have been closely involved with numerous mastering facilities around the world. For these most demanding engineers in our industry, we have created specialized products such as our all-tube tape repro head amps, digital converters, high-powered vacuum tube monitor amplifiers, through to custom monitoring consoles, all the way up to complete facility design and installation.

The Mastering Version of our Variable Mu® Limiter Compressor incorporating detented and logable steps built with 1% metal film resistors on sealed gold-contact Grayhill switches is available. NO, we will not mod a “normal” unit to the Mastering Version. The Mastering Version of the Variable Mu Limiter Compressor uses expensive Greyhill rotary switches with gold contacts where conductive plastic pots were used. The steps are determined with a large number of 1% precision metal film 1/2 watt resistors. The best conductive plastic pots only have 10% or 20% tolerance. The ten fold improvement in precision helps a great deal in left-right matching. There is a subtle audible improvement with stepped switches as well. Audiophile HI-FI often uses that technique to wring the last drop of performance out of a preamplifier. And I bet you wanted stepped switches mostly for resets.

The INPUT LEVEL is a five position switch with a generic optimum setting of “0” in the 12:00 position. Each step in either direction is 2 dB. For reference, Unity Gain is “0”.
The OUTPUT Attenuators are in half dB steps. Reference Unity is -11.5 or fully counter-clockwise. You might think of these as “gain makeup”. They are marked technically, in that “0” or fully clockwise has zero attenuation in the circuit. The tube circuit actually has 15.5 dB of gain. The Input attenuator at “0” removes 4 dB and the Output attenuator removes the last 11.5 producing “unity”. With a little compression the “gain make-up” available with the Output Attenuator is very handy.

The Threshold is in half dB steps calibrated to LIMIT mode. In Compress the steps are approximately 1/4 dB. There are 24 steps so LIMIT gets a 12 dB range and Compress has a 6 dB range of adjustment. In some cases it is common to use the Input Attenuator to find a good starting point. Some Mastering engineers find using the combination of Input, Output and Threshold to achieve a little different “drive”. Another good reason for stepped gains.

The Attack Time has been slightly extended in both directions compared to a regular Variable Mu and divided into 11 steps.
The Recovery 5 position switch is exactly the same as a regular Variable Mu.

The switches specifically are like this:

Input: -4 to +4 in 2 dB steps
Output: -11 to 0 in 1/2 dB steps (24 detents)
Threshold: -11 to +1 in 1/2 dB steps LIMIT(24 detents) and 24 x 1/4 dB steps in COMPRESS
Attack: 11 positions CW (fast) 15mS, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90mS (slow) CCW
Release: 5 positions CW (fast) 200mS, 400mS, 600mS, 4S, 8 Seconds (slow) CCW

Mastering Version engraving shown with HP SC Mods.

The High Pass Side Chain mod comes stock on all Manley Variable Mu Limiter Compressors since 12/2009, on both regular and mastering versions. This mod adds two switches to the front panel, one for each channel, so that when engaged, the side chain will not respond to frequencies lower than 100Hz. (We standardly use 100Hz as the -3dB point. Other frequencies can be custom ordered.) This HP SC Mod can be used with music with heavy bass lines or bass-heavy mixes where you don’t want the bass driving the whole action of the compressor.

The filter is a very gentle 6db per octave 1 pole filter, and will typically be down 1-3db at 100 Hz, and down 4-6db at 50Hz. As you decrease the frequency the amount of limiting will decrease also. At the extreme LF (<20Hz) there should be very little gain reduction going on. The whole intent of the filter is to keep very LF stuff (like a heavy kick drum) from activating the compression/limiting so that the overall level doesn't duck with every drumbeat. We can add the High Pass Side Chain option stock boards for an extra $200 NET to older Manley Variable Mu limiters. Units which are more than ten years old that have to be hand wired for this mod will cost slightly more in order to install the circuitry, approximately $275.

Another mod we can do for several Variable Mu’s is to accommodate Surround Sound Linking. This presumes you want to use 4 to 6 (or more) channels of Variable Mu for your 5.1 surround work, and you own two or three Stereo Variable Mu units (or more). For this we add a pair of RCA jacks on the back of each unit so that the sidechain can be easily plugged into another one or two other Stereo Variable Mu units. Then the LINK switch is replaced with a three-way switch and wired to select amongst the following three conditions:

LINK: Link the side-chains of the two channels in this chassis
EXT: Link the side-chains of the two channels in this chassis AND to the other units plugged into the RCA
linking sidechain access jacks on the back of the unit.
SEP: Do not link to any other channels

We add TWO RCA jacks to each unit for easy daisy-chaining. Just use any ol’ RCA cables you have lying around to link up the sidechains.

Then all the sidechains track each other. You still have to set up each unit as you would though, as the very act of linking the side chains does not create a master-slave situation.

We usually charge $200 NET per unit to do this Surround Linking mod.

We also offer a Mid/Side mod upgrade (aka. Vertical/Lateral or Sum/Difference) to the Variable Mu® Limiter Compressor which opens the door to stereo encoding and decoding as well as exciting image enhancement processing capabilities. For instance, setting to compress only the in-phase information allows the augmentation of the stereo image as the out-of-phase content is left untouched. Or, conversely, if you need a “more-mono” mix for broadcast, or vinyl-cutting for instance, you can set it to kill off more of the out-of-phase info which leaves more in-phase material in the final result. Read more about the MS Mod on page 12 of the Owner’s Manual.

Total mod cost for the M-S option is $500 NET added to the base cost.

In order to add both M-S and the HP SC mods, we move the power switch to the rear panel and install both channels’ HP SC switch to the center hole where the power switch was. The M-S switches then flank this switch.

Please inquire with us personally for detailed information.

Which tubes should I use in my Variable Mu® Limiter Compressor? 6386? 5670? or 6BA6 TBAR Mod?

Yes, the newer units use the 5670 tube instead of the 6386. By now the availability of the original USA GE 6386 is poor; we don’t have any left at all, and what we do have are not usable due to noise, microphonics, bad side-to-side match, etc.

Does the 5670 sound different?

Well, up to about 6db of limiting it’s about the same. After that point, the 5670 version tends to sound more “squashed” than the original 6386 version. Some like it better, some don’t– depends on what you’re trying to do. To solve all these problems, Paul came up with a really good solution: the T-Bar Mod which uses a pair of 6BA6 pentodes wired as single triodes to replace each dual triode 5670 (or 6386). the 6BA6 TBAR Mod is the preferred system to use in the Manley Variable Mu® for reasons of ability to perfectly match each phase-halve section and each stereo set, ability to select for lo-noise and lo-microphonic sets for a low cost, and because the action of the 6BA6’s so closely resemble the smooth 6386 limiting curves.

Read more about Paul’s special T-Bar Mod here.

Update 2009: There is a new production 6386 being made by JJ factory. They are very expensive, $120 each! We have not tested these tubes.

You can read an informative thread about the JJ6386 on Gearslutz to decide if you want to them instead.

Follow the directions below to convert your 5670-powered Manley Variable Mu® to use the 6386 tubes. It’s more work to do this than to install our 6BA6 T-Bar mod kits which we charge $250 for (complete and installed with four 6BA6 tubes selected for quad-perfect matching, lo-noise, and lo-microphonics).

If you want us to convert your unit for you, we can. You’ll be supplying the 6386 tubes and we will all pray to the Tube gods that they are quiet and internally matched. Maybe buy extras to be sure. Check out our service page for rates and then fill in the RA form to book the service and you can work with Paul on that.


Replace 3K57 resistors connected to pins 4 & 6 of 5670 socket with 6K8 1W.
Replace 1K resistors connected to pins 2 and 8 with 511 Ohms.
Replace 10K “balance” trimpot with 500 Ohm trimpot.
Remove 200 Ohm resistor between pins 2 and 8.
Replace 33K 1W resistor between wiper of “GR cal” trimpot and the junction of the meter with 22K 1W (resistor is located near the rear corner of the pcb next to the GR cal trimpot).
Repeat procedure for the other channel; re-calibrate as outlined in the owner’s manual.
What is the difference between turning up the INPUT level control to drive the unit into limiting with more level versus using the THRESHOLD control?

The limiting characteristics shouldn’t change much, but the distortion characteristics will. The input control on these units is located ahead of the tubes, directly “behind” the input transformer. So as you advance the input control, you’re hitting the input tube (which is the gain-varying stage) with more and more signal. Gain reduction is done by making the DC bias on this tube more negative (the threshold control determines how much). The tube has a limit as to how far it can be biased negative to reduce gain before it goes non-linear, and since your audio signal is obviously swinging both positive and negative, the big swings of the audio signal plus the negative gain control voltage will eventually push the tube into the red zone. Tubes like the 6386/6BA6/12BA6 can be pushed a lot farther before the THD builds than tubes like a 5670. Higher signal level also pushes the rest of the tubes and the output transformer harder, which will change the sound character as well.

500 Series Rupert Neve Designs 543 Compressor $995

RND 543

The Portico 543 Mono Compressor delivers the unobtrusive, musical-sounding dynamic control and brick-wall limiting made famous in the Portico 5043 to the 500 series format. The 543 features a fully controllable compressor-limiter with feed-forward / feedback modes, Peak / RMS detection and a built in side chain high pass filter. With an unrivaled heritage and a tremendous feature set, the 543 yields a combination of rich warmth, flexibility and precision that is sure to resonate in the ears of sound engineers the world over.

Built around Mr. Rupert Neve’s custom transformer designs, each 543 module encompasses individually controllable threshold, attack, release, makeup gain, ratio, side chain HPF, Feed-Forward / Feed-back selection and Peak/RMS detection modes. With the compressor inactive, the 543 may be used as a transformer-coupled, high-performance line amplifier, and two 543 may be linked for stereo operation as well. The chassis is built to standard 500 series specifications, with power and I/O provided by the rack.

How the Compressor Works

In order to control gain, a V.C.A. or Voltage Controlled Amplifier (or Attenuator) is used. There are many types of V.C. including the use of tubes, discrete and integrated solid state circuits and naturally non-linear devices, each one having its characteristic behavior that reflects sonically on the final performance, and gives it a character or signature that can be musically attractive or not! The Portico 543 makes use of a very accurate, low noise, low distortion V.C.A. having, essentially, no signature of its own.

A part of the audio signal is rectified and smoothed to produce a suitable control voltage for the V.C.A. which has to respond very quickly and have low distortion. If the response is too fast, low frequency signals will themselves, be gain controlled! If the response is too slow, the signal will overshoot and the first few cycles will not get compressed. The speed and accuracy of the response, known as the “attack”, and the time frame that gain remains under the initial control, known as “release” or “recovery” and plays a large part in the way a compressor sounds.

Feed-Forward or Feed-Back?

The 543 also has the ability to switch between feed-forward and feed-back modes. If the V.C.A. Control voltage is taken from the 543 output, (i.e. after the V.C.A.) it cannot act immediately on the V.C.A. because it has already been modified by settings of the V.C.A. and circuits through which it has passed. This is known as a “Feed-Back” compressor. The two compression characteristics are quite different; there is more “Overshoot” and both the attack and recovery ramps are changed, providing the user with powerful choices.
In most of Mr. Rupert Neve earliest designs, feed-back detection controlling the VCA with a rectified voltage from the unit output was intrinsic to the musical dynamic response. However, the very nature of a feedback compressor limits the attack time of the compression circuit. To offer faster, more technically accurate response times, feed-forward detection was implemented on Mr. Rupert Neve’s more modern designs. With the FF / FB switch, both classic and modern VCA responses are available.

RMS or Peak?

One of the more unique features of the 543 is the new Peak / RMS mode also found in the Portico II Channel. This switch allows the VCA (voltage control amplifier) to respond to both RMS (Root Mean Square) and peak levels. RMS circuits are considered to better mimic the way the ears perceive apparent loudness, while peak circuits tend to directly respond to the waveform voltage which may be more of a concern for prevention of clipping and maximizing levels. In this case, peak mode uses a combination of both methods to get the best of both worlds, and avoids the drawbacks of each method on its own.

Ratio and Threshold

Above a given “THRESHOLD” signals are reduced by an adjustable amount ranging from 1:1, (which is linear, or no reduction at all), to more than 40:1 which is a very high ratio, equivalent to that of a Limiter. RATIO is sometimes referred to as “Slope” because when depicted on a graph, the slope of the graph representing Output versus Input, is what changes.

Ratio and Threshold are closely inter-dependent. If a RATIO as high as 40:1 has been set, then if the THRESHOLD is set at 0 dBu, even when a massive signal of +40 dBu (unlikely!) is presented to the input, the output signal will only be +1 dBu. RATIOS as high as this would normally be set somewhere above 0 dBu – say at +14 dBu, in order to prevent the output signal level exceeding just over +14 dBu to protect, for example, a digital recorder. Similarly, if a RATIO of 5:1 has been set, an input signal which is 10dB above THRESHOLD will only rise by 2dB above that THRESHOLD at the output.

THRESHOLD control covers the Range from below -30dB to +22dBu. When THRESHOLD set at a low level, with a fairly high RATIO the amount of gain reduction will be considerable and it may be necessary to use some GAIN after the compressor to restore the apparent signal level.

Attack Time

The ATTACK time is the time taken for the compression circuits to start compressing. A long ATTACK time allows short duration peaks to “escape” and go through uncompressed. This may cause overload on subsequent digital circuits. A very short attack time sounds un-natural and robs the signal of “life” by removing transients. Some transients are extremely fast and have little effect on the sound quality. Setting a long attack time often means that almost no gain reduction occurs because the transient is history (!) before compression has had time to operate. However, even the fastest circuits take time to operate which means that there is always some “Overshoot”. Small amounts of “Overshoot” are musically desirable – there are exceptions, of course.

Setting the right values of RELEASE and ATTACK is what compression is all about! Once the principles are understood a Compressor-Limiter such as the 543 provides a powerful tool that actually appears to enhance the dynamic range of a recording and so provide greater musical enjoyment.

Release / Recovery

The notes above explain how the 543 handles signals of constant amplitude such as pure tones. Real program signals, however, are continually changing in level. The way in which a compressor deals with actual program material depends upon the magnitude and duration of peaks in the program level. If the RELEASE TIME is set to be very short, a short duration signal will be compressed but the gain will return to normal very quickly, giving a fluctuating and un-natural sound known as “Pumping” when the background, or other signals, are forced up and down. The gain will also tend to follow the wave form of low frequency signals. RELEASE TIME should be set long enough for the gain to remain reasonably constant between each bass note or between speech syllables