Q2 Audio F765 500 Series Compex Compressor/Limiter

The Q2 Audio F765 is the compression and limiting sections of the famous “Compex Limiter” built into a single 500 Series slot. The audio path contains the same blue Phillips type capacitors and BC series transistors as the original Compex.

Formatted after the vertical “N-Unit” with slight layout changes for enhanced usability, the F765 lives up to the reputation it’s big brother, the F760X-RS, has made over decades of being a “secret weapon” on countless records.

Q2 Audio F765 500 Series Compressor/Limiter

Q2 Audio’s F765 is a single-channel 500 module version of the compressor and limiter sections of Q2 Audio’s reissue of the two-channel ADR Compex Limiter F760X-RS that originally came out in the late ’60s. The audio path contains the same blue Phillips type capacitors and BC series transistors as the original Compex. The F765 is formatted after the vertical 700/600-series compressor/limiter “N-Unit” modules with slight layout changes for enhanced usability.

Inside are three circuit boards of entirely discrete components. There are no op-amps. Construction uses some surface mount technology utilizing the more expensive MELF-style (metal electrode leadless face) parts that sound better than the standard flat thin film. Otherwise for more critical audio sections and timing circuits, through-the-hole construction and discrete transistors like the original units are used. Totally noticeable is the CineMag CMOB-3L output transformer bulging out of the side of the unit’s all steel case/shield cover.

Key to the sound and operation, the same FET-based Vari-Loss Amp amplifier topology from the F760X-RS is deployed in the F765. The single channel F765 differs only that it does not have the expander/gate section–there is no front panel space for those extra controls!

Operationally, the fast recall ability and ease of use of the F765 is much like the 2U F760X-RS. All the controls (except for the Input and Output level controls) on the F765 are rotary or toggle switches. After you initialize and set the controls of F765 for unity throughput and a basic range of the limiter’s operation, the compressor Threshold, Ratio, Attack and Release times and whether you want the Limiter on, off or on with Pre-Emphasis are all “decisions” made with hard switch contacts.

The ‘Compex Sound’ is popular for a wide range of uses from unique compression effects (drum kit smashing as Led Zeppelin used the original modules) to more conventional and subtle dynamic range control. The F765 is available in matched pairs (as I have here for this review) that guarantees accurate stereo linking made possible through the rear edge connector and a 500 rack that supports stereo linking. The F765 also has sidechain access as specified in Radial Engineering’s Workhorse racks using their rear-panel Omniport TRS jack. You can do ducking effects, de-essing or Vocal Stressor processing.

I started my evaluation with a lead vocal processing in a Pro Tools 12HDX mix session. I inserted my two F765 on the stereo vocal output stem going to my SSL Sigma Summing System for a mix of a Rock song. The singer constantly switched back and forth from falsetto to full voice with the accompanying large level changes–too quiet while in falsetto and too loud with full voice to “sit” well within the track mix.

I tried subtle leveling using just the compressor section alone. I went with a compression ratio of 2:1–switchable compressor ratios available are 1:1, 2:1, 3:1, 5:1, 10:1 and 20:1. With Threshold switched to the -4dB position, Attack time at the slowest 25ms (.25ms and 2.5ms are the other choices) and Release time switched to 0.2ms (switchable Release times are 25ms, 50ms, 100ms, 200ms, 400ms, 800ms, 1.6ms, 3.2 seconds and Auto mode), I obtain about 5dB (indicated) maximum gain reduction on the Sifam meters.

The lead vocal remained much more constant in level with better intelligibility and, most importantly, easily sat within the mix. I noticed minimal compressor artifacts at this mellow ratio setting and somewhat conservative peak gain reduction amounts. Dulling, pumping and breathing were not heard at all.

Next I wanted to go for a more aggressive and compressor/limiter colorful “sound” on the same vocal. I switched in the Limiter section of the F765–it has a fixed Attack Time of less than .25ms for overshoot (brickwall 100:1 ratio) control but without over-limiting. Release time is 250ms and is also fixed.

It bears noting that like the original Compex, the Vari-Loss FET amplifier takes in control signals from the compressor and limiter detectors and develops a combined gain reduction action. The F765 does not just run the output of a compressor section into the input of a separate limiter or vice/versus. So using this design you can expect much more accurate processing with less deleterious artifacts, noise and distortion.

Initially I kept the previous compressor settings and engaged the Limiter. With the limiter inserted I had to make up a little more output gain and I noticed the meter moving about the same amount of GR. The red Limit LED flashed with every limiter threshold level crossing.

The F765 has the P.E. (Pre-emphasis) position that toggles in a passive, high frequency boost circuit into the limiter’s sidechain. Pre-emphasis mode tends to flatten out the high frequencies and, on vocals, can function as a subtle de-esser. My vocal track didn’t have a “S” problem but using the P.E. mode definitely took the “punch” out the sibilance–it was just too much in this case and it would be great to be able to adjust the amount of high frequency boost used in P.E. mode.

What A Lot Of Fun-Drums

I found P.E. better for keeping cymbals tamed and at a little lower level relative to the rest of the drums when using the limiter on overheads or room mics.

Increasing the Input gain control on the F765 will start to get things seriously going on with both the compressor and limiter working hard if you like. I first tried using just the Limiter on drums.

To only use the Limiter, switch the compression ratio to 1:1. The limiter on my drum top kit stem produces that kind of sound heard on early Brit-invasion records-it sounds like a tube compressor but with the limiter’s fixed super-fast attack time, there is not much adjustment possible apart from “more or much more” by turning the Input level up more and/or much more.

So I switched off the limiter and set the compressor to function as a limiter. I used a 10:1 Ratio, 25ms Attack, and 100ms Release time. Release time setting will depend very much on what your drummer is playing, the tempo and where you adjust the Threshold. I ended up with Threshold somewhere between -4dB and -8dB. The little Sifam meters were showing much more action–I’ve never seen them move like that–up to 20dB of indicated gain reduction on tom-tom hits and snare drum fills! I set this process up on a parallel stereo fader in Pro Tools so I could combine it with the original audio. (Wet/Dry)

Q2 Audio’s F765 500 Series Compressor/Limiter is a chip off the old block in so many ways. It is amazing that all that functionality and power is contained within the confines of a single slot 500 module and still have room for a GR meter! I’m sure if it were a two-space module it could also have the original’s Gate/Expander with all its controls and features too.

None-the-less, the F765 as is has the sound and color of the original 2U Compex processor. The F765 succeeds well as an excellent compressor/limiter for any source from raucous vocals to boring drum recordings. It is a well-made quality piece of gear that, after initial setup, is easy to get solid operation.

I just love my pair here in the rack and I use them on every mix.

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