Recording - August 2015 - 36

B Y PA U L V N U K J R .

Q2 Audio ADR Compex F760X-RS
This reissue of a famed multieffects processor delivers impressive performance

Today we are looking at a hardware
reissue of the classic Compex F760XRS... but just what is a Compex F760XRS? It may be one of the first multi-processors ever built, with a compressor,
expander/gate, and a limiting circuit, all
in one unit. Such a thing is commonplace
now, but not so much in the 1960s.
If the name still escapes you, just know
that its compressor-actually an early
variant that was built into the recording
console-was, along with two microphones, a stairwell and the drumming
skills of John Henry Bonham, the single
biggest contributing factor to the drum
groove found on Led Zeppelin's classic
"When The Levee Breaks".
A comp(l)ex bit-o-Brit history
The compressor that would become
known as the Compex was developed,
designed, and built in England in the late
1960s by ADR (Audio Design Recording).
Like many early audio processors, it was
also supplied as an add-in component for
Helios consoles.
Following that were a myriad of variations until about 1973, when the company essentially landed on the version we
are looking at today. Note that ADR still
exists in Great Britain as Audio Design
Reading, but they have since moved on
from the classic Compex designs.
Q2 to the rescue
Luckily for us, Tim Mead and his company
Q2 Audio have worked out a deal with ADR
to reproduce modern versions of this classic
for today's market. By way of pedigree, Tim
is the lead technician for US high-end audio
superstore Vintage King Audio. While there,
Tim became proficient at refurbishing and
rebuilding classic Compex boxes and he
36

RECORDING August 2015

eventually decided to build his own. While
Q2 is his own separate company, he still
works out of the Vintage King Detroit complex, and Vintage King is his US distributor.
Getting to know the F760X-RS
Unlike most simple vintage compressors of yesteryear, this beast has a staggering 20 controls on its faceplate, consisting of 8 knobs and 12 toggle switches. To be fair, it is a 2-channel unit that,
as mentioned, consists of a compressor
section, an expander/gate, and a limiter.
It can operate as a stereo device or as
two mono units. While early units came in a
3U rackmount box with a drastically different layout, eventually the design was refined
into a 2U version, a size replicated on the
Q2 unit. The steel enclosure is finished in
black, and all of the unit's knobs and sections are color coded in red, blue, green,
and yellow with additional white accents.
The F760X-RS is a solid-state FET
device and all three sections make use of
the same FET gain circuit. Here's a quick
rundown of what's new vs. old:
~ The original units made use of now
discontinued Sifam meters, so new equalquality replacements were needed;
~ The original units had separate A-B-C
cards whereas the new unit features a
shared PCB board across all sections;
~ The original open-frame pots have
been replaced by conductive plastic versions (the rotary switches are Swiss-made
ELMA switches, as on vintage units);
~ The ability of the unit to hold calibration has been greatly improved by
upgrading the voltage references.
Additionally Tim told me via email that
"Very little has changed circuit-wise. The
audio path utilizes the same circuitry and the
same capacitors (Phillips 021 series - Blue

Axials), the sidechain is done in thin film SMD
(resistors), and the transistors, electrolytics
and trimmers, etc., are still through-hole
types." Speaking of that sidechain, this addition is the only liberty Tim took with the unit to
"modernize" it; we will look closer at this feature in a moment.
Lastly, note that this version of the
F760X-RS is transformerless, but just as
there were transformer-equipped variants
in the 1970's, Q2 does offer an optional
Jensen transformer upgrade.
Common to both channels is a set of large
stereo input and output knobs. These drive
the unit in conjunction with an additional
input and output on each channel. Think of
the stereo ins and outs as coarse gain, and
each individual channel as fine tuning.
Each channel has its own gain reduction meter, and the threshold circuit of
each
channel's
compressor
and
expander/gate can be linked for stereo
use. Interestingly the peak limiter section
remains independent and unlinked.
On the rear of the unit are unbalanced
(yes, you read correctly) XLR ins and outs
for each channel as well as a TRS 1/4"
sidechain access (Tip return / Ring send
/ Sleeve ground). Why is the unit unbalanced? For authenticity, since the original
was. The power supply is internal and
uses a modern 3-prong IEC jack.
The unit follows a bright and easy-tolearn color scheme. The I/O knobs are
black, release knobs are green, threshold
is yellow, range is red, and ratio is blue.
The Compressor section's labels are
orangish-red, the Expander's are green,
and everything else is written in white.
The modules, feature by feature
The Compressor section has stepped
rotary controls for Threshold (-20 to 0 in 2 dB



Table of Contents for the Digital Edition of Recording - August 2015

Contents
Recording - August 2015 - Intro
Recording - August 2015 - Cover1
Recording - August 2015 - Cover2
Recording - August 2015 - 1
Recording - August 2015 - 2
Recording - August 2015 - 3
Recording - August 2015 - 4
Recording - August 2015 - 5
Recording - August 2015 - Contents
Recording - August 2015 - 7
Recording - August 2015 - 8
Recording - August 2015 - 9
Recording - August 2015 - 10
Recording - August 2015 - 11
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Recording - August 2015 - 13
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Recording - August 2015 - 16
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Recording - August 2015 - 18
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Recording - August 2015 - Cover3
Recording - August 2015 - Cover4
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