A stereo compressor offering the classic dynamic
characteristics that only an optical design can provide, by Hugh Robjohns.
Sometimes the old ways are still the best ways, and many experienced
recording engineers may well subscribe to that view when it comes
to compression. Optical compressors may seem rather inferior in terms
of their tech specs when compared to the current state of the art
in VCA and even digital dynamics processors. However, the inherent
imperfections of compressors based on optical gain-reduction cells
create a certain audio quality which is often extremely beneficial
and complementary to music recording in general, and to bass instruments
and vocals in particular. The original designs used ordinary incandescent
lamps with light-dependent resistors or photosensitive transistors,
but these days LEDs are used because they are cheaper and have an
infinitely longer life and much faster response - all of the character
and none of limitations.
There are many manufacturers with opto-compressors in their portfolio,
the newest of which is a New Zealand company, Buzz Audio, which has
recently launched a new stereo optical compressor housed in a substantial
2U rackmount case. Its user controls are wonderfully simple and intuitive
- harking back to a former era where a couple of big knobs were all
that was needed! (Stop sniggering at the back...)
Hardware Tour.
The rear panel of this large black box carries XLR sockets for the
inputs and outputs of the unit's two channels. This machine can be
used as either a dual-channel processor or as a stereo unit, courtesy
of a stereo link toggle switch on the front panel. The inputs and
outputs are electronically balanced and operate at a nominal +4dBu,
although other input sensitivities can be supplied from the factory
(including -10dBV). The handbook for the SOC 1.1 helpfully reminds
users that the screens of the connecting cables are not required for
conveying audio information, and can be disconnected at the sending
end to avoid earth loops.
The mains input is via an IEC socket, complete with integral fuse
holder, and a separate mains voltage selector is provided on the rear
panel. Taking a quick peek inside the box revealed a very tidy construction
with unusually high-quality components and ICs - particularly in the
I/O circuitry. The gain-reduction element is a light-dependent resistor
(LDR) driven by an LED, the pair sealed in a small black module, while
the entire circuitry is solid state, with high-quality operational
amplifiers used throughout. The electronics are mounted on separate
circuit boards for the two channels and their front panel controls,
with ICs in sockets and a plethora of sealed trimmers to align the
responses of the LDR/LED devices. Ribbon cables interconnect the PCBs,
and if servicing became necessary it would appear to be very straightforward
to remove and replace components or complete boards. Although it has
little bearing on reliability or audio quality, the SOC 1.1 does have
something of a hand-built appearance - both internally and externally.
While this is most evident in the poor fitting of the self-tapping
screws which hold the lid on, the internal quality is better than
that found in many mass-produced products, so don't let first impressions
put you off.
The published specifications claim a bandwidth extending between 4Hz
and 250kHz (-3dB points, and without any gain reduction applied),
so this is old-world technology with digital-age performance! Harmonic
distortion is below 0.008 percent without gain reduction, rising to
0.03 percent with 20dB of compression. The residual noise is an impressive
-100dBu (A-weighted) and crosstalk is below the noise.
The operational controls comprise variable Drive and Output knobs,
with switched attack, ratio and release parameters. The huge Drive
and Output controls are disturbingly light to the touch - the visual
impression suggests psychologically that there should be a certain
weight here, so the ease with which these knobs rotate just doesn't
seem right! The Drive control determines the amount of signal sent
to the side-chain and therefore effectively adjusts the threshold
above which gain reduction occurs. Turning this knob clockwise produces
greater amounts of compression - simple but very intuitive. The Output
knob introduces make-up gain from 0 to +15dB, compensating for the
inherent reduction in level when compression occurs.
The attack time constant is set by a three-way toggle switch. The
centre position is an Auto programme-related setting, with slow and
fast options available on either side. The slow setting is around
70ms while the fast is about 1ms - both vary slightly with the amount
of gain reduction. The release time is set by a rotary switch with
six positions marked 1, 2, 4, 8, 16 and Auto. These numbers represent
hundreds of milliseconds - ie. spanning 0.1 to 1.6 seconds - this
being the time taken to recover from 20dB of gain reduction. Again,
the actual recovery time depends on the amount of gain reduction being
applied. The Auto mode employs faster recovery times with transient
material, but slower times with more consistent signals in the usual
way, and is a safe bet for most material.
The Ratio control is another rotary switch, this time with just four
positions: 2, 5, 10 and 20. These define the compression ratio, although,
since the unit has a soft-knee characteristic, the declared ratio
occurs only after significant gain reduction has been applied - particularly
with the lower ratios. Thus much more subtle compression characteristics
can be obtained when using moderate amounts of drive, which is exactly
what you would want.
There are two final controls - both toggle switches. The lower of
the two effects a full bypass through a relay connecting the input
directly to the output without any intervening electronics. The second
switch selects the signal routed to the large backlit VU meter. The
three available options are input signal, gain reduction, and output
signal. The backlight is provided by an arc of yellow LEDs mounted
on a circuit board fixed behind the meter scale, which is unusual
but very effective. The meter is calibrated such that a +4dBu input
reads 0VU, as does zero gain reduction. Inevitably, the amount of
gain reduction shown by the meter is an approximation, because of
the slow averaging characteristic of the meter's ballistics, something
which is quite noticeable with fast transient material. The GR meter
also continues to show the intended amount of compression even when
the channel is bypassed.
The second channel features identical controls to the first, and the
right-hand side of the front panel is completed with another pair
of toggle switches. The lower one powers the unit and has an associated
LED, while the top one engages the stereo linking facility. Although
this links the side-chains of both channels in some way, it is essential
that all controls are set to identical positions, which can be tedious.
In The Studio
The SOC 1.1 reminds me how much I like optical compressors! It is
so easy to use: set the ratio for the required amount of 'squash',
then crank up the Drive control until the desired sound is achieved.
The Output control will restore the level and the Attack and Release
controls can be adjusted to taste and to minimise any pumping effects.
Largely as expected, I found the device to be fabulous on bass instruments
(provided the Release control wasn't set too fast) and extremely good
with vocals. It seemed equally effective whether delivering gentle,
subtle compression or full-on total squashing of the dynamics. In
all cases, it sounded musical - if not always transparent - and I
was always able to find a setting which worked, regardless of the
nature of the source or material. What more could you ask of a compressor?
Using the unit to compress a stereo master was successful, but more
fiddly than I would have liked, mainly because of the requirement
to set both channels identically. However, this is a minor complaint
and the results easily compensate for the small amount of operational
fiddling. This is a charming compressor in every way. It sounds sublime,
it is a no-brainer to use, and it delivers results which are nothing
if not musical.
Copyright Sound on Sound Magazine, May 2002. TOP
