GML 2020 User Manual

G e o r g e M a s s e n b u r g L a b s

GML 2020 High Resolution Discrete Input
Channel

The GML 2020 High Resolution Discrete Input Channel sets a new standard in signal
processing. Combining the features of the GML flagship 8200 Parametric Equalizer,
8300 Transformerless Microphone Preamplifier, and 8900 Dynamic Gain Control with
additional functions, this powerfully pristine single channel unit embodies the legendary
detail and accuracy for which GML is renowned.

Owner's Manual

Version 1.2

April, 2002

All materials herein © GML, LLC.

GML, LLC

P.O. Box 1366
Franklin, TN 37065

615.790.1016 (ph)

615.794.4802 (fax)

[e-mail]

INTRODUCTION

The GML 2020 High Resolution Discrete Input Channel sets a new standard in signal
processing. It combines the features of the GML flagship 8200 Parametric Equalizer,
8300 Transformerless Microphone Preamplifier, and 8900 Dynamic Gain Control into a
powerfully pristine single channel unit. The power and functionality of these
revolutionary processors is further coupled with the addition of a front panel musical
instrument input, two position high-pass filter, front panel phantom and phase switching,
and flexible routing options while maintaining the GML standard of performance and
transparency. These advanced circuits, envisioned by George Massenburg, have
benefited from over 30 years of research, limited manufacturing, critical listening
analysis, widespread usage by demanding industry professionals, and continuous
evaluation by the GML Engineering Department.

FEATURES

The GML Model 2020 High Resolution Discrete Input Channel derives its power and
flexibility in large part from its features:

• All-discrete, Class-A design; no integrated circuits to compromise the

audio path

• DC-servo and premium-grade film coupling; no interstage capacitors to

add distortion or degrade over time; no tantalum electrolytic, ceramic, or
aluminum electrolytic capacitors in the signal path

• Transformerless; precision electronically balanced inputs

• Designed with GML 9202 low-noise, low-distortion, wide dynamic range,

wide bandwidth precision discrete opamps

• DC-servo stabilized direct-coupled output

• Multi-input format: Mic, Line, or M/I (front panel musical instrument)

• Wide gain range (-10 dB to +70 dB) available on all inputs

• Flexible routing: EQ before DYN, EQ to DYN Sidechain, DYN before EQ

• High-pass filter; selectable 40 Hz, flat, 100Hz

• Front panel Phantom and Phase switches

• Link function; allows multiple units to act in concert

• Precise input signal LED meter; 5 color-coded steps

• Interstage (EQ out, DYN out) Clip/OL LED

• Illuminated push-button switches for "EQ In", "Dynamics In" and "Link"

• Carbon-film precision potentiometers, manufactured to GML exacting

standards

• GML-specified custom robust rotary switches; precious metal contacts

• Precious-metal interconnects

• High-quality XLR interconnects, Au over Ag

• Proprietary discrete VCA; excellent low-level resolution, low distortion, low

noise

• Highly accurate logarithmic Dynamics control: provides consistent

processor action across a wide dynamic range for added flexibility and
transparency

• Powerful RMS Dynamics detection: enables true musical-energy tracking;

acts upon all-important signal energy, not merely signal level

• Three independent Dynamics detectors to capture the full range of

transient content: Peak, Fast RMS and Slow RMS

• Two Dynamics modes: Soft Knee and Hard Knee

• LED indicators for Dynamics detector activity and compression ratio

• Accurate multi-segment Dynamics LED meter, displays state of VCA

• Dynamics section takes maximum advantage of modern surface-mount

technology for reliability, manufacturability, and compactness

• Multi-colored knobs, GML standard

• Stylish and durable black-anodized aluminum front panel

• Rugged chassis; black powder-coat finish over aluminum

• Reverse-anodized front panel legend, much more permanent than ink or

paint processes

• Baked-on epoxy paint silkscreen rear panel legend

• Quality PCB manufacturing, assembly, and chassis construction

OPERATION

The Model 2020 High Resolution Discrete Input Channel offers unprecedented flexibility
and sonic accuracy when dealing with a variety of signal sources and conditions.
Whether a very simple gain stage or a comprehensive processing path with filter, EQ,
and compression is desired, the Model 2020 can accommodate, quickly adapting from
one requirement to the next. Although on the surface the Model 2020 seems dauntingly
complex, this single-channel unit may be subdivided into several sections for a more
straightforward operational analysis.

I. Input Section

The input section of the Model 2020 High Resolution Discrete Input Channel is based
on the gain stage of the Model 8300 Transformerless Microphone Preamplifier, though
there are some design changes and quite a few additions to expand its capabilities
further than is realized in the highly specialized Model 8300 mic preamp.

Three input formats are accommodated on the Model 2020: on the back panel, XLR
connectors are provided for microphone and line inputs separately, while the front panel
sports a 1/4" unbalanced musical instrument connector.

The microphone input of the Model 2020 is an exact copy of the Model 8300 input
circuitry, complete with 48V phantom power and subsequent 2-pole phantom filter.
Unlike the Model 8300, the phantom power switch is conveniently located on the front
panel and includes an LED indicator to warn the user that phantom power is ON. The

Mic input is precision electronically balanced, 1 kΩ nominal input impedance, with

tremendous dynamic range and wide bandwidth.

The 20kΩ balanced bridging Line input is the simplest input path to the gain stage and

allows the Model 2020's versatility to extend well beyond tracking sessions. Indeed, the
inclusion of this input allows the Discrete Input Channel to be used at any stage of the
recording process. The Model 2020 thus proves rather desirable in mixing situations,
either as a single independent unit or in a bank of linked units (for further information on this

topic, see the Link section of this manual).

Rounding out Model 2020 input options, the front panel M/I input extends the sphere of
application to the realm of the oft-neglected musical instrument. This high impedance

unbalanced input (1MΩ nominal) offers extremely low noise and low distortion, plus a

wide dynamic range and ruler-flat extended frequency response to outclass common
instrument inputs. Especially useful for recording bass or electric guitar--in addition to
an amp, miked and routed through another Model 2020, or straight to tape--this input
can also be used for a variety of other unbalanced sources such as electronic
keyboards, samplers, etc.

The input source for the Model 2020 is selected on the lower Routing switch, located
near the lower left corner of the front panel. Only one source may be active through the
channel at a time; however, signals may be present at the other inputs since the source
select function does not ground unused inputs (see Specifications page of this manual for

separation figures). Phantom powering may remain On, if necessary, when switching

between input sources without damage to other input devices since the phantom supply
does not reach the Line or M/I inputs. It is, however, recommended that the phantom
power be turned Off when not in use to preserve power supply efficiency and eliminate
possible DC pops when changing input source selection.

An extremely wide range of gain settings (10dB of attenuation through 70dB of gain in
accurate 5dB steps) is available for all inputs of the Model 2020. This feature
accommodates a wide range of input signals, from extremely "hot" microphone inputs to
extremely "low" line or M/I inputs, which are commonly neglected by the vast majority of
peripheral audio processing devices. Indeed, the minimal 20dB of gain found in most
microphone preamplifiers can be excessive in certain circumstances, and decreases the
possibility of optimized gain staging. By the same token, real-world line level signals
often require more than the typical 10 dB of gain commonly accorded them. A high
quality rotary switch, marked Gain, is used to control the gain setting of the input
section, using a combination of discrete metal film resistors for the ultimate in accuracy,
stability, durability, and sonic integrity.

Incorporated into the input section, the Phase switch activates a relay just before the
balancing portion of the input section. No signal is present at the front panel Phase
switch--it is merely a relay control and LED indicator voltage. Engaging this switch
reverses the phase of the input signal, akin to swapping pins 2 and 3 on the Mic or Line
XLR's. In the case of an M/I input, the Phase function will change the absolute polarity
of a given input signal.

Integral to the operation of the input section is the balancing stage. This stage provides
common-mode rejection and precision balancing for the inputs, while also buffering the
input gain stage from all subsequent stages of the Model 2020. As with all sections of
the Discrete Input Channel, this stage exhibits extremely low noise and distortion, wide
bandwidth and dynamic range, and utilizes precise DC-servo coupling.

The Input Meter provides a valuable addition to the Model 2020 input section. This 5­segment discrete LED display follows the input signal source and displays an accurate
indication of signal level after the input stage and high-pass filter. It should be noted
that the range and accuracy of this particular meter exceeds many common level
indicators. Indeed, with 54dB difference between the maximum indicator (+24dBu) and
the minimum indicator (-30dBu), the dynamic range of the meter presents quite a design
challenge; however, this meter is rugged and generally accurate to within +/-1dB of the
front panel legend. Fast transient response and a decaying LED fadeout help make this
meter extremely useful and pleasant to view.

II. Filter

Immediately following the input section of the Model 2020 is an extremely useful and
transparent high-pass filter, which has three optional settings: 40Hz roll-off, flat
response, or 100Hz roll-off. Designed for maximum clarity and musicality, this active
second order Butterworth high-pass filter is implemented with premium-grade film
capacitors, precision metal-film resistors, and a pristine GML 9202 discrete opamp.
Passband response is maximally flat for both the 40Hz and 100Hz selections, while the
Model 2020's ruler-flat frequency response is preserved in the "flat" setting. A second­order high-pass filter, this circuit creates a -12dB per octave roll-off below the corner
frequency, an appropriately flexible and powerful function since many engineers use the
low-frequency band of a fully parametric equalizer--a Model 8200, for instance--in shelf
mode as a high-pass filter of sorts. The presence of this dedicated filter, then, actually
enables the user to reserve the low-frequency EQ band of the Model 2020 for other
purposes.

III. Routing

A significant portion of the Model 2020's allure is the tremendous flexibility accorded the user in determining the internal signal routing of the EQ and Dynamics sections. This functionality is incorporated in the front panel Routing control, a five-position rotary switch manufactured to GML's exacting standards.

Specifically, the Routing control allows the user to configure the Model 2020 in one of
five topologies. Comprising four of the options, the EQ section may feed the Dynamics
section or the Dynamics section may feed the EQ section--in either case, allowance has
been made to accommodate an external sidechain input to the Dynamics section via the
back panel Sidechain In XLR. Additionally, the Dynamics sidechain may source the
output of the EQ, with both sections' audio inputs receiving signal from the input stage.
The latter case disables the external sidechain, is labeled "EQ to DYN SC" on the front
panel legend, and can be useful in ducking or other frequency-dependent dynamic gain
control applications. In all cases, the main Output of the Model 2020 is the direct­coupled DC-servo corrected output of the final stage, whether that stage is the
Equalizer section or the Dynamics section. There is, therefore, no dedicated output
buffering stage, nor is there a master output level control, unless the Dynamics Output
control is used to provide this function.

IV. Parametric Equalizer

The EQ section of the Model 2020 High Resolution Discrete Input Channel offers
astonishing precision and sonic accuracy when sculpting the response of any source.
Its operational characteristics, duplicated from the Model 8200 Parametric Equalizer,
have been honed through many years of use in the most critical recording and mixing
situations and have proven both reliable and amazingly accommodating.

The basis of the parametric design topology, in general, specifies control over not only
gain or attenuation, but also over both frequency and “Q” factor in multiple user-defined
bands. In the case of parametric equalization, “Q” is defined as the center frequency of
the alteration (whether gain or attenuation) divided by the bandwidth of that alteration. It
follows, then, that low values of “Q” affect a wide range of frequencies around the cut or
boost, while high values of “Q” specify a narrow slope around the center frequency.

Another important aspect of true parametric equalization, as embodied in the Model
2020 Equalizer section, is the existence of overlapping frequency bands, which provide
great precision and flexibility.

Specifically, this EQ provides complete control over a wide frequency range and “Q”
setting, along with 15 dB of cut or boost on four individual bands, with the front panel
controls for frequency and “Q” mounted concentrically. The addition of the “Q”
characteristic empowers the user to maintain precise and musical control over the tonal
attributes of the spectrally processed signal.

The availability of up to 15 dB of gain or attenuation is significant in the audio world,
since many contemporary equalizers offer merely 12 dB of cut or boost, and usually not
over such flexible overlapping multiple bands. Thus, it is possible to achieve almost any
particular “sound” that is desired, no matter if the goal is a subtle alteration or an audible
coloration effect.

Front panel controls for the Model 2020 Equalizer section include an illuminated in/out
pushbutton switch plus four color-coded bands with amplitude and dual-concentric
frequency and "Q" potentiometers. The amplitude pots are continuously variable
between +15 dB and -15 dB, with highly accurate 0 dB settings. Each band offers a
wide array of frequencies to select on the outer knob of the dual-concentric
potentiometer, with a good deal of overlap between bands to accommodate almost any
combination of cuts and boosts. The Low and High bands offer counter-clockwise
detents for shelving curves in addition to the variable "Q" values from 0.4 to 4 found in
the other two bands--all on the central knobs of the dual-concentric potentiometers.
Frequency markings can be found along the inner, larger ring of numbers, while the
outer, smaller numbers encircling the dual-concentric potentiometer denotes "Q" value.

V. Dynamics

Note: A basic understanding of the fundamental terms and concepts regarding dynamic range devices in general is
paramount to the following discussion and is presumed to exist for the user.

In contrast to most conventional dynamic range devices, the Dynamics section of the
Model 2020 is a complex dynamic range controller that incorporates several powerful
features to provide unparalleled flexibility and musicality. Indeed, this section may be
thought of as two completely different dynamic range controllers in one package: the
simple (physically) change of the Ratio setting from Soft Knee to Hard Knee style
compression affects this complete change in the section. This difference will be further
delineated as it becomes necessary and/or appropriate throughout the following
discussion.

All input signals, whether in Soft Knee or Hard Knee operation, must first undergo
conversion to a logarithmic control signal. Logarithmic control, whether peak or RMS,
results in the same audible effect--in terms of sonic coloration--over the entire range of
compression, in contrast to many contemporary dynamic range devices, which produce
different audible artifacts varying with the amount of compression employed due to a
linear control path signal. In the case of sidechain operation (as determined by the
Routing control), this logarithmic control signal is derived from the sidechain signal
instead of the audio input signal itself, as in normal operation.

The logarithmic control signal generated by the log converter passes next to three
detectors. These independent detection circuits are optimized to affect different
transient aspects of any possible audio signal: the Slow RMS Detector acts on the least
transient (program level) signals, the Fast RMS Detector responds to more highly
transient signals, and the Peak Detector deals with the steepest transients. This control
architecture allows for individual circuit optimization in each style of detector. The
nature of utilizing RMS-style control signals more closely follows the natural response of
the human ear-brain complex, thus resulting in greater musicality and audible integrity,
even in cases of drastic compression.

The Timing control affects the actions of the detection circuits in an intuitive manner,
wherein interrelated attack and release time constants are varied simultaneously for the
Slow and Fast RMS Detectors, while timing release values are determined for the Peak
Detector. Interestingly, the release of the Fast RMS Detector corresponds to the attack
of the Slow RMS Detector, while the Slow RMS Detector's release time may be varied
independently of its associated attack time constant by engaging the Release
Hysteresis control.

PFS

Slow RMS Detector active

Fast RMS Detector active

Peak Detector active

FRONT PANEL LEDS

A Detector Compare circuit immediately follows the three detectors of the Dynamics
section. The function served by the comparison circuit is to determine how the dynamic
range will be affected in subsequent processing blocks. At this stage, the Crest Factor
controls--both Fast and Peak--determine at what level the dynamic range will be altered
for the corresponding control signals from the RMS Detectors. These "weighting
factors" allow the comparison circuit to vary the amount of dynamic range alteration
applied to the VCA. These controls may actually be seen as a type of predictor to
determine the resultant crest factor (signal peak vs. RMS) of the output signal. Higher
values of Crest Factor (clockwise potentiometer rotation) will result in more transient
preservation at the output, all other factors being equal. Interestingly, the crest factor
value for the Slow RMS Detector is fixed in the detector comparison circuit.

The most complex and difficult aspect of the operation of the Model 2020 Dynamics
section involves the interrelationship of the Ratio and Threshold controls. It is best to
conceptualize these sections in terms of two different units, each with the same control
circuitry as described above, but with distinct operational characteristics thereafter.

In terms of straightforward operation, the Soft Knee operation mode is simplest to detail.
Operation under this condition emulates the operational principle similar to that found in
La2-style compressors, wherein the Threshold control behaves as a conventional
threshold control to determine the dB threshold value for the dynamic range controller,
while the Ratio control remains in the "Soft" position. The signal is unaffected until the
threshold is surpassed, then compression begins at a moderate ratio and increases
automatically, according to a prescribed compression characteristic. Continued signal
levels increasing above the threshold result in higher compression ratios such that the
further a signal goes beyond the threshold, the more severely compressed it becomes.