George Massenburg Labs 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) gmlinc@ix.netcom.com 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 k nominal input impedance, with tremendous dynamic range and wide bandwidth. The 20k 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 (1M 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 ...