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The C-03 is Esoteric's first standalone preamplifier and as such, another novelty release synchronized with their 20th anniversary. It offers 3 RCA and 2 XLR inputs and 2 RCA and 1 XLR outputs.

Discrete zero-gain input and output buffers condition the signal, with the power buffer's +/-38V high-voltage rails adopted directly from Esoteric's reference dual-mono D-01 digital-to-analog converter [right] for speed and dynamics.

The dual-mono circuit layout includes two 200VA toroidal power transformers all housed in a 22kg/48.5lb triple-segmented chassis with a full-depth front bay for the power supply and control circuitry which is separated by a lateral divider from the two-story rear bay which contains the i/o stages connected via 8N purity copper wire.

The dual-mono, remotely steered volume control amplifier is embedded in the output stage, runs twin Burr Brown PGA 2320 processors for voltage gain, attenuation and balance and offers input-specific gain trim over a +/-18dB range in 0.5dB steps and channel balance over +/-3dB. The alphanumeric display allows for input naming, the polarity of the XLR inputs can be inverted and the touch-sensing volume knob proper was designed for pleasing torque and tactile feedback without wheel spin.

The micro-processor control circuitry hibernates between commands and communicates with the main analog circuit via opto-coupler to avoid all signal degradation and noise. Additionally, all connected components except for the actively selected one are decoupled automatically from both hot and ground connections to eliminate all ground current noise.

The C-03, it seems, was optimized for dynamics (high voltage power supply) and resolution (very low noise floor). Hence certain features like a tape loop were omitted deliberately and the volume control circuit placed at the output rather than input. Typical for Esoteric is the reliance on chassis mass and decoupled footers to address resonance control while shielding partitions segregate the power supply from critical signal-path stages.

Stated performance specs for the C-03 include a frequency response of 5Hz to 120kHz -3dB; 0.005% THD at 1kHz/2V; a sterling S/N ratio of 100dB; 40K input impedance; and 47K/100K output impedance on RCA and XLR respectively. Power consumptions is a low 18 watts, maximum voltage swing +/-7V and maximum allowable input voltage 7V so even high-output digital sources may be used. The channel balance and input trim settings are accessed through the Setup command on the remote only and stored in non-volatile memory.

The <> Input commands on the remote also select from the menu choices accessed by multi-clicking the Setup command. Options include volume display (in 99 steps or inverted, in actual dB of attenuation); output selection (can disable unused outputs); gain (0 or +12dB); input XLR phase inversion (turns pins 2 or 3 hot); RCA 3 input designation (normal or thru-put); and input naming (can also program skipping of unused inputs when the input selector knob is turned).

If the zero-gain setting is invoked and input trim selection at 0, full output of the C-03 necessarily equates to unity gain, i.e. mirrors the input signal voltage perfectly just like a pass-thru. This, for all practical intents and purposes, converts the C-03 into a fully passive buffered preamp - though it's obviously not passive in the traditional sense of eschewing a power cord or being a simple passive pot that's connected directly to the i/o ports to create impedance issues depending on setting.

But it is important to understand that the entire voltage gain of the C-03 is contained exclusively in the PGA 2320 operational volume control amplifier whose resistor ladders are actuated by the volume control encoder's position in 0.5dB increments. Turning the volume control triggers a micro processor with A/D conversion which controls the Burr Brown chips. This puts the actual volume control manipulation completely outside the signal path proper and there is no analog-to-digital conversion performed on the input signal. (See the below block diagram for a visual).

For a practical example on the gain equation: if the input trim is set to +18dB, the volume knob to -10dB and the gain option to +12dB, then the PGA2320 is instructed to deliver 20dB of gain. The aforementioned +/-38V rails are for the output buffer exclusively. Other sub circuits run on dedicated +/15V or +/-18V supplies. Added Kazutaka Tsuda, "with a maximum 7Vrms input level, peaks will be +/-9.9V and we have a lot of margin to +/-15V but a 7Vrms input already is very high for a preamplifier".

This diagram also shows how all inputs are actively switched on both hot and ground connections to completely disconnect from the preamp circuitry those components which aren't chosen by the input selector. In short, the C-03 incorporates a lot of carefully considered complexity in the name of simplicity.

Or as Esoteric puts it themselves: "Eliminating extraneous circuits such as those for tone controls and REC out has enabled us to create the shortest possible signal path. By minimizing the circuit, we've been able to achieve the optimal layout with the highest grade components. As if this wasn't enough, we've arranged the input and output audio boards one above the other in a dedicated compartment at the rear of the chassis close to the i/o ports, resulting in a still shorter signal path. The buffer amplifier uses a high definition discrete circuit configuration for the inputs and outputs. The output stage in particular delivers signals to the external power amplifier via an output buffer circuit with the same high-voltage design as the superior monaural D-01 D/A converter, delivering all music signals with power to spare."