Building from a Strong Foundation
Preamplifiers are charged with two seemingly simple tasks: source selection and volume control. The challenge, however, lies in the care with which small and delicate audio signals are treated and the degree to which they are protected from external sources of corruption.

Advanced audio circuit designs may employ premium component parts and proper construction techniques, but fail to deliver the ultimate in performance due to their environment. Commonly recognized environmental quality concerns include power supply and electromagnetic noise. Less obvious is the effect of physical vibrations, which can generate microphonic distortions in sensitive circuits. Sonically, the effect is like adding a thin veil over the sound. Since most stereo components are installed in the listening room where they are used, they are subjected to the vibrations induced by the sound of the system itself.

Power and Tranquility
Audio circuits require clean and stable power to reach their full performance potential. And while all Delta series components benefit from high-quality power supplies, the Classé CP-700 goes to an extreme. AC power connects to an external power supply chassis, where noise from the AC mains is completely isolated from the sensitive audio signals.

Inside the power supply chassis are two pc boards. The lower board handles the conversion of AC to DC, while the upper manages the regulation of DC for the CP-700. Common mode chokes are used to filter noise from the AC entering the power supply chassis. They also filter noise on the return path, preventing the CP-700 from adding noise back onto the AC mains. After the filters, two small transformers sit in parallel. One powers a circuit which monitors line conditions such as voltage and phase. The second powers auxiliary circuitry consisting of sensors and logic circuits.

The Audio circuits are powered by a third, larger transformer, rated at 135VA and constructed with five separate secondary windings. One winding is dedicated to control circuits while two each are devoted to left and right channels. The four audio channel supplies use discrete full-wave bridge rectifiers with high-speed Schottky diodes, each separately bypassed with low ESR capacitors. For left and right channels, four newly developed positive DC regulator amplifiers are used in tandem to generate the required + and - DC voltages.

This topology allows tighter voltage regulation with a smaller voltage drop than had been possible before. The small voltage drop translates into reduced ground current and results in less heat dissipation. Together, these factors further reduce the already low noise floor.

The five DC sources provided by the external supply are further refined with local regulation blocks in three separate areas: control, left channel (+ and -) and right channel (+ and -). Low ESR OSCON and film-type bypass capacitors are used throughout to ensure a fast and quiet reservoir of power. The motherboard also features unusually large power and ground traces to ensure excellent low impedance distribution of power.

Input signals are routed through high quality connectors which have been properly terminated to avoid crosstalk from unused inputs. A ground plane between adjacent connectors further ensures that each input benefits from a low noise electrical environment. Input selection is handled by special shielded, low-voltage relays with gold-plated silver contacts that are ideal for switching low-level audio signals. In addition to passing ultra-low current and low-voltage signals, these precision relays are rated to perform to their specifications for over 100 million cycles. The switching scheme disconnects all unused input signals from the chassis, giving the selected source what amounts to its own, dedicated preamp.

Single-ended sources are routed through an instrumentation-grade input buffer to generate a truly balanced signal. Unlike other single-ended to balanced conversion topologies, both legs of the resulting differential signal share the same path lengths and are truly symmetrical. Simpler topologies have one leg routed through a separate inverter/buffer stage or simply pass the ground connection along as if it were the inverted half of the balanced signal (as in the CP-500 preamp). While all of these methods do work, they involve some compromise. The best results are obtained using the more complex but higher performance topology employed in the CP-700.

Balanced Circuitry
Balanced circuits, also called differential or symmetrical, provide a means for rejecting noise and distortion whenever it appears identically on two different signals. For this reason, balanced components and interconnects are designed to handle two separate signal paths per channel. One is an inverted or mirror image of the other. When applied to a differential amplifier, the signals are subtracted, one from the other. The result is that noise and distortion that are the same on both signals (common mode) get subtracted away. The signals that are opposites of one another would add to zero if they were being added together, but since they are being subtracted, they produce a larger, clean copy that is twice the size of the original, minus anything that was the same on both signals. The signal is now larger compared with the noise (improved dynamic range) and noise and distortion common to both paths have been largely eliminated. When well-executed, balanced signal paths may significantly outperform the standard single-ended variety.

All signals, once in the balanced mode, are routed through a DC servo to remove any unwanted DC from the source. Coupling the signal in this way eliminates the need for sonically degrading DC blocking caps, while protecting speakers from the possible damage or distortion resulting from passing DC through to the amplifier.

As in the CP-500, separate stereo volume controls are used for each channel to maximize stereo separation and allow better rejection of common mode noise and distortion. The output of each channel's volume control consists of an inverted and a non-inverted signal, which are routed to the plug-in output buffer circuit boards. These four-layer circuit boards each contain a total of six identical circuits making up three discreet, three-stage differential buffer amplifiers (+ and - paths for each buffer). One is dedicated to the single-ended output. The other two are dedicated to the inverted and non-inverted signals on the balanced outputs. Because they have their own dedicated output buffers, both single-ended and balanced output stages may be used simultaneously with absolutely no degradation.

For balanced signals a further benefit applies. Any small imbalance caused by the volume control is eliminated by the configuration of the differential output buffers. Keeping in mind that the balanced output buffers consist of four identical circuits, two are employed as a differential buffer amplifier for the inverted signal and two for the non-inverted. The balanced output of the volume control drives both + and - sides of the inverted path, then with polarity reversed, drives the - and + sides of the non-inverted path. This topology assures optimum common mode rejection for amplitude errors as well as any other common mode noise or distortion artifacts.

The topology of each output buffer is executed with discrete components. Low-noise J-FETs are utilized at the inputs, gain is provided by second-stage bipolar transistors and the MOSFETs are used at the output to isolate the load from the bipolar gain stage.

As with amplifiers and other audio components, preamp performance ultimately depends on intelligent design choices and careful execution. Throughout the CP-700, parts selection, circuit topologies and layout, and construction techniques contribute to outstanding performance in both the laboratory and the listening room.

Optional Phono
The CP-700 may be fitted with optional Classé phono modules (CPM). Gain is selectable from the front panel to accommodate both low-output (MC) and high-output (MM) cartridges with 72dB and 42dB of gain respectively. The signal path benefits from clean, quiet power, supplied by local ultra-low-noise regulator amplifiers. The entire signal path, including RIAA equalization is performed in balanced mode, preserving one of the inherent benefits of phono cartridge outputs. The CPM uses discrete, low-noise output drivers and passive components are all high-grade polypropylene capacitors and 1% film-type resistors. Custom phono loading may be added to available sockets on the pc board. Installation by an authorized Classé dealer is recommended.

Control
Digital control circuitry in the CP-700 is executed on several isolated locations including the center of the motherboard, a daughter card near the AC input of the external power supply and on a separate board behind the front panel. Power for control circuits is derived from a separate winding on the main transformer as well as an independent secondary transformer/power supply. In addition to the preamp operation controls, the CP-700 monitors line voltage, frequency, AC phase and ground condition as well as internal temperature. Full control is available from the bi-directional RS-232 port on the rear panel, as well as by IR remote. DC trigger outputs enhance the system automation available from the CP-700.

Touchscreen
The front panel controls are implemented by way of a TFT touchscreen display, which simplifies the user interface, reduces display-related noise and allows users the convenience of customization. Name input buttons as you like and disable those that are not in use to remove them from the main operation screen. The touchscreen makes the preamplifier easier to use and also adds to its aesthetic value, whether used alone or together with other Classé Delta series components.