Preamplifiers
ART88 Triode Preamplifier
The ART88 Preamplifier has been designed to serve as an industry reference standard for line-stage preamplifier performance, a role filled for more than a decade by its legendary predecessor, the GAT preamplifier.
At the same time, it is notable for its convenience and ease of use. The unit features input selection and level adjustment by microprocessor-controlled relays, allowing level and balance to be adjusted in 100 steps of approximately .7 dB per step. All functions can be controlled by the hand-held wireless remote control. The ART88 Preamplifier will be available in a numbered, limited edition of 250 units which will be sold through select conrad-johnson dealers.
The ART88 is essentially a dual-monaural preamplifier, the two channels sharing only the power transformer and chassis. The circuitry is elegant in its simplicity. Each channel’s audio circuit features two high-transconductance twin triode vacuum tubes. Gain is provided by a composite triode of paralleled sections of one NOS Philips PCC88. This amplifier stage is coupled to the preamplifier outputs through a high-current buffer stage consisting of paralleled sections of a second PCC88 twin-triode. This buffer stage provides a sufficiently low output impedance to make the ART88 compatible with a wide range of interconnect cables and amplifiers. Each channel’s audio circuit is powered by its own dedicated DC power supply featuring discrete voltage regulators that isolate the audio circuits from each other and from the AC power line. Infrasonic noise is minimized by operating the tube filaments (heaters) on a DC voltage supplied by another regulated power supply.
Considerable care and attention have been paid to the physical aspects of the design as well. Microprocessor controlled relays, used for selecting sources and level setting, are located right at the input connectors, keeping signal paths short to minimize crosstalk between channels and inputs. The level control section and the audio circuitry are each shock mounted on independent daughter circuit boards to isolate the audio circuits from mechanical vibrations from the level control relays as well as from external vibrations. The elegant casework is executed in heavy gauge aluminum to assist in minimizing microphonics.
Experience has taught conrad-johnson that the full potential of any circuit can only be realized by using the finest available parts. Each part for the ART88 has been selected on the basis of both sonic performance and durability. The audio circuit (including the level control) is executed entirely with precision metal foil resistors and proprietary CJD Teflon capacitors. Tubes have been carefully hand selected for minimum noise and microphonics. The DC regulated power supplies use the same quality resistors as the audio circuits. Capacitors for the power supplies are exclusively CJD polypropylene and Teflon – there are no electrolytic capacitors in the audio circuits or their related power supplies. All signal switching is done by special relays designed for signal applications. These relays are sealed and have gold plated silver contacts. Input and output connectors are machined from oxygen free copper, and are silver plated with a Rhodium flash. The ART88 is built without compromise to achieve a new standard in musical performance, and it is built to last.
ET7-S2 Enhanced-Triode Preamplifier
The ET7-S2 enhance triode line-stage preamplifier has been designed to incorporate the advances made with the Series 2 upgrade to conrad-johnson’s flagship GAT into a more affordable model.
The circuitry of the ET7-S2 follows closely that of the GAT Series 2. The audio circuit is elegant in its simplicity. An audio amplifier stage consisting of a single-ended triode amplifier provides the voltage gain. This amplifier stage is connected to the preamplifier outputs through a high-current MOSFET buffer, which achieves a very low output impedance, making the ET7-S2 extremely flexible in the choice of interconnect cables and amplifiers. DC voltage is supplied to the circuit by the revised discrete voltage regulator developed for the GAT Series 2. It isolates the audio circuit from the power line by maintaining negligible impedance across the audio frequency band. Infra-sonic noise is minimized by operating the tube filaments (heaters) on a dc voltage supplied by a separate regulated power supply.
The ET7-S2 also boasts the GAT’s convenience and ease of use. The unit features input selection and level adjustment by microprocessor-controlled relays, with controls available both at the front panel and via a full-function remote. All control functions of the ET7-S2 operate through microprocessor-controlled relays. For the level control, the relays select combinations from a network of precision, low-noise metal foil resistors to effect loudness and balance changes in 100 steps of approximately .7dB per step. All switching and adjustment can be controlled by the unit’s remote control. A Theater Mode, which passes the input signal through at unity gain, makes it simple to incorporate the ET7-S2 into a high quality home theater system without compromising its two-channel performance.
Like its bigger brother, the GAT Series 2, the ET7-S2 is constructed from the highest quality, carefully selected parts for unrivaled sonic performance and reliability. Tubes have been selected for minimum noise and microphonics. CJD Teflon capacitors are used in the ET7-S2 for output coupling and post-regulator power supply storage. In the power supplies, all capacitors are of polypropylene or Teflon—there are no electrolytic capacitors anywhere in the audio circuits or their DC power supplies. The resistors in these circuits (including the two dozen level control resistors) are special low inductance, high-precision laser-trimmed metal foil resistors. Switches are sealed relays with gold-plated silver contacts. Input and output connectors are machined from oxygen-free copper, and are gold plated.
For four decades, the conrad-johnson design team has been dedicated to creating audio components that do credit to music performances. Through meticulous engineering, careful specification of only the finest available component parts, and a painstaking attention to construction, conrad-johnson has created what they feel is a line-stage without rival in its class.
ET6/ET6SE Vacuum Tube Preamplifiers
At conrad-johnson, development of their flagship products has traditionally led to important enhancements of their entry-level offerings.
The ET6 preamplifier is a case in point. The ET6 line-stage is a “baby” version of conrad-johnson’s flagship line-stage preamplifier, the GAT. The family resemblance is much more than skin deep.
With voltage gain provided by a triode vacuum-tube, the zero-feedback line-stage circuit of the ET6, along with its DC voltage regulators, closely replicates the circuit of the GAT.
Finally, and especially noteworthy, the ET6 features a relay operated discrete stepped attenuator level-control circuit of the type found in the GAT and ET7, absent only the balance control function found in those more costly models.
Functionally, the ET6 provides highly flexible I/O facilities with six separate line-level inputs (including one “tape” loop), plus a theater loop that facilitates use in a home theater system. In addition, the ET6 offers the convenience of infrared remote control of all functions.
For vinyl enthusiasts, the ET6 is optionally available with a vacuum-tube phono stage that borrows its zero-feedback audio circuit from conrad-johnson’s reference quality TEA1-S3 phono stage.
Customers can choose between high gain (recommended for cartridges with a rated output of 1.0 mV or less) and low gain (recommended for cartridges with rated output greater than 1.0 mV) versions of the phono stage. Input resistive loading is switch selectable over a range from 47 kOhms down to 200 Ohms. The ET6 phono stage is built to the same high quality standard found in the ET6 line-stage.
Premium quality component parts are specified to optimize performance and to sustain that performance for years to come.
Precision, metal-film resistors yield low noise performance for greatest clarity. Polypropylene capacitors, used exclusively in the audio circuits and the regulated plate voltage supplies, offer extraordinary temporal accuracy.
In fact, the ET6 is built to standards that meet or exceed those of many companies’ “flagship” offerings.
Those who wish to take the performance to yet a higher level will be thrilled by the availability of an upgraded “SE” version. The ET6SE boasts CJD Teflon capacitors and metal foil resistors in critical circuit applications.
It would be fair to say that the only thing “entry-level” about the ET6 is its price.
ET6/ET6SE Vacuum Tube Preamplifier Reviews: