Jeff Rowland Design Group Coherence preamplifier & Cadence phono stage Page 2
The six aluminum modules that form the heart of the Coherence are mounted to the chassis floor with elastomeric grommets for maximum isolation and rigidity. Indeed, this preamp showed the least sensitivity to external vibration of any electronic component I've had in my system. All modules are connected to each other, and to the I/O board on the back panel, with fine-gauge, tightly twisted Cardas hookup wire. Rowland feels that combining a small, tight loop area for the active circuitry, with twisted round wire to connect it all together, maximizes effective speed while minimizing RFI and magnetic coupling to the circuitry.
Where a single input transformer for each channel would have been adequate, Rowland went all out and used two Jensen JT-10-KBD transformers per channel, these contained within the far left and right modules. With a pair of these devices per side, the output of each forms an extra pair of cross-coupled, opposite-polarity signals to ensure a differential signal path all the way through the preamp, while also providing the vital mode conversion and resulting common-mode rejection (CMR) just before each channel feeds separate stereo attenuators. These transformers also provide isolation and compensation for the key amplifier circuitry, located on the same small circuit board as the attenuator within the second set of sealed modules.
A large Jensen JT-11-DM line-level output transformer, complete with 15MHz bandwidth and ultra-low phase distortion, is found in each of the two innermost modules. This is a very different animal from an input transformer, or the type of output transformer typically used to couple the output of a tube amplifier to a reactive loudspeaker. While they're sonically less important than at the inputs, Rowland insists that these output transformers ensure the best possible balance of the signal feeding the downstream component, while also reducing "backdoor" contamination. (See Sidebar, "Transformers and the Audio Interface.")
Unlike the MDAC often recruited for use as a volume control in other remote-controlled preamps, the Coherence employs Crystal Semiconductor CS-3310 digitally controlled stereo analog attenuators. The CS-3310 is specifically designed for superior sonic performance, yet without the inherent disadvantages of a DAC used for volume control: routing the music signal through a series of poor-quality polysilicon resistors and CMOS switches, excess "zipper noise," and coarse steps of attenuation. In addition, Rowland claims that a special synergy occurs between the input transformers and these attenuators in the Coherence.
In spite of the many advantages of the CS-3310, one of the chip's few limitations is that it can handle only a moderate input voltage (around 8V peak-to-peak) without overloading. However, since the two attenuators are preceded by transformers with a 12dB step-down, they won't be overdriven.
The Coherence's active circuitry employs an ultra-high-speed current-feedback amplifier block built around the excellent AD-815 chip from Analog Devices. This amplifier's transimpedance topology essentially comprises an input buffer followed by a current mirror, then an output buffer. An AD-815 has a large open-loop bandwidth, a slew rate of nearly 1000V/µs, and robust stability while driving reactive loads. It also offers unusually high output capability—up to a whopping 500mA—and can sustain linear current drive into widely varying loads—even down to around 8 ohms!
Furthermore, the AD-815 is not slew-rate-limited, as a voltage-feedback design can be: as the signal voltage varies, the slew rate remains stable. This leads to improved settling time and reduced energy storage, with their associated smearing effects. The Jensen input transformers inherently form a Bessel low-pass filter as well, providing the necessary bandwidth-limiting function for the signal and obviating the need for any additional signal-filtering components to protect the amplifier. Rowland claims that this feature, combined with careful compensation and bypass of the amplifier's power-supply pins, helped achieve his design goal of a blazingly fast, highly stable, and linear preamp with robust immunity from spurious high-frequency artifacts and EMI—all key parameters for superior sound quality under real-world conditions.
Any amplifier or preamp is essentially a DC power supply modulated by the AC signal. This makes the quality of the power supply a vital issue. You often hear sophisticated, well-designed AC supplies described as being "almost as good as a pure battery," and the best of these designs indeed come very close. Rowland uses the real McCoy.
Inside the chassis of the power supply, which handles both the Coherence and Cadence, a large potted toroidal transformer is mounted in a deep central well and flanked by a pair of Panasonic 12V, 7.2 amp/hour, rechargeable lead-acid batteries in their own machined corrals. These form a full-function, high-quality AC power supply that also acts as a sophisticated, microprocessor-controlled battery charger. The main technical advantage derived from a battery-powered preamp stems from its inherently low noise and very low source impedance, maintained over a wide frequency range. In addition, pure DC operation avoids the conducted and radiated noise typically produced by rectifiers and regulators.
In this design, the preamp and phono stage always use battery power. Even when in AC mode, the power supply is simply float-charging the batteries while both, in parallel, supply juice to the circuitry. As a result, even while recharging the batteries, you'll still achieve almost all the sonic benefits of battery power.
Lead-acid batteries lack the "memory" effect of nicads, but good design practice and battery longevity dictate that you not deep-cycle them. Rowland designed the system to prevent discharging below 50% of full charge. Once you've gone that far—which takes between 14 and 18 hours of pure DC operation—the charger automatically kicks in, and you're back in float mode without having skipped a beat. I found that if you run the Cadence and Coherence simultaneously, you should manually return the power supply to AC mode before going to bed to prevent automatic cycling. Otherwise, you may lose your memory settings after the charger reboots.
Step-up with Cadence
The circuit design and parts selection of the Cadence moving-coil phono stage match the Coherence in quality, sophistication, and simplicity.
The prime criterion for superb moving-coil amplification is the ability to extract and preserve the dynamic integrity of a minute, sub-millivolt signal while precisely mapping the RIAA curve. This is no trivial task, but one ideally suited for a step-up transformer—provided it's carefully designed and matched to the range of cartridge parameters it's likely to see, as well as to the following amplification stage. A moving-coil cartridge is an inherently balanced device. Rowland took advantage of this by encouraging Jensen to design a special MC step-up transformer optimized for a circuit of low impedance and very low noise. This is unlike most step-up devices, which are followed by a high-impedance amplification stage.
The resulting Jensen JT-346-AX step-up transformers sport a very wide bandwidth of over 200kHz, "almost immeasurable" phase distortion, and an extremely high Common-Mode Rejection Ratio (CMRR): from 145dB at 60Hz to 106dB at 3kHz! High CMR can be particularly important in a phono stage due to the considerable amount of common-mode noise that often contaminates the low-level signals in tonearm leads.
The two transformers are completely encased in their own triple-mu-metal, double-copper-plated nested shields and are not ground-referenced, in order to prevent ground-current leakage from contaminating such fragile signals. The transformers can also be optimized to essentially any MC cartridge on the market by choosing one of four ranges via the Cadence's front-panel button. The low-gain setting has two impedance ranges: 250 and 400 ohms, ideal for cartridges whose DC resistance ranges from 5 to 25 ohms; and from 25 to 60 ohms and above. The high-gain setting also has two levels: for cartridge resistances of 3 ohms and below, and those between 3 and 5 ohms.
The transformers are followed by an AD-797 op-amp chip, used to shape the bottom half of the RIAA frequency curve (50Hz-2.12kHz). Above this frequency, the RIAA shaping is done with a purely passive topology using top-quality components. Unlike typical, series-feedback active equalization, which tends to cause a rising response above 20kHz, passive handling at the top end of the RIAA slope allows the ultrasonic response to continue to drop off at 6dB/octave.