The second generation of integrated amplifier from Jeff Rowland Design Group, the Concentra II, features an innovative, fully balanced from input to output circuit design that is as technologically advanced as it is musically involving. In the world of high-end audio product design, integrated amplifiers are generally considered the “budget” or entry-level components from most manufacturers – not so with the Concentra II. The Concentra II spares the user the problems of excessive heat, burdensome size, and unattractive physical design while at the same time sparing no reasonable expense to provide the best possible sound quality.

The input section of the Concentra II is derived from the awesome Synergy II preamplifier and utilizes a pair of audio input transformers custom designed with the assistance of Jeff Rowland Design Group using the latest in computer aided drafting techniques (CAD). These transformers enable the input section to accomplish a number of unique goals, including insensitivity to unbalanced source impedances, high common-mode rejection extending to well beyond the audio bandwidth, and the same input impedance and gain regardless of whether the balanced or unbalanced input terminals are used. (For a more thorough description of transformer based inputs, please see the Jeff Rowland Design Group web-site at: http://www.jeffrowland.com/techtalk.htm)

Insensitive to unbalanced source impedances, the Concentra II has been designed to operate with the widest range of audio source components possible. Making the amplifier immune to the effects of unbalanced source impedances means that in the real world where source components from different manufacturers will vary wildly in their design, a Jeff Rowland Design Group integrated amplifier will never be affected by these differences but will still provide the best possible sound quality from any CD player, surround processor, digital component, phono preamp, or analog source.

In the world of amplifier specifications, common mode rejection is the ability of a balanced audio component to reject noise, EMI, RF, and other artifacts. A high common mode rejection ratio improves dynamic range as the music is produced from a quiet, silent background free of hiss, background noise, artifacts, and ground loop hum. Because of the unique abilities of its transformer-based input section, the Concentra II realizes superior common-mode rejection, letting the subtlest details, fastest transients, and powerful dynamics present themselves with ease and clarity.

With most amplifier designs, input impedance and gain will vary depending on whether the balanced or unbalanced inputs are used – generally, unbalanced inputs will have half of the output of balanced inputs. With the Concentra II integrated amplifier, there is no difference between balanced and unbalanced inputs, meaning that there will be no differing characteristic in sound quality and output level depending on which inputs are used – both will provide the same seamless, neutral presentation.

From the input section, the delicate audio signal is then handed to a control circuit that is protected and allowed to function in the best possible environment. This portion of the preamplifier section is microprocessor controlled and fully shielded from the rest of the amplifier. Within the diamond cut faceplate of the Concentra II, a form-fitting pocket is machined to house the sensitive preamplifier board. Once the board is in place, a machined cover is placed over the pocket to seal off the preamplifier board from any noise or interference from the rest of the amplifier circuitry or power supply.

The volume control within the control circuit uses the most accurate and sonically neutral system available. A digitally controlled analog volume control is used to provide exact matching between both channels at every volume setting, as well as allowing the absolute simplest signal path possible. For each volume setting, the audio signal passes unscathed through only a single resistor. This unique system never suffers from the numerous problems found in conventional potentiometer controlled volume controls, including varying impedance and tracking accuracy, and avoids the issues associated with the use of DACs as volume controls, such as the distortions created by their internal polysilicon resistors. Inside the faceplate, the volume knob is attached to a laser cut optical encoder wheel that is read by a light sensitive diode. This diode then sends the exact number of precise 0.5 dB steps to the volume control circuitry. The microprocessor within the unit then determines the knob rotation speed and raises or lowers the volume in either large or small increments for the most accurate and transparent volume control possible in the audio world today. The volume level is displayed by a soothing blue seven-segment LED display that is easily read from the listening position. When not in use for more than 5 seconds, the volume control and display circuitry revert to a sleep mode to prevent any audible interference or distortion from corrupting the musical signal.

The amplifier output section of the Concentra II contains six power amplifier circuits per channel, each capable of 150 watts of peak power, incorporated in a novel “Intelligent Power Transistor with Gain” configuration. These output circuits are used in place of standard output transistors to provide unprecedented audio performance, stability, and reliability virtually impossible to achieve with conventional discrete components.

These integrated power output devices, or transistors, despite greater cost in comparison to their more commonly used discrete transistor counterparts, occupy very little circuit “real estate,” dramatically reducing capacitive and dielectric storage effects, as well as simplifying the overall signal path. Because of these lower capacitive and dielectric effects, open-loop bandwidth is approximately ten times greater than comparable discrete circuit designs and mid and high frequency distortion is lowered an order of magnitude while output impedance is also lowered dramatically, resulting in a much more accurate and musical presentation.

These devices also allow the added benefit of extremely accurate bias control and stability, whereas most other amplifier circuits must rely on “thermal bias.” Thermal bias is a process where a temperature sensor, most often a diode or transistor mounted to the heat sink, mimics the behavior of the output transistors and adjusts the bias current accordingly as it heats or cools – less current when hot and more current when cool. The greatest drawback with thermal bias is that there is a time delay of several seconds between the time the temperature of the output transistor changes and the temperature sensor reacts, as heat from the output device must slowly travel through the heat sink before it will arrive at the sensor. Typically, under conditions such as when music is playing, where the signal is changing instantaneously and bias requirements will vary quickly and continually, the bias adjustment will almost never be optimal for low distortion and the best possible sound quality since the bias adjustment will never catch up to the changing needs of the output transistors. Within the Concentra II, the temperature sensing is all performed directly within the output transistors themselves. This system allows the device to react as soon as demands change so that any and all bias adjustments are performed instantly and optimally along with the music – there is no delay between the time the temperature of the device changes and the sensor detects the change, as the sensor is an integral part of the device itself. In this way, distortion is lowered and reliability is dramatically increased to best serve the music while at the same time saving cost.

One common question is whether the output section of the amplifier is biased into Class-A operation. Technically, the answer is “yes,” as the exceptional linearity of the output section design achieves the advantages of pure Class-A operation. Using the “Intelligent Power Transistor with Gain” system keeps the output section properly biased at all times to eliminate crossover distortion caused by the output transistors during turn-on and turn-off, as opposed to the brute force Class-A biasing techniques used in other inadequate amplifier designs to compensate for poor linearity that result in low efficiency, excessive heat, electrical waste, and the need for power management.

Yet another advantage of the output devices Jeff Rowland Design Group has chosen is that they have no inherent current limiting – only thermal limiting. Most amplifier designs use a monitoring system to detect current draw. If the transistors are drawing too much current, the output section will shut down so as not to destroy itself. Within the Concentra II, the output transistors have no limit to the amount of current they can safely draw: unless thermally challenged, the output transistors have no power limit. If the device is driven hard and detects an unsafe temperature, it will simply lower the bias until a safe temperature is reached and then resume at the optimal bias setting.

The output transistors within the amplifiers are securely clamped to a massive, non-resonant, machined aluminum heat sink via a rigid, custom machined aluminum bar. The heat sinks have been designed with a specific height, depth, and width in proportion to the front panel, rear panel, and top and bottom covers of the amplifier so that when all of the various parts of the unit are assembled, the amplifier as a whole does not resonate or vibrate. It also means that the entire chassis acts as a heat sink for better heat dissipation. This allows optimal and efficient heat transfer from the output devices and prevents any mechanical resonance or vibration under high power conditions from affecting the signal. During demanding musical passages, the amplifier is able to play cleanly and smoothly with no chance for excessive heat or ringing heat sinks to interfere with or affect sound quality.

Inside the amplifier, the law of opposite field cancellation is utilized to its fullest extent in every place possible. Positive and negative DC current is transferred to each output device by dual polarity, low inductance power busses custom manufactured for Jeff Rowland Design Group. Within these buss bars, opposite phases are carried side by side, the two signal halves separated by a tiny 9 mil insulator. These buss bars are rated for a healthy 60 amps even though they are radically smaller than the busses used in other amplifier designs. Likewise, the output terminal of each output device is also connected to an identical power buss by high precision 0.1% resistors. In addition, these busses act as board stiffeners. As they carry opposite amplifier output signal phases on one buss bar assembly, stray fields from each polarity of the signal cancel. This eliminates any distortion affects radiated from the rest of the amplifier, as well as other parts of the amplifier being immune to any signal transfer or radiation from the buss – the musical signal is not affected by other portions of the amp, and the amp is not affected by the musical signal. This radiated field cancellation has dramatically lowered the distortion of mid- and high frequencies by an order of magnitude over previous designs, resulting in a distortion figure for middle and upper frequencies that is ten times lower than what could be achieved using standard buss bars or wire. These innovative new busses are responsible for a very significant portion of the sound quality of the amplifier and contribute to a level of sonic performance that other amplifiers cannot begin to match.

The exterior of the amplifier was artistically designed with rounded corners and no sharp edges. The entire chassis is finely constructed from pieces of solid aluminum, each cut to a specific depth and thickness that, when bolted together, prevents any unwanted vibrations or resonances from reaching the audio signal within the unit. The entire chassis is also used as a heat sink to allow the unit to cool itself as efficiently as possible when necessary. The surface of the faceplate and top cover are carefully cut in a unique process that uses a diamond tipped blade. This process was refined over many years to produce an artistic and attractive finish which has been copied a number of times but never equaled. The faceplate is then finished with fine automobile polyurethane clear coat to preserve the clarity of the finish and allow the brilliant radiance of the diamond cut process to shine. Every edge of the chassis is sealed with overlapping joints to prevent dust from entering and the interior of the chassis should never need cleaning over the lifetime of the product.