Many of the most desirable speakers in the world require high levels of power to work well. AMR developed the AM-77 Reference Class Dual-Mono amplifier based on OptiGain® which is an innovative implementation of Fusion Technology.
AMR's OptiGain® is a unique circuit design approach and ingenious implementation of the valve voltage amplification circuit coupled with a near zero solid-state amplification stage using the patent-pending Feed-Forward Error Correction circuit. With an extremely low-level of distortion achieved, the AM-77 delivers enough power to comfortably drive modern speakers with the desirable characteristics of a single-ended valve amplifier.
Traditionally, amplifiers use feedback by which a proportion of the output signal is fed-back to the input of the amplifier which is then compared to the input signal. If differences are present, the amplifier will attempt to correct this error after it has already happened.
Contrastingly, AMR's patent-pending Feed-Forward Error Correction circuit uses two identical amplifiers operating as a symmetrical output stage. The Feed-Forward Error Correction circuit extracts only the error of each amplifier and feeds it into the opposite polarity amplifier.
As a result, both the original error and its fed-forward replica are available at the exact same amplitude on both terminals of the speaker. As the speaker reacts only to the difference in the signal and none is present, the error has in effect, disappeared.
Similar error-correction schemes have been used for many years in military control applications. To our knowledge this is the first consequential application in audio (please refer to OptiGain® in the Advanced Design Features section for further details).
It has been long acknowledged that switched attenuators of various types and designs offer the highest sound quality. However, they are usually limited in range, number of steps and are not easily remote-controlled.
AMR has adopted a similar approach of using switched resistors. However, the switched resistor arrays of the OptiLevel® precision volume control allow the use of far fewer contacts and resistors to achieve a much wider range of control with finer steps.
Sonically, unlike other methods of volume control, dynamics remain, even at low-listening levels.
The AM-77 volume control is in addition, microprocessor-controlled allowing for remote control as a matter of course while still offering a superior sonic performance allied with convenience.
AMR's AM-77 Dual Mono Amplifier can be used with all kinds of dynamic, electrostatic and planar speakers, provided they are efficient enough to provide the desired sound pressure levels in your room and are of a suitable impedance.
In normal-sized modern living rooms, we recommend that speakers should have a sensitivity of at least 82dB/2.83v/1m. We also recommend that speaker impedance should not drop significantly below 3 Ohm and on average, be at least 4 Ohm.
Speakers with higher sensitivity and higher impedances can be used without any problems. Indeed, as the room size increases, it is desirable to have more efficient speakers.
Please note however, that the AM-77's very high power output can, when combined with exceptionally efficient speakers, produce sound pressure levels that may impair hearing. AMR assumes no responsibility for such results.
We feel that our customers should not be constrained in their use of our products by omitting functionality that can be provided without a loss in sound quality. We equally do not feel it fair our customers pay extra for such functionality in the form of a paid-for option.
AMR felt that the kind of phono stage we would have been able to integrate inside the amplifier or to provide as an option board would not have been in keeping with the high standards set by AMR products. This is mainly a question of size and space as a true no compromise phono stage would require considerable effort, circuit and power supply resources.
For casual, occasional listening to LPs, simple and inexpensive "phono boxes" are available from a range of specialist manufacturers. For enthusiasts whose music listening is based mainly around listening to LPs, AMR is currently developing the PH-77 standalone phono stage whose implementation is in keeping with AMR's extraordinarily high standards.
The professional Speakon Speaker connector offers touch proof and airtight silver contacts of superlative contact integrity that provides significantly improved sound quality over conventional binding posts.
While sadly uncommon in high fidelity applications, AMR felt that this connector is highly desirable on grounds of sonic integrity, safety and long-term integrity.
The ubiquity of portable digital media should not be ignored. AMR therefore designed the AM-77 to offer a 3.5mm jack for the connection of portable digital media such as an iPod®.
Many portable digital media devices offer charging from a USB connector. The USB connector on the side of the front panel provides this charging voltage to such devices. It has no other function or digital connection facility.
The mechanical structure of a heatsink resembles many tuning forks and usually resonates strongly as such. As these resonances invariably find their way into the signal passing through the amplifier, AMR felt it wise to omit them. Instead, AMR has taken great pains to design their chassis so it can effectively reject the excess heat produced by the circuit through radiation and convection without adding any sonic artifacts.