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OptiGain® circuit


OptiGain®is AMR's amplification circuit of the audio signal to a desired level in the most optimal way. In order to interface signals, often some form of gain is needed. Audio equipment requires both voltage gain and power buffering - voltage gain, where levels need to be increased and power buffering where loads (like speakers) need to be driven.

In power amplifiers a common approach is to design the amplifier circuit to have very high gain (which requires a large number of amplification stages and invariably adds noise and distortion for each added stage) and a large amount of negative feedback to set circuit gain. Often several such multi-stage circuits with their attendant negative feedback circuits are connected one after another.
In the AM-77, two stages are used for the OptiGain®circuit. First, the signal is amplified to the desired voltage using the excellent NOS 5687 thermionic electron valve. This is performed with exceptional linearity and drive without the need for a negative feedback loop. This solution is much more costly and difficult when compared with the more common and cheaper use of operational amplifiers (Op-Amps). The significant sonic performance over a circuit with negative feedback places it in rarefied company. So much so that the pre-amplification stage of the OptiGain Circuit sonically embarrasses many other standalone pre-amplifier stages.

The second half of the OptiGain®circuit is the power buffer stage where solid-state technology is combined with a thermionic electron valve phase-separation circuit. Simply put, where power buffer is required, well-implemented solid-state circuitry usually surpasses the performance of even the best circuits employing the rmionic electron valves of readily available and domestically safe-use types.

The power buffer stage uses AMR's "Class X: Jikoda circuit" with bipolar output devices, which in real-time, simultaneously detects and cancels unwanted artifacts in the audio signal: none pass to effect the playback of music, exceeding the ability of even traditional well regarded Class A circuitry.

In additional, making the power buffer stage balanced and by actively driving both terminals of the speaker (as opposed to connecting the negative terminal of the speaker to some ill-defined "ground") will ensure that the amplifier remains in full and direct control of the attached speaker and that any noise or error signals impressed upon the power supply is suppressed.

Of course, this requires in effect, two complete power buffer stages for one channel of amplification, instead of the more common use of just one power buffer stage and is thus, twice as costly but the extra cost is well justified by the clearly superior sonic performance.

Finally, having a balanced power stage allows the application of a very specific form of feed-forward error correction, a unique method of reducing non-linearity that is far superior to the common use of so-called negative feedback in amplification devices.

In the AMR circuit the error between input and output signal in one of the two balanced phases is precisely extracted and applied in reverse polarity to the opposite phase section. As a result, the output is only left with two signals which are in OPPOSITE polarity to each other. With all the errors removed, the result is a perfect replica of the input signal, driving the speaker's output.
This feed-forward error correction in effect completely removes the "solid-state" sonic signature of the power buffer stage. This leaves an amplifier that behaves sonically like an optimal, extremely high power (>160W) and very linear (0.09% THD @ 160W) single-ended triode amplifier with only two stages, which also has a constant high damping factor (over 15 from 1 Hz to 100 kHz).
 

 
   
   


     
 
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