Sunday, April 21, 2013

FM Acoustics Resolution 255 MkIIR


























Remote control adjusts volume up/down and mute.

2 /pr balanced inputs
3 /pr unbalanced inputs
1 unbalanced tape loop
1 /pr balanced output

Burn-in time at the factory - 1000 test cycles - 200 hours minimum
Vibration testing - 50,000 cycles - 60 minutes minimum

The huge advantage of internal power supplies is that there are much shorter distances between the power supply and the amplification stages. The corresponding impedance is much lower than when power supplies in separate housings are used.

Rather than switched (stepped) attenuators, VCA circuits, optical encoders or relay matrix, FM ACOUSTICS employs hand-selected precision potentionmeters in the FM 255-MKIIR. With stepped attenuators the limit of resolution is determined by the individual steps. Music levels, however, do not come in steps but rather must be finely tunable to achieve realistic reproduction, especially so in high - accuracy equipment. Therefore, level and balance control's must be continuously variable.

VCA (voltage controlled amplifiers) and the circuitry of optical encoders are well known to be detrimental to audio quality and therefore must be avoided. Multiple switched relays have the same limitations as stepped attenuators, in addition to having poorer longterm reliability (partly due to a massive additional electro-mechanical component count).  While not "fancy", fine tuned precision potentiometers as used in the FM 255-MKIIR give better results all around. They are the optimal choice.

The super-wide balance control features a unique centering system. When the knob is turned  ±45° from the center, the level adjustment range is within 1 dB. Thanks to this any audio signal can be centered much more accurately than with the usual balance controls. At full turn the level of the corresponding channel is fully attenuated. This is ideal for tests and checking purposes.

Gain adjustments
New switchable true-impedance attenuators on the balanced inputs. 0DB or -10dB.

These are very useful with some of today's high level components (CD/DVD players/converters etc.) that have very high output voltages - often far above the standardized level ratings. Such excessive input levels can overload input stages (not the ones of the FM 255...) and force the user to set the Output Level on the line stage to very low settings, so low that it actually works as an attenuator instead of a preamplifier.

The new input attenuators lower the excessive levels whilst guaranteeing perfect balancing (not an easy feat). This in turn results in optimal Level control setting of the FM 255-MKIIR (around 12 o'clock position for normal listening levels).

To allow optimization of the total system gain - which differs widely from one system to another - further level switches allow tuning of the internal gain of the FM 255-MKIIR.

This allows more flexibility in system gain optimization. Any system can be set to an optimal gain structure - which often improves total system performance. -10dB or -20dB.

Phase
A phase inversion switch on the front panel performs precise 180°phase inversion of the main outputs.

This is done without any additional circuitry. The signal does not enter an additional 180°phase inversion stage. "In-phase" and "out-of-phase" signal lines are absolutely identical.

With this switch, every source signal can be reproduced in correct phase. This phase inversion is also active with the unbalanced inputs and the tape/auxailiary loop.

Specs
Bandwidth
Without filter - 1Hz-2MHz
When using filter - 1Hz-100kHz
Phase error 20 Hz-20 kHz - 0,03 dB
Channel difference output level and balance control - 0,02dB.
CMRR - 20 Hz - 20 kHz 90dB, 1 kHz 100dB
S/N - 120dB
Input impedance balanced/unbalanced 50k
Output impedance - 10 ohm
Output stage max cable length - up to 1000m
Max input/output voltage - 9V/19,5V
Rise/Fall time - 300 ns
Size - 45x9x28cm
Weight - 5kg

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