FM ACOUSTICS FM411

Commentary

This is a stereo power amplifier of the Resolution Series that has been developed over 8 years.

The power supply is equipped with a toroidal transformer made of steel with excellent properties and manufactured by a special method.
In general, the use of multiple transformers has advantages such as low crosstalk. However, only one excellent transformer is installed to improve the characteristics by the circuit, physical layout and power supply design.
As a result, by installing only one strong transformer, it is possible to play back with a margin even from a source that requires more power than a single channel such as a timpani.

The power supply section uses a custom-made high-precision capacitor to supply power with extremely low impedance.
FM acoustic capacitors use a technique that greatly reduces the ESR (Electrical Series Resistance) to increase ripple current capacity and achieve linearity across the frequency band.
In addition, the Resolution Series has a special surge protection circuit with a soft start function to limit the rush current, which reduces the deterioration of the capacitor due to the rush current and extends its life.

Low impedance is achieved by adopting the star ground method for amplifier design.
In the star ground system, each amplification stage is connected to the ground point at the center. Ground modulation and power supply impedance are suppressed and interference between stages caused by ground connection is prevented.
In addition, the chassis is designed to be connected only to the power ground and not to the audio ground to prevent distortion, oscillation and hum when connected to the XLR cable with a preamp connected to ground.

The amplifier circuit uses a unique full discrete Class A design without overall feedback, and a discrete balanced Class A input stage provides a superior common-mode rejection of 90 dB or more.
The balanced input stage consists of three precisely adjusted voltage gain stages, followed by a pre-driver stage, a driver stage, and an output stage, so that each stage can operate only at the minimum operating range, preventing excessive load and maintaining high stability over long periods of time.
In addition, pure class A operation is guaranteed for all stages before the driver stage, and the output stage is specially enhanced class AB operation. This circuit configuration and the use of carefully selected parts reduces dynamic and static distortion.

Selected products are used for transistors.
The FM Accor Stix carries out a wide range of measurements for each transistor up to a voltage of 250 V and a current of 20A or more, and finds and uses transistor pairs that have exactly the same static and dynamic characteristics.
The transistor chip is encased in a hermetically sealed ceramic housing with an average output semiconductor of approximately 10-20 mm.²46 mm vs.²It is a large one with surface area.
The semiconductor uses bipolar transistors, which are about 10 times faster than ordinary high-quality output devices, and MOSFETs are not used to avoid compression of output current due to the integrated resistance on the chip.

It is equipped with a protective system, and in the event of an error such as transient in the power line, high-frequency oscillation of connected equipment, strong RF signal, short circuit, DC offset voltage, or incomplete earth / ground, the load is immediately disconnected and turned off.
This protection system does not use fuses or other non-linear components in the circuit paths, audio output lines, DC supply rails, etc., to minimize adverse effects on the audio signal.
In the FM411 protection system, the computer samples various circuit parameters, such as both peak and average output currents, rail voltage, output voltage, temperature, and bias, at a rate of 20 sub-second per second, and compares the data with values in a practical environment.
When a short circuit occurs in the output or when the average rush current becomes very large, the output voltage disappears and the rail voltage drops. In this situation, the FM411 shuts off the output, flashes the power supply lamp and displays an error message. At the same time, another circuit measures the load impedance at a rate of 20 times per second and automatically resets the amplifier when the short circuit disappears.
In the case of DC offset, etc., when DC of 1 v or more appears, the output is cut off and DC Error is displayed. After that, the measurement is continued and when DC is removed, the amplifier automatically returns.
For high-frequency oscillation caused by the oscillation of the ground loop or connected equipment, we use a sensor that measures all frequencies above 20 kHz instead of a general high-frequency filter. When there is a continuous high-frequency signal, the output is cut off instantly and the power supply lamp flashes to display the HFError. In this case, when the high-frequency signal is removed, it returns.
In addition, it is equipped with protection mechanisms such as multi-sensor / multi-area overheat protection circuit and protection circuit for protection circuit, and has obtained very high reliability.

The internal wiring is made of a special low-loss FORCELINES cable that is soldered to the contacts, and the high-current wiring is made of materials suitable for the application so that gold is not used for the contacts.
Internal connections, ground connections and screws are tightened with a torque variable screwdriver to ensure that each screw is tightened to the correct torque.
The rail voltage (supply voltage) connection and output connection are soldered with high precision.

The front panel uses machined aluminum 900 that is anodized with high precision after laser processing.
In addition, resonance is analyzed in every part from manually rounded edges, special treatment to prevent objects from falling inside the amplifier, and isolated output terminals, and resonance is completely eliminated when resonance is found.
In addition, the finished product is mechanically damped according to the material and method necessary to suppress acoustic resonance. The parts where such resonance is likely to occur are damped with special foam, rubber such as Delrin / 2, and other materials. It is also used for mutual decoupling in the mounting of transformers and electrolytic capacitors.

Model Rating

Type	Stereo power amplifier
Output power (Output per channel)	When both channels are driven by music or tone burst 150 WRMS (8 Ω) 275 WRMS (4 Ω) 450 WRMS (2 Ω) Continuous sine wave, minimum distortion rate output at 100 hours operation 130 WRMS (8 Ω) 200 WRMS (4 Ω) 250 WRMS (2 ohm
Maximum output voltage	110Vp-p
Maximum output current	16A (Without Output Protection Fuse, Without Current / Voltage Limiter)
THD	0.006%
Band width	3 Hz ~ 60 kHz (Adjustable to lower limit of 1 Hz by internal filter setting)
Gain	29dB
Rise time	5 μ s (@ maximum output)
Hum and noise	-110dB (20 Hz to 20 kHz, vs. maximum output, tilpic) -100dB (guaranteed value)
Input sensitivity	1.6Vrms
Input impedance	40k Ω
Input stage common mode rejection	90 dB (Tipical) 80 dB or More (at 50 Hz)
Maximum input level	+20dBm(7.75Vrms)
Adaptive load impedance	1 Ω ~ 10k Ω
Protection	Short HF oscillation RF interference + / -DC Offset Abnormal temperature rise
Input terminal	XLR3pin (1 = G, 2 = C, 3 = H)
Output terminal	Special Size Banana Jack for Four Plug 200
Power consumption	100W (Idle) 600W at 30% output 1600W at maximum output
External dimensions	Width 430x Height 180x Depth 450 mm
Weight	22kg