Musical Fidelity A1 Stereo Integrated Amplifier

This 1985 Iconic Class-A Amp Is Back to the Present. Musical Fidelity created a true hit with their decision to revisit the original but give it all new components and power supply. 
Musical Fidelity A1 Integrated Amplifier
A 25 Wpc pure class-A with sliding bias based on a near-perfect 1985 circuit design but with better power supplies and components. Also with MM/MC phonostage and remote control.
First released in 1985, the Musical Fidelity A1 made waves for being an absolute beast when it came down to driving some of the most difficult speakers of that time, even though it was actually a rather low-powered integrated amplifier. The magic came from its pure class-A design and expertly laid out discrete symmetrical circuitry topology and using only highest quality components. 
Based on its huge success, the A1 has gone through multiple reissues and new versions over the years.
Now, Musical Fidelity has preserved the specifications, appearance, and circuits of the 1985 rendition for those who missed out the first time. Inside, though, it has been updated with new transformer, power supplies, and modern low-noise components.
Pure Class A 
The A1 is built around a completely discrete and symmetrical Class A topology, offering 25 Wpc with 25 A of maximum output. Fans of class-A designs will know that 25 W is a lot of power, and when needed for hard-to-drive speakers, it slides into class-B operation. The A1 has a dual mono split rail transformer. The amp stages are supplied by fully independent left and right power supplies for better power handling and stereo imaging and each power amplifier now has double supply capacity resulting in reduced ripple and noise. The transformer‘s noise and heat dissipation are reduced due to better efficiency and updated components. The main supplies are filtered for better regulation and reduced switch-on surge.

Small-Signal Wins
The low noise preamplifier circuitry is the same as the excellent original 1985 design but greatly benefits from the improved, fully smoothed, and regulated power supplies. A "direct" switch bypasses the pre-gain before the attenuator, allowing for more knob travel with high-level digital sources, or for sensitive speakers.

The volume potentiometer is a premium ALPS RK series, a high-quality component that ensures superior performance and enhanced precision. And, in a usability upgrade since 1985, the volume pot is motorized, allowing for the use of the new remote control.

The A1 has a discrete current-mode phonostage for moving coil and moving magnet cartridges. A low noise current to voltage conversion stage is used for further amplification and RIAA equalization. Automatic input impedance matching for the selected MC input, along with increased gain increases the versatility of the A1‘s phonostage.

In Hi-Fi, the Musical Fidelity A1 has become one of the most classic designs – inside and out. First released back in 1985, it quickly made waves for being an absolute beast when it came down to driving some of the most difficult speakers of that time, even though it was actually a rather low powered integrated amplifier. But: it was all done in pure Class A with excellent and expertly laid out discrete symmetrical circuitry topology and using only highest quality components. The A1 showed people that there was more to amplifiers than just high power.

Features:

• The A1 through time - Improvements to the original
• Updated transformer to more efficient “dual mono split rail” windings
• High quality and updated ALPS RK series potentiometer as volume control
• IR remote control added for volume
• Gain block before volume control totally by passable using “direct” switch (-10dB)
• All transistors including output devices are carefully selected modern equivalents
• All capacitors in signal path are polypropylene (PP) type
• Resistors are modern low noise metal film types
• Care and attention made to not change any component specification from original but to update them to modern equivalent with long lifespan
• Layout faithfully follows original within circuit sections on circuit board
• Circuit sections better laid out, in particular the separate supplies and power amplifiers
• Phono stage and preamplifier improved for less noise and hum
• 25W Pure Class A power @ 8 Ohm
• Discrete symmetrical topology
• Planar Audio transistors with 25A maximum output
• Dynamic class A, slides (NOT switches) smoothly into Class B, allowing more current as needed
• Therefore drives difficult speakers with no problems
• Fully independent left and right power supplies
• Each power amplifier now has double supply capacity
• Transformer noise and heat dissipation reduced
• Filtered main supplies
• Gain block before volume control totally bypassable using “direct” switch
• High quality and updated ALPS RK series potentiometer
• Low noise TL071 JFET preamplifier circuitry
• Discrete current-mode MM/MC phono input stage
• Low noise current to voltage converter for further amplification and RIAA equalisation
• Automatic input impedance matching (1K nominal) for MC along with increased gain
• Original (1985) A1 appearance and circuits with modern improvements
• Remote control added for volume
• Modern updated low noise components

Amplifier:

• Power: 25 Watts per channel into 8 Ohms
• Voltage: 42.5 Volts peak to peak
• Current: 25 Amps peak to peak
• Damping Factor: 150
• Gain (volume max): 32dB (Direct Mode), 42dB (Normal Mode)
• Output devices: 2 per channel
• Signal / noise ratio: 82dB
• Channel separation: 85dB
• Frequency response: + 0, –1dB, 10Hz to 40 kHz
• Line level sensitivity: 300mV rms nominal, 8V rms max
• Line level inputs impedance: 25k Ohms
• Tape Out impedance: 220 Ohms
• Pre Out impedance: 100 Ohms

Phono Preamp:

• Phono RIAA response: +/- 1dB
• MM sensitivity: 5mV nominal
• MM capacitance / impedance / gain: 100pF / 50k Ohms / 40dB
• MM signal / noise ratio: 75dB
• MC sensitivity: 450μV nominal
• MC impedance / gain: 1k Ohm / 60dB
• MC signal / noise ratio: 70dB

Inputs:

• 1x phono RCA MM/MC, 5x line RCA

Outputs:

• 1 x line level RCA fix TAPE OUT
• 1 x line level RCA var PRE OUT
• 1 x 4mm banana/binding posts SPEAKER OUT
• Max. Consumption: 130W, 0W in standby