tereophile June 2013, MCCI Phono Stage:
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PHONO MCCI is a truly balanced, global feedback free, passively equalised moving coil phono pre-amplifier with the exceptional Current Injection input and LEF single ended class-A output stages.
The vinyl disc, dinosaur of music recording formats, only barely survived the CD. Today, however, the quality of turntables, tone-arms, and cartridges have increased to unprecedented levels, which vinyl recording producers support by pressing higher quality discs in limited quantities. This means new challenges as well as new opportunities for phono amplifiers.
The only consistent way to process the delicate music signal from a moving coil cartridge is with a balanced Current Injection (CI) input as the cartridge's signal is among the most sensitive an audio amplifier circuit will encounter. Using CI, the current of the moving coil flows through the phono amplifier, including the first half of the RIAA equalization. The result is a level of dynamics, impact, spaciousness and richness of color that surpasses the performance of step-up transformers as well as the usual voltage inputs.
At the same time, Current Injection electrically damps the moving coil cartridge by letting energy flow into the CI input, instead of eliminating it in a cartridge load resistor. Transformed energy sounds different than and superior to eliminated energy.
B.M.C.'s use of distortion-free circuits with a Current Injection input and a Load-Effect Free output brings to listeners an advanced musical experience — exceptionally lifelike, open and natural.
Combining the CI input with a low impedance moving coil cartridge give new meaning to the phrase "dynamic system."
The B.M.C. PHONO MCCI is a balanced, passively equalized moving coil (MC) preamplifier lacking negative feedback. It features natural distortion-free and Load-Effect Free single-ended Class A output and a Current Injection input, which optimally reads the signal current of the moving coil pick-up.
The MC System at CI Current Input Compared with Voltage Input
When amplifying a signal from an MC pick-up, the interaction between the MC pick-up and the input is a critical element, as is optimal utilization of the very fine MC signal. Let us compare the traditional voltage input with the CI current input in terms of signal strength, damping, and amplification:
1. Signal Strength:
Moving Magnet (MM) and Moving Coil (MC) Cartridges at Voltage and Current Inputs MM cartridges have the advantage, at the voltage amplifier input, of an output voltage almost ten times higher than MC pick-ups. It is the other way round at the CI input: MC pick-ups can provide up to ten times higher currents than MM systems. Consequently, an MC system is the stronger not the weaker system at the CI current input.
2. Damping an MC Pick-Up as an Oscillating System
A phono pick-up cartridge is an oscillating electro-mechanical system. At a voltage input, the pick-up signal will be read, but the pick-up emits practically no energy to the amplifier circuit. With a high-impedance input, the oscillation energy of an MC pick-up stays undamped and is especially noticeable in the high-frequency range. Cartridge loading resistors are connected parallel to the input to damp these oscillations. Energy is always destroyed in these resistors, so trimming a system is a compromise: The resistance must be low enough to damp system oscillations to a bearable level yet high enough for the system not to sound completely de-energized.
At the other end of the transmission path, with an amplifier driving the speakers, there is good reason not to use an amplifier with a high-impedance output, and to connect parallel resistors to damp the natural oscillation of the speakers. Power amplifiers which simultaneously drive the speakers and, through low-impedance, dampen their oscillations, are a well established standard which makes perfect sense: Driving and damping the speaker as an oscillating, electro-mechanical system go hand in hand.
Making the amplifier input low-impedance is unusual but makes perfect sense with the MC input: Signal processing and damping the oscillating electro- mechanical system through an amplifier input allows the MC system's energy to flow as current into the input. Energy is discharged this way but not destroyed. In fact, it is used for signal amplification and to conjure a vitality not previously heard.
3. Use of the Original Input Signal Current
The fine original current that the MC system feeds into the low-impedance CI input flows right through the amplification stage. At its end is a defined signal voltage through the termination. The termination can be a simple resistor, or with PHONO, a part of the RIAA filter network. At the CI circuit output, you can find the same electrons that are fed into the input. The original signal is used instead of a copy. In this way, the very delicate, finely tuned MC signal experiences the most sensitive method of amplification, and is conserved in all its richness and tonal diversity.
1. Balanced Current Injection input
2. Common base circuit for highest bandwidth
3. Widely automatic gain adjustment
4. Feedback free circuit with very short signal path
5. Full balanced circuit
6. Balanced LEF single ended class-A output
The Current Injection Circuit
The Current Injection input circuit is a remarkable implementation of the principle of the common-base circuit. It is known for having the widest broadband, the least distortion, the best dynamics, and the most musical sound of the three base circuits, the benefit of not making a copy. Actually this CI is rather an I/V converter than an amplifier in the traditional sense. Dynamic losses and distortions are remarkably lower compared to common voltage amplifiers. But it has a low-impedance input and a high-impedance output. Fortunately, the circuit is a perfect match for MC pick-ups and D/A converters, our best audiophile sources. For decades, some of the best MC amplifiers in the world have used the common-base circuit. However, only unearthed balanced input allows the MC cartridge to be coupled directly with a common-base circuit without risk or comprises. The Current Injection circuit, compared with traditional circuits, significantly reduces dynamic loss, and signal distortion and discoloration.
LEF Output Stage
Because the Current Injection circuit has a high-impedance output, a stable buffer is required to drive the load. Seen dynamically, there is no alternative to single-ended amplifiers, which run in Class-A drive. In a Load-Effect Free drive, the single-ended Class-A transistor, which is decisive for the sound, is freed from the work of driving the load. This is why we say it is Load-Effect Free. The quality goes through the natural lack of distortion up and above the classical Class-A mode, and renders negative feedback needless.
Truly Balanced Circuit
In balanced Class-A mode, the supply voltage is not modulated through the music signal. One must keep Phono in mind. The output of a phono-amplifier produces a few volts at high levels, but the input has a sensitivity in the μVolt area. The balanced Class-A mode leads to a distinct increase in the inner tranquillity of the sound pattern, leaving more room for fine details, dynamic leaps, relaxation of the performance, and three-dimensional spatial illustration. In other words, an intense, in-depth music listening experience.
RIAA Equalization and Adjustment Options
1. Passive equalisation with the I/V converter
2. Neumann corrected RIAA, or traditional RIAA
3. RIAA options for low end extension and warmth
4. Subsonic filter selectable
5. 3 selectable gains
Because all B.M.C. analog circuits work without negative feedback, there is no "active" equalization for the RIAA specification. For a precise RIAA correction, the equalization takes place in a passive manner in two stages uncoupled from each other, which calls for a frequency-dependent current/voltage converter. The B.M.C. MCCI PHONO combines the neutrality of passive filtering with the avoidance of unnecessary amplification.
The classic RIAA equalization is wrong because when cutting there is no endless rise in the high tones as they are corrected. Correction takes place In the PHONO MCCI with the help of the Quasi Industrial Standard of disk-cutting lathes by the Georg Neumann company. Music made following this standard simply possesses more breath and openness. One can deactivate the Neumann correction and easily hear its advantages.
It is even possible to slightly raise the lowest deep bass as well as the "warmness" level to correct recording errors or slightly bass-poor pick-up/pick-up arm combinations.
Ideally the cartridge should match to the tonearm and the system is well damped. In other cases a soft subsonic-filter can be selected.
Although the gain between different cartridges differs less than usual, according to the construction of the MC there might be level differences. Phono-MCCI allows the setting of 3 different gain levels.
Components of the Highest Value
1. Special ultra-low-noise transistors with very high Hfe, 10 in parallel
2. “Balanced Current” capacitors for precise and colourful sound
3. Induction-free Polystyrene capacitors
4. Metal-thin-film resistors with 0,5% tolerance
5. Fully gold plated 4-layer printed board
6. Copper plated iron shield cover
Phono-MCCI's very special transistors feature a super high Hfe and a super low typical noise figure of 0,3dB which is at the limit of today's possibilities. Each functional group uses 10 in parallel which reduces the statistical noise by 10dB. There should hardly be any more quiet solution for a phono amplifier.
The “Balanced Current” electrolytic capacitors have a balanced characteristic for charge and discharge. The important ESR-bandwidth is far beyond common electrolytic capacitors. This results in an improved musical performance, precision, detail and beauty, starting from the very low frequency, covering the whole spectrum.
Induction-free Polystyrene capacitors feature the lowest losses of all film capacitors and show no resonances. Used in audio circuits they deliver very open and pleasant treble.
Metal-thin-film resistors with 0,5% tolerance represent the highest part level within this group. SMD types do not have clamped contact wires and show lowest inductance.
The core part is build on a fully gold plated 4-layer PCB. The solder is lead-free and includes silver. The sensitive core module hat its own copper plated shielding box.
Time for New Standards
The digital music reproduction over the years challenged the analogue technology to deliver better phono components. This improvement process should not be limited by old standards. In this sense an un-balanced phono input circuitry is no longer state-of-the-art. With all the limits in noise and the related detail resolution it limits in-depth listening. All aspects of the circuitry should serve dynamic fine resolution all-over the whole frequency range for really unveiling the advantages of Vinyl-reproduction. Part of this philosophy is the abandonment of global feedback also for RIAA amplifiers. Then music can blossom and catch emotionally while being relaxing; leading us into a different world, touches and may seduce us...
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