BAlabo – BC-1 MK-II Control Amplifier from Bridge Audio Laboratory
The BAlabo BC-1 MK-II control amplifier (aka linestage) sports a radical new design concept in preamplification. With power supply voltage secondaries running internal components at an amazing 120 volts, the BAlabo BC-1 MK-II offers explosive musical peaks, unbelievably nuanced micro dynamics and a silky smooth top end. As a result, soundstaging is incomparable and at minus 110 dB, its noise is essentially non-existent.
Improvement of Low Level Linearity by Post Attenuation Scheme
A typical preamplifier circuit uses the pre-attenuation method - input signals are attenuated at the first stage of an amplifier, before the signal is amplified. This method, however, degrades sound quality, since low-level signals, once attenuated, are lost in the thermal noise and fluctuations of small, board-level components. No matter how excellent an amplifier is, the original signal can never be restored once it has been attenuated at low levels.
The BC-1 MK-II uses a “Post Attenuation” scheme to avoid signal degradation – particularly low level signals and corresponding microdynamics. Line level signals are amplified at the input stage of the BC-1 MK-II prior to reaching the volume control. The high voltage power supply of the BC-1 MK-II facilitates this method and preserves dynamic range and low-level linearity. The result is nothing short of stunning. Even experienced listeners express disbelief at first.
58 Position Volume Attenuator
The BC-1 MK-II control amplifier features an extraordinary volume control of a high reliability and smooth action that consists of 8 circuits in 58 positions - accurate to 0.2 dB at every position. All resistors are custom made to BAlabo specifications by an experienced manufacturer with a long history of providing superior quality components to the audio industry. All signal contacts are sealed to prevent dust intrusion. In addition, copper plates are installed between each section of the volume control for shielding and absorbing acoustical vibration - which can sometimes be associated with electrical contact vibration and always results in deterioration of audio quality. The switch is mounted to the chassis of with additional vibration isolation materials to offer complete immunity from external vibration.
But there is much more to this volume control than amazing accuracy and purity of signal transmission. A massive 4" diameter gold plated indicator dial made of a high density alloy combines with the switch mechanism to offer an unsurpassed smooth action when advancing from one setting to the next. The inner black fingerhold dial is cut with precision, indexed grooves around its circumference to offer a superb, positive tactile sensation. Finally, the actual attenuation settings are designed with a reverse-progressive rake so that course volume increments are at the 'bottom' of the dial and finer gradations are at the 'top'. There are 25 - 1dB steps in the middle of the volume control, and 10 - .5dB steps at the top of its range. In combination with the input attenuation found on the BAlabo BP- MK-II Power amplifier, the BC-1 MK-II's volume control can be 'situated' to operate in the user's preferred operating range for the most desirable adjustment granularity. see settings
Low Impedance Power Supply with 3D Construction
In a typical control preamplifier, the power supply feeds necessary voltages to multiple points in the circuit. Therefore it is impossible to arrange a single sensing point for feedback to the voltage regulator. A feedback style voltage regulator can reduce its internal impedance by reversing additional energy. Even though reversed energy is correctly generated at the sensing point, the process can be less than optimal at the PCB board. The PCB is itself electrically separate from the sensing point because the impedance of the copper trace is not true "zero" - even though it is only a few millimeters in length.
The BC-1 MK-II does not attempt to lower the impedance of the reversed energy, but leverages the benefits of its three-dimensional structure to decrease its effective impedance. Specially designed capacitors are located in close proximity to the actual electrical load point in order to supply instantaneous power. In other words, the separate PCB with BY-PASS capacitors joins the amplifier PCB to achieve the shortest distance in each section of the amplifier by capitalizing on a three-dimensional wiring layout. Since it is designed compactly, the supply loop is minimized in length for ultra-low impedance. The BC-1 MK-II can therefore realize superior impulse response and stability over an extremely wide frequency range. One consequential benefit of this layout is that the circuit board also serves as a shielding layer.
Signal Ground Plane Connection Lug
In some instances, small differences in ground potential between
components can detract dramatically from sound quality. The
BC-1 MK-II, offers a signal grounding lug (not chassis ground)
that is intended to tie together the signal ground planes
of the BAlabo Power amplifier to its own signal ground plane
- thereby eliminating any minute differences in ground potential.
Precision Balanced Amplification
The BC-1 MK-II employs six blocks of amplifiers in each channel,
in a perfect balanced circuit with high S/N and high input/low output
impedance. The amplifier’s first stage works as a buffer amplifier
that utilizes a combination of a DIAMOND CIRCUITRY and FET source followers.
The second stage is a differential amplifier with current feedback.
The third stage uses the same design construction as the first stage.
Since signal imbalance between plus and minus polarity in a balanced
circuit degrades THD, every BC-1 MK-II is carefully tuned at the factory
with hand-matched feedback resistors, that perfectly reject common
mode noise and drive THD to an absolute minimum.
Noise Reduction for AC Line and Power Rectification
In home environments, electronic equipment like personal computers,
add noise to the power mains and degrade the AC waveform -
this can deteriorate the performance high quality audio components.
The BC-1 MK-II employs a power line filter that is a ‘closed type’ at
the location of the AC inlet. The filter cuts the external noise
and reshapes the AC wave form. The transformer of BC-1 MK-II has a very
advanced shielding scheme when compared to most conventional
transformers with toroidal cores. Most toroids have difficulty in creating
a true ‘static shield’ using conventional construction methods.
The BC-1 MK-II has adapted an insulating copper foil to act as a shield between the primary and secondary windings – and the chassis – completely shielding the transformer and eliminating electromagnetic couplings. Rectification noise is also reduced with a BYPASS capacitor that is carefully selected for quality sound. The rectifying diode is a high-speed type, selected for its very low noise. Off the shelf capacitors after the rectification stage can have a negative influence on audio quality. Therefore the BC-1 MK-II uses custom-made capacitors developed specifically for this application. Radiation noise generated by leakage flux from charging current flow in the capacitor is prevented by means of three dividing shield blocks made of aluminum with a floating mechanical support that shunts vibration away from the transformer. The result is an astonishing 20dB mechanical noise suppression at the transformer.
Illuminated Standby/On Function
A Standby/On switch is located on the front panel so the BC-1 MK-II is ready at all times to provide the optimal sound quality without the need for a warm up period. Electrolytic caps remain charged during standby. Main on/off is located on the rear of the chassis. Blue indicates On, amber indicates Standby.
Remote Control Output Jacks - Standby/On for Amplifiers
A remote Standby/On scheme is employed in all BAlabo products. The BC-1 MK-II has a pair of RJ11 jacks on the rear panel for use with BAlabo power amplifiers, offering remote control power Standby/On switching for those units. This remote system is completely isolated from all audio signals.
Custom Components Throughout
For the ultimate in sound quality, components must be designed and selected through careful listening tests. The BC-1 MK-II boasts an amazing 85% custom components, such as all electrolytic capacitors in the power supply section, fixed resistors for the audio signal, copper foil capacitors for phase compensation and the relays. As a result, audio improvements are easily heard when compared to bulk style, off the shelf components.
Tech Specs
|
Connections and Controls
|
Additional Features
Mechanical Grounding of Chassis
The four corners of the chassis are constructed with heavy-duty, extruded aluminum
for superior mechanical integrity. The chassis is designed to aid in the dispersion
of the resonant frequencies by the use of various materials (Aluminum/Steel/Corian)
each with their own properties of absorption and transmission. External vibration
is never propagated to the internals of the amplifier. Vibration attenuation
is achieved with the use of CORIAN® resin (high tech thermo-plastic material
which is hardened by the powder of oxidized aluminum) which is located along
the top and bottom edges lengthwise so that side wall, top cover and the bottom
plate are all dampened. As the result, the local resonance of each structural
member is suppressed to an absolute minimum.
Signal Switching by Relay
For switching audio signals, relays are used to optimize the internal construction
and improve operational feel. These relays are custom made and have gold plated
contacts with a mirror surface finish to prevent signal degradation from oxidized
or pitted surfaces. The L/R balance adjustment circuit has an eleven-step resistance
changeover enabled by relays. Thus, any signal deterioration by use of the
channel balance circuit is negligible.
PowerCon AC Inlet
The AC inlet connection is very important for proper operation. The BC-1 MK-II
uses PowerCon connector made by Neutrik Corporation of Germany. The PowerCon
inlet for the AC mains offers superior reliability and product safety. The
PowerCon has one-third the contact resistance of conventional connectors and
a larger maximum current rating (20A). The PowerCon is a bayonet, snap-in mechanism
and is designed to prevent unexpected disengagement of the AC cord.
Integrated Spikes for Rigid Installation
The feet of the BC-1 MK-II are coated with Teflon with an under cushion material
to prevent shipping damage. Once the BC-1 MK-II is installed, spikes (included)
can be threaded into the base of each foot for additional mechanical grounding.
