Vaccum Tube Power Amplifier
The Conrad-Johnson MV125 offers an unparalleled level of sophistication and refinement in music reproduction, but it cannot transform an inferior stereo system. Only the very best loudspeaker systems will be capable of fully revealing the capabilities of this precision instrument. Proper set-up is important in any high-fidelity system, but with state-of-the-art components it is essential. Given a system of high quality components, there remain a number of important details which must be attended to.
Absolute Phase
Musical notes are heard through the ear’s response to waves of alternating rise and fall of air pressure. Musical transients are exclusively positive: that is, the initial effect is a rise in pressure. The ear is capable of distinguishing these positive transients from the musically unnatural alternative of a negative transient (an initial fall in air pressure). In your stereo system, these transients are created by your loudspeakers. If the speakers respond to musical transients by first moving out, they are creating a rise in pressure, and the system is said to be "phase correct". If they respond by moving in, they create a fall in pressure and the system is said to be "phase inverting". Each component in the stereo system is either phase correct or phase inverting (including the cartridge and speakers-these are normally phase correct). Your MV125 is phase correct. It is of no consequence if an individual component is phase inverting, as long as the system as a whole is phase correct. This will be the case as long as the number of phase inversions is even (or zero). If your system has an odd number of inversions, then you must add one phase inversion. This is conveniently done by reversing the positive and negative connections on either your cartridge or your speakers (be sure to reverse both channels).
Circuit Design
It has been our experience that simple designs with few active stages when executed with care and great attention to detail will be both more musical and more reliable than overcomplicated alternatives. Accordingly, the audio circuit of the MV125 is remarkably straightforward. A triode input amplifier comprised of paralleled sections of a 5751 is direct coupled to a cathode coupled differential phase inverter using high
current triodes to provide balanced, low impedance drive to the push-pull output stage. The output stage features ultralinear operation of two pairs of 6550s for high power levels with reduced source impedance. As a result, the amplifier is capable of supplying the large current demands of high amplitude musical transients driven into reactive speaker loads.
A substantial main power supply of massive polypropylene capacitors enables the MV125 to easily meet these extraordinary current demands. Voltage regulators for the voltage amplifiers and phase inverters provide nearly absolute isolation of these sensitive stages from the output stages.
The MV125 uses twelve vacuum tubes of three different types: two 5751 (V1 and V2), two 6FQ7 (V3 and V4) and eight 6550s (V5 through V12).
Specifications
Power: 125 watts per channel RMS both channels driven into 8 ohms from 30Hz to 15KHz at no more than 1% total harmonic distortion or intermodulation distortion.
Sensitivity: 880 mV to full power.
Small Siganal Distortion: Less than .1% at midband
Phase: Phase correct (non-inverting)
Frequency Response: 20 Hz to 20 kHz +0, -.5 dB
Hum and Noise: 96 dB below full power output.
Input Impedance: 100 K Ω
Dimensions (W x H x D) : 19" x 20.5" x 8.75"
Weight: 80 lbs
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