B.M.C. Phono MCCI phono preamplifier
The Aqvox's designer was a hired gun: Carlos Candeias. Now he's back, this time with the Phono MCCI balanced phono preamplifier ($3890), another current-input design, and this time with his own brand. Candeias founded B.M.C. Audio GmbH in 2009, to design (in Germany), build (in China), and market (worldwide) his creations, including the Audio Amplifier C1 integrated amplifier, which I reviewed in the May 2012 issue.
The Phono MCCI is built to a far higher, more complex standard than the Aqvox, and is MC only. Like the other B.M.C. electronic components, it sports a large, round something at the center of its front panel. On the C1, that something is a power meter; on the Phono MCCI, it's a big On/Off switch. The user can adjust little on the MCCI, so the controls are simple: On/Off, Mute, front-panel Dimthat's it. Well, not exactly. A plethora of internal jumpers is found inside; more about them shortly.
The basic difference between the Phono MCCI and virtually all other phono preamps is its current-injection input, which takes advantage of an MC cartridge's very low impedance, its inherent current-generating capabilities, and its balanced, floating-ground architecture.
Instead of a traditional voltage-gain stage, the Phono MCCI's input stage is a current-to-voltage converter. According to the Candeias, the cartridge directly injects its current into a system of "balanced DC currents," creating an amplified output voltage. The resulting amplified voltage is claimed be made "directly of the original cartridge's current" with virtually no loss, and certainly less loss than is claimed for any voltage-gain circuit.
While a few other designers of phono preamplifiers have used a similar idea, Candeias claims that they use an op-amp or similar circuit to simulate, via a feedback loop, the required low-impedance input.
The CI circuit is different
The B.M.C. Phono MCCI's Current Injection input circuit uses a variation of a common-base or grounded-base topology often used in microphone preamplifiers, where the input is applied to the emitter of a bipolar transistor. This sort of circuit turns out to be equally useful for MC cartridges and is said to produce ultrawide bandwidth and very little noise.
Because the an MC cartridge's source impedance and the phono preamplifier's input impedance determine the current produced, cartridges with a surprisingly wide range of voltage outputs can be used. While a high-output MC generates a higher voltage, its higher impedance results in reduced current output. So despite wide variations in voltage output among MC cartridges, the Phono MCCI's input stage can deal with many of them, including high-output MCs with outputs similar to those of typical MM cartridges.
The Phono MCCI's entire signal chain consists of two very short, fully balanced gain stages with zero feedback. Because the second gain stage sees a relatively high voltage from the current-to-voltage stage, it's easier to correctly implement than one required to deal with the ultralow MC cartridge voltage output. And because there's no global-feedback loop, RIAA equalization is accomplished passively in two stages decoupled from one another. The first pole is in the current-to-voltage stage, while the second is part of the second voltage-gain stage.
Carlos Candeias claims that this sort of circuit has the advantages of passive and active equalization, minus the disadvantages of either. You can choose either the standard RIAA equalization curve or the controversial Neumann-corrected version preferred by Candeias, though not by John Atkinson and others (footnote 1).
The Phono MCCI's output stage is a fully balanced, Load Effect Free (LEF), single-ended, class-A design similar to what Candeias uses in his B.M.C. C1 integrated amplifier. The claimed advantage of the circuit, when applied to a single-ended class-A design, is that it avoids distortion by allowing the transistors to work only within their linear operating range.
The class-A balanced circuit is said to widely reject even-order distortion, and Candeias claims that because it presents the power supply with a consistent load, the musical signal does not modulate the power supply. In addition, the balanced design rejects common-mode power-supply disturbances. Because the input uses current instead of an MC cartridge's ultralow voltage, the input impedance is very low, less than 3 ohms. There is no need, therefore to damp the cartridge's ultrasonic resonance with energy-destroying resistors in parallel with the input.
The Phono MCCI's high-quality parts include: 10 ultralow-noise transistors in parallel for each "functional group"; "balanced-current" capacitors; inductance-free polystyrene capacitors; thin-film metal resistors with 0.5% tolerance; fully gold-plated, four-layer printed circuit boards; and a shield of copper-plated iron.
The insides of the Phono MCCI look impressive by any standard, but particularly for the price. And inside is where you'll have to go to find the various jumpers that let you select among three levels of gain (Low, High, and Very High, standard or Neumann RIAA, a subsonic filter, and Low End Corrections consisting of Linear, Bass Boost, and Bass Boost and Warmer Sound. More about these below.
Mono cartridges with common grounds need not apply, RCA-to-XLR adapters not recommended
While all of these design features sound ideal, we don't live in an ideal world. I can't speak to the possible technical disadvantages of a current-injection circuit, but the practical ones became apparent as soon as I opened the B.M.C. Phono MCCI's owner's manual.
The manual is clumsily translated from the German, with syntax sure to flummox many. It would be relatively easy to rewrite for easier comprehension and better flow; and given the many danger cautions given, clarity is of utmost importance. The main caution is that you must maintain "ground free" connections from your turntable. The shield must not be connected to any of the four cartridge wires. That means that Rega Research turntables (and Rega's separate tonearms) will need the ground connection broken between the arm and the cartridge's blue "earth" pin.
Footnote 1: For a thorough discussion of this, see Keith Howard's "Cut and Thrust: RIAA LP Equalization" in the March 2009 issue. Even if your eyes begin to glaze over, stay with it until you get to the sidebar on the so-called "Neumann 4th pole," which is easier to understand. (If I can understand it, anyone can.)