Phono preamplifier C-C-C50-MM-BA

If two otherwise identical amplifier circuits are built, one using conventional through-hole components and the other with surface-mount devices (SMDs), the smaller SMD circuit will be sonically superior due to shorter signal paths. This applies not only to modern PWM power amplifiers, which hardly work with through-hole components due to parasitic inductances, but also to preamplifiers. Consequently, we have designed the ultimate phono preamplifier for analog turntables as small as possible and built it directly into the most suitable cable. The electronics are housed in a stable aluminum casing measuring just 8.6cm x 5.4cm x 2.5cm, with the basalt-sheathed HF coaxial cables directly exiting from the ends. In such a compact design, the closed ground plane of the double-sided circuit board can be considered a single ground point for the audio frequency range, which is a significant advantage over larger designs.

The power supply is provided by a 12V linear power supply, followed by a PWM-regulated miniature switch-mode power converter with a subsequent fourth-order filtering chain for effective ripple suppression, supplying the preamplifier with clean, dual-isolated ±15V from the mains. Each amplifier stage is additionally decoupled by second-order filtering. The filtering chains consist of damping SMD ferrites and high-capacitance multilayer ceramic capacitors, which exhibit significantly smaller equivalent series resistance than bulky electrolytic capacitors.

The design includes three linear amplifiers per channel and passive RIAA equalization with hand-selected capacitors. Each capacitor in the split equalization network consists of a parallel combination of a ±1% tolerance MKC film capacitor and a smaller NP0 ceramic multilayer capacitor. The 3180μs and 318μs time constants of the RIAA curve are corrected between the first and second linear amplifiers, and the 75μs time constant correction takes place between the second and third linear amplifiers. This arrangement is both technically and sonically optimal.

A typical Moving Magnet (MM) system requires a signal gain of 40dB (factor of 100) at 1kHz, which, with a passive RIAA equalization network, necessitates a total gain of 60dB (factor of 1000), distributed across the three linear amplifiers, each providing 20dB. The first linear amplifier, which has the highest demands for low noise and precision, is a discrete OTA (Operational Transconductance Amplifier) with the low-noise precision dual JFET LSK489 at the input, whose nonlinear distortions remain below 0.001% at 10mV input voltage, and without negative feedback! The OTA (for connoisseurs: folded cascode with Wilson current mirror) achieves the best possible transient response with minimal noise and inaudible distortion because it operates without global negative feedback, which would otherwise represent a "post-correction" and require frequency compensation that slows down signal processing speed.

The second and third linear amplifiers must handle signal voltages 10 times higher and therefore cannot operate without negative feedback, as otherwise, the distortion factor would increase disproportionately. These stages are implemented with integrated operational amplifiers whose special output stages generate no crossover distortions, ensuring that the feedback operates over pre-linearized control paths. Under the conditions maintained here for the second and third amplifier stages, the audible sound quality of these op-amps cannot be surpassed, neither by other integrated nor by arbitrarily complex discrete operational amplifiers! Anyone who believes tubes would be better is mistaken. Good solid-state circuits are superior to those with tubes (for some technical background on debunking the tube myth, see: [Elektronikinfo - The Tube Fallacy](http://www.elektronikinfo.de/strom/roehrenirrtum.htm)).

To avoid coupling capacitors in the signal path, the third linear amplifier, which drives the output cable with 47Ω, includes active offset compensation. To ensure this compensation doesn't have to deal with excessive DC offset, the OTA's DC offset is manually adjusted.

The input impedance of the C-C-C50-MM-BA is 47kΩ, and the input capacitance can be precisely adjusted via miniature rotary switches in 16 steps from 50pF to 450pF to match the specific cartridge system, with the cable capacitance already factored in.

The gain of the entire circuit can be adjusted in three steps of 3dB each, providing a total gain (relative to 1kHz) of 40dB / 43dB / 46dB. Thus, high-output MC systems can also be adequately amplified at the highest setting.

As for the sound quality: "It's as if there's (almost) only a cable in between!"

For turntables with externally routed connection cables, the C-C-C50-MM-BA is also available with RCA input jacks and shorter output cables. In this case, the input capacitance setting should account for the cable capacitance. The capacitance Cx of the connection cable is Cx – 25pF + the value set on the dial.