Analog Corner #260: CH Precision P1 phono preamplifier

Although Swiss-based CH Precision (footnote 1) is a relatively young company, its core design team has been involved in high-performance audio for many years. Their website doesn’t state the company’s age, but something there did provide a hint: Among the design team’s previous activities was work for another Swiss company, Anagram Technologies.

In 2001, in a review of Camelot Technology’s Round Table DVD player, which included an Anagram Technologies DAC, I wrote: “I was so impressed with the sound that I took the Round Table downstairs to my 2-channel audio room and used it there when I wasn’t watching movies.”

I hadn’t made the connection between Anagram and CH Precision when Raphael Pasche, the latter’s electronics design engineer, visited last fall to install CH Precision’s P1 dual-mono phono stage and the optional X1, a discretely regulated, outboard linear power supply that’s claimed to exhibit ultralow levels of noise. The P1 can be ordered as a stereo preamp for $31,000; add $17,000 for the identically sized X1 power supply. Or one mono P1 can be used for each channel, for $55,000 plus $34,000 for two X1s. Expensive stuff.

Description
Inside and out, CH Precision gear is built to the highest standards. The P1 is made of aluminum alloy, with no screws visible on any of its surfaces. The bottom plate sits on four stainless-steel feet, each fitted with an elastomer ring to protect delicate surfaces. Also supplied are long-shafted hardened steel spikes that fit concentric with those feet; the spikes can be used to fine-tune leveling, by inserting a screwdriver (supplied) in the slotted tops of those shafts, which extend all the way up to the top plate. The P1 and X1 are both 17.3″ square; the P1 is 5.25″ high and weighs 44lb, the X1 5.25″ high and 55lb. CH Precision components can be securely stacked and isolated from each other using a built-in vibration-suppression system that employs those spikes and also features hi-tech discs of carbon-polymer composite. If you don’t use this system, you can hide it by screwing machined discs into the openings atop the enclosure.

The P1’s appearance is elegant and free of knobs—it’s operated via five tiny pushbuttons—from the top, Standby/Mute/Unmute, followed by four menu-navigation controls: Up, Okay, Down, and Cancel.

Metal-film resistors are used throughout the P1’s signal path, and custom film capacitors in the filtering section. The P1 offers both RIAA and enhanced Neumann pole equalization. A pair of optional boards ($1850) include the EQ curves of EMI, Columbia, Decca (ffrr), and Teldec. I passed on this option, but that’s not meant as an editorial comment!

On the P1’s rear panel are three unbalanced (RCA) and three balanced (XLR) inputs, and outputs of both types as well as BNC. Each input has its own discrete class-A gain stage and independent power supply—this costs a lot more to implement than putting an input-selection stage before the first amplification stage, but the latter degrades the signal quality.

After the first gain stage comes the input-selector circuit, immediately followed by the passive RIAA EQ. Then comes the second gain stage, which includes the high-pass filter. The signal is buffered after equalization, and provides the necessary gain because the RIAA circuit drops the signal level. While there are unbalanced and balanced inputs, the first stages of both the voltage and current inputs are unbalanced. The rest of the circuitry is balanced. Two inputs are current-amplification circuits dedicated to moving-coil cartridges.

According to Raphael Pasche, the current input’s first gain stage is a very-low-impedance (<100 milliohm, which is a “virtual ground input”), trans-impedance amplifier that converts the current delivered by the MC cartridge into a voltage, thus producing gain. MC cartridges produce weak voltages but strong currents; the lower the cartridge’s internal impedance, the greater the current output. Since the gain is related to the cartridge’s internal impedance, which varies with the cartridge, the P1’s current inputs offer six levels of gain.

Current-amplification circuits produce the best signal/noise ratios, and don’t require cartridge loading to achieve flat response. The current input noise (CH calls it the equivalent input noise, or EIN) is specified as <–135dBu without the X1 power supply, or <–138dBu with the X1, with 1 ohms termination, gain +70dB, 22kHz bandwidth.

The third input, which can be configured for either moving-magnet or moving-coil cartridges, is a more traditional voltage amplifier featuring an ultra-low-noise FET input stage. The P1’s voltage input also offers six levels of gain: 35, 40, 55, 60, 65, or 70dB. The EIN specs for the voltage gain input are also impressive: <–130dBu without X1, <–135dBu with X1 and 1 ohms termination, gain +70dB, 22kHz bandwidth. Of course, when the third input is configured for an MM, you can use your choice of step-up transformer with your MC cartridge, giving you a third option (footnote 2).

P1 owners who use the voltage-amplification input needn’t worry about calculating loading: The P1 comes with a 45rpm test record. Play it and the P1 does the work for you, using your cartridge to calculate its results. Side 1 contains a track of 250Hz–30kHz filtered pink noise designed to be used in conjunction with a “wizard” available from the P1’s menu. The wizard analyzes the frequency response of the entire system—cartridge plus tonearm plus P1 input loading—while varying the resistive loading of the P1’s voltage input.

You can test the P1’s entire loading range of 20 ohms to 100k ohms, selectable in 500 steps (load fetishists, knock yourselves out!)—or, for finer tuning, any subset of that range. The P1 automatically acquires 21 different frequency-response curves, offering input loadings evenly spread across the range selected in the previous step.

In addition to the FR curve for each loading value, the wizard provides an average level and the standard deviation of the flatness of each curve. Once the P1 has completed its calculations, at the push of a button, the front panel displays the FR curve of whatever loading you choose. You scroll through them, looking for the flattest response (assuming that’s your goal), then push the button to set that load value, which is then displayed on the P1’s screen.

One of the problems with resistive loading is that because the signal passes through the resistor, in addition to flattening the response curve, the signal’s level is decreased. The P1’s manual suggests that you make sure that the average drop in level doesn’t exceed 2–3dB relative to the 100k ohm load selection. It also suggests that you examine the curves for the most extended FR and, of course, curve flatness.

The wizard is particularly useful if you use an outboard step-up transformer, which is when the loading math can get tricky. A selectable subsonic high-pass filter is available for each input—though if you can afford a P1, your turntable probably won’t need it.

More of CH Precision’s specs for the P1: output levels of up to 8V RMS balanced and 4V RMS unbalanced; a frequency response of >400kHz (current input selected, RIAA EQ filter disconnected); and total harmonic distortion plus noise of <0.01%, 1kHz, output level 3V RMS, 22kHz bandwidth.

While the power supply is critical to any piece of analog gear, it’s especially so for a phono preamp, which must produce a great deal of gain: Providing the amplifier stages with clean DC is critical to producing a low noise floor. The power supplies in all CH Precision products are discrete, the crucial stages using a special shunt circuit. The P1 doubles the input-stage regulation, with a second stage placed in series with the first to improve voltage consistency. The linear power supply features multiple independent local regulation circuits, along with an oversized toroidal mains transformer that supplies both phono stage boards and also powers the digital section (front panel display, microcontroller, and DSP-based system monitor). Discrete, low-noise regulators are used throughout the X1 that each audio section receives the purest DC—that is, power with the lowest possible amount of noise.

Footnote 1: CH Precision Sàrl, ZI Le Trési 6D, 1028 Préverenges, Switzerland. Tel: (41) (0)21-701-9040. Fax: (41) (0)21-701-9041. Web: www.ch-precision.com

Footnote 2: Raphael Pasche provided me with an excellent explanation of why a voltage-amplification circuit requires loading a cartridge with specific impedance. But to save space here, I refer you to Hagerman Technology’s explanation, also excellent, at www.hagtech.com/loading.html, where you’ll also find calculators for MM capacitance and MC loading.

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