Original board. One available at this price.

This is the Flat Moving Coil Phono Preamp project published in the Pro Audio Design Forum. The Flat Phono Preamp is designed for the LP archivist, mastering facility, record manufacturing or any professional application requiring high quality, low-noise, no-nonsense flat playback for low output (<1 mV) low impedance moving coil cartridges. (For "high output moving coil carts use the Flat Moving Magnet preamp.)

A Flat, Non-Equalized Fully-Balanced Preamplifier Board for RAW Recording

The Flat Phono Preamp board is a fully balanced instrumentation amp designed specifically for direct coupling to low impedance, low output moving coil phono cartridges. The flat, non-equalized preamp is ideal for RAW archival transfers where RIAA or custom EQ is applied in DSP. When paired with the RIAA EQ Monitor Switcher a fully-analog path with RIAA EQ is possible.

The fully-differential input connections are designed for balanced (floating) tonearm wiring using shielded twisted pair cables. Balanced inputs provide hum rejection of common mode signals picked up by tone arm and interconnect wiring. The low differential bias current of balanced inputs eliminate the requirement for input coupling capacitors.

Jumpers on the Flat Phono Preamp select one of four resistive terminations that are typically 10Ω, 30Ω, 100Ω and 1KΩ. (Other values below 1KΩ are available by changing on-board resistors.) Jumper-selectable gain adjustments of 50, 52, 56 and 62 dB are available for setup with different sensitivity carts. A variable trim control allows low gain adjustments from +38 to +46 dB. (Unbalanced outputs are 6 dB lower in level).

The Flat Phono Preamp is fully DC-coupled from cartridge to output owing to its balanced input, for low differential bias current, and DC common mode rejection of the preamp's internal bias voltages. A fixed frequency "differential DeBoo" servo, using a single polypropylene capacitor with passive input poles, provides a low-offset output (typically < 2mV) as well as providing a first-order high pass function with a -3 dB point of 12 Hz.

The ZTX851 transistors used in the Flat Moving Coil Preamp provide very low noise operation owing to their low < 2Ω rbb' base resistance. With a 10Ω source impedance and a gain of 62 dB the preamp has an Ein of -141 dBu, a noise density of 0.48nV√Hz and a noise figure of about 1.5 dB. In the real world, with a DL-103 cart as the actual source, an A-weighted S/N ratio of 78 dB can be obtained relative to the DL-103's 5 cm/sec reference level.

A unique advantage of the Flat Preamp is its ability to be remotely located at the turntable and powered by bipolar DC. This permits cartridge leads to the preamp to remain short allowing long low-impedance output signal lines to feed the RIAA EQ/Monitor Switcher or Workstation A/D converter. The split preamp configuration is ideal for cutting lathes, archive facilities and residential settings where separation from the lathe or turntable and the workstation or transfer console is needed.