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 servo, using a single polypropylene capacitor, provides a low-offset output (typically < 2mV) as well as providing a first-order high pass function with a -3 dB point of 10 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.
An optional output pad can be installed to permit the preamp to be used as a head amp to feed MM inputs.
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
The Flat Moving Coil Preamp has the following features:
- Flat no-RIAA for RAW capture.
- Fully-balanced Moving Coil Instrumentation Amp Input.
- Fully-balanced Outputs.
- Optional -33.8 dB output pad for use as a head amp.
- Jumper-selectable Input Termination.
- Jumper-selectable Gain.
- DC-coupled input-to-output with servo-defined high-pass response.
- Cross-coupled common mode rejection stage.
Mechanical and Power Specifications
- Board Size: 4.1" x 3.1" Mounting holes: M3 on a 3.7" x 2.7" pattern. Assembled height: Approximately 0.75" not including stand-offs.
- Power Requirements: ±15 to ±18V (max). No signal Icc approximately +80 mA, Iee -50 mA.
- Inputs: Fully-balanced, DC-coupled, Instrumentation Amp.
- Differential Input Termination: 10, 30, 100Ω and 1KΩ.
- Input Common Mode Impedance: 250Ω.
- Differential Input Offset Current: <3µA typically.
- Gain: Jumper-selectable +50, +52, +56 and +62 dB. Variable trim +38 to +46 dB.
- Outputs: Balanced THAT1240. Differential output impedance 100Ω nominal. Maximum output level +27 dBu.
- Frequency Response: 10 Hz - 200 kHz +0 ,-3 dB at 61.6 dB gain. DC-coupled with single-order servo-derived high-pass filter.
- Equivalent Input Noise: 10Ω source impedance, 61.6 dB gain, Ein of -141 dBu.
- Noise Density: 0.48nV√Hz, noise figure approximately 1.5 dB. For a 0Ω source the noise density is <0.4nV√Hz. Current noise is typically 4pA√Hz.
- Signal-to-Noise Ratio: With an actual DL-103 connected balanced and terminated in 100Ω the SN relative to 5cm/s level is typically 78 dB A-weighted.
- Distortion: 61.6 dB gain, output +10 dBu into 10KΩ load, CCIF 19/20 kHz 1:1, typically <-110 dBc. (Assembled unit with matched components.)
- Common mode rejection ratio: 61.6 dB gain, CMRR 100 dB typically, (referred to input). (Assembled unit with matched components.)
Note: Noise measurements are for a 20-20 kHz bandwidth and are unweighted unless specified. Maximum level specifications are for ±15V supplies.
Additional audio specifications and performance as they become available.
To learn more about the Flat Moving Coil Preamp, visit the Pro Audio Design Forum construction thread.
The KA-Electronics Monitor Controller is sold as either a bare PC board, a "kit" and the assembled and fully-tested module shown here.
A Mouser Electronics Project Manager bill-of-materials is available for DIY builds.
Flat Moving Coil Phono Preamp With Balanced Input and Balanced Output, Bare PC Board
- Brand: KA Electronics/Rockmore-Labs
- Product Code: FMCP-PCB
- Availability: In Stock