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FS: Balanced "Flat" Phono Preamp


2004-07-18 6:01 am
FS: Balanced "Flat" Phono Preamp (now with working pics!)

Hey Guys

I know the word "flat" doesn't make much (any) sense when talking about a phono preamp, right? Well, I'll let the original designer / builder do some explaining:

This is the Flat Phono Preamp published in the Pro Audio Design Forum. The Flat Phono Preamp is for applications requiring high quality, low-noise, no-nonsense flat/RAW playback for archival recording or vinyl streaming.

As part of the Phono Transfer System, the Flat Phono Preamp provides a non-equalized, balanced input, balanced output phono cartridge interface. When used together with the RIAA EQ/Monitor Switcher, the two boards are a complete turntable playback and workstation monitoring solution. Used stand-alone, the Flat Phono Preamp provides a DAW interface for RAW, un EQ'd archival recording with RIAA EQ applied in software. (We recommend Diamond Cut 8.5 due to its exceptional RIAA frequency and phase accuracy.)

The Flat Phono Preamp board is a fully balanced instrumentation amp designed specifically for direct coupling to moving magnet phono cartridges or moving coil carts using external transformers. The fully-differential input connections are jumper-selectable for either balanced (floating) tonearm wiring using shielded twisted pair cables or unbalanced conventional grounded (RCA) coaxial interconnect. Using either the floating or grounded input configuration common mode rejection is realized on-board using a pair of high-accuracy THAT1240 line receivers in a "double-balanced" cross-coupled output. Jumpers on the Flat Phono Preamp select either 47K or 100KΩ resistive termination with capacitive terminations of 0 to 200 pF in 50 pF steps. Two sets of jumper-selectable gain adjustment are available both ranging from +14 to +40 dB. (Balanced out +20 to +46 dB). The Flat Phono Preamp is fully DC-coupled from cartridge to output owing to its low bias current balanced input and DC common mode rejection. 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 with long low-impedance output signal lines feeding 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.

The Flat Preamp can also be used as a cutterhead feedback coil amplifier.

The Flat Phono preamp may be populated with a wide variety of dual op amps including the bipolar NE5532, NJM2114, NJM2068 and FET input OPA2134. (Assembled PC boards are shipped with NJM2068DD inputs due to their slightly lower noise. Other ICs available upon request.)

Preamp Board Jumper Description

J1, J2, J3 input capacitance.
All open - No added load.
J1 is 50 pF. J1+J2 is 100 pF. J1+J3 is 150 pF. J1+J2+J3 is 200 pF.

J4 is input resistive load. Open is 99.8K, closed can be anything less than 99.8K.
With an added load of 88.7K in parallel (J4 closed) the input resistance is "47K" (46.96K).

J5 is the input configuration: Balanced or RCA coaxial.

When balanced, the input load to ground (common mode) is 49.9K/leg.
When J5 is jumpered unbalanced the "-" input is grounded and the input resistance is "100K" (99.8K). (J4 open. J4 closed 47K.)

There are two preamp gain trims for each channel.

J6 selects which gain adjustment pot is used.
This is for normal gain, typically 5 mV at 5 cm/s, and low gain for very hot, "+12 dB" dub plates.

Digital RIAA is ULTRA accurate and is not subject to shifting that passive components are (due to heat, age, etc). I used this set up to GREAT effect with my Technical SL-1200 and KAB MM cartridge to stream vinyl live with JRiver MC. Works amazingly well. A link to more info:

Pro Audio Design Forum • View topic - Phono Transfer System

The setup includes everything you need to get going including the preamp, PS, and input cable. It cost me ~$600 + shipping - now looking for $400 shipped in the U.S. Now for some pics:



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